Why the fires in Hawaii have been so bad

In the Hawaiian language, Lahaina means “cruel sun”. The north-east trade winds provide the eastern shore of Maui with ample rain and the West Maui mountains with a superfluity of it. But Lahaina, the capital of the Hawaiian kingdom in the 19th century, sits in the mountains’ rain shadow and so gets comparatively little. What is more, the rain that does fall does so almost entirely in winter: summers are hot and dry. As a result it is no stranger to fires. But those that ripped through the beautiful city on August 8th and 9th were unprecedented in their fury. As of August 11th at least 55 deaths had been confirmed (and the number was expected to rise), and the damage to the town of around 13,000 people looks well nigh irreparable. Why were these fires so powerful?

Fires need dry fuel. Various factors provided these fires with a lot of it. Hawaii as a whole has been in a drought for over a year, and in Maui conditions recently worsened considerably. American drought-watchers recognise five levels of water stress, from “Abnormally dry” to “Exceptional drought”. In April nowhere in Maui County (composed of Maui and some smaller nearby islands) was abnormally dry, let alone suffering from full-on drought. As of the week of the fires more than a third of the island was in drought conditions and most of the rest was abnormally dry, in part the result of unseasonably hot weather. In Kihei, another town on Maui, the intense sunshine melted traffic lights on their poles.

Over the past decade meteorologists have increasingly talked of “flash droughts”, periods in which dryness increases very quickly because low or no rainfall coincides with sunlight, winds and air temperature, driving evaporation from the soil and water loss through plants into top gear. The rate at which conditions on Maui worsened meets the conditions for a flash drought, Jason Otkins of the University of Wisconsin-Madison told the Associated Press.

On top of the drought, there is land-use change. Over the years a lot of agricultural land near Lahaina has been abandoned. Such land will typically see a rich growth of grasses and shrubs, including, in Hawaii, some invasive species which can outcompete the indigenous plants. According to Tom Smith of the London School of Economics the abandonment of land previously used for farming or raising livestock typically sees the fuel load on that land increase in a way that permits much more intense fires. This has been a key factor in recent deadly fires in both America and the Mediterranean, he says.

Drought and land-use change set the scene for worse fires. But they are not essential for them. There were times last year when drought conditions on Maui were far worse than they have been this year. What seems to have set these fires apart was the addition of high winds. Some of these are associated with Hurricane Dora, a category-4 storm, which passed about 1,000km (640 miles) due south of Maui on the evening of August 8th, following a path from east to west.

Hurricanes are associated with rain much more than fire, but they can bring both. In 2018 Hurricane Lane passed much closer to the Hawaiian islands, bringing record rainfall to the island of Hawaii itself. But on Maui, roughly 120km to the north-west, its winds fanned three fires, including one, the “Kaua’ula Fire”, on the outskirts of Lahaina. Those winds were dry because they were fuelled by air that had been lifted and stripped of moisture at the heart of the hurricane, returning to the surface near its edge.

Dora passed Maui at a much greater distance, but it still brought with it strong winds. And they seem to have been strengthened by the fact that while the hurricane, an area of very low atmospheric pressure, was passing to the south of the island, there was an anomalous area of high pressure and oddly dry air to the north. Winds tend to move from high-pressure areas to low-pressure ones (curving as they do so) and the high pressure to the north seems to have strengthened the winds crossing Maui. (Something similar happened in October 2017, when winds associated with Hurricane Ophelia drew hot air from the Sahara across Portugal, fanning fires which killed 50 people.)

Local topography added to the problem. Winds that come down from a mountain in the way the winds coming into Lahaina do are strengthened and warmed by their descent. The Foehn winds of the alps and the Chinooks of North America the are well-known examples. Powerful downslope winds were also a decisive factor in the Camp Fire, the most destructive American wildfire over the past 100 years, which in November 2018 killed 85 people in the California foothills of the Sierra Nevada. Gusts of wind across Maui over the days of the fire were recorded to hit speeds of 108kph.

Taken together, the flash drought, high fuel loads and powerful winds seem to account for the disaster. What, then, of change in the future? The past half century of climate change has seen Hawaii get warmer, though not as fast as many other places; warming temperature trends tend to be faster on larger land masses. It has also been seeing more drought over the past century, a trend which may well continue.

On top of that trend, there also seems to be a change in the patterns of drought. It used to be that when ENSO, a globally important climate oscillation centred on the tropical Pacific, was in its La Niña phase, Hawaii could expect more rain, replenishing its groundwater. There has for some time been evidence that this is no longer the case, and the dryness over the past three years, which have been subject to long-running La Niña conditions, seems to bear this out. Whether this means that the severe Hawaiian drought associated with the first winter of El Niño conditions, which began a couple of months ago, can be expected to turn up as in years past, or whether the whole system has gone topsy turvy, remains to be seen.

Further abroad, two lessons stand out. One is that as the world continues to get hotter, flash droughts are likely to become more common. That has implications both for farmers and for fire management. The second is that even cities that have survived for centuries should not feel safe.