26 February 2015

Climate change: irreversible but not unstoppable

Dawn Stover

Dawn Stover

Stover is a science writer based in the Pacific Northwest and is a contributing editor at the Bulletin. Her work has appeared in...


When scientific experts moved the hands of the Bulletin’s Doomsday Clock two minutes closer to midnight last month, calling current efforts to prevent catastrophic global warming “entirely insufficient,” some people responded that climate change is a far less disastrous threat than nuclear war because it is reversible. This is a common misconception.

In ongoing data collection by the Cultural Cognition Project at Yale Law School, fewer than one in four people in a general population sample in Southeast Florida understood that if human beings stopped emitting carbon dioxide tomorrow, global temperatures would continue to rise. “Believers” in human-caused global warming were just as likely as “disbelievers” to misunderstand the extent to which we are already committed to future temperature rises.

The widespread notion that the climate is something we can fix later—after more pressing priorities have been addressed—may be the biggest obstacle to actions and policies that would slow global warming, avoid some of its worst potential impacts, and allow more time for humans and other species to adapt to a changing climate. Even though scientists have repeatedly emphasized the urgency of the situation, their message isn’t getting through to the general public or to legislators who could make a difference.

What’s missing are vivid, personalized depictions of what life will be like in the future if emissions continue unabated. Human activities have already altered the climate so radically that many scientists refer to the current geologic era as the Anthropocene, from the Greek words for “human” and “new.” But that sounds friendly and progressive compared with what actually lies ahead: a climate very similar to that of Earth’s last major warm period, the Pliocene epoch of several million years ago, minus the mastodons and mammoths. And unlike nuclear war, it’s not a question of whether climate change will rock our world, only of how bad things will get.

Committed to climate change. Though we’re seeing obvious warning signs of what is to come, such as melting glaciers and steadily increasing levels of atmospheric carbon dioxide, thus far the global average surface temperature has risen by only about 0.8 of a degree Celsius (or 1.4 degrees Fahrenheit) since 1880. However, the climate system has some built-in inertia, and the impacts of past human activities will be felt far into the future. Scientists refer to these unavoidable future changes as our climate change “commitment.”

Some of the inertia comes from the elevated levels of carbon dioxide and other greenhouse gases already in the atmosphere. If humans were to cease their emissions overnight, the oceans would quickly absorb some of these gases. But the oceans also release gases back to the atmosphere, and the level of greenhouse gases in the atmosphere would not subside back to pre-industrial levels for many centuries.

Another problem is that industrial air pollution has a cooling, as well as a warming, effect. Fossil fuel combustion releases aerosols, tiny particles and droplets that reflect sunlight and enhance cloud formation, masking the impacts of greenhouse gases. If we stopped burning fossil fuels, this cooling effect—which is difficult to quantify, but probably has less than half the impact of the greenhouse warming effect—would end.

“A large fraction of climate change is largely irreversible on human time scales,” the most recent assessment report from the Intergovernmental Panel on Climate Change (IPCC) warned. Only if human emissions were “strongly negative over a sustained period”—for example, if tree planting and other activities were to sequester far more carbon than humans release—would climate change begin to be reversed. At the moment, of course, emissions are still rising rapidly.

Points of no return. If the concentration of carbon dioxide in the atmosphere can be limited to a doubling—from about 280 parts per million (ppm) in the pre-industrial era to 560 ppm in the future (we’re currently at about 400 ppm)—the IPCC assessment estimated with “high confidence” that Earth’s temperature will reach an equilibrium somewhere between 1.5 and 4.5 degrees Celsius above pre-industrial temperatures. However, the report cautioned, “some aspects of climate will continue to change even if temperatures are stabilized.”

Among some of the most likely changes: The melting of snow and ice will expose darker patches of water and land that absorb more of the sun’s radiation, accelerating global warming and the retreat of ice sheets and glaciers. Scientists agree that the Western Antarctic Ice Sheet has already gone into an unstoppable decline. Currents that transport heat within the oceans will be disrupted. Ocean acidification will continue to rise, with unknown effects on marine life. Thawing permafrost and sea beds will release methane, a greenhouse gas. Droughts predicted to be the worst in 1,000 years will trigger vegetation changes and wildfires, releasing carbon. Species unable to adapt quickly to a changing climate will go extinct. Coastal communities will be submerged, creating a humanitarian crisis.

Some of these changes may persist for hundreds or even thousands of years after the Earth’s temperature stabilizes. Scientists worry that elements of the climate system could even reach tipping points beyond which abrupt and planetary-scale changes might occur, such as the disappearance of monsoon cycles or the Amazon’s vast tropical forests.

Welcome to the Pliocene. Even if countries reduce emissions enough to keep temperatures from rising much above the internationally agreed-upon “danger” threshold of 2 degrees Celsius (which seems increasingly unlikely), we can still look forward to conditions similar to those of the mid-Pliocene epoch of 3 million years ago. At that time, the continents were in much the same positions that they are today, carbon dioxide levels ranged between 350 and 400 ppm, the global average temperature was 2 to 3 degrees Celsius higher than it is today (but up to 20 degrees higher than today at the northernmost latitudes), the global sea level was about 25 meters higher, and most of today’s North American forests were grasslands and savanna.

A mid-Pliocene climate looks comfortable, though, compared with what will happen if we continue to emit carbon dioxide at today’s rate. As noted in the Doomsday Clock announcement, the IPCC “warned that warming—if unchecked by urgent and concerted global efforts to greatly reduce greenhouse gas emissions—would reach 3 to 8 degrees Celsius (about 5.5 to 14.5 degrees Fahrenheit) by the end of the century.”

Social inertia. Is there any way to avoid Pliocene-like conditions? “If carbon dioxide emissions could be eliminated entirely,” two scientists argued in Nature Geoscience in 2010, “temperatures would quickly stabilize or even decrease over time. Future warming is therefore driven by socio-economic inertia, and is only as inevitable as future emissions.”

That is about as helpful as telling obese people that if they just stopped eating, they would lose weight quickly. At the moment, we’d be doing well to cut humanity’s diet of fossil fuels to a level that would merely prevent further weight gain. Instead what we see is a planetary binge, with increases in fossil fuel consumption that have dwarfed the development of low-carbon energy sources during the past decade.

The scientists, however, put their finger on what is needed to turn things in the right direction: socio-economic action. Changing self-destructive behaviors can be extremely difficult, as any dieter knows, and unrealistic optimism can be just as counterproductive as defeatism. In fact, these are the twin enemies of climate action. Even climate “believers” seem to feel that either there is little they can do to prevent disaster (beyond pointing fingers at “disbelievers,” of course) or, alternatively, that technology is making (or will make) speedy progress against the problem.

Those in the over-optimistic camp may think that geoengineering, for example, can turn back the climate clock in a pinch. Unfortunately, although measures such as injecting sulfate aerosols into the stratosphere merit increased research and development, they are not ready to be safely deployed at the scale necessary to combat climate change. As a National Research Council committee recently concluded, “there is no substitute for dramatic reductions in greenhouse gas emissions.” The world needs an emissions diet plan—and a full complement of socio-economic incentives and support systems to ensure its success.

Out of the fire and into the frying pan. The inevitability of climate change doesn’t mean that we don’t have a choice to make: If we act quickly and boldly, there is a small window of opportunity in which we can work to keep global warming to a minimum. Or we can keep accelerating toward catastrophe. As Richard Somerville, one of the climate scientists on the Bulletin’s Science and Security Board, recently told me: “People today, whether they realize it or not, have control of the thermostat that will set the climate for future generations.”

Humans are constantly distracted by immediate gratifications and immediate worries. To prevent nuclear war and catastrophic climate change, we have to force ourselves to take a longer view. It’s difficult to imagine the devastating, centuries-long impacts that climate change could have on human health and the environment. Still, we stopped at two nuclear bombs. We can stop at two degrees.