What if the Intergovernmental Panel on Climate Change got the science wrong? Not just some obscure metric or a digit on the far side of the decimal point. What if the IPCC badly underestimated just how hot it is going to get and wildly overestimated how much time we have to address it?

The IPCC, of course, is the United Nations body charged with surveying, evaluating, and distilling the world’s climate knowledge into an ongoing series of reports. Policymakers the world over consider the IPCC’s pronouncements to be authoritative.

Dr. Hansen's Critique

In November, though, one of the most renowned climate scientists in the world, Dr. James E. Hansen, published an article with more than a dozen coauthors that all but accuses the IPCC of malfeasance. Because they are afraid of being accused of “crying wolf,” Hansen alleges, and because their reticence is “comfortable and well-suited for maintaining long-term support,” IPCC scientists have failed in their duty to warn the world about just how dire the climate emergency has become.

Titled “Global warming in the pipeline,” this 33-page, sprawling article is unafraid to take detours into the history and sociology of science, climate policy and politics, and the unfiltered opinions of the first author. At its core, though, the paper addresses two crucial questions: How much will the Earth warm if we double the amount of carbon dioxide in the atmosphere, so-called equilibrium climate sensitivity (ECS), and how long will it take to reach that new equilibrium?

From a reevaluation of paleoclimate data, the authors estimate that ECS is about 4.8C. That’s a lot higher than the IPCC estimate of 3C and, although ECS specifically refers to a doubling of carbon dioxide, this new number from Hansen et al. suggests that the warming effects of any change in carbon dioxide concentrations will also be much larger.

Moreover, the paper argues that the climate models that the IPCC relies on have failed to adequately account for amplifying feedback loops—retreating ice sheets exposing darker surfaces that can absorb more sunlight, greenhouse gases being released from thawing permafrost, etc.

In fact, the paper alleges, about 70 percent of the total warming caused by increased CO2 concentrations will be the result of such amplifying feedbacks, while only 30 percent will be due to the direct effects of the CO2 itself. Combine these effects with a reduction in aerosols as we limit our use of the dirtiest fossil fuels and the result is an acceleration in the amount of warming we will experience in the decades to come. Under current policies, the globally averaged surface temperature will rise by 0.27 C per decade, breaching 1.5 C in the 2020s and 2 C well before 2050—a devastating scenario next to the comparatively rosy predictions of the IPCC.

Rethinking Climate Strategy

Ultimately, this terrifying forecast leads Hansen and his coauthors to a surprising and iconoclastic conclusion. Recall that two factors determine the temperature of the Earth: the amount of energy coming in and the amount of energy going out. The climate crisis has emerged because humans have reduced the amount of energy going out. Almost all of our efforts to mitigate climate change to date have also focused on that end of the equation: reducing heat-trapping emissions or even removing carbon dioxide from the atmosphere. 

Hansen et al., however, believe that the climate emergency has become so severe that it is time to consider the other side of the equation: the energy that is coming in. By injecting aerosols into the atmosphere to reflect sunlight, so-called “solar radiation management,” we could buy enough time to save ourselves from the worst effects of climate change. It is a shocking suggestion, since even talking about this sort of geoengineering has long been taboo among the climate science community for fear it will be used to justify inaction on other fronts.

Of course Hansen and his coauthors have already met with pushback. In a blog post, climate scientist Michael E. Mann declared that the paper is “very much out of the mainstream.” No doubt others will chime in soon, especially about that solar radiation management idea.

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