February 13, 2016 — A large group of climate scientists has made a bracing statement in the journal Nature Climate Change, arguing that we are mistaken if we think global warming is only a matter of the next 100 years or so — in fact, they say, we are locking in changes that will play out over as many as 10,000 years.
“The next few decades offer a brief window of opportunity to minimize large-scale and potentially catastrophic climate change that will extend longer than the entire history of human civilization thus far,” write the 22 climate researchers, led by Peter Clark, from Oregon State University.
The author names include not only a number of very influential climate scientists in general but several key leaders behind major reports from the United Nations’ Intergovernmental Panel on Climate Change, including MIT’s Susan Solomon and Thomas Stocker of the University of Bern in Switzerland.
The researchers’ key contention is that we have been thinking about climate change far too narrowly by only projecting outward to the year 2100, which the research says “was originally driven by past computational capabilities.” Rather, we should consider that the long-term consequences of human emissions for global temperatures and sea level will play out over many millennia.
“It’s a statement of worry,” said Raymond Pierrehumbert, a geoscientist at Oxford University and one of the study’s authors. “And actually, most of us who have worked both on paleoclimate and the future have been terrified by the idea of doubling or quadrupling CO2 right from the get-go.”
“In hundreds of years from now, people will look back and say, ‘Yeah, the sea level is rising; it will continue to rise; we live with a constant rise of sea level because of these people 200 years ago that used coal, and oil and gas,’ ” said Anders Levermann, a sea-level-rise expert at the Potsdam Institute for Climate Impact Research and one of the paper’s authors. “If you just look at this, it’s stunning that we can make such a long-lasting impact that has the same magnitude as the ice ages.”
The key reason for this is that carbon dioxide stays in the atmosphere for a very long time before being slowly removed again by natural processes. “A considerable fraction of the carbon emitted to date and in the next 100 years will remain in the atmosphere for tens to hundreds of thousands of years,” the study noted. Meanwhile, the planet’s sea levels adjust gradually to its rising temperature over thousands of years.
So what will the world look like in 10,000 years, thanks to us? That really depends on what we do in the next few hundred years with the fossil fuels to which we have relatively easy access. It also depends on whether or not we develop technologies that are capable of pulling carbon dioxide out of the air on a massive scale, comparable to the amount that we’re currently emitting.
But assuming that we don’t develop such technologies, here are the key factors to consider — as laid out in the new paper — about how we are shaping the planet’s very distant future.
From 1750 to the present, human activities put about 580 billion metric tons, or gigatons, of carbon into the atmosphere — which converts into more than 2,000 gigatons of carbon dioxide (which has a larger molecular weight).
We’re currently emitting about 10 gigatons of carbon per year — a number that is still expected to rise further in the future. The study therefore considers whether we will emit somewhere around another 700 gigatons in this century (which, with 70 years at 10 gigatons per year, could happen easily), reaching a total cumulative emissions of 1,280 gigatons — or whether we will go much further than that, reaching total cumulative levels as high as 5,120 gigatons. (It also considered scenarios in between.)
In 10,000 years, if we totally let it rip, the planet could ultimately be an astonishing 7 degrees Celsius warmer on average and feature seas 52 meters (170 feet) higher than they are now, the paper suggests. There would be almost no mountain glaciers left in temperate latitudes, Greenland would give up all of its ice and Antarctica would give up almost 45 meters worth of sea level rise, the study suggests.
Still, anyone observing the world’s recent mobilization to address climate change in Paris in late 2015 would reasonably question whether humanity will indeed emit this much carbon. With the efforts now afoot to constrain emissions and develop clean energy worldwide, it stands to reason that we won’t go so far.
“With Paris, it does get us off the exponential growth, and we might level off at 2,000, 3,000 gigatons,” said Pierrehumbert.
Still, what’s striking is that when the paper outlines a much more modest 1,280-gigaton scenario — one that does not seem unreasonable, and that would only push the globe a little bit of the way beyond a rise of 2 degrees Celsius over pre-industrial temperature levels — the impacts over 10,000 years are still projected to be fairly dramatic.
In this scenario, we only lose 70 percent of glaciers outside of Greenland and Antarctica. Greenland gives up as much as four meters of sea level rise (out of a potential seven), while Antarctica could give up up to 24. Combined with thermal expansion of the oceans, this scenario could mean seas rise an estimated 25 meters (or 82 feet) higher in 10,000 years. There is, to be sure, “a big uncertainty range on that prediction,” Pierrehumbert said by email.
Once again, a key factor that could mitigate this dire forecast is the potential development of technologies that could remove carbon dioxide from the air and thus cool down the planet much faster than the Earth on its own can through natural processes. “If we want to have some backstop technology to avoid this, we really ought to be putting a lot more money into carbon dioxide removal,” Pierrehumbert said.
Pierrehumbert said he believes that we will manage to develop such a technology in coming centuries, so long as human societies remain wealthy enough — but he added that we don’t know yet about how affordable it will be.
The new study fits into a growing body of scientific analysis suggesting that human alteration of the planet has truly brought on a new geological epoch, which has been dubbed the “anthropocene.” Taking a 10,000-year perspective certainly reinforces the geological scale of what’s currently happening.
The ability to carry an analysis out so far into the future, Levermann said, is really the result in recent years of several key scientific developments. One is that “we are now in a better position to model the ice sheets, really,” he said.
At the same time, scientists have also recently begun to calculate so-called carbon budgets that describe how much we can emit and still hold the planet to a variety of temperature thresholds.
All of this coming together means that a conversation about increasingly long-range forecasts, and about the millennial-scale consequences of today’s greenhouse gas emissions, is growing within the scientific world. The question remains whether a similar conversation will finally take hold in the public and political one.
by Chris Mooney | The Washington Post