Central Africa receives the most flashes of lightning per square kilometer, while the polar regions receive the least. This global map of lightning flash density was created with data from the Lightning Imaging Sensor (LIS) aboard the Tropical Measuring Mission (TRMM) and the Optical Transient Detector (OTD) aboard the Microlab-1 spacecraft. (Credit: Jeff De La Beaujardiere, Scientific Visualization Studio)

With satellites, they have discovered that more than 1.2 billion lightning flashes occur around the world every year. (Rwanda has the most flashes per square kilometer, while flashes are rare in polar regions.) Laboratory and field experiments have revealed that the core of some lightning bolts reaches 30,000 Kelvin (53,540 ºF), a temperature hot enough to instantly melt sand and break oxygen and nitrogen molecules into individual atoms.

And then there is this: each of those billion lightning flashes produces a puff of nitrogen oxide gas (NOx) that reacts with sunlight and other gases in the atmosphere to produce ozone. Near Earth’s surface, ozone can harm human and plant health; higher in the atmosphere, it is a potent greenhouse gas; and in the stratosphere, its blocks cancer-causing ultraviolet radiation.

In 1827, the German chemist Justin von Liebig first observed that lightning produced NOx — scientific shorthand for a gaseous mixture of nitrogen and oxygen that includes nitric oxide (NO) and nitrogen dioxide (NO₂). Nearly two centuries later, the topic continues to attract the attention of scientists.

Fossil fuel combustion, microbes in the soil, lightning, and forest fires all produce NOx. Scientists think lightning’s contribution to Earth’s NOx budget — probably about 10 percent — is relatively small compared to fossil fuel emissions. Yet they haven’t been sure whether global estimates of NOx produced by lightning are accurate.

“There’s still a lot of uncertainty about how much NOx lightning produces,” said Kenneth Pickering, an atmospheric scientist who studies lightning at NASA’s Goddard Space Flight Center in Greenbelt, Md. “Indeed, even recent published estimates of lightning’s global NOx production still vary by as much as a factor of four. We’re trying to narrow that uncertainty in order to improve the accuracy of both global climate models and regional air quality models.”

Using data gleaned from aircraft observations and satellites, Pickering and Goddard colleague Lesley Ott recently took steps toward a better global estimate of lightning-produced NOx and found that lightning may have a considerably stronger impact on the climate in the mid-latitudes and subtropics — and less on surface air quality — than previously thought.

According to a new paper by Ott and Pickering in the Journal of Geophysical Research, each flash of lightning on average in the several mid-latitude and subtropical thunderstorms studied turned 7 kilograms (15.4 pounds) of nitrogen into chemically reactive NOx. “In other words, you could drive a new car across the United States more than 50 times and still produce less than half as much NOx as an average lightning flash,” Ott estimated. The results were published July.

When the researchers multiplied the number of lightning strokes worldwide by 7 kilograms, they found that the total amount of NOx produced by lightning per year is 8.6 terragrams, or 8.6 million metric tons. “That’s somewhat high compared to previous estimates,” said Pickering.

More remarkable than the number, however, is where the NOx is produced. A decade ago, many researchers believed cloud-to-ground lightning produced far more NOx per flash than intracloud lightning, which occurs within a cloud and far higher in the atmosphere.

The new evidence suggests that the two types of lightning produce approximately the same amount of NOx per flash on average. But since most lightning is intracloud, this suggests a great deal more NOx is produced and remains higher in the atmosphere. Compounding this effect, the research also shows that strong updrafts within thunderstorms help transfer lower level NOx to higher altitudes in the atmosphere.

“We’ve really started to question some of our old assumptions as we’ve gotten better at measuring lightning in the field,” said Ott.

The observations spring out of field projects conducted in Germany, Colorado, Florida, Kansas, and Oklahoma between 1985 and 2002. For example, in a NASA field campaign called the Cirrus Regional Study of Tropical Anvils and Cirrus Layers Florida — Florida Area Cirrus Experiment (CRYSTAL-FACE) aircraft flew headlong through anvil-shaped thunderheads to measure the anatomy of the thunderstorms. Sensors sampled the pressure, humidity, temperature, wind, and the amount of trace gases such as NOx and ozone.

Later, Ott input this data, as well as additional data from the U.S. National Lightning Detection Network and NASA’s Total Ozone Mapping Spectrometer (TOMS), into a complex computer model that simulated the six storms and calculated the amount of NOx that the average flash of lightning produced. With that number, she could then estimate the amount of NOx that lightning produces globally each year.

“One of the things we’re trying to understand is how much ozone changes caused by lightning affect radiative forcing, and how that might translate into climate impacts,” said Pickering.

There’s a possibility that lightning could produce a feedback cycle that accelerates global warming. “If a warming globe creates more thunderstorms,” Pickering noted, “that could lead to more NOx production, which leads to more ozone, more radiative forcing, and more warming,” Pickering emphasizes that this is a theory, and while some global modeling studies suggest this is indeed the case, it has not yet been borne out by field observations.

The new findings also have implications for regional air quality models. Scientists from the U.S. Environmental Protection Agency (EPA), for example, are already plugging the new numbers into a widely-used air quality model called the Community Multi-scale Air Quality Model. “Lightning is one of the smaller factors for surface ozone levels, but in some cases a surge of ozone formed from lightning NOx could be enough to put a community out of compliance with EPA air quality standards during certain times of the year,” said Pickering.

Pickering offered one important caveat to the findings: The value of 7 kilograms per flash was derived without consideration of lightning from storms in the tropics, where most of the Earth’s lightning occurs. Only very recently have data become available for tropical regions, he noted.

The adjacent foothills hold lakes that were not scoured by glaciers during late Quaternary glaciations, and thus preserve exceptionally long sedimentary records of climate change. (Credit: Jason Briner, Courtesy Queen's University)

While environmental changes at the lake over the past millennia have been shown to be tightly linked with natural causes of climate change — like periodic, well-understood wobbles in Earth’s orbit — changes seen in the sediment cores since about 1950 indicate expected climate cooling is being overridden by human activity like greenhouse gas emissions. The research team reconstructed past climate and environmental changes at the lake on Baffin Island using indicators that included algae, fossil insects and geochemistry preserved in sediment cores that extend back 200,000 years.

“The past few decades have been unique in the past 200,000 years in terms of the changes we see in the biology and chemistry recorded in the cores,” said lead study author Yarrow Axford of CU-Boulder’s Institute of Arctic and Alpine Research. “We see clear evidence for warming in one of the most remote places on Earth at a time when the Arctic should be cooling because of natural processes.”

The study was published Oct. 19 in the Proceedings of the National Academy of Sciences. The study included researchers from CU-Boulder, the State University of New York’s University at Buffalo, the University of Alberta, the University of Massachusetts and Queen’s University in Kingston, Ontario.

The sediment cores were extracted from the bottom of a roughly 100-acre, 30-foot-deep lake near the village of Clyde River on the east coast of Baffin Island, which is several hundred miles west of Greenland. The lake sediment cores go back in time 80,000 years beyond the oldest reliable ice cores from Greenland and capture the environmental conditions of two previous ice ages and three interglacial periods.

The sediment cores showed that several types of mosquito-like midges that flourish in very cold climates have been abundant at the lake for the past several thousand years. But the cold-adapted midge species abruptly began declining in about 1950, matching their lowest abundances of the last 200,000 years. Two of the midge species adapted to the coldest temperatures have completely disappeared from the lake region, said Axford.

In addition, a species of diatom, a lake algae that was relatively rare at the site before the 20th century, has undergone unprecedented increases in recent decades, possibly in response to declining ice cover on the Baffin Island lake.

“Our results show that the human footprint is overpowering long-standing natural processes even in remote Arctic regions,” said co-author John Smol of Queen’s University. “This historical record shows that we are dramatically affecting the ecosystems on which we depend.”

“The 20th century is the only period during the past 200 millennia in which aquatic indicators reflect increased warming, despite the declining effect of slow changes in the tilt of the Earth’s axis which, under natural conditions, would lead to climatic cooling,” notes the University of Colorado’s Dr. Axford.

The ancient lake sediment cores are the oldest ever recovered from glaciated parts of Canada or Greenland. Massive ice sheets during ice ages generally scour the underlying bedrock and remove previous sediments.

“What is unique about these sediment cores is that even though glaciers covered this lake, for various reasons they did not erode it,” said study co-author Jason Briner of the University at Buffalo. The result is that we have a really long sequence of sediment that has survived Arctic glaciations.”

The study was funded by the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada and the Geological Society of America.

A study published in the journal Science last month that involved CU-Boulder researchers and reconstructed past temperatures in the Arctic using ice cores, tree rings and lake sediments concluded that recent warming around the Arctic is overriding a cooling trend caused by Earth’s periodic wobble. Earth is now about 0.6 million miles further from the sun during the Northern Hemisphere summer solstice than it was in 1 B.C. — a trend that has caused overall cooling in the Arctic until recently.

INSTAAR researcher and CU-Boulder geological sciences Professor Gifford Miller was a co-author on both the PNAS study and the recent Science study.