The coastal waters of temperate regions once supported vast underwater forests of giant kelp, whose fronds could reach more than thirty metres from the seabed to the surface. These forests, dominant along the west coasts of North and South America, around New Zealand and Tasmania, and across stretches of the South African and European Atlantic coasts, are among the most productive ecosystems on Earth. A single hectare of mature giant kelp produces as much biomass in a year as a hectare of tropical rainforest, and the structural complexity of the forest supports an equally complex community of fish, invertebrates, and marine mammals. Yet the global area covered by kelp has declined by roughly forty percent since the middle of the twentieth century, and the pattern of decline has accelerated in the last two decades.

The biological basis of kelp productivity is straightforward. A kelp frond is covered in small gas-filled bladders that keep it buoyant, so each individual can extend from a holdfast anchored on the seabed up into the sunlit surface waters. The holdfast, a mass of root-like branches called haptera, resists the pull of waves and currents. Growth proceeds from the base of each frond at a rate that can exceed fifty centimetres a day in favourable conditions; Macrocystis pyrifera, the giant kelp of the Pacific, is among the fastest-growing organisms known. The productivity supports, directly or indirectly, many of the fisheries of the Pacific coast, and the forests themselves protect coastlines by dissipating the energy of storm waves before they reach the shore.

The principal threats to kelp forests are now well-understood. Warming waters reduce the supply of dissolved nutrients that upwell from deeper, colder layers, starving the forest of the nitrogen and phosphorus it needs for rapid growth. Marine heatwaves, abrupt increases in sea temperature lasting weeks to months, can kill entire forests outright. In 2011, a heatwave off western Australia eliminated hundreds of kilometres of kelp in a single season, and the affected areas have not recovered because the warm water has supported a different, simpler community of reef fish that actively graze any new kelp seedlings. The South African marine biologist Thandi Mokoena has described such transformations as regime shifts, arguing that they can be difficult or impossible to reverse even if water temperatures return to their former range.

A second major threat comes from sea urchins. Urchins are natural grazers of kelp, and healthy forests support large numbers of them in balance with predators such as sea otters and certain fish. When the predators are overfished or removed by disease, the urchin population can explode, consuming the kelp canopy and leaving behind an underwater landscape known as an urchin barren. Once an urchin barren has established itself, it can persist for decades, because the surviving urchins continue to graze any new kelp seedlings before they reach a size at which they could regrow the forest. The disappearance of sea otters along much of the North American Pacific coast in the nineteenth century, driven by the fur trade, produced urchin barrens that persisted in some areas until otters were reintroduced in the late twentieth century.

Restoration efforts have attempted to address both threats. Mokoena's laboratory has worked on an urchin-removal programme on the South African west coast, in which divers physically remove urchins from selected areas to allow kelp to re-establish. The approach is labour-intensive and covers only a small area, but pilot studies have shown that a few months of removal can allow kelp to return to a harvestable density, at which point the restored forest can be self-sustaining. A more speculative line of research involves cultivating kelp strains selected for heat tolerance and re-seeding heat-damaged areas with them. The Australian marine biologist Kirra Davis has warned that such programmes must be carefully designed, because introducing non-local genotypes to a wild population risks disrupting the local genetic structure in ways that might compromise long-term resilience.

The cultural and economic implications of kelp decline vary by region. Along parts of the Pacific coast, kelp forests underpin a commercial abalone industry that has contracted sharply as the forests have retreated. On the South African coast, kelp beds supply a traditional small-scale fishery and a growing industry harvesting kelp for alginate production, a thickening agent used in food and pharmaceuticals. In Tasmania, where the giant kelp forests have declined by more than ninety-five percent since the 1940s, the loss has affected recreational diving, commercial fishing for sea urchins, and the cultural identity of several coastal communities. Mokoena has argued that the value of a kelp forest cannot be captured by a simple commercial calculation, and that its decline has consequences that are not fully reflected in any market price.

Carbon cycling provides a further reason for attention. Kelp absorbs carbon dioxide as it grows, and although most of the resulting biomass is consumed by grazers or by decomposition, a fraction of it is exported to deep water where it can remain sequestered for centuries. Estimates of the total carbon sequestration by global kelp forests vary widely, but the lower bound is large enough to be of practical interest in climate policy. Davis has argued that maintaining existing kelp forests, and restoring damaged ones where possible, should be considered part of the general programme of natural climate solutions alongside forest protection and peatland restoration. Whether the argument attracts the funding it requires will depend on the integration of marine ecosystems into national climate accounts, a process still at an early stage.

The retreat of kelp forests has attracted less public attention than the loss of tropical coral reefs, despite covering a roughly comparable global area. The disparity reflects, in part, the fact that kelp is harder to photograph than coral and that its fauna is less immediately appealing to a popular audience. Mokoena has observed that one of the tasks facing her discipline is to communicate the significance of what is happening underwater, in a medium most people cannot see, before the changes become too extensive for economic restoration to be feasible.