A recent study published in Geophysical Research Letters has challenged the conventional understanding of the relationship between global warming and ocean evaporation. A research team from the Institute of Geographic Sciences and Natural Resources Research of the Chinese Academy of Sciences discovered a phenomenon that despite rising sea surface temperatures, global ocean evaporation has decreased over the past decade.

Ocean evaporation plays a central role in Earth’s hydrological cycle, contributing more than 85% of atmospheric water vapor. Historically, higher sea surface temperatures were expected to increase evaporation rates. However, observations since the early 2000s have revealed a puzzling slowdown in the growth of global water vapor, prompting scientists to re-examine how ocean evaporation responds to rising temperatures.

To investigate this phenomenon, the researchers analyzed advanced satellite-based ocean heat flux data to assess long-term trends in global ocean evaporation. Their findings revealed a significant shift in the evaporation trend over recent decades.

The study exhibited that while global ocean evaporation showed an upward trend from 1988 to 2017, this trend reversed in the late 2000s. “Since then, two-thirds of the world’s oceans have experienced a reduction in evaporation, resulting in a slight decline in global evaporation rates between 2008 and 2017. This contradicts what we would typically expect in a warming climate,” said Dr. MA Ning, the lead author of the study. 

The researchers identified a key factor behind this unexpected trend: a decline in wind speed, a phenomenon known as “wind stilling.” They suggest that wind stilling is likely linked to shifts in atmospheric circulation patterns, particularly the Northern Oscillation Index, which has recently transitioned from a positive to a negative phase.

“Changes in wind speed may be associated with decadal variations in Earth’s climate system,” Dr. MA explained. “The recent decline in ocean evaporation should not necessarily be interpreted as evidence of a weakened hydrological cycle, as it may instead reflect natural climate oscillations.”

While the decreasing trend in ocean evaporation may seem counterintuitive in the context of global warming, it highlights the complexity of Earth’s climate system and the intricate feedback mechanisms that govern the planet’s hydrological processes.