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 pre-industrial levels — something that’s possible if greenhouse gas emissions keep rising at their current rate.

The Intergovernmental Panel on Climate Change 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 made unusable by rising seas.

And 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 combining.

”[G]iven 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.”