If we take bold, coordinated, global action now — in this decade — we can limit climate change to a tolerable level. But if we stay on our present course, then heaven help us all.

That is the stark message of a new report, taking stock of the latest research, from the United Nations Intergovernmental Panel on Climate Change (IPCC). The only good news is that wealthy nations, including the United States, are taking steps to curb their emissions of carbon dioxide and other greenhouse gases, though they need to do more. The bad news is that emissions from China, India and other developing nations continue to rise.

And the worse news is that the world will likely blow past what scientists see as a crucial benchmark — a rise in global average temperatures of 1.5 degrees Celsius (roughly 2.7 degrees Fahrenheit) over preindustrial levels — by the “first half of the 2030s.” On the cooler side of that threshold, we see and feel the effects of climate change but can manage them. On the warmer side, which is where we are heading, those impacts threaten to become overwhelming.

Already, according to the report, we have raised the planet’s surface temperature by 1.1 degrees Celsius (or 2 degrees Fahrenheit) since we began burning coal, oil, natural gas and other fossil fuels on a massive scale. And of all the extra carbon dioxide and methane we have sent into the atmosphere since 1850, roughly 40 percent has been spewed since the IPCC’s first study on climate change in 1990. We can’t claim that scientists didn’t warn us.

Yet China — by far the world’s biggest carbon emitter — last year approved the construction of 168 new power plants fired by coal, the dirtiest of the fossil fuels in terms of the amount of carbon released. China has talked about reaching net-zero carbon emissions by 2060, but that will not be good enough. What we need, according to the IPCC report, is “rapid and deep and, in most cases, immediate greenhouse gas emissions reductions in all sectors this decade.”

If we allow ourselves to zoom past 1.5 degrees Celsius of warming, we won’t like it. The IPCC predicts with “high confidence” that there will be “irreversible adverse impacts on certain ecosystems with low resilience, such as polar, mountain, and coastal ecosystems, impacted by ice-sheet, glacier melt, or by accelerating and higher committed sea level rise.”

If we allow ourselves to go all the way to 2 degrees Celsius of warming, we really won’t like it. There will be life-threatening heat waves, widespread coastal flooding, social unrest and increased migration because of failed crops and chronic water shortages, as well as an increase in the number and intensity of extreme weather events. It will be, for our grandchildren, a very different, much crueler world. Given current national policies and the existing fossil-fuel-based infrastructure, that is the future humanity is creating — unless we hasten the transition to clean energy.

And if we allow ourselves to fall into that hellscape, it would be hard to climb back out of it. Our carbon emissions would have to be more than net-zero; they would have to be net-negative. Whatever clever schemes and new technologies we devise to achieve that end would have to overcome the countereffects of “increased wildfires, mass mortality of trees, drying of peatlands, and permafrost thawing,” which all would weaken natural systems by which carbon is sequestered. Reversing the temperature rise would be like trying to climb a greased pole.

I am tempted to wallow in despair, but there is no time for that. The scientists and activists who have sounded the alarm on climate change and pushed for action have made a real difference. The most dystopian scenarios, in which temperatures climbed more than 4 degrees Celsius before the end of the century, now are seen as much less likely than they once appeared. The flip side, though, is that researchers now realize that their projections of when some adverse impacts would kick in were conservative. The increase in extreme weather events, for example, was expected to be seen at 1.5 degrees Celsius of warming. But it’s happening already.

The realistic goal has to be to keep warming as far below 2 degrees Celsius as possible. The market-driven solution would be a carbon tax that incentivizes the switch to clean energy. Until that becomes politically possible, governments must continue to encourage the transition. And even as the United States competes with China economically and strategically, we need to find a way to work with Beijing on climate change, which threatens the Pearl River Delta as much as the Mississippi River Delta.

We see the disaster looming. Failure to head it off is not an option.