If the Earth warms by two degrees Celsius, we’re in trouble. Well, by mid-century we’ll probably blow past that limit as we are on pace to warm the planet by three or four degrees.
If 2°C looks increasingly out of reach, then it’s worth looking at what happens if we blow past that and go to, say, 3°C or 4°C.
Four degrees (or 7.2° Fahrenheit) may not sound like much. But the world was only about 4°C to 7°C cooler, on average, during the last ice age, when large parts of Europe and the United States were covered by glaciers. The IPCC concluded that changing the world’s temperature in the opposite direction could bring similarly drastic changes, such as “substantial species extinctions,” or irreversibly destabilizing Greenland’s massive ice sheet.
In 2013, researchers with the World Bank took a look at the science on projected effects of 4°C warming and were appalled by what they found. A growing number of studies suggest that global food production could take a big hit under 3°C or 4°C of warming. Poorer countries like Bangladesh, Egypt, Vietnam, and parts of Africa could see large tracts of farmland made unusable by rising seas.
But what seemed to unnerve the authors of the World Bank report most was all of the stuff we don’t know. Most climate models currently make predictions in a linear fashion. That is, they basically assume that the impacts of 4°C of warming will be twice as bad as those of 2°C. But that might be wrong. Impacts may interact with each other in unpredictable ways. Current agriculture models, they noted, don’t have a good sense for what will happen to crops if heat waves, droughts, new pests and diseases all combine together.