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The islands of Hawaii are world renowned for their generally pleasant and tranquil weather. However, the was a stark reminder that Hawaii also can experience drought and hot, dry, windy weather, providing the conditions for destructive fires.

Hawaii has seen a generally in the amount of each year as the . Climate change was to Maui’s wildfire catastrophe, and rising temperatures and associated rainfall changes are expected to increase the islands’ fire risk. These changing weather patterns will also affect Hawaii’s ecosystems and freshwater resources.

at the University of Hawaii, and I have worked with colleagues to develop sophisticated that over the 21st century.

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.

The in particular have reason to be concerned about the available for residential, commercial and agricultural uses. In addition, changes in rainfall are expected to affect the , harming some unique native species and increasing some that enhance fire danger.

Average Rainfall Drops Sharply In The Rain Shadow

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, . The town of Kehaka, 15 miles to the southwest and in the rain shadow, on average.

These sharp differences over short distances have made projecting future climate change in Hawaii a particularly daunting challenge.

Computer approximate the atmospheric wind, temperature and humidity at discrete points on a regular grid. The horizontal spacing between grid points in global climate models is . To put that in perspective, Maui is only at its widest.

Predicting Hawaii’s Changing Climate

We created a model that and is able to capture those variations, including the rain shadow effect.

Using that model, we 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 Celsius). Such a scenario is quite plausible and even demands some significant reduction in current emission rates, but still pushes well beyond the internationally agreed 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.

This overall “wet gets wetter and dry gets drier” trend . Our computer model shows that it also applies to the rainfall gradations over the very short distances relevant for Hawaii.

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 .

Developing Better Projections To Help Prepare

Rainfall rates will still vary year to year. Indeed, Hawaii rainfall is known to display , due in large part to the influences of and the , both natural climate patterns. Such natural variations are expected to coexist with the overall centurylong trend toward drier or wetter conditions.

Simulating rainfall in climate models still has many uncertainties, and there are particular challenges in representing the fine geographical details of the rain in Hawaii. Another produced results broadly consistent with ours but projects an overall stronger drying trend in the islands.

While further research should help reduce the uncertainties in climate projections, our results suggest that, in the long term, Hawaii needs to prepare for more extreme conditions, including a heightened risk of wildfires.

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