Environmental Change Institute, University of Oxford

Photo: Adobe Stock / Kagenmi

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Professor Myles Allen writes about his latest assignment - to explain the history and science of climate change to a judge in a San Francisco court - in a case brought by American cities against the oil companies.

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A couple of weeks ago, I had an unusual — and challenging — assignment: providing a one-hour “tutorial" on the basic science of human-induced climate change to a Federal District Court in San Francisco. Judge William Alsup had requested this tutorial to bring him up to speed on the fundamental science before proceedings begin in earnest in a case brought by the cities of San Francisco and Oakland, on behalf of the people of California, against a group of major fossil fuel companies, addressing the costs of climate change caused, they argue, by products those companies have sold.

The format was straightforward — two hours each for the plaintiffs and the defendants, and the judge had provided us with a series of questions on the essential physics that he wanted addressed, as well as requesting a timeline of how our understanding of climate change has evolved over the past 150 years. My presentation was followed (also speaking for the plaintiffs) by Professors Gary Griggs, showing detailed projections of sea-level rise and its impacts on California, and Don Wuebbles, presenting key findings from the latest US National Climate Science Special Report. Between Gary and Don, the Court heard from Theodore Boutrous, a lawyer speaking on behalf of Chevron, one of the defendants.

The case was fairly widely covered, (here’s an example) and most of the attention was, understandably, on what the oil companies had to say: the fact that Gary, Don and I agreed with the Intergovernmental Panel on Climate Change was hardly ever likely to be newsworthy. But I’ve had a few requests since about what I presented — including from some students who spotted that a carefully compressed summary of climate change science might be quite handy revision material. So, with exam season nearly upon us, here it is — or at least, here is what I would have presented if I’d got through it all: in preparing this material, I had completely failed to anticipate the number and depth of Judge Alsup’s questions, so we only got as far as the Charney Report.

Prior to the hearing, Andrew Dessler on Twitter, Gavin Schmidt at RealClimate and Oliver Milman at the Guardian all had a crack at the judge’s questions, so you might like to compare our answers.

I was definitely more ambitious — one journalist called my presentation “dense”, which I think is pretty derogatory coming from a journalist — but there was no question Judge Alsup wanted to go deeper than the kind of material that (in my experience) most journalists are prepared to tolerate. I go into more detail than Gavin, Andrew and Oliver on how attribution works, partly because that’s what I do, but also because just telling the judge “the IPCC says the warming is pretty much all human-induced and 80% of that is CO2” would have been a bit circular, having been involved myself in those IPCC assessments since the 1990s.

My contribution had its ups and downs — a low point was definitely when Judge Alsup declared “your chart sucks” in response to a powerpoint slide which showed an artist’s impression of the Nimbus 4 satellite at the expense of a graph of how the spectrum of outgoing long wave radiation changed in response to rising greenhouse gases between 1970 and 1997. Frustratingly, the chart he wanted (from John Harries’ 2001 paper) was hidden under the pretty picture, but we were already late and I chickened out of breaking open the powerpoint to move figures around in a live courtroom. But the high point came just a few seconds later, when he asked “so, how much did the temperatures [of carbon dioxide molecules emitting energy to space in those critical wavelengths of the infrared] fall over those 27 years?” — showing that, after only half-an-hour, and despite my obscure charts, he had already got a better grasp of the basic mechanism of the enhanced greenhouse effect than many a lifelong specialist in climate change communication.

I’ve restored that spectrum to its rightful place in this version, as well as adding some more material on molecular dipoles at the beginning, since Judge Alsup (and others since) had questions about how it was that carbon dioxide molecules could act on infrared radiation over a much larger volume than the molecules themselves actually occupy. I’ve also added some more material later on to address other questions that came up. This is just provided for your interest: the material I actually covered at the time is all available on the court record.

The edited presentation runs for just under 45 minutes, and I’ve broken it up into five segments. I’ve also put up the powerpoint in case you want to use some of the graphics in your own teaching. I hope it’s useful.

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Tutorial: The basic science of human-induced climate change

"... after only half-an-hour, and despite my obscure charts, he had already got a better grasp of the basic mechanism of the enhanced greenhouse effect than many a lifelong specialist in climate change communication."