For generations of divers, the ocean felt like a constant—a reliable canvas of vibrant life or silent history. Yet, the writer observes that the impacts of climate change are accelerating, rewriting the script of our most beloved dive sites, from the lush, temperate forests to the tropical resting places of history's great naval tragedies.

Part I: The Ghost Forests and the Tropical Takeover

The most visible change for many divers is the radical ecosystem shift known as tropicalization, where warming waters act as an unstoppable poleward current, driving cold-water species out and inviting new, warm-water species in.

The Tragedy of the Urchin Barrens

Along the coasts of Northern California, the once-majestic Giant Kelp forests are a stark example of this shift. These towering ecosystems, which provide food and shelter for hundreds of species, have been devastated.

The Chain Reaction: A series of marine heatwaves (prolonged, intense warming periods) in recent years caused mass mortality among key predators, most notably the Sunflower Sea Star—a major consumer of sea urchins.

• The Result: With their primary predator gone, Purple Sea Urchin populations exploded. These voracious grazers stripped the remaining kelp, converting hundreds of miles of vibrant, productive kelp forest into barren, underwater deserts of rock, covered only by urchins. The diver who plunged into a shadowy, swaying kelp cathedral a decade ago now drops onto an empty, brightly lit moonscape where the only life is a carpet of spines.

  New Neighbors in Cold Waters

  Conversely, in traditionally cool regions like the coasts of the UK or Northern Japan, divers are encountering what were once exotic sights: tropical species. As ocean temperatures rise, warm-water fish, like parrotfish and certain species of rabbitfish, are expanding their range.⁴ While colorful, this "tropicalization" poses a new threat:

• these tropical herbivores often graze at higher rates than native temperate species, leading to overgrazing and the consumption of kelps and sea grasses that form the foundation of the cool-water ecosystem.⁵ This compounding effect accelerates the destruction of temperate habitats, demonstrating that the challenge is not just the loss of species, but the introduction of new ones with disruptive eating habits

   

  Part II: The Silent Chemical Threat to History

• While the changing life in the water is easy to see, a more insidious change is threatening the very structure of the ocean floor: the corrosion of historical shipwrecks due to ocean acidification (OA).

  The Fate of Iron and Steel Wrecks

• For the iconic iron and steel shipwrecks cherished by the diving community, this increased acidity is a catastrophe.

• In normal seawater, the iron in a wreck reacts with saltwater to form a protective layer of iron oxide and other compounds, often cemented by marine life into a hard concretion. This layer acts like a natural shield, slowing down the decay of the metal hull.

• However, increasingly acidic water accelerates the chemical process of corrosion and can actively strip away this protective concretion. This leaves the metal structure vulnerable to faster disintegration.

• Truk Lagoon and the Race Against Time

  The writer could consider the fate of the WWII fleet in Truk Lagoon (Chuuk), one of the world's most famous wreck diving destinations. The warm, tropical waters there already accelerate natural corrosion.

• Combine this natural decay with the increasing threat of ocean acidification, and the clock is ticking faster than ever. Archaeologists are in a race to document these sites, not only to preserve history but also to mitigate environmental risk. The accelerated structural decay increases the chance of a catastrophic oil leak from the remaining fuel bunkers within these sunken vessels, turning a historical tragedy into an ecological one.

• The vibrant kelp forests and the silent, cargo-filled decks of the great wrecks are both testaments to how profoundly climate change is altering the submerged world. For the diving community, understanding these changes is the first step toward advocating for the conservation of the fragile, dynamic environments that fuel their passion.