One idea that often gets brought up in discussions of climate change is that it doesn’t matter what Europe and the United States do as long as India and China keep growing at their furious pace and emitting heat-trapping greenhouse gases without end.

There’s some truth to this — Asia now emits roughly twice as much carbon dioxide as North America — but it’s worth breaking down the split in responsibility more precisely. Here’s a chart from the Center for American Progress, based on an EPA analysis, that’s worth unpacking:

What we have here are three different emissions scenarios and the temperature risks they pose. In the first, “reference” scenario, the whole world keeps polluting at its current pace, and the odds are overwhelming that we could warm the Earth by more than 3°C by 2100 — and a very real chance we could surpass 5°C, a shocking outcome in which the giant ice sheets in Greenland and Antarctica would eventually disappear, pushing sea levels up some 70 meters over many centuries, and widespread extinction would ensue. (Several different energy agencies have warned that we’re currently on pace for this fate, transforming the planet irrevocably.)

So that’s doomsday. Meanwhile, under the chart’s middle scenario, wealthy countries — the United States, Japan, European nations — would cut emissions sharply, but countries like China and India would dither and delay, merely restraining the growth of their emissions through 2050. In this scenario, the world would only have an 11.1 percent chance of staying under the 2°C threshold, but the most likely outcome would be something less than 3°C. The bulk of the evidence suggests that 3°C could create a lot of dire, hard-to-manage outcomes — extreme heat waves, droughts, sharp sea-level rise — but again, it beats an end-of-the-world-style 5°C rise. Which means it’s hardly pointless for the United States and Europe to act on their own.

Now, the safest scenario on the chart is the third one, titled “full participation.” In this case, wealthy countries would reduce their emissions 80 percent by 2050, while developing nations would cap their emissions by 2025 (at the latest) and ratchet down their carbon pollution to 26 percent below 2005 levels by 2050. If the countries of the world agreed to follow this path, then we’d have a 75 percent chance of staying under that agreed-upon 2°C threshold.

Yet this “safe” scenario is looking less and less likely by the day. Wealthy countries would need to start reducing their pollution immediately, while China’s emissions would have to peak very soon. (Some experts have suggested that China could peak by 2020, though that’s a fairly optimistic prediction.) In addition, the United States, Europe, and Japan would probably need to chip in money to help fund clean-energy projects in the developing world — a topic that’s under discussion at the U.N. climate talks in Durban, South Africa, right now.

So fast-growing countries like China and India do play a pivotal role in the climate picture. But it’s also not useless for places like Europe and the United States to act, even if China and India are moving more sluggishly.