IPCC: Here’s how bad global warming could get

These stories are referring to a massive new report from the UN’s Intergovernmental Panel on Climate Change (IPCC), which sorts through thousands of scientific studies and details all the ways that rising temperatures are messing with our farms, fisheries, and coastlines. (This is the second of three reports from the IPCC.)

The panel also warned that global warming will cause even more trouble if temperatures keep rising. Crop failures. Water shortages. Deadly heat waves. And so on.

So how worried should we be? Below is a basic primer on some of the broader issues raised by the 2,600-page IPCC report. Yes, global warming’s going to be a real problem — especially by making it harder to grow food. It’s not going to wipe out humanity, and there are things we can do to adjust. But there are also real limits to our ability to adapt, especially in poorer nations.

All those greenhouse gases that we’ve put into the atmosphere by burning fossil fuels have already warmed the planet 0.8°C (or 1.4°F) since the 19th century. That’s enough, the IPCC notes, to start melting glaciers and ice caps, push up sea levels, hurt agriculture, damage coral reefs, and increase deadly heat waves.

The map above shows some of the areas of the world where global warming is already having a tangible impact — from the melting of Arctic sea ice to the wilting of fruit trees in the Sahel to damage to Australia’s Great Barrier Reef.

In some of these cases, however, rising temperatures are only part of the story. In the Kenyan highlands, for instance, malaria is on the rise. That’s partly attributed to hotter temperatures that expand the range of the disease. But there are other key factors at play, too — changes in population, drug resistance, and vaccination rates also matter here.

The same goes for the Great Barrier Reef: pollution and tourism are mixing with higher temperatures to cause damage. Global warming often isn’t the only problem.

Future global warming is likely to be even more dramatic, imposing higher costs on every corner of the planet. But how bad things get will ultimately depend on us.

If the world cuts its greenhouse-gas emissions immediately and drastically, we could possibly limit total global warming to around 2°C or less (that’s 3.6°F). The IPCC says this would still create problems. Sea levels could rise another two feet, threatening the 345 million people who will live in vulnerable coastal cities by mid-century. Agriculture will start suffering. Heat waves will get more severe. But at least we’d contain the damage.

On the flip side, if greenhouse-gas emissions keep rising indefinitely, then we start getting more drastic warming — say, 4°C or more (that’s 7.2°F). That, the IPCC says, could lead to things like “substantial species extinction” and “large risks to global and regional food security,” and the risk of irreversibly destabilizing Greenland’s massive ice sheet, which would cause ocean levels to rise significantly for future generations. Bad news.

As economist Richard Tol notes, humans are a tough species. We survived the Ice Age with primitive tools, and it’s unlikely that global warming will finish us off. But that’s not necessarily a comforting thought. The Vietnam War didn’t kill everyone, either, but it still inflicted plenty of damage.

That said, while global warming can make the world a more dangerous place, it doesn’t have to lead to mass death. The report makes clear that climate is often only one factor in a variety of maladies. Hotter temperatures, for example, can mean more deadly heat waves. But public health programs can help counteract that.

So, more plausibly, we will have to find ways to adapt to a warmer climate. The relevant questions are things like: How much harm will we suffer along the way? How much will it cost us? How much risk do we want to take on? Will all countries be able to adapt or just the wealthy ones? How many other species will vanish? And so on.

Mild global warming can have some benefits. Fewer people are dying of extreme cold in the winter, for instance. Or another example: As the oceans have warmed, many fish have been migrating toward the poles. That means less fish in some regions but more fish in others.

But there’s a catch: The downsides of global warming clearly start exceeding the benefits once global warming climbs past 1°C, the IPCC estimates. Deadly heat waves, rising sea-levels, and extinctions all start greatly outweighing the upsides of mild winters. And the costs get even more severe as the planet gets hotter and hotter.

Will global warming cause food shortages? This is one alarming possibility, though not certain. Recent studies have found that even a 2°C rise in temperatures could damage crops and hurt our ability to grow food.

Global warming can length the growing season, but it also increases the number of extremely hot days, which can damage crops like corn. In the tropics, these negative effects already appear to be winning out. And this is expected to worsen over time: More and more studies now suggest that agricultural output for key cereal crops could actually decline worldwide by 2030. Under worst-case scenarios, Africa’s crop output could get cut in half.

That’s a problem: An estimated 842 million people worldwide already suffer from chronic hunger. And food demand is only going to rise as the world’s population swells from 7 billion today to 9.6 billion by 2050. Climate change is going to make the task of feeding everyone much, much harder.

But there are also things we can do to soften the blow. As the University of Minnesota’s Jonathan Foley explains here, crop yields are stagnating in many parts of the world because of insufficient investment in modern farming techniques — poor access to fertilizer, for instance. People can also do things like paring back the amount of meat in their diets or cutting down on food waste. We don’t have to stand by helplessly.

This isn’t as clear-cut as some headlines have suggested. Studies have found that climate can be a factor in civil conflicts; say, if a drought leads to dispute over scarce water resources. But the IPCC notes that climate is rarely the only factor in conflicts — or even the most important one. So war is hardly inevitable.

Still, the IPCC makes clear that global warming is likely to displace people and increase food and freshwater scarcity. Combined with other factors, particularly in weak states, those changes could prove volatile.

Let’s take another look at what the IPCC says about how Africa will deal with the risk of crop failures as a result of global warming.

“We still don’t know what the exact limits of adaptation are,” says Stanford’s David Lobell, who helped lead one of the IPCC’s chapters on agriculture. A lot will depend on whether crop scientists can help breed plants that can withstand hotter temperatures. “But we can also just look around and see that in desert areas, plants aren’t very productive — and they’ve had thousands of years to do better. So there’s an evolutionary argument against crops being able to adapt to extremely high temperatures.”

But the panel isn’t very confident in this estimate and says the true number could be higher. For one, it notes, economic estimates of global warming “depend on a large number of assumptions, many of which are disputable.” Economic models have a hard time quantifying the value of, say, species that could go extinct.

What’s more, these numbers don’t account for the risk of catastrophic losses or sudden. There aren’t very good economic estimates for 3°C or 4°C of global warming because we still don’t have a great understanding of what will unfold if temperatures rise that high.

Phrasing the problem this way can also obscure the cost of humanitarian disasters in poorer countries. Because these nations have low standards of living, they’re not a huge part of global economy. The poorest 20 countries in Africa only make up 0.12 percent of global income, for example. But widespread misery and destruction in these countries would still be a disaster.