Sunday, November 8, 2020

The Great Pause Week 34: Dragon Dimming

"Pandemic lockdowns curtailed jet travel, closed shops and schools, and reduced global dimming by more than 20%. So what happened then?"


 There was a famous experiment on May 21, 1946 that cost the Manhattan Project physicist Louis Slotin his life. In 2016, Alex Wellerstein revisited it for The New Yorker

Slotin’s procedure was simple. He would lower a half-shell of beryllium, called the tamper, over the core, stopping just before it was snugly seated. The tamper would reflect back the neutrons that were shooting off the plutonium, jump-starting a weak and short-lived nuclear chain reaction, on which the physicists could then gather data. Slotin held the tamper in his left hand. In his right hand, he held a long screwdriver, which he planned to wedge between the two components, keeping them apart. As he began the slow and painstaking process of lowering the tamper, one of his colleagues, Raemer Schreiber, turned away to focus on other work, expecting that the experiment would be uninteresting until several more moments had passed. But suddenly he heard a sound behind him: Slotin’s screwdriver had slipped, and the tamper had dropped fully over the core. When Schreiber turned around, he saw a flash of blue light and felt a wave of heat on his face.

Subsequent calculations put the total number of fission reactions at about three quadrillion. This radioactivity excited the electrons in the air, which, as they slipped back into an unexcited state, emitted high-energy photons — a blue flash. Slotin swept the tamper aside to end the reaction, but it was already too late. He had been machine-gunned by a billion atomic bullets and a wave ray outside the visible spectrum. His bone marrow was already reacting like microwave popcorn. The man who assembled the heart of the Trinity device died nine days later, at the age of thirty-five.

In 2020, humans tickled the dragon’s tail in a different way. A paper published in Nature Communications in 2019 calculated that as little as a 20% reduction in industrial activity will drive a 1°C spike in temperature within days or weeks by removing the cooling effect of aerosol pollution (global dimming). The estimate, taken from improved satellite instruments, doubled the risk cited in the most recent IPCC report (AR5) so was greeted with skepticism. Then came Covid.

Pandemic lockdowns in the Spring of 2020 curtailed jet travel, closed schools, stores and restaurants, brought birdsong back to cities, and converted previously busy superhighways to cannonball race tracks. It also reduced global dimming by more than 20%.

According to pre-pandemic knowledge as epitomized by the 2019 Nature Communications paper, losing global dimming should have brought an average global surface temperature rise of 0.5 to 1.1°C, and precipitation increases of 2.0 to 4.6 percent, along with an uptick in extreme weather. This would all happen within weeks to months, not years to decades. 

Alarmingly relevant, a research note from Hansen and Sato on October 14 reported a steeper than expected rise in global surface temperature for the past 5 years. After eliminating the usual suspects — greenhouse gases, solar gain, ocean warming, ice melt — they concluded “that the warming acceleration must be due to the one other large climate forcing, atmospheric aerosols” (specifically, their diminishment). In other words, dimming had already been underway due to pollution controls or economic downturn prior to Covid and warming was picking up as a result.

But then the October issue of Nature contained an article by 14 climate scientists who studied global dimming between early February and the end of June. Google mobility trends which track phone movements indicated that more than 80% of the population in the 114 countries in the dataset (4 billion people) reduced their travel by more than 50%. Home energy use increased 4% while air pollution from industry declined. The Nature paper reported that while reduced sulfur emissions weakened the cooling effect by 20%, which should have been a cause for concern, the drop in nitrogen emissions (NOx) reduced greenhouse warming as much as 30%, cancelling the SO2 effect and possibly making the Earth a little cooler than it otherwise might have been. Because of built-in lag times, they expect further reductions of upper atmospheric CO2 around 2 ppm in two years’ time. If we see that in the Mauna Loa data it should validate this finding. The paper went on to discuss the difference to climate change depending on whether the economic recovery to Covid-19 is driven by a green stimulus package or an increase in fossil fuel use.

The pandemic lockdown tickled the dragon’s tail only to discover fears of the McPherson Paradox (kicking the fossil habit speeds warming) are perhaps overblown.

We came away considerably better for this experiment than Louis Slotin did in 1946. Once the US election is decided, we can go full steam ahead with the Green New Deal.


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