The most recent Intergovernmental Panel on Climate Change (IPCC) report left no doubt about the future of the world if we don't slow the rate at which we release heat-trapping gases into the atmosphere. In a word, it's going to get bad.
But exactly how bad is still an open question, and a lot depends not only on how we react, but how quickly. The rate at which humans cut down on greenhouse gas emissions--if we do choose to cut them--will have a large bearing on how the world turns out by 2100, the forecasts reveal.
This graphic from the World Resources Institute gives a sense of the dynamics at play. It presents four "emissions pathways," ranging from the very optimistic to the highly pessimistic.
The first "Low Emissions" scenario is for a 66% drop in greenhouse emissions by 2050 compared to 2010 levels. It's what we might call a soft landing, because under those conditions scientists believe we'll be relatively safe. The world would have warmed only by 2 degrees C over pre-industrial levels (the level set by various international agreements). Still, almost of the quarter of the world would suffer depleted groundwater supplies by 2080, and many more people will face extreme flooding, the WRI says. So, life wouldn't be peachy.
The "Medium Emissions" scenario sees increases in emissions until 2040 and the world exceeding its "carbon budget"--the level at which it should stay within the 2 degrees limit--by 2056. By 2100, the planet has warmed by 2.9 degrees, and economic productivity has fallen by 20%. By the 2080s, six times as many people are experiencing catastrophic flooding as the 1980s.
The "High Emissions" scenario doesn't see emissions peaking until 2080, while global temperatures jump 3.7 degrees C by 2100. The carbon budget is exhausted in 2057. The impact on agricultural production is so heinous that adaption is no longer viable, the WRI predicts.
As if that's not bad enough, there's one last "Highest Emissions" scenario (they should have called it the Doomsday Scenario, really). It sees the carbon budget obliterated in 2045 and global temperatures increasing a whopping 4.8 degrees by century's end. Many animals have become extinct and farming in some places, like southern Brazil, has become impossible.
But won't we adapt to the new conditions, you might ask? Well, maybe. The scenarios here assume flat technology development, not the leaps forward in innovation that we can hope for. We could have drought-resistant crops and new ways of recycling and desalinating water, for instance, that could make these predictions less forceful.
The easier course, though, is to cut emissions. To have a fighting chance of coping with climate disorder, we have to cut greenhouse gases quickly, not just wait until it's convenient.