Severe thunderstorm season is upon us and dangerous storms are forecast to develop over the next few days in the Plains and Midwest. Global warming is often connected with increased extreme weather, but will it make warm season thunderstorms more severe? On balance, yes – the latest research concludes: models and theory point to an increase in severe thunderstorms as the climate warms.
Severe thunderstorms require two primary ingredients:
1) Energy or fuel supplied from hot, humid air to feed violent vertical storm motions (updrafts and downdrafts). A common metric for this is a quantity called CAPE, convective available potential energy.
2) Turning winds with altitude, or wind shear to help storms spin. Wind shear is driven by temperature contrasts and is a necessary ingredient for tornadoes.
As the atmosphere warms up, climate models predict an uptick in CAPE. But they also simulate decreasing wind shear because high latitudes are forecast to warm more than mid-latitudes, reducing temperature contrasts.
If CAPE – fuel for thunderstorms - increases, but shear – which helps thunderstorms spin – decreases, what’s the net effect? In a research article published last week, Harold Brooks, a researcher on climate change and severe storms at the National Weather Service, says the expectation is for more severe storms overall but, perhaps, fewer tornadoes:
….simulations, in general, suggest that the increase in CAPE will more than offset the decrease in [SHEAR] over the US, leading to more frequent environments favorable for severe thunderstorms.
…tornadoes are much less likely to increase and, in fact, are more likely to decrease, for a particular decrease in the mean shear.
Brooks writes damaging winds in thunderstorms in a warming world would come more from non-tornadic storms (i.e. straight line winds and microbursts) but that if “favorable environments” increased enough, tornadoes might not decrease.
A 2007 study in the Proceedings of the National Academy of Sciences developed some maps geographically describing the general increase in CAPE, decrease in wind shear and overall increase in severe thunderstorms days projected for the end of the 20th century (but note some regional differences). NASA republished these last week, and I provide them here.
Here’s how NASA describes the changes shown:
The maps above show the results of a model comparing the summer climate in 2072–2099 with the climate from 1962–1989. CAPE (top map) is predicted to rise enough to overwhelm a slight decrease in vertical wind shear (middle map), leading to an increase in severe thunderstorms (third map), especially in Missouri and coastal North and South Carolina. The modeling suggests that the increase in CAPE will be the strongest in the Southeast and the decrease in wind shear strongest in the Mountain West. The eastern United States will see more of an increase in days favorable to severe thunderstorm formation than the western part of the country.
These projections notwithstanding, forecasting exactly how thunderstorms change in a warming world remains an enormous challenge. As such, Brooks’ discussion of proposed thunderstorm characteristics in a warming world is full of qualifiers.
“The signals in the climate models and our physical understanding of the details of storm-scale processes are sufficiently limited to make it extremely hazardous to make predictions of large changes or to focus on small regions,” Brooks writes.
He also cautions against linking individual thunderstorms event to climate change.
“By their very nature, severe thunderstorms are rare events at any location … The presence of singular, or near-singular events, in the record cannot be used as evidence of trends,” Brooks writes.