Climate scientists react to Trump | ft. ‪@ClimateAdam‬

The World Meteorological Organization reported this month that 2024 is likely to average 1.55 C hotter than the late 1800s, the first time a full-year average will pass the 1.5 level. Several one-month intervals have also surpassed that level in recent years.

This month, researchers from the U.K. published a study in Nature Geoscience that suggests Earth has warmed at least 1.39 C since that same period, and even more—as much as 1.49 C—since the 1700s, when humans began burning fossil fuels in earnest.

Both findings have ignited concern amongst scientists and climate policy experts. But neither means that the 1.5 C goal has yet been surpassed, formally.

Because surprisingly, Betts says, nowhere in the Paris Agreement does it define how to measure the Earth's increasing temperature.

The authoritative science organization known as the Intergovernmental Panel on Climate Change (IPCC), has long held that a single month, or even a whole year, of temperatures averaging above 1.5 C isn't sufficient to demonstrate that level of warming.

That's because temperature rise doesn't happen smoothly. Even without global warming, some years are hotter or colder than others. Weather patterns like El Nino can skew some years hotter than expected, for example. To account for that natural temperature wobble, the IPCC suggests looking at averages over a 20-year period. That requires looking backwards at years of global average temperature data, like the WMO report, while also looking forward using climate models to predict future rise.

Using those methods, scientists calculate that 2023 was 1.31 C hotter than the pre-industrial period.

But there's a problem, says Nathan Gillett, a climate scientist at Environment and Climate Change Canada: that approach is inherently backward-looking. Even if warming progresses past 1.5 C, "We won't be able to say that until after we passed it," he says.

That approach could obscure the true amount of warming, says Andrew Jarvis, a climate scientist at Lancaster University and an author of the new Nature Geoscience analysis.

Even using those metrics, Jarvis says, it's likely "we're going to exceed one and a half degrees in the next ten years." The only way to prevent that increase, he says, is to implement significantly more aggressive climate action immediately.

Is staying below 1.5 C possible?

Ryna Cui, a climate scientist at the University of Maryland, echoes that sentiment.

"Even with a really rapid pace, [temperatures] may not be able to peak below 1.5," Cui says. "I do think we are looking for an overshoot," a period of time in which global temperatures surpass 1.5 C warming before coming back down below that value.