We have seen a quantum jump in extreme weather events in the Northern Hemisphere in the last several years. Droughts, deluges, and heat waves are increasingly getting “stuck” or “blocked,” which in turn worsens and prolongs their impact beyond what might be expected just from the recent human-caused increase in global temperatures.
A growing body of research ties that unexpected jump to a weakening of the jet stream — in particular to “more frequent high-amplitude (wavy) jet-stream configurations that favor persistent weather patterns,” as a new study puts it.
Much of this new research ties the weakening jet stream to “Arctic amplification (AA) — defined here as the enhanced sensitivity of Arctic temperature change relative to mid-latitude regions,” in the words of the new study, “Evidence for a wavier jet stream in response to rapid Arctic warming” by Jennifer Francis and Stephen Vavrus. But that is no by no means a universally accepted explanation. I’ll review some of the evidence in this post.
Reinsurer Munich Re has the most comprehensive database of global natural catastrophes Their 2010 analysis, “Large number of weather extremes as strong indication of climate change,” concluded “it would seem that the only plausible explanation for the rise in weather-related catastrophes is climate change. The view that weather extremes are more frequent and intense due to global warming coincides with the current state of scientific knowledge.” For instance, a 2010 Journal of Climate study that found “global warming is the main cause of a significant intensification in the North Atlantic Subtropical High (NASH) that in recent decades has more than doubled the frequency of abnormally wet or dry summer weather in the southeastern United States.”
In 2011, Dr. Peter Höppe, Head of the Geo Risks Research Department at Munich Re explained what had persuaded him of the causal link:
For me the most convincing piece of evidence that global warming has been contributing already to more and more intense weather related natural catastrophes is the fact that while we find a steep increase in the number of loss relevant weather events (about tripling in the last 30 years) we only find a slight increase in geophysical (earthquake, volcano, tsunami) events, which should not be affected by global warming. If the whole trend we find in weather related disaster should be caused by reporting bias, or socio-demographic or economic developments we would expect to find it similarly for the geophysical events.
And that was before two years of off-the-charts extreme weather catastrophes, particularly in North America (in 2011, the head of NOAA said the record dozen billion-dollar weather disasters was “a harbinger of things to come.”
Then, in an October 2012 study, Munich Re linked the rapid rise in North American extreme weather catastrophes to manmade climate change: “Climate-driven changes are already evident over the last few decades for severe thunderstorms, for heavy precipitation and flash flooding, for hurricane activity, and for heatwave, drought and wild-fire dynamics in parts of North America.”
At the same time non-climatic events (earthquakes, volcanoes, tsunamis) have hardly changed, as the figure shows. Höppe said at the time: “In all likelihood, we have to regard this finding as an initial climate-change footprint in our US loss data from the last four decades. Previously, there had not been such a strong chain of evidence. If the first effects of climate change are already perceptible, all alerts and measures against it have become even more pressing.”
That same month a study led by the National Oceanic and Atmospheric Administration (NOAA), “The recent shift in early summer Arctic atmospheric circulation” concluded global warming was driving changes in extreme weather in North America. As NOAA explained at the time:
“Our research reveals a change in the summer Arctic wind pattern over the past six years. This shift demonstrates a physical connection between reduced Arctic sea ice in the summer, loss of Greenland ice, and potentially, weather in North American and Europe,” said [NOAA’s James] Overland, an oceanographer who leads the laboratory’s Coastal and Arctic Research Division.
The shift provides additional evidence that changes in the Arctic are not only directly because of global warming, as shown by warmer air and sea temperatures, but are also part of an “Arctic amplification” through which multiple Arctic-specific physical processes interact to accelerate temperature change, ice variability, and ecological impacts.
Arctic amplification is often explained this way: Warming melts highly reflective white ice and snow, which is replaced by the dark blue sea or dark land, both of which absorb far more sunlight and hence far more solar energy. But a key point, as NOAA indicates, is that it actually consists of multiple, synergistic effects, as I discussed here.
“Enhanced warming of the Arctic affects the jet stream by slowing its west-to-east winds and by promoting larger north-south meanders in the flow,” as NOAA explained. “The researchers say that with more solar energy going into the Arctic Ocean because of lost ice, there is reason to expect more extreme weather events, such as heavy snowfall, heat waves, and flooding in North America and Europe but these will vary in location, intensity, and timescales.”
Prof. Jennifer Francis of Rutgers — coauthor of the 2012 NOAA-led piece (and lead author of the new 2015 study) — said at the time, “What we’re seeing is stark evidence that the gradual temperature increase is not the important story related to climate change; it’s the rapid regional changes and increased frequency of extreme weather that global warming is causing. As the Arctic warms at twice the global rate, we expect an increased probability of extreme weather events across the temperate latitudes of the northern hemisphere, where billions of people live.”
Francis explained her findings in this 2013 video:
The path of the jet stream “typically has a meandering shape, and these meanders themselves propagate east, at lower speeds than that of the actual wind within the flow. Each large meander, or wave, within the jet stream is known as a Rossby wave.”
That brings us to an August 2014 study from a team of scientists from the Potsdam Institute for Climate Impact Research (PIK). That study offered a specific mechanism for why we’re seeing this quantum leap in extreme weather — some Rossby waves are stalling out for extended periods of time: “the study shows that in periods with extreme weather, some of these waves become virtually stalled and greatly amplified.”
As the PIK explained at the time:
Weather extremes in the summer — such as the record heat wave in the United States that hit corn farmers and worsened wildfires in 2012 — have reached an exceptional number in the last ten years. Man-made global warming can explain a gradual increase in periods of severe heat, but the observed change in the magnitude and duration of some events is not so easily explained. It has been linked to a recently discovered mechanism: the trapping of giant waves in the atmosphere. A new data analysis now shows that such wave-trapping events are indeed on the rise.
For a longer discussion of the details of that mechanism, see here.
Not every study comes to the same conclusion as NOAA, PIK, and Francis (see, for instance, here). One 2014 study claims to “disconfirm the hypothesis that deep tropospheric warming in the Arctic during OND [October, November. December has resulted substantially from sea ice loss.” But as Francis explained to me, the authors of that 2014 study “state that the first link in the ‘chain’ connecting rapid Arctic warming with a wavier jet stream, as proposed in our 2012 paper, is sea-ice loss — but in fact it is Arctic amplification (Arctic warming faster than mid-latitudes). While sea-ice loss is one of the factors contributing to Arctic amplification (AA), it is certainly not the most important factor — only 20% according to this study.” Francis also points out “their modeled response to sea-ice loss is presented as time-averages, so any signal of jet-stream wave amplification will not be detected unless the ridge/trough system occurs in the same place every time, which it often does not.”
Clearly the interactions between global warming and Northern Hemisphere weather are complex. We still have much more to learn about “Recent Arctic amplification and extreme mid-latitude weather,” as made clear in a recent Nature Geoscience paper (with that title) written by several of the leading researchers in the field, including Francis.
But the evidence is mounting that we have entered a new regime of extreme weather thanks to our as-yet unrestricted emissions of greenhouse gas. The latest 2015 study, by Francis and Vavrus, concludes:
These results reinforce the hypothesis that a rapidly warming Arctic promotes amplified jet-stream trajectories, which are known to favor persistent weather patterns and a higher likelihood of extreme weather events. Based on these results, we conclude that further strengthening and expansion of AA in all seasons, as a result of unabated increases in greenhouse gas emissions, will contribute to an increasingly wavy character in the upper-level winds, and consequently, an increase in extreme weather events that arise from prolonged atmospheric conditions.