253-year coral core from Olowalu records land runoff rising with West Maui's sugarcane era

Corals build their skeletons in increments, the way trees add rings, and elements from the surrounding seawater are locked into each layer as it forms ^[6]. Measure the chemistry along a core and you have a dated log of the water over the reef. This one came from a single massive colony, analyzed by a team from the University of Hawai'i Sea Grant program, the university's School of Ocean and Earth Science and Technology and the U.S. Geological Survey ^[1].

What I like about the design is what the team set beside that chemistry: the coral's own growth, historical records of land-use change and large-scale climate patterns such as El Nino ^[7]. Seawater chemistry moves with ocean climate as well as with what washes off the land. Without the climate series, a run of unusual years could pass for a new field upslope. According to the release, the coral recorded both natural climate variability and major changes within the watershed ^[8].

The sugarcane connection is a match in timing. The chemical changes are dated to the period "as sugarcane agriculture expanded" ^[4], and the release describes the late-1960s turn in coral growth as "concurrent with" sugarcane expansion ^[5]. One core can show that two things moved together. It cannot rank the causes of a change on a coast where several things were changing at once.

Most of the series shows a coral growing faster, with calcification rising through much of the record ^[5]. The decline covers roughly the final 45 years of 253, under a fifth of the series ^[13]. The release did not disclose the size of that decline or of the 20th-century chemical changes, or what drove the earlier rise in growth.

I think the reference half of the result stands on firmer ground than the causal half. Chemistry laid down before Western contact and plantation agriculture is a direct measurement of the reef in an earlier state ^[3]. The core cannot show whether keeping sediment on land now would bring calcification back. It records what arrived with the fields, and it stops in 2013 ^[2].

Samuel Kekuewa is a Hawai'i Sea Grant postdoctoral affiliate researcher ^[12]. "One of the important lessons from this study is that some of the changes that occurred on land more than a century ago can still provide important context for what we see on reefs today," he said ^[10]. He also pointed to the restoration already under way: across Hawai'i, Kekuewa said, communities are already doing a large amount of work to restore watersheds, retain sediment on land and protect the downstream coral reefs. "Records like this can help provide the long-term context needed to support and guide that work," he said. ^[11]