The Ocean’s Hidden Role: How Sea Currents Turn Reforestation’s Cooling Into Distant Warming

Planting trees has become one of the most celebrated weapons in the fight against climate change, enshrined in net-zero pledges from more than a hundred countries and championed by initiatives such as the Bonn Challenge, under which 115 nations have committed to restoring a thousand million hectares of land by 2040. The logic seems airtight: trees pull carbon dioxide out of the atmosphere, so planting more of them must cool the planet. But a new modeling study published in Earth System Dynamics reveals a troubling complication. When researchers at Simon Fraser University traced the full Earth system response to large-scale reforestation over multiple centuries, they found that the ocean does not merely passively absorb the climate consequences of new forests. Instead, ocean dynamics actively amplify a remote warming effect that spreads across the globe, intensifies for centuries after the trees have regrown, and is strongest in the very high-latitude regions where climate change is already most alarming.

The study, led by Pierre Etienne Banville with Alexander J. MacIsaac and Kirsten Zickfeld, tackles a long-standing blind spot in climate science. Forests influence climate in two distinct ways. Beyond storing carbon, they alter the physical properties of the land surface, changing how much sunlight is reflected, how much water evaporates, and how much heat is transferred to the air. These biogeophysical effects operate locally, at the site of the forest itself, and non-locally, in distant regions connected through atmospheric and oceanic transport. Previous work had established that large-scale forestation tends to produce non-local warming, particularly at high latitudes where dark forest canopies mask bright snow. But most studies focused on land-atmosphere interactions alone, leaving the ocean’s role largely unexplored, especially over the long timescales on which the ocean actually operates.

To isolate that role, the team ran paired multi-century simulations with the University of Victoria Earth System Climate Model, an intermediate-complexity model that couples a dynamic ocean, a thermodynamic-dynamic sea ice model, and a dynamic global vegetation model. The experimental design was elegant in its simplicity. The researchers first spun the model up for ten thousand years under pre-industrial conditions, then deforested the globe by allowing only grasses to grow, and finally let forests regrow on either 50 percent or 25 percent of land grid cells in a checkerboard pattern. This checkerboard arrangement is the key to separating local from non-local effects: over reforested cells, the temperature difference from the deforested baseline contains both local and non-local signals, while over cells that remained grassland, any difference must be entirely non-local. Crucially, one set of simulations used a fully dynamic ocean, while a second set prescribed fixed sea surface temperatures, effectively switching off ocean circulation and ocean-atmosphere and ocean-sea ice feedbacks.

The results were striking. In both sets of simulations, reforestation cooled the surface locally, most strongly in the tropics where broadleaf trees with large leaf areas dominate, and warmed it non-locally, most strongly at high latitudes. But when the dynamic ocean was included, the non-local warming became far larger in magnitude and covered a much greater geographic area. The mechanism begins over land. Forests darken the surface, increasing absorbed shortwave radiation, and alter the turbulent heat fluxes, raising the sensible heat flux that warms the overlying air. This warmer air is advected away from reforested regions, raising air temperatures globally. Through the Clausius-Clapeyron relationship, warmer air holds more water vapor, and that additional vapor strengthens the greenhouse effect, increasing incoming longwave radiation and warming sea surfaces everywhere.

From there, the ocean takes over and escalates the process. Warmer sea surface temperatures increase evaporation over the ocean, pumping even more water vapor into the atmosphere and reinforcing the greenhouse effect in a classic positive feedback. At high latitudes, the warmer ocean melts sea ice, exposing darker water that absorbs more sunlight, which warms the ocean further and melts more ice. The simulations also revealed a temperature-vegetation feedback: warmer conditions increase leaf area in both reforested and remaining grassland cells, darkening the surface and amplifying the warming. A surface energy balance decomposition showed that in the dynamic ocean runs, the increase in incoming longwave radiation driven by the water vapor feedback accounts for most of the non-local warming, whereas it plays only a modest role when sea surface temperatures are pinned in place.

Not every oceanic response pushes in the same direction. The warming of sea surfaces reduced the density of surface waters and, combined with freshwater input from melting sea ice in the Labrador Sea, weakened the Atlantic Meridional Overturning Circulation, the great conveyor belt that carries heat northward. During the initial phase of forest regrowth, this slowdown dampened high-latitude warming by reducing meridional heat transport, partially counteracting the water vapor and sea ice-albedo feedbacks. But over the following five centuries, the circulation recovered almost to its pre-reforestation strength, and the resumption of poleward heat transport contributed to the continued rise of sea surface temperatures at high latitudes, adding yet another layer to the amplification.

Perhaps the most consequential finding is temporal. By the time vegetation had regrown and stabilized, roughly five hundred years after reforestation began, the local effects had essentially settled. In the prescribed sea surface temperature runs, the non-local effects had stabilized too. In the dynamic ocean runs, however, the non-local warming kept strengthening for another five hundred years. The culprit is the ocean’s enormous thermal inertia: it takes centuries for the ocean to equilibrate with the atmosphere, as heat is slowly absorbed, mixed downward, and later resurfaced by currents. This sustained rise in sea surface temperature feeds the amplifying feedbacks, committing the planet to additional non-local warming long after the forests themselves have reached maturity. The authors draw a direct analogy to the committed warming from past carbon dioxide increases, which is likewise driven by oceanic processes.

The researchers stress that these results are not an artifact of an extreme, planet-scale experiment. A sensitivity analysis with reforestation covering 25 percent of land grid cells, closer to the extent of historical deforestation, reproduced the same geographic and temporal patterns at roughly half the magnitude, with ocean dynamics driving a similar proportion of the non-local warming. The team also acknowledges limitations. The University of Victoria model uses a simplified single-layer atmosphere that cannot represent cloud feedbacks or atmospheric circulation changes, and its land surface scheme produces a decrease in evapotranspiration with reforestation that runs counter to most other models. Still, comparisons with the CMIP6 model ensemble under the Land Use Model Intercomparison Project suggest that including clouds would likely weaken tropical non-local warming while strengthening it at high latitudes, leaving the ocean’s amplifying role fundamentally intact.

The policy implications are uncomfortable but important. Net-zero frameworks typically treat a tonne of carbon stored in trees as equivalent to a tonne of carbon not emitted from fossil fuels, yet the full Earth system response to forestation unfolds over timescales far longer than most policy horizons. Large-scale forestation projects that appear beneficial when judged on carbon sequestration and local biogeophysical effects may carry a hidden, growing debit in the form of committed non-local warming that intensifies for centuries, particularly in the Arctic. The authors argue that decision-making frameworks must consider the complete Earth system response over sufficiently long timeframes, and they suggest that geological net zero, in which emitted carbon is permanently stored in geological reservoirs, would sidestep these biogeophysical complications entirely. Forests still deliver enormous benefits for biodiversity, ecosystem services, and carbon storage, but this study makes clear that counting on them to offset fossil fuel emissions requires accounting for an ocean that remembers, amplifies, and answers back on its own slow schedule.

Subject of Research: Ocean-mediated non-local biogeophysical warming effects of large-scale reforestation over multi-century timescales

Article Title: Ocean dynamics amplify remote warming effects of reforestation

Article References: Ocean dynamics amplify remote warming effects of reforestation. (n.d.). https://doi.org/10.5194/esd-17-1315-2026

Image Credits: AI Generated

DOI: 10.5194/esd-17-1315-2026

Keywords: reforestation, ocean dynamics, climate modeling, biogeophysical effects, sea surface temperature, AMOC, water vapor feedback, sea ice-albedo feedback, net-zero policy, carbon sequestration, Earth System Dynamics, thermal inertia