Over the past two decades, NASA instruments have documented a growing imbalance in Earth’s solar energy budget, with more energy entering than leaving the planet. This imbalance is largely attributed to greenhouse gas emissions, but new research suggests a significant role for declining cloud coverage. Using data from the long-lived NASA Terra satellite, researchers have identified a 1.5% per decade reduction in global cloud cover, particularly in equatorial and midlatitude regions. These changes appear to be driven by shifts in atmospheric circulation rather than reductions in pollution. The implications of this trend are profound, with potential feedback effects that could accelerate global warming. However, uncertainties remain regarding the persistence of these trends, as conflicting patterns in global atmospheric circulation complicate predictions. Researchers stress the urgency of understanding these dynamics to predict future climate impacts.
For more than 20 years, NASA instruments in space have tracked a growing imbalance in Earth’s solar energy budget, with more energy entering than leaving the planet. Much of that imbalance can be pinned on humanity’s greenhouse gases emissions, which trap heat in the atmosphere. But explaining the rest has been a challenge. The loss of reflective ice, exposing darker ground and water that absorb more heat, isn’t enough to explain the deficit, and the decline in light-reflecting hazes as countries clean up or close polluting industry falls short as well. “Nobody can get a number that’s even close,” says George Tselioudis, a climate scientist at NASA’s Goddard Institute for Space Studies. But Tselioudis and his colleagues now think they can explain the growing gap with evidence collected by a remarkably long-lived satellite. They find that the world’s reflective cloud cover has shrunk in the past 2 decades by a small but tangible degree, allowing more light in and boosting global warming. “I’m confident it’s a missing piece. It’s the missing piece,” says Tselioudis, who presented the work last week at a meeting of the American Geophysical Union.
Climate scientists now need to figure out what’s causing these cloud changes. They also need to tackle a more alarming question: whether the trend is a feedback of climate change that might accelerate warming into the future, says Michael Byrne, a climate dynamicist at the University of St. Andrews. Although some models have predicted the cloud changes, Byrne says, “I don’t think we can answer this question with much confidence.” Clouds come in all shapes and sizes, but two of the most consistent cloud swaths are formed by Earth’s large-scale airflow patterns. One band, near the equator, stretches around the planet like a belt. It forms as trade winds of the Northern and Southern hemispheres converge, forcing moist air upward to cool and condense into clouds. Another band occurs in the midlatitudes, where jet streams usher large swirls of stormy weather around the planet.
In August, Tselioudis and his co-authors reported that over the 35 years covered by weather satellite imagery, the equatorial cloud bands had narrowed, while the tracks of midlatitude storms had shifted toward the poles, hemming in the region in which they can form and shrinking their coverage. But the result, published in Climate Dynamics, was stitched together from many different satellites, each with its own quirks and errors, which made it hard for the researchers to be sure the small trends they detected were real. Now, the team has turned to a single satellite, NASA’s Terra, which has been monitoring the planet for nearly a quarter-century. Looking at the same cloud systems, the team found exactly the same trends, with cloud coverage falling by about 1.5% per decade, Tselioudis says. “It’s only now that the signal seems to be coming out of the noise.” Bjorn Stevens, a climate scientist at the Max Planck Institute for Meteorology, says a couple percentage points may not sound important. “But if you calculate these trends, it’s massive,” he says. “This would indicate a cloud feedback that’s off the charts.” The team also found that 80% of the overall reflectivity changes in these regions resulted from shrinking clouds, rather than darker, less reflective ones, which could be caused by a drop in pollution. For Tselioudis, this clearly indicates that changes in atmospheric circulation patterns, not pollution reductions, are driving the trend.
The new work doesn’t stand alone. Earlier this year, in Surveys in Geophysics, a group led by climate scientist Norman Loeb at NASA’s Langley Research Center also traced the gap in the energy imbalance to declining cloud coverage. But Loeb, who leads work on the set of NASA satellite instruments called Clouds and the Earth’s Radiant Energy System, which tracks the energy imbalance, thinks pollution declines may be playing an important role in the cloud changes, especially in the Northern Hemisphere. “The observations are telling us something is definitely changing,” he says. “But it’s a complicated soup of processes.” If global circulation changes are at work, an urgent question is whether they will continue, says Tiffany Shaw, a climate dynamicist at the University of Chicago. The same models that predict a narrowing of the equatorial storm belt also suggest climate change will cause air over the eastern Pacific Ocean to warm faster than the west, weakening an important branch of the large-scale circulation. But for the past few decades, the eastern Pacific has actually been cooling, strengthening these winds instead. Other observations, meanwhile, suggest the rest of the circulation is weakening. The confusion makes it hard to know whether the cloud banks will continue to shrink as the world warms. “With a lot of this,” Shaw says, “the real world will show us the answer.” Stevens, for one, is increasingly worried. “If the trend holds up, we’re in trouble,” Stevens says. “We hope, hope it changes its direction tomorrow.”
Sources:
SCIENCE
https://www.science.org/content/article/earth-s-clouds-are-shrinking-boosting-global-warming .
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