Global Warming and Tornadoes

Last year was a record year for severe weather and this year is on track to be as bad or even worse. Although the northeastern US does not see too many tornadoes (see image #1), well-publicized images of the mass destruction and of 18 wheeler trucks flying around in the air in many parts of the US are causing many to ask if human-caused climate change is responsible.

Tornadoes need two ingredients: 1) warm, moist air and 2) wind shear.

As humans keep dumping massive amounts of heat-trapping gases such as carbon dioxide into the air, global temperatures have been rising rapidly. As a result of this warmer climate, there is more heat available for these thunderstorms. Warmer air is less dense than surrounding cooler air so increasing air temperature will increase the chances that air will rise and form clouds. As the planet warms, more water will evaporate from the surface which adds moisture to the air. Moisture provides the fuel for thunderstorms to grow. As air rises, the water vapor releases its heat which drives the air upward even higher and faster. This is why thunderstorm clouds (cumulonimbus clouds) are very tall. (Think of moisture as the gasoline and the cloud as the engine. Stepping on the gas increases the car’s power.) It is clear that human-caused global warming is increasing the heat and moisture of the atmosphere so that thunderstorms can get stronger and wetter.

Ingredient #1 is definitely increasing. Severe thunderstorms were expected to increase in a warmer world and they have.

Munich RE is one of the world’s largest reinsurers (they insure the insurance companies) so this company carefully monitors global disasters because it greatly affects their profits. Note image #2 from Munich RE which shows an increase in insured losses in the US from thunderstorms. As climate has warmed, losses have increased.

Image #3 from Munich RE shows how weather and climate-related insured losses have increased dramatically compared to other disasters such as earthquakes, volcanoes and tsunamis.

As humans keep warming the climate it is like injecting steroids into the weather. This video shows how warming the planet is like a baseball player injecting steroids. He may not hit a home run every time but his chances of doing so are greatly increased. There are more severe storms and they are becoming more intense.

So what about ingredient #2: wind shear? 

Wind shear, as demonstrated in this video, is the “twist” in the atmosphere created by winds moving from different directions and different speeds with increasing height in the air. It is this shear that causes the rotation in a thunderstorm which may result in a tornado. The latest research suggests that human-caused climate change may be weakening the average wind shear which means there may be less opportunity for thunderstorm rotation and tornadoes.

Ingredient #2 may be decreasing. A warmer world might cause less wind shear meaning fewer tornadoes.

Unfortunately, there is not enough data to determine which ingredient will “win” out in the long run. Scientists use models to project the future. Climate models work very well on a large scale and have been quite accurate in projecting global temperature trends but given the very small size of tornadoes, today’s models cannot simulate tornadoes well enough to have any confidence in future projections.

A very good summary of our current understanding regarding a link between severe thunderstorms and tornadoes can be viewed here: A brief assessment of the impact of large-scale climate change on severe thunderstorms and tornadoes. This document was prepared for The White House by members of the Climate Science Rapid Response Team which I co-founded in 2010. The conclusions of this report are:

Human activities have made a substantial contribution to the recent warming of the Earth’s surface and moistening of the atmosphere. We know this with high confidence. It is very likely that these large-scale changes in climate have influenced – and will continue to influence – many different types of extreme event, such as heavy rainfall, heat waves, and flooding. Large-scale climate change is also likely to affect small-scale phenomena like severe thunderstorms and tornadoes. The nature and the degree of influence are very uncertain, particularly for tornadoes.

Model simulations can provide us with useful information about likely changes in the large-scale environmental conditions which affect severe thunderstorms and tornadoes. At present, such simulations cannot actually represent these small-scale phenomena.

Although we do not see too many tornadoes around these parts, Long Islanders are not immune from the impacts of climate change, particularly rising sea levels and more intense hurricanes. Mark your calendars for Friday April 27. I will be presenting a talk on how climate change will affect us here in New York. The talk is open to the general public and will be held at Suffolk County Community College in Selden. Location: Smithtown Science Bldg. Room T-109 (Location D on map). Time: 7 – 8 PM.