Using satellite data, researchers measured what is known as Earth’s energy imbalance — the difference between how much energy the planet absorbs from the sun, and how much it’s able to shed, or radiate back out into space.
When there is a positive imbalance — Earth absorbing more heat than it is losing — it is a first step toward global warming, said Stuart Evans, a climate scientist at the University at Buffalo. “It’s a sign the Earth is gaining energy.”
That imbalance roughly doubled between 2005 and 2019, the study found. “It is a massive amount of energy,” said Gregory Johnson, an oceanographer for NOAA’s Pacific Marine Environmental Laboratory and co-author of the study. Johnson said the energy increase is equivalent to four detonations per second of the atomic bomb dropped on Hiroshima, or every person on Earth using 20 electric tea kettles at once. “It’s such a hard number to get your mind around.”
The Earth takes in about 240 watts per square meter of energy from the sun. At the beginning of the study period, in 2005, it was radiating back out about 239.5 of those watts — creating a positive imbalance of about half a watt. By the end, in 2019, that gap had nearly doubled to about 1 full watt per square meter.
Oceans absorb most of that heat, about 90 percent. When researchers compared satellite data to temperature readings from a system of ocean sensors, they found a similar pattern. The agreement between the data sets surpassed expectations, Loeb said, calling it the “nail in the coffin” for the imbalance results.
“The fact that they used two different observational approaches and came up with the same trends is pretty remarkable,” said Elizabeth Maroon, a climatologist at the University of Wisconsin at Madison unaffiliated with the study. “It lends a lot of confidence to the findings.”
The biggest outstanding question is what is driving the acceleration.
The study points to decreases in cloud cover and sea ice, which reflect solar energy back into space, and an increase in greenhouse gases emitted by humans, such as methane and carbon dioxide, as well as water vapor, which trap more heat in the Earth, as factors in the imbalance. But it is difficult to discern human-induced changes from cyclical variations in the climate, the researches said.
“They are all kind of blended together,” said Loeb, who added that further research is needed to determine the factors.
The period studied overlapped with fluctuations in the climate that may have played a significant role in the acceleration, including a strong El Niño event from 2014 to 2016, which led to unusually warm waters. The Pacific Decadal Oscillation is a longer-term, El Niño-like fluctuation, and around 2014 that also switched from a “cool” phase to a “warm” phase.
But, Johnson says, that doesn’t let humans off the hook. “We’re responsible for some of it,” he said. It’s just unclear how much.
Kevin Trenberth, a distinguished scholar at the National Center of Atmospheric Research, said the results of the study aren’t particularly surprising given these climactic variations. But 15 years is not enough time to establish a trend, he said.
“Certainly you’d like to see another 10 years or something like that to see how this behaves,” he said. “The question is: Will this continue?”
That too is unclear, Johnson said. The imbalance could shrink in some years compared to others, he said, but the general trajectory appears to be upward, especially if the Pacific Decadal Oscillation stays in a warm phase.
“The longer we observe it,” he said, “the more certain we become of the trend.”
Tracking Earth’s energy imbalance will also help scientists better understand climate change, Johnson said. Other common metrics, such as air temperature, only catch a fraction of the effect of the sun’s heat. The imbalance, he said, measures “the full amount of heat that goes into the climate system.”
Regardless of the magnitude or reasons for the accelerated imbalance, the fact that it is positive is crucial, said Trenberth. “It’s the sign that matters here,” he said. “The fact that it’s positive means that global heating is happening.”
That extra heat, especially in the oceans, will mean more intense hurricanes and marine heat waves.
“I hope the heating doesn’t keep going at this clip,” Loeb said. “It’s not good news.”