The boat leaves Monterey at 6:15 in the morning. Dr. Lena Morales, a marine ecologist with the Monterey Bay Aquarium Research Institute, sits on the stern rigging her primary regulator with the practised speed of someone who has done this two thousand times.

The destination is a stretch of coast off Garrapata State Park, about thirty nautical miles south, where she has maintained a long-term monitoring transect since 2012. The site is one of fourteen along the central California coast included in the PISCO programme, the Partnership for Interdisciplinary Studies of Coastal Oceans, a multi-university collaboration that has tracked rocky reef ecology from Oregon to Baja California for more than two decades.

Morales is forty-six. She has dived this transect, by her own count, 187 times.

Day one. The visibility is fifteen metres, unusually good. The water temperature at depth is 11 degrees Celsius. She descends along the anchor line into a forest of Macrocystis pyrifera, the giant kelp, whose stipes rise from the rocky bottom in straight green columns toward the surface canopy fifteen metres above.

The kelp here was, until 2014, the densest she had ever measured. A marine heatwave that year, known as the Blob, raised offshore temperatures by 2 to 3 degrees for nearly two seasons. The kelp thinned. A second heatwave in 2016 and a sustained period of warm water through 2019 thinned it further.

By 2020 the transect, which Morales had recorded at 14.2 kelp stipes per square metre in 2013, held just 2.7.

Day two. The dive plan calls for fish counts along the southern arm of the transect. Morales moves slowly, slate in hand, recording species and size class in five-metre intervals. Blue rockfish, kelp greenling, painted greenling, a juvenile lingcod hiding under a ledge.

She notes, with a small tap on the slate, the absence of sunflower stars. Pycnopodia helianthoides, once a common predator on the central coast, was effectively eliminated in 2013 and 2014 by sea star wasting disease. Its disappearance contributed to a population explosion of purple sea urchins, Strongylocentrotus purpuratus, which graze on kelp holdfasts.

“She notes, with a small tap on the slate, the absence of sunflower stars.”

Without sunflower stars to control them, the urchins formed dense barrens along much of the northern California coast. South of Monterey, sea otters provided an alternative control. Morales is not subtle in her affection for the otters.

Day three. The wind comes up and the dive is short. She surveys urchin density only, in three permanent quadrats along the inshore edge. Counts are 14, 9, and 22 individuals per square metre. The third quadrat is in a small barren patch that has persisted since 2018. The other two suggest that urchin grazing pressure has eased in recent years.

On the boat ride back, Morales eats an apple and looks at the data. The recovery, she says, is uneven, but it is happening.

Day four. The first thick canopy of new growth. Morales surfaces inside a patch of Macrocystis that has reached the surface and laid its blades flat in the sun. The light filters down through them in shifting green shafts. A school of juvenile rockfish moves through the stipes.

She measures stipe densities at three points and records 8.4, 11.2, and 9.7 per square metre. The recovery from the 2020 low is real and substantial.

The reasons are not entirely clear. A cooler oceanographic regime since 2022. The return of some sea star species, including a small recovery in Pycnopodia populations from a captive-bred release programme run by the Sunflower Star Laboratory. Continued otter foraging on urchins in shallow water. A series of strong winter storms that physically dislodged urchin barrens.

Day five. A long dive. Morales surveys the northern arm of the transect and finds a small population of red abalone, Haliotis rufescens, that she had not recorded since 2016. The species is protected from harvest and has declined dramatically along the central coast. Six adults in a single dive is, she says quietly into her dictaphone on the surface, the best news she has had this year.

Day six. The boat takes a longer route, past the Point Sur Lighthouse and into open water. A pod of Risso's dolphins crosses the bow, slow and grey and scarred. Morales does not bother to count them. They are not her species.

She is thinking, she says, about the longer arc. The kelp forests of the central California coast have always fluctuated. They are not stable systems. They respond to ocean temperature, nutrient upwelling, El Nino cycles, storms, and a complex food web that includes otters, urchins, abalone, and dozens of fish species.

What is new, she says, is the speed and frequency of the warm-water episodes. The system can recover from one heatwave. The question is whether it can recover from a heatwave every three or four years for the rest of the century.

Day seven. The final dive of the week. Morales sets a transect tape from the anchor and begins her slow counting traverse. The kelp is dense overhead. A harbour seal materialises out of the green, looks at her with an expression she will not anthropomorphise, and is gone.

She finishes the count at 9.1 stipes per square metre. The number goes into a spreadsheet that has been kept, in continuous form, since 2012.

On the boat back to Monterey she watches the coast slide past. The mountains of Big Sur drop straight into the sea. Somewhere below the green surface, the kelp she has counted for fourteen years is doing the only thing it knows how to do, which is grow toward the light.