After decades of timid proclamations, the new report of the Intergovernmental Panel on Climate Change, released earlier this year, finally concluded with "very high confidence" that humans are causing global warming. And yet, the range of possible values for just how far that warming will go is roughly unchanged from the last report, made six years ago. In fact, we're as hazy on this issue as we were almost 30 years ago. Ask the most basic questions of a climate scientist today— How hot will it get? Will we bake and flood the planet beyond recognition, or will we just have balmier Novembers?—and you'll get pretty much the same answers you would have gotten when Jimmy Carter was sporting cardigans in the White House.
Recently, an article in the journal Science titled "Why Is Climate Sensitivity So Unpredictable?" addressed this notable lack of progress. The authors, Gerard Roe and Marcia Baker, explore the fact that the most probable range for climate sensitivity has remained around where it is now—between 2.2 degrees Celsius and 4.5 degrees Celsius of warming—for decades, despite great strides in our understanding of the climate and our ability to simulate it with computer models. They also look at why we haven't been able to rule out very high outcomes, either—like 7 degrees Celsius, 10 degrees Celsius, or even 15 degrees Celsius. According to the IPCC, these seemingly small differences could have tremendous impacts: A real temperature change of 3 to 4 degrees Celsius would likely lead to a rise in sea level, disruptions in rainfall patterns, and an increase in extreme weather; an uptick of 5 or 6 degrees, on the other hand, could change the face of the planet. So, which scenario should we expect? According to Roe and Baker, we're nowhere near a firm answer.
Why does the uncertainty persist? And why does the climate-sensitivity graph retain its stubborn asymmetry—with a long tail of possible outcomes on the side of catastrophic extremes? The pattern reflects two fundamental properties of the climate: First, gradual warming of the Earth sends the climate into feedback loops that in turn increase the greenhouse effect. As the planet's surface heats up, the atmosphere takes on more water vapor (a greenhouse gas much more powerful than CO2), snow cover and sea ice diminish (reducing how much sunlight reflects back into space), and more clouds form (blanketing the heat of the planet's surface). We don't fully understand these processes, but we're quite confident that, taken together, they make the Earth a hotter place. Second, at low levels of warming, the climate is still fairly stable and can dampen these feedbacks. But at warmer and warmer temperatures, the climate becomes inherently less stable and more sensitive to feedbacks. It's so sensitive, in fact, that we may never be able to know precisely what's going to happen—no matter how much we learn about our planet.
Some climatologists aren't as pessimistic, arguing that evidence from ice cores and the fossil record will help us to rule out the most extreme estimates for climate sensitivity—the figures of 7 degrees Celsius and up. But the Roe-Baker paper invites us to consider the fact that, for some time to come, we simply may not know more precisely than we do now what kind of world we live in—one with climate sensitivity of 2 degrees of warming, or one closer to 10.
Ultimately, this warming value in degrees Celsius may just not be a useful measure for policy-makers. It is, after all, the product of a thought experiment: Climate sensitivity is a term of art, referring to a theoretical scenario where we've first doubled the concentration of CO2 in the atmosphere and then instantly curbed all additional CO2 emissions. The calculation also assumes that the oceans have had decades to adjust to the new atmospheric makeup and new temperatures, the concentration of water vapor in the air has recalibrated, and the ice sheets have receded as far as they're going to go. Once all of these feedbacks have reached a new equilibrium, how much hotter will the planet be? It's a useful question, and it does give us some insight into warming rates—the higher climate sensitivity turns out to be, the faster temperatures will rise this century in real life. But it isn't a projection of global temperature nor of the on-the-ground impacts of climate change. True temperatures will be influenced by many other factors—like our continuing CO2 emissions, changes in the carbon cycle and vegetation distribution, rapid ice melt, and the contribution of aerosols.
But emissions goals—the numbers set by various international protocols and governing bodies—are based largely on our climate-sensitivity predictions. And that means that our politicians are going to need to get a lot more comfortable with uncertainty—as much as this might complicate traditional accords to reduce carbon emissions by a certain amount and a specific date. In a companion paper from the same issue of Science, physicists Myles Allen and David Frame make an argument that might sound more natural coming from a pair of economists: Given all the uncertainty, these emission-cap agreements are dangerous. It would take quite a bit of political effort and capital to lock the governments of the world into a goal of, say, twice the CO2 levels of preindustrial times. But even in our perfect thought experiment, we don't know if those levels would ultimately lead to a dangerous temperature increase of 3 degrees Celsius or an apocalyptic one of 8 degrees Celsius. So, what happens if after 15 years of diplomacy, innovation, and behavioral change, we're on target for our emissions cap, but climate sensitivity turns out to be much worse than we expected? The treaty-making would have to begin again.
If ignoring uncertainty is dangerous, trumpeting the extreme outcomes is at least unhelpful. Bad, rigid policy seems to follow when worst-case scenarios hang over all decisions. (The war on terror, with all its abuses, is predicated on the notion that we'll never know if, when, or where the next strike will occur.) So, we shouldn't expect the highest temperatures, but we can't rule them out, either. Instead, we need a policy that's flexible enough to respond to what we see happening on the ground (and in the air, and in the oceans). Allen and Frame suggest that an adaptive approach to global warming might work best, where we're constantly measuring how we're doing and updating our projections. This would require a very high tolerance for uncertainty from our leaders—an awareness of the "known unknowns," perhaps, although coupled with the kind of flexibility the author of that phrase never mustered. Will we get that from our next president? This, too, remains uncertain.