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 images of Hawaii over four decades and concluded the islands have endured significant “browning,” or declines in vegetation health.

“There’s been an incredible amount of change over the last 40 years,” said Austin Madson, an assistant professor at the University of Wyoming, who 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 certain ranges of the electromagnetic spectrum.

The evidence, published in the journal Environmental Management, suggests that climate change is making its imprint in the islands even if the changes are gradual and may not be noticeable from year to year, decade to decade.

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

The paper, published in November, is titled “A Near Four-Decade Time Series Shows the Hawaiian Islands Have Been Browning Since the 1980s.”

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

Globally, the trend is that increases in greening as warmer temperatures and plant growth have accelerated at higher latitudes and elevations. Hawaii and other Northern Hemisphere tropical areas apparently are bucking the trend with long-term decreases in rainfall and more drought.

Overall, there was a significant decline in plant health across the Hawaiian Islands from 1982 to 2019, according to the study. The islands of Lanai and Hawaii experienced the greatest decreases 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 indicated no significant overall change in NDVI for the small and low-lying islands of Niihau and Kahoolawe. But Kauai, Oahu, Molokai, Lanai, Maui and Hawaii island all experienced significant browning between 1984 and 2019.

For all months except January, significant declines in NDVI occurred more frequently than increases, especially from February to November, according to the study.

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

While there were relatively few significant changes in plant health for December, most islands experienced significant greening in January. During March, however, most islands experienced declines in NDVI.

The researchers also looked at different land classes over time: urban areas, agriculture, bare ground, non-native vegetation, native vegetation and alpine/subalpine. The results show that plant health significantly decreased in all the land classes for most months of the year, and especially during March.

Native vegetation cover across all islands experienced significant declines in plant health, especially on the leeward sides, with natives in the Kona region of Hawaii island experiencing the greatest declines.

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

NDVI declines, the study said, are exacerbated by grazing expansion, the spread of non-native species, wildland fires and changes in agriculture. For example, the study’s time period included the precipitous decline of sugar and pineapple in the islands.

Can anything be done to reverse the browning trend?

Although natural resources managers can’t control the impacts of climatic changes or drought, they may be able to stabilize the health of native ecosystems by working to reduce human-caused fire and grazing, or by removing the non-native or invasive species that affect native vegetation, according to the study.

Scientists from Hawaii contributing to the study were Lucas Berio Fortini of the U.S. 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 is planning a follow-up study comparing bird counts and diversity to changes in vegetation health.