U.S. Lightning Strikes May Increase 50% Due to Global Warming

A warmer world will have much more dangerous cloud-to-ground lightning capable of igniting more forest fires, according to a study published Thursday in the Journal Science. The research found that for each degree Centigrade (1.8°F) of global warming, lightning in the U.S. is expected to increase by 12%. This would result in about a 50% increase in lightning by the year 2100, assuming business-as-usual emissions result in a world that is 4°C (7°F) warmer. Main author David Romps of the University of California-Berkeley said in a press release, “This has to do with water vapor, which is the fuel for explosive deep convection in the atmosphere. Warming causes there to be more water vapor in the atmosphere, and if you have more fuel lying around, when you get ignition, it can go big time…the faster the updrafts, the more lightning, and the more precipitation, the more lightning.” The study looked at U.S. lightning statistics for the year 2011, and discovered that a simple measure of atmospheric heat and moisture--the precipitation rate multiplied by the stability of the atmosphere (expressed as the Convective Available Potential Energy, or CAPE)--could describe 77% of the variation in lighting. By applying this simple measure to predicted levels of heat and moisture in a future warmer world, the scientists came up with their predictions for more lightning. The study makes sense from basic principles, and brings up three major concerns about the impacts of a future world with more lightning:

1) More lightning-caused fires
2) More lightning-caused ozone pollution and thus global warming
3) More lightning direct strike deaths and damages


Figure 1. Lightning sparks a grass fire near Granite, Oklahoma on June 8, 2008. Image credit: wunderphotographer Glenn Patterson.

The costs and death toll from lightning-caused fires in the U.S. and Canada
Over the ten years from 2003 - 2012, 42 U.S. firefighters were killed as a result of lightning-caused fires. An additional 19 firefighters were killed by the lightning-caused Yarnell Hill Fire in Arizona in 2013. U.S. wildfire fighting costs averaged $1.8 billion annually during 2009 - 2013, according to Headwaters Economics. Although only 15% of U.S. wildfires were ignited by lightning between 2001 - 2010, these accounted for approximately 60% of the acres burned, and much of the annual costs of firefighting, according to the National Interagency Fire Center. For example, in 2012, the Whitewater-Baldy Complex Fire, the largest fire in New Mexico history, and the Rush Fire, the 2nd largest in California history, were both triggered by lightning strikes. Lighting also causes building fires through direct strikes. The National Fire Protection Association says that lightning-caused fires that are responded to by local fire departments in the U.S. killed an average of nine people per year and did $451 million in direct property damage per year between 2007 - 2012.

Environment Canada estimates that lightning strikes are responsible for 45% of all wildfires in Canada and 81% of the total area burned. The cost of lightning-related damage and disruption to the Canadian economy was estimated to be between $600 million and $1 billion each year (Mills et al. 2009).


Figure 2. Smoke rises from the uncontrolled northern front of the lightning-ignited Gap fire on July 5, 2008 near Goleta, California. President Bush declared a state of emergency for all of California in July 2008 in response to more than 1,400 fires that were mostly started by dry lightning storms on June 20, 2008. More than 19,000 firefighters from 42 states battled the California wildfires. (Photo by David McNew/Getty Images)

Death and damages due to direct lightning strikes
In addition to killing people in lighting-caused fires, lightning kills people with direct strikes. In 2006 - 2013, an average of 33 people per year died as a result of lightning strikes, according to NOAA. So far in 2014, 25 people have been killed. Fishing, camping and boating were the three highest risk activities for people dying from lightning strikes, according to a 2013 NWS study. The insured costs of direct lightning strikes have been rising in recents years, due to an increase in valuable home electronics that get fried in a strike. These damages were approximately $1 billion per year in 2010 - 2011, according to the Insurance Information Institute.

Lightning-caused forest fires may increase at a lesser rate
Climate models show that the increase in instability (higher CAPE) due to global warming is not expected to be uniform over the U.S., with strong increases over the Southeast U.S., and little increase over the Western U.S., where the majority of lightning-caused fires originate. The 12% increase in lighting per °C of global warming the study found is averaged over the entire U.S., and the increase in lightning is likely to be much lower over the Western United States--perhaps a factor of six less. A 2007 study by Del Genio et al. found that increasing the global temperature by 2.7°C would cause drying over the Western U.S. that would lead to fewer thunderstorms overall. However, the strongest thunderstorms increased in number by 26%, leading to a 6% increase in the total amount of lighting hitting the ground each year, or about a 2% increase per °C of global warming.

Increased lightning will create more ozone pollution and more global warming
Lightning creates nitrogen oxides, which in turn react to make significant amounts of ozone in the lower atmosphere--a dangerous pollutant that seriously impacts human health and crop growth. Ozone is also a greenhouse gas, so global warming-caused increases in lightning could potentially cause additional global warming of a few percent. How much is uncertain, as estimates of lightning-produced nitrogen oxides vary by up to a factor of four. Lower-atmosphere ozone was responsible for about 12% of human-caused global warming due to greenhouse gases in 2011, according to the 2013 IPCC report. However, increased ozone due to lightning could be offset somewhat by the fact that lightning-created nitrogen oxides trigger chemical reactions that help destroy methane, another potent greenhouse gas.


Video 1. ‪Every Lightning Strike in America in 2011, In One Minute‬. Data from the National Lightning Detection Network, UAlbany; animation by David Romps, UC Berkeley, and Phil Ebiner, UC Berkeley Public Affairs. Thursday's study in Science studied lightning over the U.S. in 2011 to come up with a simple way to represent lightning frequency based on how much heat and moisture is in the atmosphere.

Jeff Masters