Last year was the at an alarming rate, and scientists struggled to understand exactly why it had happened.
They know that the extraordinary heat was fueled by a number of factors, mainly planet-warming pollution due to the burning of fossil fuels and the natural weather pattern. But these factors alone do not explain the unusually rapid rise in temperature.
Now, a new study published Thursday in the journal Science says the missing part of the puzzle has been identified: clouds.
To be more specific, the rapid increase in warming was fueled by a scarcity of low clouds over the oceans, according to the research. This finding may have alarming implications for future warming.
In simple terms, fewer low, bright clouds mean the planet has “darkened,” allowing it to absorb more sunlight, said Helge Goessling, an 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.
Earth’s albedo has been declining since the 1970s, according to the report, due in part to melting light snow and sea ice, exposing darker land and water that absorb more energy from the sun, warming the planet.
Low clouds also contribute to this effect, as they reflect sunlight.
Scientists analyzed NASA satellite data, weather data, and climate models and found that declining low clouds reduced the planet’s albedo to record lows last year. The study determined that some areas, including .experienced a particularly significant drop.
Last year fits into a decade-long decline in low cloud cover, Goessling told CNN.
What the study can’t yet explain for sure is why this happens. “It’s a very complex beast and very difficult to unravel,” Goessling said.
He believes it’s likely the result of a combination of factors. The first is a 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 was also helping to cool the planet by making the clouds glow.
Natural variations in climate, including changes in ocean patterns, may also have contributed, but Goessling points to a third, more alarming factor: global warming itself.
Low clouds tend to thrive in a low, cold, and moist atmosphere. As the planet’s surface warms, this can cause them to thin or dissipate entirely, leading to a complicated feedback loop in which low clouds disappear due to global warming and their disappearance then drives further warming.
If this happens, projections of future warming could be underestimated, and “we should expect pretty intense warming in the future,” Goessling said.
Mark Zalinka, an atmospheric scientist at Lawrence Livermore National Laboratory who was not involved in the study, said that “the fact that clouds play a key role in the story makes sense, as they essentially act as Earth’s sunscreen.”
Small changes in cloud cover can “dramatically change the albedo of the Earth,” 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 substantial amount of change in cloud cover, “we could see stronger global warming than previously predicted.”
Clouds may seem simple, even mundane, but they are infinitely complex and scientists are still far from unraveling how they behave. They’re “one of the biggest headaches” in climate science, Goessling said.
But determining how clouds will respond to global warming is key, Zalkina said. “It literally determines how much future warming awaits us.”