Top Scientists Just Ruled Out Best-Case Global Warming Scenarios

Source: By Eric Roston, Bloomberg • Posted: Wednesday, July 22, 2020

A new research paper narrows a key range for future heating. We ask one of the authors what it means.

A major new study of the relationship between carbon dioxide and global warming lowers the odds on worst-case climate change scenarios while also ruling out the most optimistic estimates nations have been counting on as they attempt to implement the Paris Agreement.

A group of 25 leading scientists now conclude that catastrophic warming is almost inevitable if emissions continue at their current rate, even if there’s less reason to anticipate a totally uninhabitable Earth in coming centuries. The research, published Wednesday in the journal Reviews of Geophysics, narrows the answer to a question that’s as old as climate science itself: How much would the planet warm if humanity doubled the amount of CO₂ in the atmosphere?

 That number, known as “equilibrium climate sensitivity,” is typically expressed as a range. The scientists behind this new study have narrowed the climate-sensitivity window to between 2.6° Celsius and 3.9°C.

That’s smaller than the current range accepted by  the United Nations-backed Intergovernmental Panel on Climate Change, which has for almost a decade used a spread between 1.5°C to 4.5°C—a reading of climate sensitivity that has changed little since the first major U.S. climate science assessment in 1979. Improving these estimates is “sort of the holy grail of climate science,” says Zeke Hausfather, director of climate and energy at the Breakthrough Institute and one of the study’s authors.

Climate sensitivity is one of the most iconic numbers in climate science, but it’s not necessarily intuitive. The range isn’t a projection; it’s more like a speed limit that influences projections. “It informs all the other things—like 2100 warming projections, for example—that depend on the sensitivity of our models, and our scenarios,” Hausfather says.

What gave the authors confidence is that three independent lines of evidence—the modern temperature record, geological evidence, and the latest Earth systems models—all agreed on the same answer. Kate Marvel, a research scientist at NASA’s Goddard Institute for Space Studies and Columbia University’s applied math and physics department, also contributed to the new paper. She answered questions for Bloomberg Green about the scope and meaning of the new work.

What is “equilibrium climate sensitivity,” and why is it so important?

It’s basically answering this question: How hot is it going to get? People are sometimes really surprised. They’re like, “You guys have one job like, why do you not know this?”

The number one determinant in how hot it’s going to get is what people are going to do. If we gleefully burn all the fossil fuels in the ground, it’s going to get very hot. If we get extremely serious about mitigating climate change—cutting our emissions, moving off fossil fuels, changing a lot about our way of life—that will have a different impact on the climate. As a physical scientist, “What are we going to do?” is totally above my pay grade.

The really interesting thing is that, even if you remove all of the uncertainty about what people are going to do, we still don’t know exactly how hot it’s going get. As it gets warmer, the climate changes. It’s not just the temperature but rainfall patterns, the water vapor in the atmosphere, cloud cover, ice melt, land surface changes. Those changes can themselves affect how hot it gets and how rapidly. Some of these feedback processes are not very well constrained simply because we haven’t done this before.

Why not? What’s been stopping us from answering those questions?

It’s not like everybody else was stupid before this. It’s just that we are getting more and more data. We’ve made really big strides in understanding the ways in which the recent past may be different from the far future—the ways in which it is a good analog and the way it’s not a good analog. We’ve kind of made strides on understanding what we don’t understand. Does that make sense?

Yes, I think so! Why was the team was able to rule out the lower levels of climate sensitivity?

Really, really low sensitivities come about if the planet has some sort of protective mechanism, where it organizes itself to slow down the warming. What we’re saying is we don’t see any evidence for this kind of natural protective mechanism. We are much more confident that climate sensitivity is not super-low than we are that climate sensitivity is not super-high. If there is this kind of mechanism kicking in to save us, why hasn’t it happened yet? Why hasn’t there been any evidence of it?

Our most likely value of climate sensitivity is what people have thought for a long time now, around 3°C. That’s really scary. That’s not great. There is a tendency to overemphasize really large values, as if a climate sensitivity of 3°C wasn’t a catastrophe.

So why are you able to limit the odds of super-high sensitivity?

There are some models in the newest generation that are giving very high values of climate sensitivity, above 4°C, 5°C. In light of all of the different evidence, those are not very likely, but they’re not ruled out.

How would you describe all this to a policymaker?

I am super sympathetic to people saying this doesn’t really matter for policy. Right now this is kind of a theoretical quantity. But if things continue the way they’re going, this is not going to be theoretical for that long.

Most policy people know carbon dioxide is a greenhouse gas, they know where it comes from. They know how it’s getting in the atmosphere, and they know that it has to be reduced as quickly as possible. And I don’t think this changes any of that.

We can’t rule out really, really, really wildcard catastrophic, very dangerous outcomes. That’s something to keep in mind as policymakers move forward. There’s a tendency to try to put the perfect numbers on things, to say we have 12 years to save the planet. Honestly, we have, like, negative 30 years to save the planet.

Bonus question for readers who’ve made it this far: Clouds have been the great unknown for scientists. Simulating clouds has led to quirks in some of the newer models. Why are clouds always so beguiling to climate scientists?

Clouds are super, super important in setting the thermostat of the planet. There are a lot of them, and they have this dual role. They trap infrared radiation from the Earth’s surface, but at the same time, clouds block sunlight and make it cooler than it would be otherwise. If you look at its two competing effects, right now the cooling power of clouds wins. So if we got rid of all the clouds tomorrow—which I do not think we should do—the planet would get a lot hotter. It’s really, really difficult to get clouds right in a model of the entire planet.

Transcript has been edited for length and clarity.