Even if all burning of fossil fuels was stopped today, why will temperatures continue to increase

Over the years, scientists have raised a key question about future climate change: When, exactly, does the warming stop?

Do temperatures stop rising as soon as people cease emitting greenhouse gases? Or is some level of future warming already guaranteed, or “locked in,” by our past emissions?

Recent studies generally suggest good news on that front. The vast majority of future warming depends on future emissions. There shouldn’t be much future warming locked in from the past.

Now, a new study suggests that’s mostly true — but with some caveats. It’s important to look at all kinds of human emissions in the atmosphere, the research suggests, not just the main greenhouse gases like carbon dioxide.

When accounting for all of them, there may be a brief give-and-take before global temperatures finally stabilize. Global temperatures could continue to rise for a few years, or a few decades, after all emissions stop, and then they may fall back down again as the climate system stabilizes.

That means past a certain point, the world may not be able to avoid temporarily overshooting the Paris Agreement’s temperature targets.

In fact, the study suggests that even if all emissions halted today, the planet would still have around a 42 percent chance of overshooting the target of 1.5 degrees Celsius, even if temperatures later dropped back below it.

Still, the study emphasizes, it remains true that the vast majority of future warming depends on future emissions. And the faster those emissions spiral down to zero, the less warming there will be.

The study, released yesterday in the journal Nature Climate Change, used a specialized model to investigate how the Earth’s climate system would respond if emissions were to suddenly stop.

The study looked at a range of possible future climate scenarios assuming different levels of emissions, some higher, some lower. For each possible scenario, the researchers examined a series of hypothetical stop dates — the point at which emissions suddenly vanish — between now and 2080.

The study makes a point of accounting for a wide variety of different human-caused emissions, not just the heavy hitters like carbon dioxide and methane.

That’s important, the authors say, because some kinds of emissions actually have a cooling effect on the planet — and as a result, they’re masking a small portion of the warming that’s being caused by greenhouse gases. That means when these emissions vanish, this masking effect will also stop, and temperatures may temporarily rise beyond what might be expected when considering greenhouse gases alone.

Certain types of air pollution associated with the burning of fossil fuels are a prime example. Some of these particles are highly reflective and help beam sunlight away from the Earth, slightly cooling the climate. If the planet stops burning fossil fuels, some of these pollutants will also disappear, taking their small masking effect with them.

In the short term, the researchers suggest, this means that global temperatures may actually rise slightly for a few years after all emissions halt. If emissions suddenly stopped today, for instance, the study suggests that the planet may continue to warm by a few tenths of a degree for several years.

Later, some of this effect is likely to reverse itself. Temperatures may begin to fall again as certain types of potent but short-lived greenhouse gases — like methane — begin to disappear from the atmosphere over time.

That’s where the concept of overshoot comes in.

The study finds that there’s a 42 percent chance of overshooting the 1.5 C target even if emissions stopped today. At the same time, there are much lower odds of the Earth’s final temperature stabilizing above 1.5 C — probably around 5 percent or so.

And the 2 C target still remains firmly in reach at this point. The study estimates only about a 2 percent chance of overshoot if emissions halted immediately, and zero odds that the Earth’s stable temperature would exceed 2 C in the long run.

Far more important to the fate of the planet, the study says, is what happens in the next few years.

The risk of overshoot rises substantially the longer humans continue emitting greenhouse gases. The study estimates that the risk of exceeding the 1.5 C target rises to 66 percent by 2029, assuming moderate emissions over the next few years.

And the risk of permanently overshooting the temperature targets also rises dramatically the longer emissions are released.

The research has a few important caveats. On one hand, it assumes that human-caused emissions suddenly stop all at once. That’s not how it would work in the real world, and changes in the Earth’s temperatures may be different in real life.

On the other hand, the study doesn’t account for certain kinds of potential climate feedbacks that could speed up future warming — things like thawing permafrost and rapid ice loss. That means that some of its estimates of future warming could be on the conservative side.

The study’s general warning, however, is clear. And it echoes the warnings issued in the newest reports from the U.N. Intergovernmental Panel on Climate Change. Meeting the Paris targets is only possible with immediate and dramatic reductions in global greenhouse gas emissions. And even then, the 1.5 C target is rapidly closing in.

The latest IPCC reports also warn the world not to underestimate the dangers of overshooting global temperature goals. Even if they later stabilize below 1.5 or 2 C, some climate impacts can’t be so easily reversed. Certain types of damage to glaciers and ice sheets, global sea-level rise, and the extinction of plant and animal species, for instance, are essentially permanent on human time scales.

At the same time, multiple recent studies have warned that current global climate policies are still not enough to put the world on track to meet the Paris climate goals.

“Our findings make it all the more pressing that we need to rapidly reduce emissions,” Michelle Dvorak, lead study author and a Ph.D. student at the University of Washington, said in a statement.

AUTHOR

Richard B. Rood

Professor of Atmospheric, Oceanic and Space Sciences at University of Michigan

Imagine the smokestacks without the billowing clouds of greenhouse gas pollution.

Earth’s climate is changing rapidly. We know this from billions of observations, documented in thousands of journal papers and texts and summarized every few years by the United Nations’ Intergovernmental Panel on Climate Change. The primary cause of that change is the release of carbon dioxide from burning coal, oil and natural gas.

International climate talks in Lima this week are laying the foundation for next year’s UN climate summit in Paris. While negotiations about reducing emissions grind on, how much warming are we already locked into? If we stop emitting greenhouse gases tomorrow, why would the temperature continue to rise?

Basics of carbon and climate

The carbon dioxide that accumulates in the atmosphere insulates the surface of the Earth. It’s like a warming blanket that holds in heat. This energy increases the Earth’s surface average temperature, heats the oceans and melts polar ice. As consequences, sea level rises and weather changes.

Since 1880, after carbon dioxide emissions took off with the Industrial Revolution, the average global temperature has increased about 1.5F (0.85C). Each of the last three decades has been warmer than the preceding decade, as well as warmer than the entire previous century.

The Arctic is warming much faster than the average global temperature; ice in the Arctic Ocean is melting and the permafrost is thawing. Ice sheets in both the Arctic and Antarctic are melting. Ecosystems on both land and in the sea are changing. The observed changes are coherent and consistent with our theoretical understanding of the Earth’s energy balance and simulations from models that are used to understand past variability and to help us think about the future.

Slam on the climate brakes

What would happen to the climate if we were to stop emitting carbon dioxide today, right now? Would we return to the climate of our elders? The simple answer is no. Once we release the carbon dioxide stored in the fossil fuels we burn, it accumulates in and moves amongst the atmosphere, the oceans, the land, and the plants and animals of the biosphere. The released carbon dioxide will remain in the atmosphere for thousands of years. Only after many millennia will it return to rocks, for example, through the formation of calcium carbonate – limestone – as marine organisms’ shells settle to the bottom of the ocean. But on time spans relevant to humans, once released the carbon dioxide is in our environment essentially forever. It does not go away, unless we, ourselves, remove it.

If we stop emitting today, it’s not the end of the story for global warming. There’s a delay in temperature increase as the climate catches up with all the carbon that’s in the atmosphere. After maybe 40 more years, the climate will stabilize at a temperature higher than what was normal for previous generations.

This decades-long lag between cause and effect is due to the long time it takes to heat the the ocean’s huge mass. The energy that is held at the Earth by the increased carbon dioxide does more than heat the air. It melts ice; it heats the ocean. Compared to air, it’s harder to raise the temperature of water – it takes time, decades. However, once the ocean temperature is elevated, it adds to the warming of the Earth’s surface.

So even if carbon emissions stopped completely right now, as the oceans catch up with the atmosphere, the Earth’s temperature would rise about another 1.1F (0.6C). Scientists refer to this as committed warming. Ice, also responding to increasing heat in the ocean, will continue to melt. There’s already convincing evidence that significant glaciers in the West Antarctic ice sheets are lost. Ice, water, and air – the extra heat held on the Earth by carbon dioxide affects them all. That which has melted will stay melted – and more will melt.

Ecosystems are altered by natural and manmade occurrences. As they recover, it will be in a different climate from that in which they evolved. The climate in which they recover will not be stable; it will be continuing to warm. There will be no new normal, only more change.

[youtube https://www.youtube.com/watch?v=qwvWH5tn1Tk?wmode=transparent&start=0]

Best of the worst case scenarios

In any event, it’s not possible to stop emitting carbon dioxide today, right now. Despite significant advances in renewable energy sources, total demand for energy accelerates and carbon dioxide emissions increase. I teach my students that they need to plan for a world 7F (4C) warmer. A 2011 report from the International Energy Agency states that if we don’t get off our current path, then we’re looking at an Earth 11F (6C) warmer. Our current Earth is just over 1F warmer, and the observed changes are already disturbing.

There are many reasons that we need to essentially eliminate our carbon dioxide emissions. The climate is changing rapidly; if that pace is slowed, the affairs of nature and human beings can adapt more readily. The total amount of change, including sea-level rise, can be limited. The further we get away from the climate that we have known, the more unreliable the guidance from our models and the less likely we will be able to prepare. The warmer the planet gets, the more likely reservoirs of carbon dioxide and methane, another greenhouse gas that warms the planet, will be released from storage in the frozen Arctic permafrost – further adding to the problem.

If we stop our emissions today, we won’t go back to the past. This is not reason, however, to continue with unbridled emissions. We are adaptable creatures, with credible knowledge of our climate’s future and how we can frame that future. We’re already stuck with some amount of guaranteed climate change at this point. Rather than trying to recover the past, we need to be thinking about best possible futures.

via What would happen to the climate if we stopped emitting greenhouse gases today?.

DISCLOSURE STATEMENT

Richard B. Rood receives funding from government and foundation research grants.

The Conversation is funded by Howard Hughes Medical Institute, Robert Wood Johnson Foundation, Alfred P Sloan Foundation and William and Flora Hewlett Foundation. Our global publishing platform is funded by Commonwealth Bank of Australia.