The winter solstice is almost upon us: Saturday, December 21, is the shortest day of 2019 for anyone living in the Northern Hemisphere.

If pagan rituals are your thing, this is probably a big moment for you. If not, the official first day of winter is neat for other reasons, too.

Technically, the solstice occurs when the sun is directly over the Tropic of Capricorn, or 23.5° south latitude. In 2019, this will occur at 11:19 pm Eastern time on Saturday.

Below is a short scientific guide to the solstice and the longest night of the year.

1) Why do we have solstices?

The winter and summer solstices, the seasons, and the changing length of daylight hours throughout the year are all due to one fact: Earth spins on a tilted axis.

The tilt — possibly caused by a massive object hitting Earth billions of years ago — means that for half the year, the North Pole is pointed toward the sun (as in the picture below). For the other half of the year, the South Pole gets more light. It’s why we have seasons.

Here’s a time-lapse demonstration of the phenomenon shot over the course of a whole year from space. In the video, you can see how the line separating day from night (called the terminator) swings back and forth from the poles during the year.

And here’s yet another cool way to visualize the seasons. In 2013, a resident of Alberta, Canada, took this pinhole camera photograph of the sun’s path throughout the year and shared it with the astronomy website EarthSky. You can see the dramatic change in the arc of the sun from December to June.

(You can easily make a similar image at home. All you need is a can, photo paper, some tape, and a pin. Instructions here.)

In the Northern Hemisphere, “peak” sunlight usually occurs on June 20, 21 or 22 of any given year. That’s the summer solstice. By contrast, the Southern Hemisphere reaches peak sunlight on December 21, 22 or 23 and the north hits peak darkness — that’s our winter solstice.

In the Northern Hemisphere, the winter solstice occurs when the sun is directly overhead the Tropic of Capricorn, or 23.5° south latitude. Like so:

2) Is the solstice really the first day of winter?

Well, depends: Are you asking a meteorologist or an astronomer?

Meteorologically speaking, summer is defined as the hottest three months of the year, winter is the coldest three months, and the in–between months are spring and fall.

Here’s how NOAA breaks it down:

Meteorological spring includes March, April, and May; meteorological summer includes June, July, and August; meteorological fall includes September, October, and November; and meteorological winter includes December, January, and February.

Astronomically speaking, yes, winter begins on the winter solstice.

3) How many hours of sunlight will I get on the solstice?

That depends on where you live. The farther north from the equator you are, the less sunlight you’ll get during the solstice — and the longer the night will be. Alaska climatologist Brian Brettschneider created this terrific guide for the United States:

Here in Washington, DC, we will get nine hours and 26 minutes of sunlight.

On the off-chance you live near the Arctic Circle, you’ll barely get any daylight during the solstice. Fairbanks, Alaska, for instance, will get three hours and 41 minutes. (If you live north of the circle, you’ll get none at all.) You can check out how much light you’ll get on the solstice on TimeAndDate.com.

The timelapse below shows the eerie scene in Fairbanks — the sun basically skims the horizon for a brief while and then vanishes:

4) Is the winter solstice the earliest sunset of the year?

Not usually. Just because December 21 is the shortest day of the year for the Northern Hemisphere, doesn’t mean every location has its earliest sunset or latest sunrise on that day.

Brettschneider also created this map showing earliest sunset/latest sunrise for different parts of North America:

If you live in DC, you missed the earliest sunset — it happened during the week of December 5. But you can still catch the latest sunrises on January 4, 5, 6, and 7th. If you like sleeping in, that’s arguably the most exciting day of the winter. No annoying sun bothering you in the morning.

5) Is the winter solstice the coldest day of the year?

Not usually! It’s true that the Northern Hemisphere gets the least direct sunlight on the winter solstice (which is officially the first day of winter). But the coldest months are yet to come — usually in January or February, depending on where you live.

A big reason for this “seasonal lag” is that the Earth’s massive oceans absorb much of the sun’s energy and release it slowly, over time, so there’s a delay between when there’s the least sun and when the air temperatures are actually coldest. The same thing happens in summer — there’s a delay between when solar insolation is at its maximum (the summer solstice in June) and when the hottest months are (usually July or August).

This seasonal lag varies greatly from place to place — during the summer, it’s pretty extreme in San Francisco, which is surrounded by water on three sides and where temperatures don’t typically peak until September. Likewise, places far from large bodies of water, like Iowa, can often see sharper swings in temperature than places like Rome that are surrounded by ocean.

6) What does the winter solstice have to do with Stonehenge?

No one knows for sure why Stonehenge was built some 5,000 years ago (at least we don’t, sorry). But one strong possibility is that it was used to mark solstices and equinoxes. That’s because the structure is directly aligned toward the sunset during the winter solstice. (The sun also rises directly over the Heel Stone during the summer solstice.)

Why was the winter solstice a big deal? Here’s Teresa Wilson of the American Astronomical Society to explain: “While the summer solstice draws a larger crowd, the winter solstice may have been more important to the ancient builders. At this time, cattle were slaughtered so the animals did not need to be fed through the winter, and wine and beer made previously had finally fermented.”

Nowadays, humans still gather to pay homage the winter solstice at Stonehenge — they just use modern technology.

At least the winter solstice at Stonehenge seems like a top-rate party:

The Wikipedia entry on Stonehenge is absurdly detailed, so read up on that if you want more. And Vox’s Joss Fong created this video to explain how Stonehenge was (probably) constructed.

7) Is the solstice night the longest night in Earth’s entire history?

Probably not, although it’s close. And the reason is quite interesting. Joseph Stromberg did a fantastic deep dive into this topic for Vox, but here’s the two-minute version.

Ever since the Earth has had liquid oceans and a moon, its rotation has been gradually slowing over time due to tidal friction. That means that over very, very long periods of time, the days have been getting steadily longer. About 4.5 billion years ago, it took the Earth just six hours to complete one rotation. About 350 million years ago, it took 23 hours. Today, of course, it takes about 24 hours. And the days will gradually get longer still.

Given that, you’d think the winter solstice of 2016 would be the longest night in all of history. But while it’s certainly up there, it doesn’t quite take top honors.

That’s because tidal friction isn’t the only thing affecting the Earth’s rotation — there are a few countervailing factors. The melting of glacial ice, which has been occurring since the end of the last ice age 12,000 years ago (and is now ramping up because of global warming) is actually speeding up Earth’s rotation very slightly, shortening the days by a few fractions of a millisecond. Likewise, geologic activity in the Earth’s core, earthquakes, ocean currents, and seasonal wind changes can also speed up or slow down the planet’s rotation.

When you put all these factors together, scientists have estimated that the longest day in Earth’s history (so far) likely occurred back in 1912. That year’s summer solstice was the longest period of daylight the Northern Hemisphere has ever seen. And, conversely, the 1912 winter solstice was the longest night we’ve ever seen.

Eventually, the effects of tidal friction should overcome all those other factors, and Earth’s days will get longer and longer as the planet’s rotation keeps slowing (forcing timekeepers to add leap seconds to the calendar periodically). Which means that in the future, there will be plenty of winter solstices that set new records as the “longest night in Earth’s history.”

8) Are there solstices on other planets?

Yes! All the planets in our solar system rotate on a tilted axis and therefore have seasons, solstices, and equinoxes. Some of these tilts are minor (like Mercury, which is tilted at 2.11 degrees). But others are more like the Earth (23.5 degrees) or are even more extreme (Uranus is tilted 98 degrees!).

Below, see a beautiful composite image of Saturn on its equinox captured by the Cassini spacecraft (RIP) in 2009. The gas giant is tilted 27 degrees relative to the sun, and equinoxes on the planet are less frequent than on Earth. Saturn only sees an equinox about once every 15 years (because it takes Saturn 29 years to complete one orbit around the sun).

9) Ugh, I hate winter. Is there anything about the season for me to enjoy?

Yes, winter is cold and dark. But at least, it gives us beautiful sunsets.

Sunsets are, in fact, better during the colder months of the year. NOAA meteorologist Stephen Corfidi says peak sunset season for the middle latitudes (think the northeastern United States) is November through February, and it has to do with the confluence of a few meteorological factors. Namely: cold, non-humid air is clearer than the hot muggy air of summertime, which allows sunset colors to reach our eyes undiluted.

Making things even better in the winter: A lot of the air we’re breathing has descended down from the Arctic, where there are fewer trees and industrial activities to produce aerosols (which make the sky hazy). So not only is the air clearer due to low humidity, it’s also a bit clearer to begin with.

One last bonus: As we approach the winter solstice, the time the sun takes to set lengthens, due to the angle the sun takes in setting into the ground. During the equinoxes, the sun pretty much sinks into the ground at a 90-degree angle. Nearer the winter solstice, the sun sets on more of an angle, drawing out the time it takes to set. Which is to say: Sunset colors linger closer to the winter solstice, which allows us to enjoy them for longer.

10) I clicked this article accidentally and really just want a cool picture of the sun

We got you:

The image above was taken by NASA’s Solar Dynamics Observatory, a spacecraft launched in 2010 to better understand the sun.

In 2018, NASA launched the Parker Solar Probe, a spacecraft that will come within 4 million miles of the surface of the sun (much closer than any spacecraft has been before). The goal is to study the sun’s atmosphere, weather, and magnetism and figure out the mystery of why the sun’s corona (its atmosphere) is much hotter than its surface. Still, even several million miles away, the probe will have to withstand temperatures of 2,500 degrees Fahrenheit.

Happy solstice!

This explainer was updated by Umair Irfan in December 2018 and draws heavily from a card stack written by Brad Plumer in 2015. Brian Resnick contributed the section on the Paris climate accord in 2017.

There’s a vast and growing gap between the urgency to fight climate change and the policies needed to combat it.

In 2018, the United Nations’ Intergovernmental Panel on Climate Change found that it is possible to limit global warming to 1.5 degrees Celsius this century, but the world may have as little as 12 years left to act. The US government’s National Climate Assessment, with input from NASA, the Environmental Protection Agency, and the Pentagon, also reported that the consequences of climate change are already here, ranging from nuisance flooding to the spread of mosquito-borne viruses into what were once colder climates. Left unchecked, warming will cost the US economy hundreds of billions of dollars.

However, these facts have failed to register with the Trump administration, which is actively pushing policies that will increase the emissions of heat-trapping gases.

Ever since he took office, President Donald Trump has rejected or undermined President Barack Obama’s signature climate achievements: the Paris climate agreement; the Clean Power Plan, the main domestic policy for limiting greenhouse gas emissions; and fuel economy standards, which target transportation, the largest US source of greenhouse gases.

At the same time, the Trump administration has aggressively boosted fossil fuels: opening unprecedented swaths of public lands to mining and drilling, attempting to bail out foundering coal power plants, and promoting hydrocarbon exploitation at climate change conferences.

Trump has also appointed climate change skeptics to key positions. Quietly, officials at these and other science agencies have been removing the words “climate change” from government websites and press releases.

Yet the evidence for humanity’s role in changing the climate continues to mount, and its consequences are increasingly difficult to ignore. Atmospheric carbon dioxide concentrations now top 408 parts per million, a threshold the planet hasn’t seen in millions of years. Greenhouse gas emissions reached a record high in 2018. Disasters worsened by climate change have taken hundreds of lives, destroyed thousands of homes, and cost billions of dollars.

The big questions now are how these ongoing changes in the climate will reverberate throughout the rest of the world, and what we should do about them. The answers bridge decades of research across geology, economics, and social science, which have been confounded by uncertainty and obscured by jargon. That’s why it can be a bit daunting to join the discussion for the first time, or to revisit the conversation after a hiatus.

To help, we’ve provided answers to some fundamental questions about climate change you may have been afraid to ask.

1) What is global warming?

In short: The world is getting hotter, and humans are responsible.

Yes, the planet’s temperature has changed before, but it’s the rise in average temperature of the Earth’s climate system since the late 19th century, the dawn of the Industrial Revolution, that’s important here. Temperatures over land and ocean have gone up 0.8° to 1° Celsius (1.4° to 1.8° Fahrenheit), on average, in that span:

Many people use the term “climate change” to describe this rise in temperatures and the associated effects on the Earth’s climate. (The shift from the term “global warming” to “climate change” was also part of a deliberate messaging effort by a Republican pollster to undermine support for environmental regulations.)

Like detectives solving a murder, climate scientists have found humanity’s fingerprints all over the planet’s warming, with the overwhelming majority of the evidence pointing to the extra greenhouse gases humans have put into the atmosphere by burning fossil fuels. Greenhouse gases like carbon dioxide trap heat at the Earth’s surface, preventing that heat from escaping back out into space too quickly. When we burn coal, natural gas, or oil for energy, or when we cut down forests that usually soak up greenhouse gases, we add even more carbon dioxide to the atmosphere, so the planet warms up.

Global warming also refers to what scientists think will happen in the future if humans keep adding greenhouse gases to the atmosphere.

Though there is a steady stream of new studies on climate change, one of the most robust aggregations of the science remains the Intergovernmental Panel on Climate Change’s fifth assessment report from 2013. The IPCC is convened by the United Nations, and the report draws on more than 800 expert authors. It projects that temperatures could rise at least 2°C (3.6°F) by the end of the century under many plausible scenarios — and possibly 4°C or more. A more recent study by scientists in the United Kingdom found a narrower range of expected temperatures if atmospheric carbon dioxide doubled, rising between 2.2°C and 3.4°C.

Many experts consider 2°C of warming to be unacceptably high, increasing the risk of deadly heat waves, droughts, flooding, and extinctions. Rising temperatures will drive up global sea levels as the world’s glaciers and ice sheets melt. Further global warming could affect everything from our ability to grow food to the spread of disease.

That’s why the IPCC put out another report in 2018 comparing 2°C of warming to a scenario with 1.5°C of warming. The researchers found that this half-degree difference is actually pretty important, since every bit of warming matters. Between the two outlooks, less warming means fewer people will have to move from coastal areas, natural weather events will be less severe, and economies will take a smaller hit.

However, limiting warming would likely require a complete overhaul of our energy system. Fossil fuels currently provide just over 80 percent of the world’s energy. To zero out emissions this century, we’d have to replace most of that with low-carbon sources like wind, solar, nuclear, geothermal, or carbon capture.

Beyond that, we may have to electrify everything that uses energy and start pulling greenhouse gases straight from the air. And to get on track for 1.5°C of warming, the world would have to halve greenhouse gas emissions from current levels by 2030.

That’s a staggering task, and there are huge technological and political hurdles standing in the way. As such, the world’s nations have been slow to act on global warming — many of the existing targets for curbing greenhouse gas emissions are too weak, yet many countries are falling short of even these modest goals.

2) How do we know global warming is real?

The simplest way is through temperature measurements. Agencies in the United States, Europe, and Japan have independently analyzed historical temperature data and reached the same conclusion: The Earth’s average surface temperature has risen roughly 0.8° Celsius (1.4° Fahrenheit) since the early 20th century.

But that’s not the only clue. Scientists have also noted that glaciers and ice sheets around the world are melting. Satellite observations since the 1970s have shown warming in the lower atmosphere. There’s more heat in the ocean, causing water to expand and sea levels to rise. Plants are flowering earlier in many parts of the world. There’s more humidity in the atmosphere. Here’s a summary from the National Oceanic and Atmospheric Administration:

These are all signs that the Earth really is getting warmer — and that it’s not just a glitch in the thermometers. That explains why climate scientists say things like, “Warming in the climate system is unequivocal.” They’re really confident about this one.

3) How do we know humans are causing global warming?

Climate scientists say they are more than 95 percent certain that human influence has been the dominant cause of global warming since 1950. They’re about as sure of this as they are that cigarette smoke causes cancer.

Why are they so confident? In part because they have a good grasp of how greenhouse gases can warm the planet, in part because the theory fits the available evidence, and in part because alternate theories have been ruled out. Let’s break it down in six steps:

1) Scientists have long known that greenhouse gases in the atmosphere — such as carbon dioxide, methane, or water vapor — absorb certain frequencies of infrared radiation and scatter them back toward the Earth. These gases essentially prevent heat from escaping too quickly back into space, trapping that radiation at the surface and keeping the planet warm.

2) Climate scientists also know that concentrations of greenhouse gases in the atmosphere have grown significantly since the Industrial Revolution. Carbon dioxide has risen 45 percent. Methane has risen more than 200 percent. Through some relatively straightforward chemistry and physics, scientists can trace these increases to human activities like burning oil, gas, and coal.

3) So it stands to reason that more greenhouse gases would lead to more heat. And indeed, satellite measurements have shown that less infrared radiation is escaping out into space over time and instead returning to the Earth’s surface. That’s strong evidence that the greenhouse effect is increasing.

4) There are other human fingerprints that suggest increased greenhouse gases are warming the planet. For instance, back in the 1960s, simple climate models predicted that global warming caused by more carbon dioxide would lead to cooling in the upper atmosphere (because the heat is getting trapped at the surface). Later satellite measurements confirmed exactly that. Here are a few other similar predictions that have also been confirmed.

5) Meanwhile, climate scientists have ruled out other explanations for the rise in average temperatures over the past century. To take one example: Solar activity can shift from year to year, affecting the Earth’s climate. But satellite data shows that total solar irradiance has declined slightly in the past 35 years, even as the Earth has warmed.

6) More recent calculations have shown that it’s impossible to explain the temperature rise we’ve seen in the past century without taking the increase in carbon dioxide and other greenhouse gases into account. Natural causes, like the sun or volcanoes, have an influence, but they’re not sufficient by themselves.

Ultimately, the Intergovernmental Panel on Climate Change concluded that most of the warming since 1951 has been due to human activities. The Earth’s climate can certainly fluctuate from year to year due to natural forces (including oscillations in the Pacific Ocean, such as El Niño). But greenhouse gases are driving the larger upward trend in temperatures.

And as the Climate Science Special Report, released by 13 US federal agencies in November 2017, put it, “For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.”

More: This chart breaks down all the different factors affecting the Earth’s average temperature. And there’s much more detail in the IPCC’s report, particularly this section and this one.

4) How has global warming affected the world so far?

Here’s a list of ongoing changes that climate scientists have concluded are likely linked to global warming, as detailed by the IPCC here and here.

Higher temperatures: Every continent has warmed substantially since the 1950s. There are more hot days and fewer cold days, on average, and the hot days are hotter.

Heavier storms and floods: The world’s atmosphere can hold more moisture as it warms. As a result, the overall number of heavier storms has increased since the mid-20th century, particularly in North America and Europe (though there’s plenty of regional variation). Scientists reported in December that at least 18 percent of Hurricane Harvey’s record-setting rainfall over Houston in August was due to climate change.

Heat waves: Heat waves have become longer and more frequent around the world over the past 50 years, particularly in Europe, Asia, and Australia.

Shrinking sea ice: The extent of sea ice in the Arctic, always at its maximum in winter, has shrunk since 1979, by 3.3 percent per decade. Summer sea ice has dwindled even more rapidly, by 13.2 percent per decade. Antarctica has seen recent years with record growth in sea ice, but it’s a very different environment than the Arctic, and the losses in the north far exceed any gains at the South Pole, so total global sea ice is on the decline:

Shrinking glaciers and ice sheets: Glaciers around the world have, on average, been losing ice since the 1970s. In some areas, that is reducing the amount of available freshwater. The ice sheet on Greenland, which would raise global sea levels by 25 feet if it all melted, is declining, with some sections experiencing a sudden surge in the melt rate. The Antarctic ice sheet is also getting smaller, but at a much slower rate.

Sea level rise: Global sea levels rose 9.8 inches (25 centimeters) in the 19th and 20th centuries, after 2,000 years of relatively little change, and the pace is speeding up. Sea level rise is caused by both the thermal expansion of the oceans — as water warms up, it expands — and the melting of glaciers and ice sheets (but not sea ice).

Food supply: A hotter climate can be both good for crops (it lengthens the growing season, and more carbon dioxide can increase photosynthesis) and bad for crops (excess heat can damage plants). The IPCC found that global warming was currently benefiting crops in some high-latitude areas but that negative effects are becoming increasingly common worldwide. In areas like California, crop yields are estimated to decline 40 percent by 2050.

Shifting species: Many land and marine species have had to shift their geographic ranges in response to warmer temperatures. So far, several extinctions have been linked to global warming, such as certain frog species in Central America.

Warmer winters: In general, winters are warming faster than summers. Average low temperatures are rising all over the world. In some cases, these temperatures are climbing above the freezing point of water. We’re already seeing massive declines in snow accumulation in the United States, which can paradoxically increase flood, drought, and wildfire risk — as water that would ordinarily dispatch slowly over the course of a season instead flows through a region all at once.

Debated impacts

Here are a few other ways the Earth’s climate has been changing — but scientists are still debating whether and how they’re linked to global warming:

Droughts have become more frequent and more intense in some parts of the world — such as the American Southwest, Mediterranean Europe, and West Africa — though it’s hard to identify a clear global trend. In other parts of the world, such as the Midwestern United States and Northwestern Australia, droughts appear to have become less frequent. A recent study shows that, globally, the time between droughts is shrinking and more areas are affected by drought and taking longer to recover from them.

Hurricanes have clearly become more intense in the North Atlantic Ocean since 1970, the IPCC says. But it’s less clear whether global warming is driving this. 2017 was an exceptionally bad year for Atlantic hurricanes in terms of strength and damage. And while scientists are still uncertain whether they were a fluke or part of a trend, they are warning we should treat it as a baseline year. There doesn’t yet seem to be any clear trajectory for tropical cyclones worldwide.

5) What impacts will global warming have in the future?

It depends on how much the planet actually heats up. The changes associated with 4° Celsius (or 7.2° Fahrenheit) of warming are expected to be more dramatic than the changes associated with 2°C of warming.

Here’s a basic rundown of big impacts we can expect if global warming continues, via the IPCC (here and here).

Hotter temperatures: If emissions keep rising unchecked, then global average surface temperatures will be at least 2°C higher (3.6°F) than preindustrial levels by 2100 — and possibly 3°C or 4°C or more.

Higher sea level rise: The expert consensus is that global sea levels will rise somewhere between 0.2 and 2 meters by the end of the century if global warming continues unchecked (that’s between 0.6 and 6.6 feet). That’s a wide range, reflecting some of the uncertainties scientists have in how ice will melt. In specific regions like the Eastern United States, sea level rise could be even higher, and around the world, the rate of rise is accelerating.

Heat waves: A hotter planet will mean more frequent and severe heat waves.

Droughts and floods: Across the globe, wet seasons are expected to become wetter, and dry seasons drier. As the IPCC puts it, the world will see “more intense downpours, leading to more floods, yet longer dry periods between rain events, leading to more drought.”

Hurricanes: It’s not yet clear what impact global warming will have on tropical cyclones. The IPCC said it was likely that tropical cyclones would get stronger as the oceans heat up, with faster winds and heavier rainfall. But the overall number of hurricanes in many regions was likely to “either decrease or remain essentially unchanged.”

Heavier storm surges: Higher sea levels will increase the risk of storm surges and flooding when storms do hit.

Agriculture: In many parts of the world, the mix of increased heat and drought is expected to make food production more difficult. The IPCC concluded that global warming of 1°C or more could start hurting crop yields for wheat, corn, and rice by the 2030s, especially in the tropics. (This wouldn’t be uniform, however; some crops may benefit from mild warming, such as winter wheat in the United States.)

Extinctions: As the world warms, many plant and animal species will need to shift habitats at a rapid rate to maintain their current conditions. Some species will be able to keep up; others likely won’t. The Great Barrier Reef, for instance, may not be able to recover from major recent bleaching events linked to climate change. The National Research Council has estimated that a mass extinction event “could conceivably occur before the year 2100.”

Long-term changes: Most of the projected changes above will occur in the 21st century. But temperatures will keep rising after that if greenhouse gas levels aren’t stabilized. That increases the risk of more drastic longer-term shifts. One example: If West Antarctica’s ice sheet started crumbling, that could push sea levels up significantly. The National Research Council in 2013 deemed many of these rapid climate surprises unlikely this century but a real possibility further into the future.

6) What happens if the world heats up more drastically — say, 4°C?

The risks of climate change would rise considerably if temperatures rose 4° Celsius (7.2° Fahrenheit) above preindustrial levels — something that’s possible if greenhouse gas emissions keep rising at their current rate.

The IPCC says 4°C of global warming could lead to “substantial species extinctions,” “large risks to global and regional food security,” and the risk of irreversibly destabilizing Greenland’s massive ice sheet.

One huge concern is food production: A growing number of studies suggest it would become significantly more difficult for the world to grow food with 3°C or 4°C of global warming. Countries like Bangladesh, Egypt, Vietnam, and parts of Africa could see large tracts of farmland turn unusable due to rising seas. Scientists are also concerned about crops getting less nutritious due to rising CO2.

Humans could struggle to adapt to these conditions. Many people might think the impacts of 4°C of warming will simply be twice as bad as those of 2°C. But as a 2013 World Bank report argued, that’s not necessarily true. Impacts may interact with each other in unpredictable ways. Current agriculture models, for instance, don’t have a good sense of what will happen to crops if increased heat waves, droughts, new pests and diseases, and other changes all start to combine.

“Given that uncertainty remains about the full nature and scale of impacts,” the World Bank report said, “there is also no certainty that adaptation to a 4°C world is possible.” Its conclusion was blunt: “The projected 4°C warming simply must not be allowed to occur.”

7) What do climate models say about the warming that could actually happen in the coming decades?

That depends on your faith in humanity.

Climate models depend on not only complicated physics but the intricacies of human behavior over the entire planet.

Generally, the more greenhouse gases humanity pumps into the atmosphere, the warmer it will get. But scientists aren’t certain how sensitive the global climate system is to increases in greenhouse gases. And just how much we might emit over the coming decades remains an open question, depending on advances in technology and international efforts to cut emissions.

The IPCC groups these scenarios into four categories of atmospheric greenhouse gas concentrations known as Representative Concentration Pathways. They serve as standard benchmarks for evaluating climate models, but they also have some assumptions baked in.

RCP 2.6, also called RCP 3PD, is the scenario with very low greenhouse gas concentrations in the atmosphere. It bets on declining oil use, a population of 9 billion by 2100, increasing energy efficiency, and emissions holding steady until 2020, at which point they’ll decline and even go negative by 2100. This is, to put it mildly, very optimistic.

The next tier up is RCP 4.5, which still banks on ambitious reductions in emissions but anticipates an inflection point in the emissions rate around 2040. RCP 6 expects emissions to increase 75 percent above today’s levels before peaking and declining around 2060 as the world continues to rely heavily on fossil fuels.

The highest tier, RCP 8.5, is the pessimistic business-as-usual scenario, anticipating no policy changes nor any technological advances. It expects a global population of 12 billion and triple the rate of carbon dioxide emissions compared to today by 2100.

Here’s how greenhouse gas emissions under each scenario stack up next to each other:

And here’s what that means for global average temperatures, assuming that a doubling of carbon dioxide concentrations in the atmosphere leads to 3°C of warming:

As you can see, RCP 3PD is the only trajectory that keeps the planet below 2°C of warming. Recall what it would take to keep emissions in line with this pathway and you’ll understand the enormity of the challenge of meeting this goal.

8) How do we stop global warming?

The world’s nations would need to cut their greenhouse gas emissions by a lot. And even that wouldn’t stop all global warming.

For example, let’s say we wanted to limit global warming to below 2°C. To do that, the IPCC has calculated that annual greenhouse gas emissions would need to drop at least 40 to 70 percent by midcentury.

Emissions would then have to keep falling until humans were hardly emitting any extra greenhouse gases by the end of the century. We’d also have to remove carbon dioxide from the atmosphere.

Cutting emissions that sharply is a daunting task. Right now, the world gets 87 percent of its primary energy from fossil fuels: oil, gas, and coal. By contrast, just 13 percent of the world’s primary energy is “low carbon”: a little bit of wind and solar power, some nuclear power plants, a bunch of hydroelectric dams. That’s one reason global emissions keep rising each year.

To stay below 2°C, that would all need to change radically. By 2050, the IPCC notes, the world would need to triple or even quadruple the share of clean energy it uses — and keep scaling it up thereafter. Second, we’d have to get dramatically more efficient at using energy in our homes, buildings, and cars. And stop cutting down forests. And reduce emissions from agriculture and from industrial processes like cement manufacturing.

The IPCC also notes that this task becomes even more difficult the longer we put it off, because carbon dioxide and other greenhouse gases will keep piling up in the atmosphere in the meantime, and the cuts necessary to stay below the 2°C limit become more severe.

9) What are we actually doing to fight climate change?

A global problem requires global action, but with climate change, there is a yawning gap between ambition and action.

The main international effort is the 2015 Paris climate accord, of which the United States is the only country in the world that wants out. The deal was hammered out over weeks of tense negotiations and weighs in at 31 pages. What it does is actually pretty simple.

The backbone is the global target of keeping global average temperatures from rising 2°C (compared to temperatures before the Industrial Revolution) by the end of the century. Beyond 2 degrees, we risk dramatically higher seas, changes in weather patterns, food and water crises, and an overall more hostile world.

Critics have argued that the 2-degree mark is arbitrary, or even too low, to make a difference. But it’s a starting point, a goal that, before Paris, the world was on track to wildly miss.

Paris is voluntary

To accomplish this 2-degree goal, the accord states that countries should strive to reach peak emissions “as soon as possible.” (Currently, we’re on track to hit peak emissions around 2030 or later, which will likely be too late.)

But the agreement doesn’t detail exactly how these countries should do that. Instead, it provides a framework for getting momentum going on greenhouse gas reduction, with some oversight and accountability. For the US, the pledge involves 26 to 28 percent reductions by 2025. (Under Trump’s current policies, that goal is impossible.)

There’s also no defined punishment for breaking it. The idea is to create a culture of accountability (and maybe some peer pressure) to get countries to step up their climate game.

In 2020, delegates are supposed to reconvene and provide updates about their emission pledges and report on how they’re becoming more aggressive on accomplishing the 2-degree goal.

However, many countries are already falling behind on their climate change commitments, and some, like Germany, are giving up on their near-term targets.

Paris asks richer countries to help out poorer countries

There’s a fundamental inequality when it comes to global emissions. Rich countries have plundered and burned huge amounts of fossil fuels and gotten rich from them. Poor countries seeking to grow their economies are now being admonished for using the same fuels. Many low-lying poor countries also will be among the first to bear the worst impacts of climate change.

The main vehicle for rectifying this is the Green Climate Fund, via which richer countries, like the US, are supposed to send $100 billion a year in aid and financing by 2020 to the poorer countries. The United States’ share was $3 billion, but with President Trump’s decision to withdraw from the Paris accord, this goal is unlikely to be met.

The agreement matters because we absolutely need momentum on this issue

The Paris agreement is largely symbolic, and it will live on even though Trump is aiming to pull the US out. But, as Jim Tankersley wrote for Vox, “the accord will be weakened, and, much more importantly, so will the fragile international coalition” around climate change.

We’re already seeing the Paris agreement lose steam. At a follow-up climate meeting this year in Katowice, Poland, negotiators forged an agreement on measuring and verifying their progress in cutting greenhouse gases, but left many critical questions of how to achieve these reductions unanswered.

But the Paris accord isn’t the only international climate policy game in town

There are regional international climate efforts like the European Union’s Emissions Trading System. However, the most effective global policy at keeping warming in check to date doesn’t have to do with climate change, at least on the surface.

The 1987 Montreal Protocol, which was convened by countries to halt the destruction of the ozone layer, had a major side effect of averting warming. In fact, it’s been the single most effective effort humanity has undertaken to fight climate change. Since many of the substances that eat away at the ozone layer are potent heat-trappers, limiting emissions of gases like chlorofluorocarbons has an outsize effect.

And the Trump administration doesn’t appear as hostile to Montreal as it does to Paris. The White House may send the 2016 Kigali Amendment to the Montreal Protocol to the Senate for ratification, giving the new regulations the force of law. If implemented, the amendment would avert 0.5°C of warming by 2100.

Regardless of what path we choose, the key thing to remember is that we are going to pay for climate change one way or another. We have the opportunity now to address warming on our own terms, with investments in clean energy, moving people away from disaster-prone areas, and regulating greenhouse gas emissions. Otherwise, we’ll pay through diminished crop harvests, inundated coastlines, destroyed homes, lost lives, and an increasingly unlivable planet. Ignoring or stalling on climate change chooses the latter option by default. Our choices do matter, but we’re running out of time to make them.

Further reading:

Avoiding catastrophic climate change isn’t impossible yet. Just incredibly hard.

Reckoning with climate change will demand ugly tradeoffs from environmentalists — and everyone else

Show this cartoon to anyone who doubts we need huge action on climate change

It’s time to start talking about “negative” carbon dioxide emissions

A history of the 2°C global warming target 

Scientists made a detailed “roadmap” for meeting the Paris climate goals. It’s eye-opening.

On Tuesday, the Environmental Protection Agency and the US Army Corps of Engineers presented the Trump administration’s proposal to undo a major Obama-era environmental regulation, the Clean Water Rule.

The rule defined the “Waters of the United States,” a.k.a. WOTUS. These are the rivers, streams, and lakes that fall under federal jurisdiction and forms the foundation of a massive piece of environmental regulation, the Clean Water Act.

The Obama rule, first published in 2015, was meant to clarify which streams and wetlands fall under federal clean water protections — a question that had been causing legal frustration for years.

But it triggered fierce blowback from farm and industry groups across the country. “Opponents condemn it as a massive power grab by Washington,” Politico reported, “saying it will give bureaucrats carte blanche to swoop in and penalize landowners every time a cow walks through a ditch.” Some of those criticisms were overblown (it doesn’t cover puddles, for instance), but the rule was widely cited by conservatives as a perfect example of EPA overreach under President Obama.

Last year, President Trump signed an executive order directing the EPA to begin the long process of repealing the Clean Water Rule and replacing it with … something else.

Earlier this year, the EPA suspended the Obama rule. And on Tuesday, the agency revealed its replacement, one it said will smooth over the problems that made the 2015 regulation so contentious.

“For the first time, we are clearly defining the difference between federally protected waterways and state protected waterways,” said EPA acting Administrator Andrew Wheeler in a press release. “Our simpler and clearer definition would help landowners understand whether a project on their property will require a federal permit or not, without spending thousands of dollars on engineering and legal professionals.” 

However, environmental groups said the new definition would cut the number of waterways the federal government must regulate, leaving them vulnerable to pollution. It excludes, for example, waterways that flow only for parts of the year, like after rainstorms while snow melts.

“This sickening gift to polluters will result in more dangerous toxic pollution dumped into waterways across a vast stretch of America,” said Brett Hartl, government affairs director at the Center for Biological Diversity, in a statement. “The Trump administration’s radical proposal would destroy millions of acres of wetlands, pushing imperiled species like steelhead trout closer to extinction.”

EPA officials said they didn’t know just how many waterways would be excluded from federal jurisdiction under the new proposal. However, a document obtained by E&E News showed that the EPA and the Army Corps had estimated last year that 18 percent of streams and 51 percent of wetlands would not receive federal protections under the revisions.

Tuesday’s announcement is only the beginning of a long regulatory and legal process. The EPA is taking comments on the proposal for 60 days and will host a listening session in Kansas City, Kansas, in January. In the meantime, it’s worth understanding how WOTUS became so controversial and what it means for huge swaths of the country. Here’s what you need to know.

What the Waters of the US rule actually does

To understand this rule, we need to go back to 1972, when Congress passed the Clean Water Act. That law features dozens of regulations and permitting requirements for anyone discharging pollution into the “waters of the United States” in a way that could affect human health or aquatic life. These rules apply to factories, power plants, golf courses, new housing developments — and much, much more.

For example, under the law, a facility storing oil that could leak needs to prepare a spill prevention plan aimed at minimizing discharges. If the facility is far away from any “waters of the United States,” however, it doesn’t face these requirements.

But here’s the tricky part. The Clean Water Act doesn’t precisely define what “waters of the United States” means. That’s left to the EPA and the Army Corps of Engineers. And it’s a hard question. For instance, the law is clear that major navigable rivers and lakes and any connected waterways should be protected. That includes major rivers like the Mississippi River, the Colorado River, and the Ohio River. But what about waterways that are only loosely connected? What about the 60 percent of streams that are dry for part of the year but then connect when it rains? Any pollution dumped into those waters could affect key ecosystems. Should they be regulated?

In the 2000s, this uncertainty led to a pair of Supreme Court decisions that only ended up creating more bewilderment. In a split decision in Rapanos v. United States in 2006, Justice Anthony Kennedy argued that Clean Water Act protections applied to wetlands that “significantly affect the chemical, physical, and biological integrity of other covered waters.” But Justice Antonin Scalia argued that protections only applied to wetlands “with a continuous surface connection” to navigable water — a far smaller number of wetlands. And it wasn’t totally clear which opinion took precedence.

“The short answer is that the state of post-Rapanos wetlands jurisdiction is a mess,” Richard Frank of the University of California Davis told Greenwire in 2011. In the ensuing years, whenever a dispute arose over whether a landowner — be it a housing developer, a golf course, a farm, or what have you — needed a Clean Water Act permit or not, courts had to resolve it on a case-by-case basis.

So under Obama, the EPA and Army Corps of Engineers tried to bring clarity to the matter. They sifted through more than 1,200 scientific papers to figure out which types of bodies of water were important to aquatic ecosystems and therefore deserved protection, per Kennedy’s opinion.

The final Waters of the US rule, published in June 2015, outlined which bodies of water were automatically covered by the Clean Water Act — requiring permits for discharges or dredging or dirt fill — and which ones still needed to be dealt with on a case-by-case basis. For instance:

• In the past, tributaries of navigable rivers were evaluated on a case-by-case basis. But under the new rule, they’re automatically protected if they have a bed, a bank, and a high-water mark. This includes many streams that are dry for part of the year. Waterways without these features are still dealt with case by case.
• Wetlands and ponds are now automatically covered if they’re within 100 feet or within the 100-year floodplain of a protected waterway. Otherwise, it’s case by case.
• Certain “isolated” waters that are not connected to navigable waters now get automatic protection if they have a “significant nexus” to protected waters — like the vernal pools of California.

The rule also explicitly exempted a number of bodies of water often found on farms, such as puddles, ditches, artificial ponds for livestock watering, and irrigation systems that would revert to dry land if irrigation were to stop. Here’s a graphic:

For its part, the EPA argued that this rule didn’t significantly expand the waters under its jurisdiction. Rather, it created more certainty for about 3 percent of the nation’s waterways — to avoid bringing cases to court every time there was a legal gray area. According to the EPA, the rule offered clearer protection to upstream bodies of water that contribute to drinking supplies for one-third of the population.

Before the rule came out, few who worked on it expected widespread blowback. “This rule will provide the clarity and certainty businesses and industry need about which waters are protected by the Clean Water Act,” Obama said when the final rule was announced. But things turned out very differently.

Why the Waters of the US rule became so controversial

Opponents of the rule — particularly farming and ranching groups — clearly didn’t buy the EPA’s line that this was only a technical update. Nor were they comforted by the EPA’s exemptions for agriculture. Instead, they called it a power grab.

“The agency is making it impossible for farmers and ranchers to look at their land and know what can be regulated,” argues the American Farm Bureau Federation on its site. “EPA has vastly expanded its authority beyond the limits approved by Congress and affirmed by the U.S. Supreme Court.”

Some Western farmers, for instance, fretted about the open, unlined canals they use to irrigate their lands during the growing season. These systems divert water from streams, serve as water sources for wildlife, and can connect to larger bodies of water elsewhere. As Reagan Waskom and David Cooper of Colorado State University explain, farmers and ranchers feared that these canals would fall under the rule’s definition of “tributary” and might have to be replaced by costly pressurized pipes. Or, alternatively, that fertilizer use near these waterways would be more strictly regulated.

Defenders of the rule dismissed these scenarios. Jon Devine, a lawyer with the Natural Resources Defense Council, pointed out that the Clean Water Act has always regulated agriculture lightly. “This rule doesn’t really change those exemptions,” he says. Indeed, one recent study found that the EPA’s jurisdiction over farms actually shrank under the new rule.

The EPA was also pretty explicit that it wouldn’t target farmers. “We will protect clean water without getting in the way of farming and ranching,” then-EPA Administrator Gina McCarthy told the National Farmers Union in 2015. But few farmers or ranchers believed her. Their argument was that the rule was vague enough that the EPA could crack down on them if it chose. It’s basically a question of trust. And at the moment, conservatives are not particularly inclined to trust the EPA.

Joni Ernst, a Republican senator from Iowa, made that clear in former EPA Administrator Scott Pruitt’s confirmation hearing in 2017. “My constituents tell me the EPA is out to get them rather than work with them and there is a huge lack of trust between many of my constituents and the EPA,” she said. “If we take a look specifically at the WOTUS rule, Iowans truly feel that the EPA ignored their comments and concerns, threw them under the rug and then just moved forward.”

However the backlash started, it took on a life of its own. Trump began citing the water rule on the campaign trail as an example of EPA overreach, earning cheers from rural audiences. In signing his executive order in March, he called it a “destructive and horrible rule.”

Why it will be difficult — but not impossible — for the EPA to undo the Obama-era WOTUS rule completely

Pruitt delayed the Obama WOTUS rule’s implementation for two years, buying the EPA time to come up with an alternative.

But implementing a new definition for WOTUS requires proposing a new rule that’s supported by extensive scientific and legal arguments, opening up the proposal for public comments, responding to those comments, and then defending the final rule in court as a superior approach. This could take years.

And much of the ambiguity around which waterways deserve Clean Water Act protection still holds even if you repeal the Obama rule. Which wetlands are covered? How do you deal with streams that flow part of the year? How do you interpret that mess of a Supreme Court decision in 2006?

In his executive order, Trump asked the EPA to consider Scalia’s opinion in Rapanos, which extended protection to wetlands only if they had a “continuous surface connection” to navigable waterways and extended protection to streams only if they were “relatively permanent.” So it’s not surprising that acting EPA Administrator Wheeler’s replacement rule would cover far fewer waterways — leaving out, for instance, many of the 60 percent of streams that don’t flow year-round.

Environmental groups say that’s a problem. Devine argues that polluters could take advantage of a weaker rule with less certain protection for streams and waterways. As long as there’s ambiguity about where the Clean Water Act applies, it would be harder for citizen groups or the Department of Justice to bring a case against companies dumping chemicals or other pollutants into smaller bodies of water upstream.

“Without this rule, enforcement has been unpredictable,” Devine says. “The EPA has mainly been focused on big rivers and lakes so that they wouldn’t have to litigate to the ends of the earth about whether the Clean Water Act applied to waters upstream. But if you can only regulate the biggest rivers and lakes — and the pollution problem is much farther upstream — then you’re not effectively protecting the receiving water or the watershed.”

Still, it’s not clear that the EPA can scale back the water rule significantly. Federal courts have typically embraced Kennedy’s more expansive interpretation of the Clean Water Act rather than Scalia’s, and any rollback of Obama’s rule would still leave plenty of legal gray areas where the courts will need to decide on a case-by-case basis whether the Clean Water Act applies. “It’s going to be incredibly complex to figure this out,” says Richard Revesz, a professor of environmental law at New York University.

Further reading:

• Repealing the clean water rule is only step one for Trump. He’s also targeting Obama’s signature climate policy, the Clean Power Plan. Read here for more on how he might do that.
• The EPA is also rolling back restrictions on major sources of toxic air pollution.

Thursday at 3 pm, President Trump will announce whether or not the United States should stay in the Paris climate deal — the key international treaty to tackle global warming.

Update: President Trump pulls US out of the Paris climate agreement

The stakes are high: The Paris accord is a nonbinding treaty that will depend on persuasion and cooperation to succeed. Nearly every country on Earth has submitted a voluntary (if woefully inadequate) plan to restrain its greenhouse gas emissions, and the 2015 Paris deal created a formal process by which leaders could help one another ratchet up ambitions over time and push for stronger action at future meetings. If the world’s most powerful country steps back, that entire architecture could erode.

At the moment, many onlookers are betting Trump will pull out of Paris. But some of Trump’s advisers, like Secretary of State Rex Tillerson, have warned of severe diplomatic blowback if America pulls out. And a variety of coal, oil, and gas companies have urged Trump to stay in, noting that there’s no real harm in sticking with a nonbinding treaty and that it’d be better to try to shape global climate negotiations from within.

During the campaign, Trump repeatedly vowed to pull out, and key advisers like Stephen Bannon are lobbying him to follow through. Scott Pruitt, the head of the Environmental Protection Agency and an ardent opponent of climate action, has argued that the US should exit, calling it a “bad deal” for the country.

A decision either way, experts say, could have major implications for the future of climate talks — and, ultimately, the future of Earth’s climate itself. So let’s break down the options.

1) What happens if Trump decides to leave Paris?

If the Bannon wing prevails, it would be easy enough for Trump to exit Paris. After all, the accord was never ratified by the Senate, and it is basically nonbinding.

To leave, the United States could invoke Paris’s formal withdrawal mechanism, which would technically take four years, although US officials could stop participating in any future talks immediately. (More radically, Trump could pull out of the underlying UN Framework Convention on Climate Change, which would take just one year and signal that the US is abandoning all international efforts on climate change.)

By far the biggest question at that point would be how other countries react. Leaders in Europe, China, and India have insisted they would carry on addressing climate change without the US. But the future arc of climate negotiations — in which countries try to ramp up their climate efforts in the hopes of limiting global warming below 2°C — might look very different.

China, the world’s largest emitter, would be poised to assume a dominant role in future talks, and its leaders have tended to argue for looser oversight and accountability mechanisms within the deal than the US has. “China has historically been a little more wary of strong international procedures and institutions,” David Victor, a political scientist with the University of California San Diego, told me recently. “You might see the treaty become more decentralized and less formalized over time.”

A bigger concern is that if the US steps back, other countries could decide to scale back their own individual efforts to tackle global warming, says Andrew Light, a former senior climate negotiator at the State Department who is now at the World Resources Institute. “If the US pulls out altogether, the chances increase that developing countries like Brazil or India back away from their own commitments and say, ‘Why should we bother doing this if the world’s biggest historical emitter is completely out of the game now?’” he said. If that were to happen, the chances of avoiding severe global warming would start to look far more dire.

Of course, no one knows for sure what will happen. It’s possible that a US withdrawal could have a galvanizing effect on the rest of the world, and other governments would redouble their efforts to promote clean energy and curb emissions. Most nations still have a vested interest in avoiding drastic temperature increases. But there’s a real risk that momentum for stronger action would be blunted.

There’s also the prospect that the US could face serious diplomatic repercussions for leaving. Europe, China, and other countries could threaten to withhold cooperation on other issues the US cares about. In the most extreme scenario, other countries could threaten to impose carbon tariffs on the US, sparking a trade war. That’s why many Trump allies, like Sen. Bob Corker (R-TN), have argued it may prove smarter to stay in.

2) What happens if Trump decides to stick with Paris?

Now imagine Trump decides to stay in the Paris accord. After the initial fanfare subsides, there’s the crucial question of whether the Trump administration would try to fulfill the Obama administration’s commitments under the deal — like the pledge to cut US greenhouse gas emissions 26 to 28 percent below 2005 levels by 2025.

Odds are, they wouldn’t. Trump has ordered the repeal of many of Obama’s domestic climate policies, including the Clean Power Plan to curtail emissions from power plants, and various regulations around methane leaks from oil and gas operations. If those policies get dismantled, it will be nearly impossible to meet Obama’s pledge.

One possibility is that the Trump administration simply revises the US emissions goal downward — something they could probably get away with, though it would flout the spirit of the accord, in which countries are supposed to enhance ambition over time. A move like that might, in turn, lead other countries to reconsider their own climate plans. If the United States isn’t taking its pledge seriously, why should they?

On top of that, a core part of the Paris deal involves the US pledging $3 billion in aid to poorer countries to help them expand clean energy and adapt to droughts, sea level rise, and other global warming calamities. The Obama administration already chipped in $1 billion. But Republicans in Congress and the Trump administration have insisted they have no intention of delivering the rest.

Some experts think that Trump abandoning US commitments on aid could be nearly as damaging to international climate efforts as withdrawing from Paris altogether. “For the developing countries, this will be a sign that America is unreliable and that the benefits of staying engaged in climate negotiations are fleeting,” Victor wrote in an essay in November.

That said, Light thinks this outcome could prove less disruptive than total withdrawal. Assume Trump stays in Paris, revises down the US pledge, and dismantles the Clean Power Plan. Even then, Light told me in March, there’d still be enough action from states that you could imagine the US making at least some headway on emissions, even if it’s far less than Obama promised. “And that would still leave the possibility that the US could rebound [on climate action] after one term of Trump,” he says.

3) If Trump sticks with Paris, how might US negotiators shape future agreements?

The 2015 Paris deal was only a first step in the long, grinding process of dealing with climate change — and a woefully inadequate step at that. If you add up all the country pledges worldwide, they don’t come close to keeping us below 2°C of global warming. They add up to a severe 3°C or more, depending on which analysis you trust:

The big hope with the Paris accord was that these individual national pledges would be strengthened over time, as countries cooperated and pushed each other to pursue deeper emissions cuts. So a key question here is what role US climate negotiators might play in this ratcheting process.

For example: Over the next two years, negotiators are meeting at the UN to hash out rules around how to review individual country pledges and policies. This “transparency mechanism” could prove contentious. The Bush and Obama administrations had long pushed for strict, uniform transparency standards. China and other developing nations, by contrast, have in the past preferred a “bifurcated” system that holds them to somewhat looser reporting requirements.

If Trump does stay in Paris, one question is whether the US will try to hold developing countries to the strictest transparency standards possible. And it’s worth asking how much leverage the US will really have if it’s also reneging on its emissions targets and aid promises elsewhere.

This sounds like an obscure issue, but for many observers it’s critical. After all, climate change is a collective action problem. Countries are less likely to take the plunge and push for emissions cuts unless they know everyone else will jump with them. “To me, the essence of this agreement is what it can do to strengthen confidence that everyone’s doing their fair share, primarily through greater transparency,” says Elliot Diringer, executive vice president of the Center for Climate and Energy Solutions. “With greater confidence, everyone can do more. Weaker transparency rules would make it harder to strengthen confidence and ambition over time.” 

Beyond transparency, the world’s nations are also supposed to formally take stock of their progress by 2018 and then submit new — and ideally stronger — NDCs by 2020. But this process of strengthening global pledges could get bogged down if the US is weakening its stated ambitions. It’s not hard to imagine India or China feeling less pressure to step up its efforts if the richest country on Earth is backsliding.

It’s also possible that the Trump administration may push to reorient global climate talks in an entirely new direction — say, by placing a greater focus on working with China to promote carbon capture technology for coal and natural gas. Some coal companies have been urging Trump to stay in the Paris deal to do exactly that.

“My guess is that the Trump administration would come out swinging against the idea of targets and regulations and instead argue that technology is the only way to solve this problem,” says Paul Bledsoe, a senior energy fellow at the Progressive Policy Institute. “They might argue that a crash program on carbon capture is the best way forward here.”

The larger point here is that the Paris climate deal isn’t guaranteed to succeed just because Trump sticks with the agreement. These nuances of what happens to a process that’s already well underway really do matter, and there are lots of ways the US could potentially change the deal — or weaken it — from within.

Further reading:

• Here’s our primer on how the Paris climate deal works. And here’s a scientific “roadmap” showing what countries would actually need to do to meet the Paris goals. It’s daunting.
• Amy Harder of Axios has a nice piece on why many companies — including many coal companies — would prefer that Trump stick with the Paris climate deal.
• Last fall, Neil Bhatiya and Tim Kovach wrote much more on what it would mean for China to take the lead on global climate policy — particularly in places like sub-Saharan Africa.
• How carbon capture could become a rare bright spot on climate policy in the Trump era.