Welcome to The Hill’s Sustainability newsletter, we’re Saul Elbein and Sharon Udasin — every week we follow the latest moves in the growing battle over sustainability in the U.S. and around the world.

Healthy kelp forests protected California’s undersea ecosystems from the ravages of the mid-2010s heat wave, a study has found. 

The denser and more sheltered the kelp beds, the more they insulated the habitats beneath from the knock-on effects of warming, according to findings published on Tuesday in Proceedings of the Royal Society B. 

Undersea forests: Kelp is a family of seaweeds that grows in thick stands in shallow seas and oceans. 

• These underwater formations function much like forests on land — though unlike trees, kelp technically aren’t plants but communities of single-celled organisms.
• These dense kelp growths provide food and shelter for everything from shrimp to seabirds and seals. 

Feedback loop leads to dieback: During the heat wave that stretched from 2014 to 2016, however, kelp forests along California’s central coast began a process of severe decline. 

The reason? An El Niño event-fueled heat wave spiked ocean temperatures. 

Urchins unleashed: The hotter ocean temperatures killed off sea stars — allowing a population boom in their usual prey, sea urchins, according to the study.

• Sea urchins, in turn, munched on kelp, and as their numbers rose, the undersea canopy died.
• As the kelp canopy shrank, more sunlight broke through, driving ocean temperatures underneath higher — perpetuating the warming cycle. 

Dramatic losses: In the years after the heat wave, kelp density plummeted by more than half — and by 2020, those declines increased to nearly three-quarters. 

Hungry sea urchins left behind what’s known as “barrens,” or bare, rocky reefs in many areas that had once housed kelp forest. 

Resisting the decline: Scientists from the Monterey Bay Aquarium found that some kelp forests resisted the decline. 

• “We found that larger stands of giant kelp prevented shifts in sea urchin foraging behavior,” lead author Joshua Smith, a research scientist at the Monterey Bay Aquarium, said in a statement.
• These “persistent forests” were located in “typically less productive” seas, marked by gradual downward slopes and a geography that protected against waves. 

Those features “enabled them to become densely packed with kelp prior to the marine heatwave,” Smith said.  

Otters to the rescue: Kelp forests could also count on an additional reinforcement: urchin-eating sea otters, according to a study published in January in PLoS One. 

• “Where sea otters are absent, kelp forests have declined dramatically,” said Teri Nicholson, also of the Monterey Bay Aquarium, in a statement at the time.
• Over the past century, the density of sea otter populations was “the strongest predictor of change in kelp canopy coverage,” Nichols added. 

Reversing a persistent decline: The authors of Tuesday’s study expressed hope that their findings could help scientists better target the sheltered habitats where a declining kelp population can hang on in a warming world.

• Co-author Pete Raimondi, a marine ecologist at the University of California, Santa Cruz, stressed the importance of locating “areas where kelp can persist on its own.”
• Doing so, he added, could help identify where kelp restoration efforts have the best chance at success.