CNN
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Last year was the hottest ever recorded, oceans boiled, glaciers melted at an alarming rate, and it left scientists struggling to understand exactly why.
They know that the extraordinary heat was caused by a number of factors, mainly pollution from the burning of fossil fuels and the natural climate pattern. El Nino. But these alone did not explain the unusually rapid temperature rise.
Now a new study, published Thursday in the journal Science, says it has identified the missing piece of the puzzle: clouds.
To be more specific, the rapid increase in warming, according to the study, was amplified by a lack of low-lying clouds over the oceans – findings that could have alarming implications for future warming.
Simply put, fewer bright, low clouds mean the planet is “blacked out,” allowing it to absorb more sunlight, says Helge Goessling, author of the report and a climate physicist at the Alfred Wegener Institute in Germany.
This phenomenon is called “albedo” and refers to the ability of surfaces to reflect the sun’s energy back into space.
According to the report, Earth’s albedo has been declining since the 1970s, partly due to the melting of light-colored snow and sea ice, exposing darker land and water that absorbs more solar energy, harming the planet warms up.
Low clouds also contribute to this effect because they reflect sunlight.
The scientists searched NASA satellite data, weather data and climate models and found that the decline in low cloud cover was slowing The planet’s albedo reached a low point last year. Areas included parts of the North Atlantic Ocean experienced a particularly significant decline, the study found.
Last year fits into a decade-long decline in low cloud cover, Goessling told CNN.
What the research cannot yet explain with certainty is why this happens. “This is such a complex beast and so difficult to untangle,” Goessling said.
He thinks this is likely due to a combination of factors. The first is a reducing pollution from shipping due to regulations aimed at reducing harmful sulfur emissions from the industry. While this has been a victory for human health, this type of pollution also cooled the planet by brightening clouds.
Natural climate variabilities, including changing ocean patterns, may also have contributed. But Goessling points to a third, more alarming factor: global warming itself.
Low clouds usually thrive in a cool and moist lower atmosphere. As the planet’s surface warms, this could cause them to thin out or disappear completely, creating a complicated feedback loop where low clouds disappear due to global warming, and their disappearance then leads to further warming.
If this happens, future warming projections could be underestimated and “we should expect quite intense warming in the future,” Goessling said.
Mark Zalinka, an atmospheric scientist at Lawrence Livermore National Laboratory who was not involved in the research, said that “the fact that clouds play a key role in the story makes sense, because they essentially act as the Earth’s sunscreen.”
Small changes in cloud cover can “dramatically change the Earth’s albedo,” he told CNN.
Tapio Schneider, a climate scientist at the California Institute of Technology, said the worrying implication of the research is that if global warming is responsible for a significant amount of change in cloud cover, “we could see stronger global warming than previously predicted.”
Clouds may seem simple, even mundane, but they are endlessly complex and scientists are still far from unraveling how they behave. They are “one of the biggest headaches” in climate science, Goessling said.
But figuring out how clouds will respond to global warming is critical, Zalkina said. “It literally determines how much future warming awaits us.”