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The trade winds are back. But they might not last for long.

According to a University of Hawaii study, Hawaii’s northeast trade winds have decreased by almost a third over the past 40 years.

“This change is very dramatic,” said Pao Shin Chu, who heads the Hawaii state climate office in addition to teaching at the university.

Many Hawaii residents rely on trade winds to cool their homes. Civil Beat reported in September that Aiea residents were so concerned about trade winds reaching their homes that a developer commissioned a study to reassure neighbors that its proposed tall buildings wouldn’t block the winds. Trade winds are also an important source of rainfall, which helps sustain the state’s water supply and maintain the islands’ lush environment.

“[The trade winds are] the primary source of precipitation on the windward side and extend to leeward side,” Chu said.

The study found that between 1973 and 2009, northeast trade winds decreased from 291 days per year to just 210 days per year.

During the same period, the east trade winds increased in frequency. But Chu said it’s not enough.

“[The increase in east trade winds is] not sufficient enough to offset the decrease in northeast trade winds,” Chu said.

Chu attributed the trends to changes in subtropical high pressure, a large-scale atmospheric phenomenon that may be caused by global climate change.

Potential Impact on Hawaii’s Animals, Water Supply

If trade winds continue to decrease in frequency, Hawaii’s environment may have to pay the price.

“If you look just at the winds [it seems okay],” Chu said. “But translate that into precipitation — that’s a different story.”

Professor Robert Cowie, head of the Ecology, Evolution and Conservation Biology graduate program at the University of Hawaii, said that the trend could have significant implications for Hawaii’s ecology.

“If this is a real trend and if it’s a big enough trend and if that trend is associated with a reduction in rainfall, then it could impact species of plants and animals that depend upon current rainfall, particularly at higher elevations,” he said.

Tom Giambelluca, a professor in the University of Hawaii’s Department of Geography and an expert in rainfall in Hawaii, said that previous studies have shown that the amount of rainfall in Hawaii has been decreasing over the years.

“There have been some pretty big ups and downs in rainfall in Hawaii over the past almost a century back to about 1920,” Giambelluca said. “Overall I would say that there is reasonably good evidence for a slow decline overtime underlying those ups and downs.”

He said that there is ongoing research to determine the cause of the decline, which some scholars attribute to global warming. He did not know of any study linking the trends in rainfall to those of trade winds.

No matter its cause, less rainfall could hurt Hawaii’s animals and population.

“Decreases in precipitation could have a lot of negative consequences for Hawaii including effects on natural ecosystems and on our water supply and our agriculture,” Giambelluca said.

Lack of rainfall could shrink the forest habitat of Hawaii’s endangered birds. The decrease could also affect the islands’ water supply.

“In many parts of the state we are using all or nearly all the available water resources so if we have a decline in rainfall that would lead to less water supply,” he said.

Giambelluca said a decrease in precipitation would probably come with sunnier weather, which would increase evaporation and demand for water.

“Those things put together would have negative consequences,” he said.

You can read the study in the .