Human caused greenhouse gas pollution has warmed the planet. Global warming means more extreme weather. Many meteorologist who watch the weather every day see this. More and more research shows that greenhouse gas pollution changes the weather in a way that causes even more change in the weather. Changing weather systems means more lightning, increased high precipitation events in certain regions like the US Northeast, including more frequent large snow storms.
Global warming has had uneven effects. The Arctic has warmed relatively more than most of the rest of the planet. The major movements of air masses are driven by a combination of the rotation of the Earth and the movement of extra heat from the Equator towards the poles, a process that sets up the trade winds and the jet stream. But the additional warming in the Arctic has changed this pattern measurably, resulting in these and other changes in weather patterns.
A new study out in Science, by Dim Coumou, Jascha Lehmann, and Johanna Beckmann, “The weakening summer circulation in the Northern Hemisphere mid-latitudes,” demonstrates that storm activity in much of the Northern Hemisphere has changed in a way that matters to our weather, and is likely to change more in the future. From the abstract:
Rapid warming in the Arctic could influence mid-latitude circulation by reducing the poleward temperature gradient. The largest changes are generally expected in autumn or winter but whether significant changes have occurred is debated. Here we report significant weakening of summer circulation detected in three key dynamical quantities: (1) the zonal-mean zonal wind, (2) the eddy kinetic energy (EKE) and (3) the amplitude of fast-moving Rossby waves. Weakening of the zonal wind is explained by a reduction in poleward temperature gradient. Changes in Rossby waves and EKE are consistent with regression analyses of climate model projections and changes over the seasonal cycle. Monthly heat extremes are associated with low EKE and thus the observed weakening might have contributed to more persistent heat waves in recent summers.
The study has been written up by Chris Mooney, in the Washington Post, and Roz Pidcock at Carbon Brief. From the study’s press release,
“When the great air streams in the sky above us get disturbed by climate change, this can have severe effects on the ground,” says lead-author Dim Coumou. “While you might expect reduced storm activity to be something good, it turns out that this reduction leads to a greater persistence of weather systems in the Northern hemisphere mid-latitudes. In summer, storms transport moist and cool air from the oceans to the continents bringing relief after periods of oppressive heat. Slack periods, in contrast, make warm weather conditions endure, resulting in the buildup of heat and drought.”
“Unabated climate change will probably further weaken summer circulation patterns which could thus aggravate the risk of heat waves,” says co-author Jascha Lehmann “Climate simulations for the next decades, the CMIP5, remarkably show the same link that we found in observations. So the warm temperature extremes we’ve experienced in recent years might be only a beginning.”