The Good and Bad News about Rising Temperatures

Scientists believe they can better pinpoint how much future climate change the world can expect.

There's some good news: It won't likely be as bad as the worst-case predictions. The bad news: It'll probably be worse than the rosiest projections.

New research published yesterday in the journal Nature claims to have substantially narrowed down the scientific estimate of how much warming would occur if carbon dioxide concentrations in the atmosphere were to double from their preindustrial levels. It's a metric known as "equilibrium climate sensitivity," or ECS, and it's a key component of scientists' estimates about how future greenhouse gas emissions may alter the climate.

Unfortunately, it's also a metric that comes with significant uncertainty. Dozens of studies have attempted to calculate it, with their findings falling across a broad range of possible warming scenarios. So far, the best estimate presented by the Intergovernmental Panel on Climate Change suggests that the range of warming likely falls between 1.5 and 4.5 degrees Celsius.

It's a broad range with a worrying upper limit. Preindustrial carbon dioxide concentrations hovered at around 280 parts per million. A doubling of those levels would amount to 560 ppm—and the planet is already nearly halfway there. Current estimates place carbon dioxide concentrations at around 405 ppm. Under the IPCC's most severe projection, the Earth would already be locked into at least 2 degrees of warming. World leaders are striving to keep the rise in global temperatures within a 2-degree threshold.

The new study, led by Peter Cox of the University of Exeter, suggests the IPCC's worst projections may be too high. It also suggests its lowest projections may not be severe enough.

The research places the likely range of warming, under a doubling of carbon dioxide concentrations, between 2.2 and 3.4 degrees Celsius. There's less than a 3 percent chance the warming would be less than 1.5 degrees, the researchers note—and less than a 1 percent chance it would be greater than 4.5 degrees.

"If the upper limit of ECS can truly be constrained to a lower value than is currently expected, then the risk of very high surface-temperature changes occurring in the future will decrease," said climate scientist Piers Forster of the University of Leeds, who was not involved with the research, in a commentary also published yesterday in Nature. "This, in turn, would improve the chances of keeping the temperature increase well below 2°C above pre-industrial levels."

But as Forster also points out, the new study doesn't completely agree with all recent research on the subject—meaning there's still room for debate among climate scientists about where the most likely range actually falls.

For its part, the new research takes a different approach than many other studies. Previous estimates have often relied on historical warming trends to inform their predictions about the future. The new study instead focused on how present-day temperatures naturally vary from one year to the next, regardless of the long-term warming trend, and how those variations may influence the sensitivity of the climate system. Testing the idea on historical climate simulations, it found that these annual variations were useful in helping to predict equilibrium climate sensitivity.

"Helping me to sleep a little easier"

If the findings are correct, they could be taken as a bit of good news for the climate.

Although the new study's lower limit is higher than that of previous estimates, it also suggests that we may avoid the kind of catastrophic climate change associated with the IPCC's worst-case scenario of 4.5 degrees warming.

That said, another study published just last month, also in Nature, came to a different conclusion. That study used a similar method to evaluate future warming and suggested that the most accurate climate models—the ones that best reflect real-life observations of climate change—are the ones that make the most severe predictions about future global warming (Climatewire, Dec. 7, 2017).

In his commentary, Forster acknowledges that more research will be required to resolve the debate but notes that he believes the latest study may carry more weight, being "based on simpler physical theories of climate forcing and response."

But not all scientists are necessarily convinced. In a statement to the independent Science Media Centre, Steven Sherwood of the Australian Research Council's Centre of Excellence for Climate System Science said he felt "it is not clear whether to put more weight on this study, or the previous ones suggesting even higher sensitivity."

But he added that, nonetheless, "one thing all the studies agree on is that climate sensitivity is not very low"—that is, central estimates of the equilibrium climate sensitivity always fall above 2 degrees Celsius. That means that even under the more moderate findings in the new study, it's imperative for world leaders to prevent carbon dioxide levels from doubling if they want to meet the climate goals outlined in the Paris climate agreement.

That doesn't make continued study of the ECS any less important, though. On the contrary, continuing to narrow down the estimated range will give scientists and policymakers a better and better idea of what climate effects to expect from current emissions trajectories and how aggressively they must act to change those outcomes.

Though more debate is likely to come, the new study for now provides renewed hope that exceeding the 2-degree threshold is not yet an inevitable end. And to Forster, at least, that's some relief.

"I should thank Cox and colleagues for helping me to sleep a little easier in my bed at night," he wrote.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.