The amount of heat-trapping carbon dioxide in the atmosphere hit a record high in May. That's because humanity kept emitting a prodigious amount of carbon, even through the worst pandemic in a century. But if civilization does begin to significantly cut emissions, global temperatures won't promptly start going down, like flipping a climate switch.
In research published Tuesday in the journal Nature Communications, scientists at the Center for International Climate Research in Norway determined how Earth's temperature would respond to different types of emission cuts. At best, the first signals of cooling wouldn't be apparent for some 25 years. And that's assuming a potential — though extremely ambitious — reality where global society immediately starts reducing emissions by five percent each year. (Right now, emissions are still going up each year.)
That's because, due to national economies largely run on fossil fuel burning, humanity has loaded the atmosphere with the highest levels of carbon dioxide in at least 800,000 years, but more likely millions of years. Just starting to turn this big ship around requires decades of significantly decreased emissions. And then, fully stabilizing the temperatures in line with the Paris climate agreement (at around 3.6 degrees Fahrenheit, or 2 Celsius, above pre-Industrial temperatures) may take a century, or longer.
"The climate system is operating on timescales of multiple decades to centuries to even thousands of years," Matthew Lachniet, a climate scientist at the University of Nevada at Las Vegas who had no role in the research, told Mashable. It took us over a century to build up a massive "pile" of CO2 in the atmosphere. "It will take at least as long to get out of this mess," Lachniet said.
Slow climate response
Why would the climate be "slow," from the human perspective anyway, to respond to emission cuts?
It's because our yearly carbon emissions are like a big bank deposit, constantly adding more carbon dioxide to the atmosphere's (already large) bank account of CO2. We're still going to make deposits, but perhaps increasingly smaller deposits, for decades.
"Much of the CO2 we emit initially piles up in the atmosphere, so even if we reduce emissions, we’re still adding to the cause of the warming," explained Bjørn Samset, an author of the study and a climate scientist at the Center for International Climate Research. "A five percent cut per year means five percent less of an increase annually, but an increase all the same."
What's more, Earth's climate is still baking from our previous emissions, and the natural processes that take CO2 out of the atmosphere act relatively slowly, said Samset. The ocean and forests gradually soak up about half the carbon emissions humans emit. And, critically, the ocean has absorbed nearly unfathomable amounts of heat over the last three decades. So even if we magically cut all of our carbon emissions today, the seas would still slowly release this heat back into the atmosphere, which would continue to heat the planet.
"The oceans hold onto that warmth like a battery," said Lachniet, referencing how they gradually give off energy.
Lastly, it's challenging to detect with certainty when the planet might stop cooling. That's because, year-to-year, Earth's global temperatures naturally jump around due to normal recurring climate patterns like El Niño, wherein warmer ocean surface temperatures heat up the overall globe. This adds "noise" to global climate measurements. It's why there are dips and valleys within Earth's current, relentless warming trend.
"Even if a five percent cut per year would reduce the rate of warming immediately, in principle this noise makes it hard to detect the reduction," said Samset.
The big picture
To see how different emission scenarios would eventually show a cooling signal, Samset and his team used a climate model (found at http://live.magicc.org/) to show how different greenhouse gases or particles in the atmosphere impact the planet (CO2, however, is the most important greenhouse gas humans emit). Then, they used another model to simulate how natural weather variability would impact these different emissions scenarios. This research showed substantial differences in yearly temperatures, making it difficult to pinpoint exactly when Earth will start cooling as emissions are slashed. (A trend will eventually become clear, just like Earth's relentless warming trend has.)
It's important that political leaders understand that the benefits of slashing emissions won't be promptly apparent. Confirming the first hints of cooling requires patience.
"Given time, the reduction will of course be visible, but 'time' in this case means at least a couple of election cycles," noted Samset.
The researchers found that — in a fictional world where humanity promptly cut all greenhouse gas emissions — the first cooling signal wouldn't become apparent until 2033. And if humanity managed to slash emissions enough to satisfy what the United Nations considers to be the lowest and most ambitious emissions pathway (technically called RCP 2.6, wherein CO2 emissions decline after 2020 and fall significantly until later this century), a cooling signal would appear sometime around 2047, almost three decades later.
Perhaps the most critical takeaway is the first evidence of a cooling trend doesn't mean the climate has stabilized. It would only mean we're making progress. It's similar to the coronavirus pandemic: Just because infections precipitously fell in New York doesn't mean the job's over. New Yorkers must mask and social distance until there's a reliable vaccine, or the welcoming infection trend will reverse.
"The big focus is what the climate is like in 2100 or 2200," emphasized Lachniet. "Have we stabilized or have we melted the West Antarctic ice sheet and flooded the East Coast of the United States?"
Lachniet refers to the melting of glaciers like Thwaites in Antarctica. The Florida-sized glacier alone could raise sea levels by over two feet — but may unleash up to eight more feet of sea level rise from its glacial neighbors. It has already destabilized, as warmer ocean waters eat away at its undersides. Thwaites is now retreating by about half a mile each year.
"Thwaites is the one spot in Antarctica that has the potential to dump an enormous amount of water into the ocean over the next decades," Sridhar Anandakrishnan, a professor of glaciology at Penn State University, told Mashable in February.
Letting the climate continue to warm this century could spell environmental doom.
"We're trying to avoid a collapse of ice sheets," said Lachniet. "There’s no putting that genie back in the bottle."
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