Coal-burning heating system in central Beijing. (David Gray/Reuters)

To see why, check out the graph below from David Hone. One way to think about avoiding drastic climate change is that we need to keep the total accumulation of carbon in the atmosphere at below 1 trillion tons in order to have a 50-50 shot at avoiding 2°C. We’re about halfway there right now. Think of it like a bathtub — the longer the faucet runs, and the higher the water level gets, the more quickly you’ll have to turn off the tap to avoid overflow. And with carbon, too much dithering can greatly affect how quickly the world will have to turn off the tap:

By contrast, the red and blue lines show what would need to happen if a new legal agreement on carbon doesn’t come into effect until 2020. If that new agreement is extremely aggressive and starts cutting immediately, then the world would suddenly have to start cutting emissions at 3 percent per year to avoid 2°C (the red line). If the new post-Durban treaty delayed cutting global emissions until 2030, then we’d need to cut emissions by 4.5 percent per year (the blue line).

And both of those scenarios would prove quite arduous. As Dave Roberts has noted, the Stern Review and other studies have found that “emissions reductions of 3 to 4 percent a year are the maximum compatible with continued economic growth.” (Indeed, the only time we’ve seen truly drastic emissions cuts came after the Soviet Union collapsed — the former communist countries saw their emissions fall some 43 percent between 1990 and 2000, or about 5 percent per year.) Meanwhile, there are scientists like Tyndall’s Kevin Anderson who are even less optimistic than Hone and think that further delay would entail even sharper cuts.

In any case, that’s not an argument that avoiding drastic climate change is possible. There are plenty of smart people thinking through how this task can be done. But as the IEA found, every year of delay makes the task a lot harder and more expensive, especially as the world keeps building coal plants and other energy infrastructure that are expected to operate for decades.