(Revises "last week" to exact date in first paragraph)
By Gloria Dickie, Clare Trainor, Daisy Chung and Travis Hartman
LONDON (Reuters) -The wildfire that ripped through Lahaina on Aug 8., reducing what had once been the jewel of the historic Hawaiian kingdom to rubble, was decades in the making, scientists say. Still, it would take a unique combination of the elements to produce America's deadliest wildfire in more than a century.
In the days before the wildfire started on Aug. 8, temperatures in Lahaina simmered in the low 30s Celsius (high 80s Fahrenheit) — about average for the time of year.
But it was drier than usual. Southeastern Maui has been enduring a moderate-to-severe drought all summer, according to the U.S. Drought Monitor.
The state normally relies on the La Nina climate pattern to deliver quenching rains during winter. But the three-year La Nina that ended in 2022 didn't deliver as much rain as expected — continuing a 30-year trend which has recorded rainfall declining by about 30% during Hawaii's wet season.
"Recent La Ninas have been much, much drier than we expected, as we've seen multi-year droughts getting more severe," said climatologist Abby Frazier at Clark University in Massachusetts, who has spent more than a decade working in Hawaii.
Amid this arid backdrop came the wind.
Over Aug. 7 to 9, gale-force wind gusts reached 67 miles per hour (108 kilometres per hour) in Maui County, according to the National Weather Service. The fierce winds uprooted trees and roiled seas.
At first, some meteorologists blamed Dora — a Category 4 hurricane spinning some 700 miles (1,100 km) south of Honolulu — for whipping up the tempestuous winds. However, Honolulu-based meteorologist John Bravender said his analysis suggests that Dora likely played a more minor role in the fire.
"Dora, even though it was a major hurricane, had a very small wind field, and it's very far away from the state," said Bravender, who works with the National Oceanic and Atmospheric Administration's (NOAA) Central Pacific Hurricane Center. But it did cause warm air around the storm to fall lower in the atmosphere, closer to the ground.
At the same time, a strong high pressure system to the north of Hawaii sent a prevailing east-northeast wind called Moa'e or A'eloa that swept down and across the leeward side of Maui.
The winds from this high pressure system — known as the North Pacific High — likely combined with the warm air layer, called the inversion layer, to push warm, dry air across the volcanic peaks towering over Lahaina, Bravender said.
Such events occur a few times each year, but "this was extreme in the magnitude of it," he said.
As the winds moved down the slopes to lower elevations, the descending air compressed, causing it to heat up. At the base of the mountains — about one mile (2 km) from town — the winds met with dry grasses and parched earth, rather than the native shrubs and dry forests that once grew in a tangle of tropical trees, ferns, mosses and lichens before being replaced by sugar plantations in the 19th and 20th centuries.
The dry winds sapped the drought-stressed grasses of what little moisture they still had.
While climate change, which is driven by fossil fuel use, continues to warm the planet's atmosphere, wildfires such as those burning in Canada this month have grown worse in northern and mid-latitude forests worldwide.
But warmer temperatures weren't the driving factor in Maui, which saw only "a small background signal of climate change," said the climatologist Frazier.
Instead, she said, the invasive grasses were "the largest factor at play with this fire."
A NEW FUEL
When American missionaries arrived in Lahaina in the early 19th century, they transformed the tropical region by building over wetlands and Hawaiian fish ponds, and turning the port into an international hub for whale oil.
The colonisers replaced local customs with their own, and many native Hawaiians died from diseases introduced by the missionaries to which they had no natural immunity.
During this time, wildfires were less common — and those that occurred were often sparked by lightning or lava and burning ash spewed from volcanic eruptions.
By the mid-1800s, another commodity had taken priority. Sugarcane, brought to the islands by early Polynesian migrants, became a key Lahaina export. The town's first sugar company, Pioneer Mill, developed the dry forest and native shrubland around Lahaina into plantations. Other companies joined in, and by the 1930s sugar plantations covered more than 250,000 acres (100,000 hectares) of Hawaii.
Cheaper labour markets in India, South America and the Caribbean in the following decades led most Hawaiian sugar companies to end production by the 1990s, including Pioneer Mill in 1999, and the plantation lands were largely abandoned.
But the lush forest and native shrubland did not return.
The once-rich soils had lost much of their nutrient value and eroded away.
"Once you disturb an ecosystem like that and replace it with plantations, it does not return to its former state," said fire scientist Thomas Smith at the London School of Economics and Political Science.
And so African grasses took over, including buffel grass and guinea grass, which had been introduced to the islands as pasture for livestock. Today, over 90% of Hawaii's native dry forests have disappeared, and non-native grasses cover roughly a quarter of the state, according to scientists.
Hawaii is particularly vulnerable to plant invasions, as the remoteness of the islands meant that native species evolved without much competition or defenses, said fire ecologist Jennifer Balch at the University of Colorado Boulder who studies grass fires.
The grassland expansion over the last century has coincided with a roughly 400% increase in wildfires, according to the Pacific Fire Exchange group, a fire communication project led in part by the University of Hawaii.
These grasses are "plants that, when you see them dry up, you just think 'wildfire'," said botanist Mike Opgenorth, director of the National Tropical Botanical Garden’s Kahanu Garden and Preserve on Maui.
On the other hand, "a well-established forest system is able to buffer those moments of dry weather and high winds," he said, with dead tree logs and forest leaves still holding more moisture than finer fuels like grasses.
Strong winds can also move faster over a grassland than they would through a forest, where they face friction against trees.
Investigators have yet to determine what first sparked the Lahaina fire on Aug. 8, but scientists say it is clear how flames managed to rush so quickly across the grasslands, through the plantation-era wooden buildings and up to the harbor in just a few hours.
"It was an incredibly flammable landscape surrounding a very flammable town," Smith said.
(Reporting by Gloria Dickie in London and Clare Trainor; Edited by Julia Wolfe, Katy Daigle, Simon Scarr and Josie Kao)