The primary focus of international climate negotiations this week in Baku, Azerbaijan, is how to pay for the costs of cutting global climate pollution and adapt to the impacts of climate change.
But there's another issue lurking: whether climate change has already heated the planet near, or past, 1.5 Celsius (2.7 degrees Fahrenheit) beyond preindustrial temperatures. That's one of the major goals laid out in the 2015 Paris Agreement.
But several new analyses, scientific studies, and international reports suggest the goal of keeping warming to below 1.5 C is becoming further away from possible. This is an outcome of countries delaying, walking back, or failing to implement ambitious efforts to cut fossil fuel emissions — moves that would most effectively stave off further warming, according to many scientific analyses.
Because of those delays, "it's a matter of when, not if," that level of warming will be surpassed, says Richard Betts, a climate scientist at the University of Exeter in the U.K.
Betts stresses that surpassing the 1.5 C level of warming should not be a reason to slow or abandon climate efforts like those being discussed at COP29. Instead, he says, it should spur more ambitious action to prevent even further warming. He compares it to surpassing a speed limit.
"It doesn't mean that driving at 68 to 69 miles an hour is safe and that driving at 71, 72 is going to kill you," he says. But the risks and consequences from a crash at higher speeds—or higher global temperatures—increase substantially.
Has Earth already warmed 1.5 C?
The signs all point in one direction: it's likely the planet is quickly approaching 1.5 C of warming.
But there are real open questions about when that will happen and how to measure it.
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.
"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.
Cui's group recently released an analysis showing that if most major economies ramp up climate ambition in the next few years and achieve the goal of hitting net-zero emissions by 2050, it is still possible to keep global temperature rise to 1.7 or 1.8 C.
"From the 1.7 peak, there are still opportunities for us to return back to 1.5," she says.
In October, the latest release of the annual U.N. Emissions Gap report, which quantifies the difference between climate goals and reality, found that the 1.5 C goal was still technically possible—if unlikely. Global emissions would need to drop rapidly by 2030, falling 42% from 2019 levels, to keep warming below 1.5 C.
To achieve the goal, some 60% of the world's electricity would need to come from renewable sources by 2030—roughly quadrupling the current capacity. A recent report from the International Energy Agency suggests the world isn't on track for that goal, forecasting about 43% renewable energy generation by 2030.
It's not technically impossible, says David Victor, a climate policy expert at the University of California, San Diego. But nor is it likely.
But "there's a huge political cost to being the first government or the first major firm to say that the goals are no longer achievable. And so no one wants to bear that political cost," Victor says.
What happens after 1.5 C?
Betts says surpassing 1.5 C level isn't like driving off a cliff: Earth's climate won't immediately become irreversibly damaged.
But science does suggest the risks associated with further warming can become much more pronounced beyond that temperature level.
"The immediately tangible impacts are the increase in the intensity and frequency of extreme events," says Lila Warszawski, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany.
Already, extreme weather like hurricanes and heat waves have been intensified by climate change. Hurricane Helene's rainfall in late September was an estimated 10% heavier than it otherwise would have been without human-caused climate change. Most heat waves—including the one that engulfed Europe in 2022 and killed tens of thousands of people—are also intensified by climate change. Such impacts are forecast to become even more pronounced as temperatures increase.
At 2 degrees Celsius of warming in the U.S., precipitation on the wettest days, like during Helene, could increase by 20, 30, or even 40% in some parts of the country, overwhelming flood control systems and endangering people's lives. The number of days over 95 degrees Fahrenheit could grow by a full month.
"In the U.S., you've already been feeling it over the last months with increasing mid-latitude storms and hurricanes," says Warszawski. Then, imagine worse, she says.
The longer temperatures stay high — and the higher they go — the more likely it is Earth could end up "exceeding hardwired thresholds in the earth system," Warszawski says. These thresholds, sometimes called "tipping points," represent changes that become essentially irreversible and even self-perpetuating, she says. And scientists don't yet know the exact temperature thresholds that could trigger such changes. But scientists think many of those thresholds could fall somewhere between 1.5 and 2 C.
But if global heating were halted or reversed, many of those risks would stop getting worse, in many cases quickly. Even if the 1.5 C goal is breached, it might still be possible to bring temperatures back down below that level within decades with continued aggressive climate action.
Will COP29 help?
Negotiations at this year's COP29 climate talks in Baku, Azerbaijan, are primarily focused on how to pay for transitioning away from burning fossil fuels and how to equitably support adaptation to the problems climate change brings.
But by next year, most countries are scheduled to release their next Nationally Determined Contributions, or NDCs, country-level roadmaps that outline plans for the next five years to cut greenhouse gas emissions. Cui says it's critical the new NDCs outline aggressive, realistic actions.
"It is going to take a lot of dramatic effort to…make sure we don't have a high overshoot" in temperature beyond 1.5 C, says Cui. "We had better make that process as short as possible."
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