全球变化知识资源中心

Even on the fairest of days, without a single cloud in sight, an electric current flows from the sky to the ground. Driven by the difference in electrical potential between Earth’s surface and the ionosphere, it is a crucial component of the global electrical circuit (GEC), which connects many electrical processes in the atmosphere.

Lightning pumps charge into the atmosphere, as do galactic cosmic rays. Electrified clouds that don’t produce lightning shoulder a share of the burden equal to that of lightning. Dust, pollutants, and other particles in the lower troposphere also play a role in the global electrical circuit, as does the changing of the seasons.

“You’re looking at the total integrated effects of all the electrified weather across the globe,” said Michael Peterson, a staff scientist at Los Alamos National Laboratory in New Mexico who has studied the circuit with satellite lightning detectors. “People have described it as the electrical heartbeat of the planet.”

Researchers are paying more attention to that heartbeat these days. They are measuring the GEC in more detail, determining the roles of everything from layer clouds to the Sun’s magnetic cycle, and looking at incorporating the electrical circuit into global climate models. “Research on some questions was getting a bit stalled, but now we can use new technology, new methods, and new instruments to push it forward,” said R. Giles Harrison, a professor of atmospheric physics at the University of Reading in the United Kingdom.

Direct Currents, Alternating Currents

Like the Time Lords of Doctor Who, Earth actually has two (electrical) heartbeats. A direct current (DC) circuit operates continuously across the entire planet, driven by everything from lightning to fair-weather currents. An alternating current (AC) circuit, on the other hand, is driven exclusively by lightning, which creates electromagnetic waves that circle the planet. Scientists are studying the relationship between the two circuits.

The GEC (DC version) was first proposed in 1920 by Scottish physicist C. T. R. Wilson, who later won the Nobel Prize for his invention of the cloud chamber. He suggested that Earth’s surface and the base of the ionosphere, a zone of ionized air at an altitude of 50–80 kilometers, formed the conductive shells of a spherical capacitor. The air served as a “leaky” insulator, allowing electric current to flow between the nested shells. Thunderstorms, Wilson wrote, served as the primary generator for this system. Electrified shower clouds, which maintain an electric charge but produce no lightning, also contributed to the circuit.

“The conceptual framework is that you’re allowing the charge generated by disturbed-weather regions to flow around the planet and find its way back to the ground through fair-weather regions,” said Harrison. “The amount of charge is about the same in the fair-weather regions as in thunderstorms. A large part of the 20th century was spent working out that balance sheet. We have a saying that what comes down must have gone up. In other words, if we see current flowing down in fair-weather regions, there must have been a charge going up.”

Through lightning, sprites, jets, and other transient phenomena, thunderstorms cause electric currents to flow up and over clouds to the bottom of the ionosphere. Electrified shower clouds contribute an equal charge to that layer, which captures and distributes the charge around the globe, keeping the “battery” juiced up. (Thanks to those clouds, if thunderstorms suddenly disappeared, the strength of the DC circuit would be cut roughly in half but wouldn’t disappear completely.)

Under fair-weather conditions, the positive ionospheric charge filters back toward the negatively charged ground. The difference in the electrical potential between the ionosphere and the surface averages about 250 kilovolts, producing a downward flowing fair-weather electric field of about 100–300 volts per meter.

Thunderstorms transport negative charge from the cloud base to the ground through lightning strokes, charged rain, and other means, completing the circuit.

The total current flowing in the global circuit, and therefore the total reaching the surface, is about 1,800 amperes. The potential of the upper atmosphere is about 300 kilovolts compared with the surface. The total power in the global circuit is roughly 1 gigawatt—“the equivalent of a modest[-sized] biomass-burning power station at best,” said Harrison.

“Measuring the Global Circuit Is a History of Failure”

Although the atmosphere is a relatively efficient insulator, it leaks because it contains clusters of ions. Some of the ions are created when molecules are zapped by galactic cosmic rays, particles accelerated to high speed in such energetic environments as supernova remnants or accretion disks around black holes. Near the surface, air is mostly ionized by radon created by the decay of radioactive elements in the crust. Other sources involve dust particles, atmospheric pollutants, or other aerosols that carry their own electric charge.

Those contaminants make it hard to measure the GEC, especially over land. “The history of measuring the global circuit is a history of failure,” said Earle Williams, a research scientist at the Massachusetts Institute of Technology. “You have to be in clean air. It can’t be contaminated by pollution or changes in air mass. If only we could get a Radio Shack meter and put one probe in the upper atmosphere and one on Earth’s surface and monitor it continuously. Unfortunately, that’s too complicated.”
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The best measurements are made from the oceans, where the air is relatively clean. In fact, much of the early evidence for the global circuit was compiled by the R/V Carnegie, operated by the Carnegie Institution of Washington, which measured the global electric field during a series of cruises from 1915 to 1929. (The vessel was destroyed in a fire in 1929.)

Its observations revealed that the global DC circuit does not exhibit a single constant value. Instead, it waxes and wanes over a 24-hour cycle. Known as the Carnegie curve, when averaged over a period of years, the cycle peaks at around 19:00 coordinated universal time (UTC) and bottoms out at around 03:00 UTC, regardless of where on Earth it’s measured.

That cycle reflects the peak of global thunderstorm activity, which feeds three electrical “chimneys”: over the Americas, Africa, and the Maritime Continent (an expanse of islands and seas at the intersection of the Indian and Pacific Oceans, from Southeast Asia to Australia). Most thunderstorms take place over land, where solar heating creates convection and rising air currents that drive cloud formation. The Americas have the most active thunderstorm seasons, so they dominate the Carnegie curve.

Africa, however, appears to dominate the AC circuit, which is driven only by lightning. The lightning discharges produce low-frequency radio waves that race around the planet, guided by the cavity formed by the charged ionosphere and the surface.

The waves combine and amplify each other, producing an electromagnetic effect known as a Schumann resonance. The primary resonance is at a frequency of about 8 hertz—eight trips around the planet per second. “It’s like sitting inside a ringing bell,” said Harrison. The resonance is maintained by the combined effect of all the lightning flashes on Earth—between 40 and 50 per second.

“There’s a paradox involving the DC and AC circuits,” said Williams. “America wins the DC circuit, which always peaks [at] around 19:00 UT. But if you look at global lightning, Africa wins, with a peak [at] around 14:00 UT. We’re trying to get a handle on the reason for that paradox.”

Shifting Baselines

That requires more extensive observations of global lightning and the Carnegie curve, one of the main challenges facing those who study the GEC. “Monitoring long-term trends in lightning is difficult,” said Keri Nicoll, an associate professor at the University of Reading and the University of Bath. “Lightning monitoring networks are constantly being updated to provide more and better measurements. This means that the baseline is constantly shifting.”

To help researchers assess lightning’s role in the global circuit, Nicoll and her colleagues established a database of observations from 19 lightning networks, primarily in Europe but stretching from India to Argentina to Antarctica. Although project funding for the Global Coordination of Atmospheric Electricity Measurements has ended, the database is still available to researchers and continues to accumulate observations from several networks. Williams and his colleagues plan to conduct their own observations to address the problem of the global circuit paradox.

The aircraft will make four sets of measurements during each flight, with each set starting at the aircraft’s peak altitude and then dropping toward the ocean surface. Scientists will compare the observations to lightning data compiled by surface networks and by sensors on board orbiting satellites.

The data should help scientists choose between two possible explanations for the differences in the American and African chimneys. “One is that there are more electrified shower clouds in the Americas than in Africa, and they’re boosting the DC circuit without affecting the AC circuit,” Williams said. “The other is that Africa might get a boost from more aerosols, which move condensates from the lower atmosphere up to the lightning-producing region,” enhancing thunderstorm formation.

Researchers are using observations old and new, often made with balloons or drones, to address several questions related to the global circuit.

“One area of research is looking at different types of storm structures,” said Los Alamos’s Peterson. “When you think of the global circuit, most people think of convection, which is the major driver of lightning. But the clouds outside the thunderstorm core are important because they become electrically active and they have a different charge structure. For example, stratiform clouds, which form behind massive lines of thunderstorms, often have inverted polarity structures (as compared to that of thunderclouds, where positive charge regularly resides above negative) which can either charge or discharge the global circuit. Where do these clouds occur, how often do they occur, [and] what kinds of charges do they actually produce? There are still a lot of questions about that.”

“The role of the global circuit in climate change is a standard essay question,” said Harrison with a bit of a chuckle. “And there are a wide range of answers. But if there’s an increase in thunderstorms because of higher temperatures, then we’d certainly expect an increase in both the AC and DC circuits.”

Higher air temperatures increase evaporation, providing more water vapor to fuel thunderstorms. More and bigger thunderstorms would produce more lightning, which could alter the intensity of the global circuit, modify the timing of the Carnegie curve, or cause other changes. Increased monitoring of the GEC would allow scientists to note those changes and use the circuit parameters as indicators of increased climate change.

Williams and his colleagues are using “thunder day” observations—the number of days that thunder was recorded at meteorological stations around the world—since the late 1800s as proxies for lightning observations or measurements of the global circuit.

Little work had been dedicated to modeling climate feedbacks from the GEC. Researchers are looking at some aspects of that feedback, such as whether the vertical current flow in the circuit might change cloud formation or structure.

“The other difficulty is how best to model the GEC and to incorporate this with climate models,” said Nicoll. “For this we need to know how to predict GEC parameters such as the global charging current, ionospheric potential, and fair-weather conduction current from a climate model….This is an ongoing area of research and one which shows great promise for the future.”

Such models could tell us much more about our changing climate and about our planet’s double electrical heartbeat.

—Damond Benningfield, Science Writer