That limit is the optimistic goal of the Paris Climate Agreement : to keep global average temperatures to 1.5 degrees Celsius above preindustrial levels, and to avoid 2 degrees of warming. The new report notes that the temperature has already crept up by 1.1 degrees, and is on track to hit 1.5 sometime in the early- to mid-2030s if things don’t change. That’s a significant update from a previous IPCC report that predicted that the planet would hit the 1.5 milestone at around the year 2040, says Zeke Hausfather, a climate scientist and the director of climate and energy at the Breakthrough Institute, who wasn’t involved in the report. “Similarly, we're passing 2 degrees somewhere between the early 2040s and early 2050s as a most likely estimate in the higher-emission scenarios,” he says, referring to one of the five outcomes modeled in the new report.
As part of the Paris accord, countries agreed to adopt nationally determined contributions, or NDCs, which outlined the steps they would take to limit greenhouse gas emissions within their borders.UN Secretary General Guterres called for countries to commit to carbon neutrality by 2050, and 60 nations signed on.
Why does that half a degree matter so much? “There's a big difference between 1.5 and 2,” in terms of the worsening of droughts , heat waves , storms , floods , ice melt , and sea level rise , says Janos Pasztor, executive director of the Carnegie Climate Governance Initiative and former UN assistant secretary-general for climate change, who wasn’t involved in the report. “Two gets a lot worse. And that beyond 2 gets a lot, lot worse. And there are chances, of course, that we will be getting in that direction.”
The report lays out projections for what would happen in five different greenhouse gas emissions scenarios: These imagine a future in which humanity is producing varying levels of carbon, from very low to very high. (In the lowest scenario, emissions drop to net zero around the year 2050 and keep falling. In the highest, they double by that year.) In other words, it’s predicting what the climate will look like depending on the speed with which our civilization decarbonizes.
The report’s accompanying color-coded graphics also show what would happen to global temperatures and to precipitation rates depending on how much the climate warms, and lays out how many world regions have experienced increases in extreme heat, precipitation, and drought. (Hint: It’s nearly all of them.)