Satellite study shows subtle signs of climate change in Hawaii

Paradise just isn’t as lush as it used to be.

Researchers have examined satellite imagery of Hawaii for four decades and have concluded that the islands have suffered significant “browning,” or deterioration in vegetation health.

“There’s been an incredible change in the last 40 years,” said Austin Madson, an assistant professor at the University of Wyoming who has teamed up with 13 other scientists from Hawaii and the mainland to sift through data going back to 1982 .

The team examined satellite data using the Normalized Difference Vegetation Index, an indicator of vegetation health based on how plants reflect specific regions of the electromagnetic spectrum.

Evidence, published in the journal Environmental Management, suggests that climate change is leaving its mark on the islands, even if the changes are gradual and imperceptible from year to year, decade to decade.

“It’s really difficult for us humans to see long-term trends,” Madson said. “But you don’t forget the data.”

The paper, published in November, is titled “A time series spanning nearly four decades shows the Hawaiian Islands turning brown since the 1980s.”

Hawaii’s vegetation isn’t literally turning brown, Madson said. In scientific circles, Browning refers to the general decline in plant health and is the opposite of Greening.

Globally, the trend is for greening to increase as warmer temperatures and plant growth have accelerated at higher latitudes and elevations. Hawaii and other tropical areas of the northern hemisphere appear to be bucking the trend with long-term declines in rainfall and more droughts.

Overall, there was a significant decline in plant health in the Hawaiian Islands from 1982 to 2019, according to the study. The islands of Lanai and Hawaii saw the largest declines in NDVI of 44% or more.

The team also compared two similar El Nino neutral years (1984 and 2019) and found similar results.

A comparison of islands showed no significant overall change in NDVI for the small and low-lying islands of Niihau and Kahoolawe. But Kauai, Oahu, Molokai, Lanai, Maui, and the island of Hawaii all saw significant browning between 1984 and 2019.

According to the study, significant decreases in NDVI occurred more frequently than increases in all months except January, particularly from February to November.

Oahu, Lanai, Molokai, and Hawaii experienced the most browning overall from 1982 to 2019.

While there were relatively few significant changes in plant health in December, most islands experienced significant greening in January. However, in March, most islands saw a decrease in NDVI.

Researchers also looked at different land classes over time: urban, agricultural, bare ground, non-native vegetation, native vegetation, and alpine/subalpine. The results show that plant health declined significantly in all land classes for most months of the year and particularly in March.

Native vegetation cover on all islands saw significant declines in plant health, particularly on the leeward sides, with natives in the Kona region of the island of Hawaii experiencing the largest declines.

The long-term trends in total annual precipitation and drought severity show correlated declines, according to the study.

NDVI declines are exacerbated by expansion of grazing, spread of alien species, wildfires and changes in agriculture, the study says. For example, the period of the study included the precipitous declines in sugar and pineapple on the islands.

Can anything be done to reverse the tanning trend?

Although natural resource managers cannot control the effects of climate change or drought, they may be able to stabilize the health of native ecosystems by working to reduce human-caused fires and grazing, or by removing the non-native or invasive species that are migrating affect native vegetation, the study says.

Hawaii scientists involved in the study were Lucas Berio Fortini of the US Geological Survey’s Pacific Island Ecosystem Research Center in Honolulu, Kapua Kawelo of the Army Natural Resources Program at Schofield Barracks, Matt Keir of the State Division of Forestry and Wildlife, and Tamara Ticktin and David W. Beilman of the University of Hawaii at Manoa.

Madson said the team plans a follow-up study comparing bird numbers and diversity to changes in vegetation health.