Will a Warmer World Be Stormier?
Ed. note: A longer version of this review article, with many references, first appeared in the on-line magazine Earthzine.
Among the most important potential impacts of anthropogenic climate warming are changes in extreme weather. Wet regions will become rainier while arid and semi-arid regions expand and become drier. Equally important are the winds, hail, lightning, and fires that result from storms. Will we see more frequent storms in a warmer climate? Stronger storms? Will storm damage escalate? These questions are much harder to answer. Each different type of storm occurs for different reasons under different conditions. Changes in weather can only be inferred indirectly from climate models from changes in larger-scale environmental conditions. Unlike precipitation, for which long and reliable historical records exist, similar records for other aspects of weather are too short and/or observationally biased to detect trends.
No single extreme event is evidence of climate change. Yet taken together, unusual weather has given the public a sense that climate is changing, as demonstrated in a recent Yale-George Mason poll. We describe here some of what the scientific community has learned about how climate change will affect storms, and what still is not understood.
Thunderstorms receive less attention than other types of extreme weather because they are so common and the damage they cause so localized. Perhaps because they are frequent and taken for granted, lightning strikes are the second leading cause of weather-related deaths in the United States (after floods). U.S. lightning mortality has decreased by almost a factor of 3 over the past 3 decades, probably due to better forecasts and societal/demographic factors rather than because of any change in lightning occurrence to date.
Thunderstorms are due to convection: Heating of Earth's surface by sunlight and infrared radiation causes water to condense as buoyant air rises. When updrafts are vigorous, water drops are carried above the freezing level, a necessary ingredient for lightning. As CO2 increases the land surface warms, making stronger updrafts that are more likely to produce lightning. In a doubled CO2 climate, one model estimates that the western U.S. will see fewer lightning storms overall, but 25% more of the strongest storms, with a 5% increase in lightning.
Lightning damage should also increase because of its role in igniting forest fires (Fig. 1). In western North America, wildfire area burned has increased dramatically in recent decades. The primary reason is drying as the temperature has risen, creating more "fuel" for fires. Climate models show the southwest U.S. drying in the 21st Century as the tropical Hadley circulation expands poleward. A recent GCM study projects tens to hundreds of additional fire counts per year per 4°×5° latitude-longitude area of western North America. Only 12% of U.S. wildfires are ignited by natural causes, but these account for 52% of the acres burned, so even a small climate change in lightning may have important consequences.
A small subset of thunderstorms occurs when wind shear — a change in wind speed and/or direction with height — is strong. Wind shear creates a horizontal "tube" of rotating air. When a thunderstorm arises in shear, rising air tilts the rotating tube into the vertical. If the thunderstorm and wind shear are strong, the result is a "significant severe thunderstorm" with large hail, strong wind gusts, or a tornado of strength F2 or greater.
Wind shear is controlled by the rotation of the Earth and by horizontal temperature differences. As climate warms, the temperature difference between low and high latitudes decreases, reducing wind shear. This competes with the strengthening of thunderstorms — which one wins? A recent study finds that the central and eastern United States will have more days per month favorable to severe storms in a doubled CO2 climate. A more conservative estimate finds that although thunderstorms will be stronger, wind shear will more often be below the threshold for severe storms. However, combined occurrences of the strongest wind shears and updraft speeds will increase, suggesting more of the most significant severe thunderstorms.
The end of the spectrum of severe thunderstorms is tornadoes (Fig. 2), which though much more rare than thunderstorms, account for almost as many fatalities and much more damage. Tornadoes are more difficult to predict than other severe thunderstorms, even climatologically. A record spanning the past half century shows a dramatic increase in U.S. tornado reports but no trend in the number of days with tornadoes and a small decrease in strong (category F2-F5) tornadoes. A possible clue to the discrepancy is that the U.S. population increased at the same time, suggesting that it may be an artifact of greater tornado detection due to increases in population density, awareness of severe weather threats, and modern technological advances such as Doppler radar. At the current time it is not possible to anticipate the sign of any climate change in tornado occurrence or strength.
Hurricanes and other tropical cyclones take energy in by evaporating warm ocean water, and eject it at a colder temperature after air rises and water condenses in the eyewall. The long-term record of Atlantic tropical cyclones is biased because many that never made landfall were not detected before the satellite era. Nonetheless, an upward trend in Atlantic and West Pacific tropical cyclone power dissipation in recent decades is correlated with a warming sea surface. Extrapolating to a warmer climate suggests a dramatic increase in hurricane destructiveness. Recent research suggests, however, that the relevant factor may not be how warm a given ocean is, but how warm it is relative to other ocean basins. In the late 20th Century the Atlantic was anomalously warm, but this is not expected to continue in the long-term. Projections of relative rather than absolute sea surface temperature changes suggest little future change in hurricane destructiveness. One model study concludes that there will be fewer Atlantic tropical cyclones overall but more category 4-5 hurricanes, but one of the four models examined disagrees.
Midlatitude synoptic storms, associated with low pressure centers and warm and cold fronts, arise from horizontal temperature differences. Since the polar regions should warm more than the tropics in the future, synoptic storms might weaken. However, at higher altitudes, the reverse is true, and one model suggests that this causes the storm tracks to intensify. Also, since warmer air contains more water vapor, latent heat released when water condenses may intensify future synoptic storms. Most climate models simulate fewer synoptic storms overall, but more of the strongest storms, as the climate warms, but a recent higher-resolution model study disagrees.
Are recent unusual winters related to climate change? It is not clear whether warming implies more snow (because of more precipitation) or less snow (because storms will more often produce rain). Winter weather on the east coast is affected by the North Atlantic Oscillation (NAO), a seesaw in pressure between the Icelandic low and Azores high. When the NAO is in its negative phase cold air outbreaks and snow are more likely. This may be responsible (along with the phase of El Niño) for the snowy 2009-2010 and 2010-2011 winters (Fig. 3). This past winter, when conditions were abnormally mild, NAO phase was mostly positive. Future warming will be superimposed on whatever the NAO and El Niño bring in a given winter.
A common, but tentative, narrative thread has emerged for all types of storms: As climate warms, we might experience fewer storms overall, but more of the strongest storms. What the public cares about most, though, is storm damage, which depends on more than just changes in the storms themselves. Regardless of whether lightning increases with warming, drying in western North America will likely lead to more fire damage. Regardless of whether hurricanes and synoptic storms intensify with warming, sea level rise and increased population and development imply more flooding damage to coastal areas from storm surges.
Del Genio, A. 2011. Will a warmer world be stormier? earthzine (retrieved Apr. 20, 2012).
Leiserowitz, A., E. Maibach, C. Roser-Renouf, and J.D. Hmielowski, 2012: Extreme Weather, Climate & Preparedness in the American Mind. Yale University and George Mason University. New Haven, CT: Yale Project on Climate Change Communication (retrieved Apr. 20, 2012).