There are a few different ways that global warming could unfold in the decades ahead. The world might heat up steadily and predictably, giving humans and other species time to adjust. Or we could see large, abrupt changes that are extremely difficult to adapt to.
It's that latter prospect in particular that worries many climate experts. Back in 2002, the National Academy of Sciences published a report titled "Abrupt Climate Change: Inevitable Surprises." Their conclusion? Rapid climate shifts — occurring in just a few decades or less — were entirely plausible. Ice sheets could collapse, say, and push sea levels up unexpectedly fast.
Similar sorts of abrupt changes have occurred in the past. Some 12,000 years ago, at the end of the Younger Dryas, a millennium-long cold period ended in just a few decades, and some 72 percent of large-bodied mammals in North America went extinct. But scientists weren't yet sure how likely similar disruptions are now that humans are warming the planet. More research was needed.
That research is now piling up. On Tuesday, the National Research Council published a brand new report, "Abrupt Impacts of Climate Change: Anticipating Surprises," that lays out what scientists have learned since 2002 about the possibility of sudden climate shifts. There are still plenty of troubling uncertainties, but researchers have learned a fair bit.
The upshot? Earth is already seeing some abrupt changes, like the fast retreat of summer Arctic sea ice. There's also a real risk that other rapid and drastic shifts could follow in the coming decades if the Earth keeps warming — including widespread plant and animal extinctions and the creation of large "dead zones" in the ocean.
On the flip side, other drastic changes "are now considered unlikely to occur this century." That includes shifts in Atlantic ocean circulation patterns that could radically alter Europe's climate, as hyped in the disaster flick "The Day After Tomorrow." Also unlikely this century: Collapsing ice sheets in West Antarctica that would push sea levels up very quickly, as well as sudden methane eruptions from the Arctic that could heat the planet drastically. Those doomsday scenarios are left to future generations.
The authors do emphasize, however, that scientists still don't fully understand all the different ways the Earth's climate can change in short order. There are lots of unknowns here. "Some surprises in the climate system may be inevitable," they conclude, "but with improved scientific monitoring and a better understanding of the climate system it could be possible to anticipate abrupt change before it occurs and reduce the potential consequences."
Here's a longer rundown of some of the abrupt changes the new National Research Council report explores, as well as how probable they are to occur this century (I've ordered them from most likely to least likely):
— Sharp increases in extinction rates. A recent study in Science found that the world is on track to warm much faster than it has in the past 65 million years. That could require some species to shift habitats at an unprecedented rate.
This concept is known as the "velocity of climate change," and the map on the right shows two different estimates of how quickly species would have to shift in order to maintain the climates of their current habitats (assuming they needed to).
Some species will be able to keep up, others likely won't: There's only so far up a mountain that pikas can climb to stay cool, for instance. And coral reefs will have difficulty adapting if the oceans keep warming and become more acidic. Add it up, and it raises the prospect of extinctions for many species.
Likelihood this century: Moderate. When you toss in other pressures that many plant and animal species are facing — deforestation, for instance — the report concludes that a mass extinction event "could conceivably occur before the year 2100," Coral reefs in particular get singled out here: "some models show a crash of coral reefs from climate change alone as early as 2060 under certain scenarios."
However, the report adds that scientists still need to develop a better understanding of how many species will react to these shifting climates. "It is an open question whether the climatic tolerances of local populations can evolve fast enough to keep up with rapid climate change."
The report also explores the possibility of an abrupt "collapse" of the Amazon rain forest due to a combination of climate change and deforestation (say, by creating a self-sustaining cycle of fires and dryness). The report concludes that some of these scenarios are "plausible," but they're still subject to much intense debate and are very difficult to model the likelihood.
— An abrupt decrease in ocean oxygen. Scientists expect the oxygen content of the ocean to decline as the world warms, due to various chemical and biological changes. And that raises a concern: In some parts of the ocean, it's possible that this process could accelerate abruptly, creating large "oxygen minimum zones" that are virtually uninhabitable for fish and other organisms.
Likelihood this century: Moderate. Similar "dead zones" are already popping up in many coastal areas around the world, mainly caused by fertilizer run-off and improperly treated wastewater. When combined with other changes in the warming ocean, "the decrease in oxygen availability might become non-linear."
— Destabilization of the West Antarctic ice sheet. The current scientific consensus is that the world will likely see between 0.4 and 1.2 meters of sea-level rise (1 to 4 feet) by century's end, depending on how fast emissions rise. This assumes the oceans will expand as they warm and ice caps and glaciers melt at a predictable pace.
But what about surprises? The report notes that the West Antarctic Ice Sheet carries enough ice to raise sea levels by 3 to 4 meters (10 to 13 feet). Right now, that massive ice sheet looks stable. But the geological record that these sheets are capable of shifting very quickly, particularly at the boundary between sea ice and land ice.
"Locations where meltwater forms on the ice shelf surface can wedge open crevasses and cause ice-shelf disintegration—in some cases, very rapidly."
Likelihood this century: Unknown but probably low.The report notes that current computer models don't capture all of the physical processes of ice sheets perfectly, so it's tough to say how likely this all is. Greenland's ice sheet is not expected to destabilize this century. By contrast, an abrupt change in the West Antarctic Ice Sheet is judged "to be plausible, with an unknown although probably low probability." (And the odds go up if the planet keeps warming past 2100.)
— Carbon or methane "bombs" released from the Arctic. There's a lot of carbon that's locked in frozen permafrost at high latitudes. There's also a lot of methane stored in the northern oceans, trapped in lattice-like structures known as clathrates. All told, there may be more carbon stored in permafrost and ocean hydrates than their are in known fossil-fuel reserves (see the chart on the right).
So what if the Earth heated up enough that the permafrost melted, the oceans warmed, and these greenhouse gases suddenly got released into the atmosphere? That could, in theory, trigger an extremely large climate shift.
Likelihood this century: Low. A sudden massive release looks unlikely this century. The report concludes that as the Arctic warms, it will gradually release more carbon and methane into the atmosphere, which will "amplify" existing warming. But a very large release is unlikely to happen a short span, say, just one or two decades.
The report cautions, however, that "this conclusion is based on immature science and sparse monitoring capabilities." Scientists still need better assessments of the long-term stability of those carbon stores. Not out of the clear yet. And the odds here also keep going up if the planet keeps warming after 2100.
— A chaotic disruption of Atlantic ocean circulation patterns. Ever wonder how Western Europe manages to stay relatively warm despite being so far north? Some scientists give partial credit to the Atlantic Meridional Overturning Circulation (AMOC), an ocean pattern that transports warm water into the North Atlantic and Nordic seas. The pattern also plays many other vital roles, like maintaining the ocean's ability to absorb carbon from the atmosphere.
Back in the early 2000s, scientists raised the prospect of a nightmare climate scenario here. Paleoclimate evidence suggests that the AMOC has changed abruptly in the past due to an influx of cool melting freshwater.
So what if, say, Greenland's ice sheets melted quickly enough to disrupt this circulation? Would we get a "The Day After Tomorrow" style scenario in Europe, where some coastal areas cool down very rapidly? (Some scientists have argued that a disruption in Atlantic ocean heat circulation may have led to such a cold spell roughly 12,900 years ago.)
Likelihood this century: Low. Fortunately, this doomsday scenario now seems unlikely anytime soon. Climate models broadly agree that an abrupt change to the AMOC "will not occur this century." Greenland would have to melt at a far faster rate than even the worst-case scenarios. The report does suggest, however, that "it is important to keep a close watch on this system," to understand both the impact of smaller changes and keep an eye on the remote possibility of big, drastic shifts.
— The full National Research Council study can be accessed here. It was sponsored by the National Oceanic Atmospheric Administration, the National Science Foundation, and the U.S. Intelligence community.