Climate Change May Bring Hawai`i More Storms, Reduced Cloud Cover

posted in: October 2007 | 0

“We were, at every step in the process — in regard to sea surface temperature, potential for storm generation, as well as changes regarding precipitation — staggered at what we were seeing,” says Stephen Miller, an ecologist with the U.S. Fish and Wildlife Service who, with colleague Jeff Burgett, recently studied how climate change might affect Hawai`i’s natural environment.

Hawai`i’s rare forest birds are already struggling with the spread of malaria to higher elevations, a situation caused by increasing summer temperatures. If climate change continues as predicted, Hawai`i will not only become hotter and drier, it will experience stronger, longer, and more frequent storms, according to Miller, a presenter at the Hawai`i Conservation Conference, held last July. Because tropical storms, as well as changes in temperature and precipitation, could derail the recovery of Hawai`i’s many endangered species, Miller called on resource managers to incorporate measures addressing potential climate change impacts into their management programs.

For the FWS, identifying climate researchers who can assist resource managers here is an “immediate, near-term goal,” Miller told Environment Hawai`i. In the meantime, Miller and Burgett, a biologist, have come up with some predictions based on historic storm data and simple physical principles of cloud formation.

Most storms that hit Hawai`i originate in the eastern Pacific storm basin, Miller said in his presentation at the conference. Storms originating in the eastern Pacific tend to dissipate when they hit cooler waters (about 26 degrees C) east of the island chain. Those cooler waters, which begin at a latitude just below the island of Hawai`i and extend upward toward California, act as a wall, blocking tropical storms that, according to meteorologists, require a sea surface temperature of at least 27 degrees C to grow in strength. When Hawai`i does get hit with an eastern Pacific storm, it’s usually because it slipped around that wall of colder water that normally protects the islands.

Climate change may alter that pattern, Miller argued.

Like El Niño Southern Oscillation, where the climate in the tropical Pacific switches to a warm or cold period every four to seven years, the climate just north of Hawai`i changes temperature every 20 to 25 years. Scientists have labeled this cycle the Pacific Decadal Oscillation (PDO). To determine how climate change might affect storms in Hawai`i, Miller and Burgett decided to look at the storm patterns over time by comparing two PDO periods.

“You can pick an arbitrary break like decades. I picked a break based on a natural sea surface process….It’s just a nice, clean way of evaluating past and current events,” Miller told Environment Hawai`i.

In studying storm patterns in the Pacific, Miller and Burgett found that, like El Niño, the PDO affects storm distribution. The researchers looked at the tracks of some 600 Pacific storms between 1965 and 2006 and found that during a warm period between 1977 and 1998, 12 storms reached Hawai`i, compared to only six during a cool period between 1947 and 1976.

While the 2007 Fourth Assessment Report by the Intergovernmental Panel on Climate Change (IPCC) predicts that hurricanes worldwide will be more intense as a result of climate change, it does not state that hurricanes will necessarily become more frequent in Hawai`i. It does, however, state that regional changes in storm frequency will depend on sea surface temperature changes.

Based on their findings, Miller argued that the increase in ocean temperature associated with climate change would weaken the PDO, which would produce stronger storms and more hurricanes, and increase the liklihood of those storms reaching Hawai`i. He noted that the spike in storm activity during warm PDO periods was the result of a 0.3 to 0.4 degrees C change in sea surface temperature, which is merely a fraction of the temperature increases climate change experts are predicting.

The IPCC has stated that, at a minimum, the sea surface temperature around Hawai`i will increase 1.5 degrees C. The average increase is predicted to be a 2.3 degrees C and the maximum could be as high as 3.7 degrees C, he said.

If storms do increase in intensity and frequency, an IPCC report on small islands states, “forest diversity could be severely affected, as adaptation responses on small islands are expected to be slow, and impacts of storms may be cumulative.”

The report cited a recent study of tropical forests in Puerto Rico, which found that hurricane-induced mortality of trees after 21 months was 5.2 percent a year, “more than seven times higher than background mortality levels during the non-hurricane periods. [The study authors] show that resistance of trees to hurricane damage is not only correlated with individual and species characteristics, but also with past disturbance history, which suggest that individual storms cannot be treated as discrete, independent events when interpreting the effects of hurricanes on forest structure.”

According to Miller, in addition to threats from hurricanes, climate change impacts on cloud formation and rainfall also imperil Hawai`i’s ecosystems. Currently, the cloud base in Hawai`i begins at about 600 meters above sea level. If the air temperature at sea level increases, in order for that air mass to cool to the point that it forms clouds, it has to rise to a certain elevation. Using this principle, Miller predicted that a temperature increase of 1.5 degrees C would push the cloud base up to about 831 meters, a 2.3 degree C increase would cause it to rise to 943 meters, and a 3.7 degree C increase would result in a cloud base at 1,169 meters. “The mountain area that produces rain will be reduced,” and less rain in a warmer environment could increase the evapotranspiration rate, as well, he said.

Based on his assumptions of a more elevated cloud base, Miller suggested that Hawai`i island could see a reduction in its cloud-producing area of between 9 and 27 percent; Maui, a 26-48 percent decrease; Kaua`i, a 34-79 percent decrease; and Moloka`i, a 48-91 percent decrease. Lana`i and O`ahu could see their clouds disappear altogether: Lana`i (where the rainforest’s ability to capture moisture is already threatened by feral ungulates) could experience a decrease of between 80 and 100 percent in its cloud-producing area; O`ahu’s reduction could be between 90 and 99 percent.

Determining how these reductions will affect rainfall is the ultimate goal.

“That’s what we’re trying to get to,” Miller says. “Knowing that precipitation is one of the main drivers in declining vegetation communities in Hawai`i…we need to figure out how precipitation is going to change over time as a result of global warming. We don’t have good information on that.”

He says the FWS eventually wants to hire climate modelers to help resolve that, adding that he and Burgett chose to use cloud cover as a surrogate for precipitation data for now. Once the FWS gets a better idea of how climate change will affect precipitation in Hawai`i, it will apply that to predict how plant communities, which rely on certain levels of precipitation, may change over time, he says.

At the conference, Miller urged the conservation community to evaluate, design and implement conservation programs that address the predicted impacts of climate change.

“We need to maintain rare and native species in full geographic ranges,” he said, adding that resource managers also needed to start focusing more on transition ecology, seed storage, captive propagation, and adaptive management.

He says that the USFWS, in its long-term planning for conservation in the state, is beginning to look at climate change impacts, as are many other federal agencies. While he says that, eventually, all of Hawai`i’s conservation agencies will have to deal with this issue, he notes that he has received no feedback from resource managers since he presented his results at the conference.

“None. Literally, none. I’m not sure why, but no one commented after the talk,” he says.

— Teresa Dawson

Volume 18, Number 4 October 2007

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