James Hansen says we're underestimating global warming acceleration — is anyone listening?

If climate models are incomplete in the ways Hansen suggests, seas may rise very quickly over the next few decades

I am more than twenty years younger than James Hansen, who told the Senate Energy and Natural Resources Committee in June 1988 that the earth was "warmer in 1988 than at any time in the history of instrumental measurements" and that the "greenhouse effect" had "been detected, and it is changing our climate now." I have never met Hansen. But I suspect that for him the nearly 37 intervening years have passed quickly. He continues to try to alert the public about the physical risks posed by accelerating climate change, using scientific tools and evidence. Through the dry prose and blocky illustrations of his publications, you can hear the straining voice of someone who knows his remaining years are few.

And so as the recent LA fires vanish from national headlines and focus shifts to daily head-spinning news from DC, it is worth your while to pay attention to what Hansen is saying in a recent report, just for a moment.

His basic point is that common climate models greatly underestimate what will happen when global greenhouse gases in the atmosphere double in concentration from what they were in pre-industrial times (so, from 280 ppm to 560. We're already at 427 this week).* He takes a few steps to explain why—his explanation has to do with consensus climate models failing to take into account use of aerosols, particularly in the developing world, that continued until 2005—but the consequences of these low estimates are profound.

The metric of “what happens after doubled GHG concentration” is a shorthand. Its nickname is "climate sensitivity." It gives communicators a chance to say "what if" using a constant baseline: "What will the new normal be, the new equilibrium average temperature be, if these numbers double?" Hansen says that new normal is likely to be ~4.5 degrees C above pre-industrial times (or 8 degrees F). That's a bit higher than consensus estimates that suggest a range of up to 4.1 degrees C of change.

But for those alive now, the shock of this paper (one that fits the very high observed temperatures of 2023-2024) comes from its implications for sea level rise in the next few decades—not centuries from now.

If the UN's climate models are incomplete in the ways Hansen suggests (which include failing to realistically simulate what's going on with ocean currents and ice), then they may be failing to warn the world that the current acceleration of global warming in the North Atlantic and North Pacific makes it likely that the rate of polar ice melt will now accelerate as well. In turn, that accelerating melting makes it much more likely that key global ocean currents will slow more quickly. This melting and slowing will suddenly send large amounts of sea level rise into global coastal communities. (I've explained this here.)

This redrawing of coastlines can happen very quickly. We know from paleoclimate evidence that sea levels can rise several meters within a century. We know that the last time global temperatures were 2 degrees C higher than pre-industrial measures, sea levels were 50-80 feet higher than they are now. Given current rates of warming, we will be at that temperature level as a globe in ~2045, just twenty years from now.

We are not, as a nation, adequately putting these highly disruptive risks on the table and dealing with them. The insurance industry sees them and property values are beginning to reflect these risks, but tinkering with insurance—which focuses on transferring risk—is not the same thing as actually lowering physical risks to actual people and communities.

Many scientists (example) criticize Hansen. I am not a scientist. He says that a kind of groupthink and "scientific reticence" (explained here) is driving researchers to depend on complex global climate models that run on supercomputers without adequately questioning those models. "Young researchers," he says, "note that heavy emphasis on global climate models tends to crowd out a wel-balanced focus on underpinning science issues, critical thinking, theoretical comprehension, and communication." In other words, there are gatekeepers signaling ranges of acceptable discourse that are fundable and respectable. Even in my shorter, nonscientific life, I've seen that cultural model play out in several disciplines, and I am sympathetic to people who show their work and do their best to engage.

Here, Hansen is saying "if you look closely at the world and observe what is actually happening, the consensus models are not serving us well." He says that there are "some pretty awful models" out there and urges other scientists to examine carefully the range of assumptions and treatments of different physical processes included in those models. He describes a kind of "model fog" which leads to faulty models being treated as if they are "a probability distribution for the real world or even a sharp tool useful for climate analysis."

Critics' first response to Hansen's recent paper was to assert that there is no significant acceleration of global warming. Hansen's response: "This is an issue where it seems best to let others and the real world provide the response."

Meanwhile, January 2025 was the hottest on record.

Critics also say, essentially, "if warming is accelerating, our models adequately capture what's going on, so there is no good reason to think that anything is happening outside our assumptions."

I am not qualified to say Hansen is right. I do think humans are likely to rely on complex mathematical models whose assumptions may be wrong. More fundamentally, I do think the physical risks we confront are real, are growing swiftly, and that we would be wise to face them now. No country is doing this well at the moment.

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* According to Hansen's paper, about 40 percent of this upward temperature climb will happen within ten years following the GHG doubling. It will happen over the lands on which we all live. It will take longer for the ocean to similarly warm, because it is so large (it has "great thermal inertia," in Hansen's language). But people live on land.

Climate will change by about 8 degrees C over 1,000 years (orange line) following CO2 doubling, Hansen's recent paper suggests. Forty percent of this warming will happen after 10 years, 60 percent in 100 years, and 90 percent in 1,000 years.

When might this doubling happen? If emissions continue at their current rate, perhaps thirty to fifty years. Worst case: We could be at 3.2 degrees C above pre-industrial temperatures just forty years from now. How old are your children? Your grandchildren?