We're scientists. We know the climate's changing. And we know why.

It was reported in the New York Times (gift link) that U.S. Treasury Secretary Scott Bessent recently claimed that scientists don't know why the climate is warming, calling the reasons "difficult to deconstruct" and dismissing climate change as a belief held by the "elite." This is not a new argument. In 2018, when Trump made basically the same claim, the Houston Chronicle asked Dan Cohan and me to write an oped responding to it. It remains one of my favorite opeds, and since the argument clearly hasn't gone away, I thought it was worth reprinting here.

At this point, just about everyone recognizes that the climate is changing. Even Donald Trump says, “I think something’s happening.” Now, the question being debated is why the climate is changing.

Though there may be a public debate, there’s no debate among scientists like us — decades of research have demonstrated that human activities, primarily the emission of carbon dioxide from the combustion of fossil fuels, are driving the climate change we are experiencing.

To understand why we are so confident, it’s useful to think about climate change as a whodunit. Climate does not change by itself, so scientists are detectives trying to solve the mystery of what has been warming the Earth for the past century.

Because we know that the climate varied before humans were burning fossil fuels, there are clearly other mechanisms besides humans that can cause change. So, the first thing that scientists do is study these mechanisms to see if they could be the culprit.

One possible non-human mechanism is the brightness of the Sun. If the Sun has been getting brighter, then that could explain the warming. The Sun, however, has an airtight alibi — we have direct measurements of the output of the Sun from satellites, and we observe that the Sun has not gotten any brighter. One suspect down.

Another possibility is the orbit of the Earth. We know that ice ages are paced by small wobbles in the Earth’s orbit, so one might wonder whether this could be causing the present warming. However, Earth’s orbit changes too slowly and is now in a phase that should be slowly cooling temperatures. Another suspect down.

Volcanoes can cool the atmosphere for a year or two. But that can’t explain decades of warming. Another suspect down.

There is an entire list of suspects that scientists have looked at, and they have not identified a single viable one. With one exception — greenhouse gases.

Police shows sometimes feature the “world’s dumbest criminal” — who doesn’t wear gloves, leaves fingerprints all over the house, drops his wallet at the crime scene, is caught on videotape exiting the crime scene, brags to his friends that he committed the crime — and when he is finally arrested has evidence of the crime in his pockets.

Carbon dioxide is like the world’s dumbest criminal — it leaves evidence all over the place that it’s guilty. First, the laws of physics tell us that adding carbon dioxide, or any other gas that absorbs infrared radiation, to the atmosphere should warm the planet.

Second, we are 100 percent sure that humans are adding carbon dioxide to the atmosphere. Just based on that, Nobel Prize-winning scientist Svante Arrhenius predicted in the late 19th century that humans would warm the climate. And, sure enough, the climate is warming.

The timing of warming, beginning just after the industrial revolution, and the magnitude of the warming, match our theories almost exactly. The figure below shows that the rapid warming of last few decades was accurately predicted in 1975.

Observed temperatures are from NASA GISS; predicted temperatures are from W.S. Broecker, “Climatic Change: Are We on the Brink of a Pronounced Global Warming?,” published in Science, 189, 460-463, 1975. The dotted line shows when the prediction was made, 1975.

Such predictions are the gold standard of science — if you can make a non-obvious prediction about some physical system, then it means that you understand something fundamental about it. This prediction shows that we really understand the warming of the climate system.

Finally, the geologic record is filled with evidence that greenhouse gases impact the climate. For example, during an event about 55 million years ago known as the Paleocene-Eocene Thermal Maximum, a huge amount of greenhouse gases was released into the atmosphere. At the same time, temperatures spiked. Then, as the greenhouse gases were removed from the atmosphere during the following 100,000 years, temperatures slowly returned to what they were before.

This is why scientists are so confident that human emissions of carbon dioxide are warming the climate — there is a mountain of evidence supporting that explanation and no plausible alternative suspects. In this whodunit, you would have no choice but to arrest carbon dioxide for warming the planet.

related stuff

Gavin Schmidt published an oped (gift link) in the New York Times the same day ours came out, which is also worth reading. He took a similar approach, investigating the warming as if it were a crime and evaluating the suspects.

In the years since this oped was written, the evidence connecting increasing carbon dioxide to a warming climate has gotten a lot stronger. In particular, a lot of terrific work has been done on the paleoclimate record. As Zeke wrote a while back on The Climate Brink, it confirms that carbon dioxide is indeed a primary controller of the climate.

If you’re interested in more details about this, I would point you to a talk by Richard Alley entitled “The biggest control knob”. It’s a really terrific explanation of how the paleoclimate record supports the primacy of carbon dioxide as THE most important regulator of our climate.

I was in the room at the 2009 AGU meeting for this and watched it live. It remains one of the best scientific talks I’ve ever seen.

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