Earlier this week, we took a crack at answering a federal judge’s questions about climate science for readers who may have asked similar ones. And on Wednesday, Judge William Alsup finally got his answers during a five-hour hearing.

The case was brought by the cities of San Francisco and Oakland against a handful of oil companies that those cities feel should help pay for the effects of sea-level rise. This claim centers on the idea that internal documents show the companies knew climate change was human-caused even as they publicly campaigned otherwise in the 1980s and 1990s.

Before the case gets rolling, Judge Alsup wanted each side to put their climate science cards on the table, establishing any disagreement on how our knowledge has evolved over time. He also wanted a few questions of his own answered about why we are certain humans are responsible for global warming.

Approach the bench

San Francisco and Oakland brought in a trio of climate scientists to do their presenting for them. First up was Oxford’s Myles Allen, who tried to summarize more than a century’s research on greenhouse gases and sources of CO 2 emissions in an hour. As Allen walked through physics and history, the judge frequently interjected clarifying questions, seemingly soaking up the education.

Allen then handed off to UC Santa Cruz researcher Gary Griggs, who focused on global sea-level rise and its impact on California. Griggs put the recent (accelerating) sea-level rise trend in the context of the last 20,000 years, and he highlighted the effect of short-term regional sea-level events coming on top of an ever-rising baseline sea level.

(We'll come back to the third scientist.)

The defendants, on the other hand, had Chevron lawyer Theodore Boutrous do all the talking. (After Boutrous surprisingly stated that he would speak only for Chevron, the judge gave the other defendants two weeks to decide whether or not they agreed with everything presented.) As expected, Boutrous did not deny that humans have caused climate change, but he did seek to paint climate science as much more uncertain in the recent past.

This likely sets out the strategic foundation for the case. Boutrous said Chevron accepted the conclusion of human responsibility in the last two Intergovernmental Panel on Climate Change reports (going back to 2007), but the company attempted to paint earlier reports as fundamentally uncertain.

To do this, the attorney cherry-picked some of the cautious language on uncertainty going back to the first report in 1990, which said we could not yet calculate how much warming had been caused by humans. But that report also clearly said that greenhouse gas emissions were causing warming and would have to be greatly curtailed to avoid dangerous climate change and sea-level rise. And as the plaintiff’s presentation pointed out, our estimate of how much warming results from greenhouse gas emissions hasn’t really changed since a 1979 National Academy of Sciences report on the topic.

By sticking to the IPCC reports, the defendants seem to be planning to argue that oil companies could fairly disagree about climate change as long as climate scientists were saying some uncertain things. To do this, though, they’ll at least have to do a better job of accurately representing the reports.

Because this case focuses on the impacts of sea-level rise, Boutrous turned next to uncertainties in those projections from the 2013 IPCC report, characterizing sea-level rise as still unclear and hard to quantify (without denying that some of it is human-caused).

Finally, Boutrous argued that it’s the use of fossil fuels—rather than the production and sale of them—that causes climate change. He pointed out that the IPCC report discusses population growth, land use, and energy use but not fossil fuel extraction. This is the obvious argument for the companies to make, despite the fact that the case explicitly targets their behavior as sellers.

Boutrous said Chevron didn’t necessarily agree with everything in the sections of the reporting on impacts or mitigation of climate change, but he did call out IPCC descriptions of climate change as a global collective-action problem.

The rest of the story

When Boutrous was finished, the plaintiff's side had one more presenter: Don Wuebbles of the University of Illinois. Wuebbles helped write the last two IPCC reports, as well as the 2017 US National Climate Assessment, so he had a few things to say about Boutrous’ remarks.

Wuebbles emphasized that climate science didn’t stop in 2012, when the last IPCC report was being put together. Specifically, research on sea-level rise has allowed for clearer projections in the last few years. That includes distinctly possible scenarios of much higher sea-level rise than the heavily caveated, conservative IPCC numbers.

As the case plays out, the defendants’ differing version of climate science’s certainty over time will play a role. The lawyers for San Francisco and Oakland, on the other hand, will likely try to counter that internal documents show these companies weren’t befuddled by scientific uncertainty, rather they were consciously combatting scientific certainty.