Our results suggest that as the planet warms, Hawaii’s dry regions will get drier, heightening the fire risk. At the same time, its wet areas will become wetter.
While Hawaii is home to some of the wettest spots on earth, it also has regions that receive little rain.
The very steep mountains on each of the main Hawaiian islands block the prevailing northeast trade winds. This results in abundant rain on the slopes facing the windward direction and dry “rain shadows” in the leeward areas. Maui’s west coast tourist communities, including Lahaina, are in one of those rain shadows.
Hawaii is remarkable for its exceptionally strong gradations in the average rainfall rates over very short distances. The summit of Mt. Waialeale in central Kauai, known as one of the rainiest places on Earth, receives an average of about 450 inches of rain per year. The town of Kehaka, 15 miles to the southwest and in the rain shadow, receives less than 20 inches per year on average.
These sharp differences over short distances have made projecting future climate change in Hawaii a particularly daunting challenge.
We created a model that zooms in on Hawaii and is able to capture those variations, including the rain shadow effect.
Using that model, we simulated the Hawaiian climate at the end of the 21st century under a scenario in which global greenhouse gas emissions from human activities continue at a rate that drives a global increase of temperature of about 4 degrees Fahrenheit (2.2 C). Such a scenario is quite plausible and even demands some significant reduction in current emission rates, but still pushes well beyond the internationally agreed goal of keeping global warming under 3.6 F (2 C) compared with preindustrial levels.
We found that in the wet windward areas of Hawaii, rainfall is projected to increase substantially. That includes increasingly frequent extreme downpours. On the other hand, rainfall is predicted to decrease substantially over much of the rain shadow regions.
The “dry gets drier” aspect is particularly important for formulating Hawaii’s plans to adapt to climate change. The soil in already dry regions may become even drier as rainfall decreases and warmer air promotes more evaporation from the surface. That includes Maui’s highly developed west coast and agricultural areas that until recently were home to large sugar cane farms.
In dry areas, fields of dry grasses, like these near Waimea on the Big Island of Hawaii, can spread a wildfire quickly on a windy day. AP Photo/Caleb Jones
Any of these things in isolation — the drought, the winds from the high-pressure system, the passing hurricane — could have created a problem for Maui. Happening all at once, they created what climate scientist Michael Mann, one of the originators of the “hockey stick” temperature graph depicting global warming and the director of the Penn Center for Science, Sustainability and the Media at the University of Pennsylvania, called “a ‘compound’ climate catastrophe.”
One lesson we must learn from Maui is that combinations of circumstances that we think of as unlikely might no longer be unlikely at all. And bad luck won’t be confined to small islands far away. – The insurance industry is already making an adjustment that we all soon will feel. A report last week by the reinsurance giant Swiss Re calculatedthat severe thunderstorms in the United States accounted for 68 percent of insured natural catastrophe losses worldwide in the first half of this year. Reinsurance companies will pass along those costs to the primary insurers who cover your home and your car. Primary insurers will eventually pass along those costs to you — though imagine facing the random violence of extreme weather without insurance at all.
As individuals and as communities, we need to think more about worst-case scenarios and actively plan for them. We have an old hemlock tree in front of our house that’s near the end of its life span. I love it, but we’re going to have to take it down and plant a replacement — before a storm brings it down.
