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Historical emissions caused the climate crisis. But it’s what we do today that will make or break it, study shows

By Rachel Ramirez, CNN

Humans’ historical greenhouse gas emissions have caused the climate crisis the world is in today. But it’s the amount emitted now and in coming years that will determine whether humanity can avert catastrophic climate changes.

That’s the main finding of a new study published in the journal Nature Climate Change on Monday, which says that even if the world cut emissions to zero today, there would still be a 42% chance of hitting 1.5 degrees Celsius of warming above pre-industrial levels within a decade. That probability rises to 66% if the world waits until 2029 to reach zero emissions.

Nevertheless, achieving zero emissions this decade is shaping out to be a pipe dream at this point, considering global emissions are still climbing, and set to continue for the next several years. A UN report showed that new and updated pledges on emissions will only cut around 7.5% from current rates by 2030. Additionally, China, the world’s largest emitter, is not set to reach zero emissions until 2060.

The study also shows that if the world emitted nothing today, there is only a 2% chance of breaching the higher, more severe, 2 degrees Celsius warming threshold. But that too increases to much more likely than not — 66% — if the world waits another 35 years before hitting zero emissions.

If zero emissions were achieved today, temperatures would peak within a decade, the study shows, and as the concentration of greenhouse gases in the atmosphere slowly decline, long as they remain low, that peak will be followed by a long and slow period of cooling.

According to the study, “future warming is governed primarily by future emissions rather than by past emissions, and thus society is not geophysically committed to exceeding key global warming levels before reaching them.”

The findings send both a sobering message that the world needs to expedite its goal of reaching net zero, where as little greenhouse gas is emitted as possible, and the rest is offset. But it also sends a hopeful message: that there is still a good chance of keeping the climate crisis in check if deep, sustained cuts to emissions are made. And the sooner the better.

What this study has taught me is that the window of opportunity to reduce our emissions to avoid temporary exceeding 1.5 degrees is rapidly closing,” Kyle C. Armour, co-author of the study and associate professor of oceanography and atmospheric sciences at the University of Washington in Seattle, told CNN. “We have to [slash emissions] almost immediately.”

Armour adds that he believes focusing on limiting 1.5 degrees C as a policy goal may no longer be “realistic,” and that the world has a better shot at addressing 2 degrees.

“That’s my personal view, just because that window of opportunity is so short — and it really has to happen this decade,” he said. “Reducing emissions to zero is not practical at the global scale. The flip side is I’m very optimistic about holding warming to near or below two degrees because we have several decades to get this figured out.”

Early this year, the UN Intergovernmental Panel on Climate Change released its latest scientific evidence showing that if the world gets 1.5 degrees hotter than levels before industrialization — even temporarily — some impacts could take thousands of years to reverse or, in some case, be irreversible altogether.

That includes highly vulnerable ecosystems in the Arctic, mountains and on the coasts. Ice sheet and glacier melt, for instance, will accelerate sea level rise. Forests, peatlands, and permafrost — places where greenhouse gas is naturally absorbed and stored — will also be at risk of becoming emitters, leading to far more warming.

At 2 degrees C of warming, as many as 18% of all land species will be in danger of extinction. For humans, as many as 3 billion people globally will experience chronic water scarcity, according to the IPCC.

The world is already at least 1.1 degrees C warmer than pre-industrial levels.

The scientists who authored the study used climate modeling to estimate the Earth’s temperature change if emissions were reduced to zero in 2021 and every year after until 2080, under different scenarios.

Researchers also took into account the role of different greenhouse gases and their concentrations in the atmosphere, as well as aerosols and ozone depletion. While burning fossil fuels is the main driver of climate change, the aerosols — tiny particles of air pollution that are emitted — also have a cooling effect, though not enough to counteract the negative impact of coal, oil and gas use.

Although much of the global focus has been on slashing carbon dioxide (CO2) emissions, which is critical to ending the climate crisis, Armour said solutions need to also take into account short-lived “climate forcers,” such as the greenhouse gas methane and aerosols.

“Because we are committed to a few tenths of a degree more warming as we lose our aerosols, those aerosols are currently masking that warming,” he said. “What that means is that we’re geophysically committed to these temperature levels, like 1.5 and 2 degrees, about five or 10 years before we would actually reached those levels, if we continue to our emissions; because as we continue our emissions, we continue our aerosol pollution.”

Kim Cobb, director of the Global Change Program at the Georgia Institute of Technology, said the study is proof that understanding the evolution of the different types of emissions matters.

“With respect to our ability to predict the timing and magnitude of peak warming, this paper shows that the devil is in the detail,” Cobb, who is not involved with the study, told CNN. “That may seem fairly ‘in the weeds’, but when you’re talking about a few tenths of a degree Celsius, that’s a big deal in terms of global warming levels.”

Cobb said the latest study not only builds on existing climate research, but also underscores what many in the scientific community already knew to be true.

“We only have a handful of years to slash emissions for any hope of limiting global warming to 1.5C,” Cobb said. “This opportunity is slipping through our fingers with each additional year of inaction.”

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