written 29 September 2019
published 6 October 2019
Risk assessment considers both the magnitude of consequences and the likelihood of occurrence. For example, a large asteroid impact is relatively rare, but the consequences can be devastating. An asteroid just 6 miles in diameter hit the Earth 65 million years ago, releasing energy equivalent to 100 million megatons of TNT, driving 75% of all life to extinction. In 2029, an asteroid 370 meters in diameter will come within 20,000 miles of Earth, a very near miss. This size impacts Earth about once every 80,000 years, releasing the energy equivalence of 1,200 megatons of TNT, 200 times the largest hydrogen bomb blast. Because of the severity of impact consequence, despite the rarity of the event, billions of dollars are spent every year to monitor the skies for incoming objects. If a large one on an intersection course is detected, plans have been made to respond, requiring years of preparation and investment of tens of billions of dollars.
Abrupt climate change has similar life extinction consequences, and should be treated with the same concern. While this destruction unfolds over a longer time frame than an asteroid impact, the odds are much shorter, estimated at 1 in 5 over the next century. We have had warnings for over 50 years, and evidence of increasing change is in the news every week. While some people think this is a hoax, or a slow moving problem to worry about in the future, the climate is a massive, complex, highly fed back, non-linear system, which has tipping points resulting in abrupt, irreversible changes.
For many years Arctic sea floor methane has been identified as one of the tipping points. The Arctic is warming 2-3 times faster than the rest of the planet, and summer ice covers less of the Arctic Ocean each year. Multi-year ice, thicker and more resistant to melting, is disappearing. The ice-free ocean is dark, absorbing heat which melts ice faster. The Arctic is a very shallow sea, with almost 2,000 gigatons of frozen methane in the sea floor sediments. At some point, heating resulting from decreased Arctic ice will raise sea floor temperatures enough to release seafloor methane, heating the Arctic more, releasing more methane, in a runaway scenario. Over the short run, methane is 80 times more potent than carbon dioxide as a greenhouse gas, and a sudden release of 50 gigatons would cause a 1°C temperature rise within months.
In the last 30 years humans have burned half of all the fossil fuels ever extracted. Roughly half the resulting carbon dioxide has remained in the atmosphere, a quarter has been taken up by plants, and the remaining quarter has been absorbed in the ocean, where is changes to carbonic acid, increasing ocean acidity 1,000 times faster than previous geologic rates. A recent MIT report described a mechanism of an ocean acidification tipping point, derived from geologic records.
Increased ocean acidity makes it difficult for organisms to form calcium carbonate shells. Normally, when these animals die, the weight of the shells causes them to drop to the ocean floor, sequestering carbon in the process. Without this shell driven carbon flux to the ocean floor, acidity at the surface rises, increasing the lethality of the water and reducing the capacity for the water to absorb more carbon dioxide. Eventually increased acidity crashes the ocean ecosystem, source of food and oxygen, increases atmospheric carbon dioxide levels, and traps more heat.
Once we pass extinction tipping points, it is too late. The longer we delay action, the shorter the odds of avoiding this fate. Climate deniers have wasted so much time that any effective effort will now require massive and sustained investment, like the economic mobilization during WW2. To preserve our species, a declaration of a climate emergency is necessary to galvanize sufficient economic resources. In California alone, 24 counties or cities have already passed such a declaration. For more information go to: https://www.theclimatemobilization.org.
Increasingly erratic weather events, and recent climate strikes mobilized by students, are shifting public opinion. Younger people have less commitment to the status quo and are more likely to live with the unfolding disasters. Despite Republican and corporate attempts to denigrate them, we should be inspired by their youthful passion and recognize we are fighting for our lives.
