What does it mean that (once rare) atmospheric rivers and bomb cyclones are becoming more frequent?

Atmospheric rivers and bomb cyclones are becoming increasingly frequent and intense parts of the North American meteorological landscape. Atmospheric rivers can cause enormous flooding events across the country. Bomb cyclones can combine hurricane conditions with Arctic cold to produce enormous personal risk from exposure to potentially deadly cold. Sometimes they occur at the same place at the same time. When that happens, as in December of 2022, they complement each other — arguably producing combined impacts that are greater than the sum of their singular impacts taken one at a time in isolation.

The undeniable increase in frequency and intensity of one or the other or both of these extreme meteorological extremes are, at very least, perfectly compatible with projections of climate change driven by our warming our planet — especially when the North American experience is considered in a global context that includes thousands of record high temperature reading across Europe at the very same time. Whatever explains data from these events must be global is scope

Given that strong finding based on honest science, there is simply not enough evidence to support any contrarian hoax view: the flawed belief that the connection between global warming driven by human emissions of greenhouses gases and increasing climate change impacts is a hoax. It is impossible to prove a negative, and here is an example of why.

This is especially true because the impacts of atmospheric rivers and cyclones reach across the entire continent whenever they occur. The incidence of deadly flooding events from atmospheric rivers has extended from the West Coast into Kentucky. The frigid cold from Arctic bomb cyclones extends as far south as Texas, as far west as the Rocky Mountains and as far east as coastal New England coastline.

Add the European heat wave to the equation and it is clear that whatever is causing these extremes must be explained in terms of a continental phenomenon — something like the jet stream. See, for example, the global map and discussion from National Oceanic and Atmospheric Administration (NOAA). The common thread is that a weakening of the jet stream that has been caused by the exaggerated warming of the Arctic in comparison with the modest increases in temperatures across the tropics. The diminishing temperature difference from pole to equator is a signature of global warming.

With regard to the continental climate, this diminishment weakens the jet stream and allows Arctic vortices to be push the cold close to the Gulf of Mexico and makes way for atmospheric rivers to invade from the western Pacific Ocean. It also allows tropical heat to push its way northward in western Europe. Nothing else can “switch signs” like that.

So, what is the extra harm? Extreme weather happens all of the time. Yes, but atmospheric rivers have hitherto been so rare that few locations are prepared. Winter polar vortexes have started to become the norm in North America, and nobody foresaw the December heat wave in Europe.

In Kentucky, 39 people died in a flash flood in 2021 due to an inadequate warning system. Flooding stranded 1,000 people in Death Valley. California was ravaged by enormous rainfall events whose intensity has been connected to climate change. A massive December 2022 winter storm driven by an expanding polar vortex across the middle of North America affected over 200 million Americans. At least 3,000 flights were canceled due to dangerously cold conditions.

Many eyes were on Texas, where 210 people died in the bomb cyclone of February 2021. Many blame the state’s self-contained and isolated electric grid that completely failed. Some Texans had no power for more than a week depending on location. In 2022, Texas dodged a bullet because it had a few some adaptive changes. The grid held this time, but barely.

Combining these lines of evidence, it is clear that both atmospheric rivers and bomb cyclones in 2022-23 should not be taken as one-off weather anomalies. That they are a part of a global fingerprint of climate change that cannot be disproven, especially when they are combined with the thousands of record temperatures across Europe at the same time. They are likely fingerprint of climate change. They are harbingers of what we can expect with increasing frequency as the planet moves toward a new normal (sometime in the next century when things settle down). One can only imagine what terrible things will happen in the meantime. (Such cautious language was carefully chosen.)

Benjamin Santer, a member of the United States National Academy of Science, Engineering and Medicine and global recognized as a climate science expert, has written that it is as dangerous to:

• Express too much certainty in the links between warming and the confluence of atmospheric river and a polar vortex cyclone bombs as it is

• Argue that that event is evidence that climate change driven by global warming is a hoax

Attribution science at the continental scale is in its infancy, but the consequences of these events are undeniably enormous. So even with a modest “compatible with global warming” finding that leads to medium confidence in the connection between global warming and these events, principles of risk management teaches a simple lesson: Pay attention, people.

This could only get worse as the planet lurches to a new and hotter normal that will be established sometime in the next century.

Gary Yohe, Ph.D., is the Huffington Foundation professor of Economics and Environmental Studies, Emeritus at Wesleyan University.