When muddy rainwater rose from Hawaii’s sheer coastal mountains and flooded residential communities last month, the damage caused by flooding was evident: homes were destroyed, businesses flooded, landslides covered highways, and raging rivers and creeks became clogged with debris.
But extreme rain events, predicted to be more common with man-made global warming, aren’t just wreaking havoc on land – the runoff from these increasingly severe storms threatens Hawaii’s coral reefs as well.
“These major events are the ones that do the most damage because they shed most of the sediment and nutrients on the reef,” said C. Mark Eakin, senior coral advisor for the National Oceanic and Atmospheric Administration and former director of the Coral Reef Watch program Agency.
Warmer climates tend to amplify existing weather patterns, said Hawaii’s state climatologist Pao-Shin Chu, noting that the islands overall have a humid climate and that strong storms are likely to be more frequent.
“Given this climate change, or global warming, as we’ve seen for the past hundred years, atmospheric water vapor pressure is increasing,” Chu said. “We have some evidence that we already have increasing, very intense rain.”
Coral reefs make up much of Hawaii’s coastal ecosystem and are vital to the state’s economy.
Hawaii’s reefs protect populated coastlines from massive ocean waves and storm surges from tropical storms – a benefit that the US Geological Survey generates more than $ 860 million annually.
The state’s reefs are valued at more than $ 33 billion, according to a study funded by NOAA.
The March floods were caused by a weather system that stalled over the islands, causing extremely heavy rain for two weeks.
On the north coast of Oahu, “a very large tidal wave” fell from the mountains and flooded the city of Haleiwa, said National Weather Service hydrologist Kevin Kodama.
“This is a big challenge in Hawaii where we have small, steep watersheds,” said Kodama. “Most of the basins in the state will cause flash floods.”
The last month had 11 flash floods and was the wettest March in 15 years, he said.
The drainage problem is manifold. Deforestation and sorting on construction sites and farms lead to increased runoff. Wild animals such as goats, pigs, and deer have clear vegetation that causes erosion and excessive sedimentation on the reefs. Constant low-level runoff carries gasoline and oil from roads, household chemicals, trash and pesticides into the ocean.
Any significant change in marine conditions, such as B. the influx of freshwater alone can adversely affect coral health. Land debris and soil accumulate on reefs and can suffocate and kill the corals. Scientists say that suspension of material in the water can also block the need for sunlight corals to survive.
One of the biggest problems for Hawaiian reefs is sewage. There are around 88,000 cesspools on the islands, many of them in coastal areas.
“Cesspools are essentially a hole in the ground that is left untreated before sewage is released into the environment,” said Jamison Gove, a research oceanographer at NOAA who lives on the north coast of Oahu.
Cesspools leak into the groundwater – and when it rains, they overflow, sending pathogens and other harmful contaminants into the ocean.
In the town of Pupukea, where professional surfers compete on the famous Banzai Pipeline reef break, more than 1.25 billion liters of sewage spill into the ocean every year – enough to fill hundreds of Olympic-sized swimming pools.
On the north coast, during the recent flood, “brown, polluted water just covered the entire city,” Gove said. “You could just smell it anywhere.”
More than half of the state’s cesspools are located on the Big Island, which is home to some of the most extensive and pristine coral reefs in the state. And Gove said some areas have shown significant decreases in coral cover where wastewater routinely ends up in the ocean.
A reef off the town of Puako – a highly monitored site – has suffered significant losses, he said. The coral cover there has decreased by about 70% since 1975.
“This is probably one of the more dramatic examples as coral cover isn’t that high in many places,” he said. “But since we don’t have this type of data everywhere, we can’t say for sure that this isn’t a more common story.”
NOAA is providing data to the state on the matter, and efforts to remove cesspools and change infrastructure to slow and redistribute floods could help Hawaii’s reefs.
The state has banned cesspools in new construction and is trying to remove the existing ones by 2050.
Although coral reefs around the world are threatened by global warming, including marine heat waves that bleach and kill coral, storm runoff could prove to be a more serious and immediate threat to reefs in Hawaii.
“In Hawaii, I would rate runoff a lot higher than ocean heat waves when it comes to driving coral decline,” said Greg Asner, director of the Center for Global Discovery and Conservation Science at Arizona State University.
In 2019, Asner and his team used aircraft imaging technology in conjunction with satellite data to create new detailed maps of all living corals in the Hawaiian Islands. The data, now used by federal and state scientists, shows a correlation between land-based pollutants from runoff and coral health.
“More runoff affects reefs, mainly by mobilizing more chemicals and sediment on land,” Asner said. “The increasing chemical pollution and sedimentation is a major reason for the decline in corals.”
The March floods were not the first of their kind.
A 2018 rainstorm on Kauai caused widespread flooding that separated a community for weeks. The storm set a new US record for precipitation in a single day at 50 inches.
Ku’ulei Rodgers, a coral reef ecologist at the Department of Marine Biology at the University of Hawaii, studied the 2018 flood and 2002 flood in the same area. The 2002 rains swept earth from a construction site into the ocean and “nearly killed an entire reef,” Rodgers said.
After the 2018 flood, a review found that fish abundance at a nearby reef was reduced by 20% and sea urchins, which help cleanse the reefs and maintain coral health, have been reduced by 40%.
When making political decisions about protecting reefs, it is important to understand that land and oceans are intertwined.
“(Local) Hawaiians knew there was a link between the two because anything they did in the highlands would affect their fishing downstream,” she said. “The better the watershed, the better the reef and vice versa.”