Natural carbon sinks are threatened by droughts and heat waves

Landscapes in southwest Europe are losing their knack for soaking up carbon dioxide. A string of hotter, drier summers is shrinking the margin between what plants absorb during growth and what they release back to the air, affecting Europe’s natural carbon sinks.

When the weather turns extreme, that slender gain can vanish, turning once helpful forests and fields into temporary carbon sources. The warning comes from the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA‑UAB) and partner groups.

The study combines satellite light measurements, ground‑level carbon‑flux towers, and a revamped vegetation model that now tracks soil moisture. It covers Portugal, Spain, southern France, and Italy from 2001 to 2022.

How plants act like living lungs

Plants pull carbon from the atmosphere during photosynthesis and lock much of it into wood, leaves, and roots. At the same time, they respire – sending part of that carbon back out as gas.

The net balance – called net ecosystem exchange – shows whether a landscape acts as a carbon sink or a source. In a stable climate, Europe’s lush spring and mild autumns keep that balance on the sink side.

Warmer springs now arrive earlier. Autumn frosts come later. That extra time boosts raw photosynthesis by roughly three to six grams of carbon per square meter each year across the study area.

The catch is that respiration grows almost as fast because heat speeds up plant metabolism and powers soil microbes that digest dead matter. On average the net annual gain is less than one gram. A single heat wave can erase it.

Historic temperature records

The summer of 2022 set historic temperature records and followed months of scant rain. Soil dried out through winter, spring, and into the next autumn.

Across the region, net carbon uptake fell by more than one‑quarter compared with the long‑term average.

Researchers calculated that, during those scorching months, vegetation failed to absorb more carbon than the nation of Spain emitted in the entire year. That demonstrates how a single season can upend national climate budgets.

Soil and plants depend on rain

The team’s updated model points to soil moisture as the hidden lever. In humid Atlantic and continental belts, plants depend on steady rainfall. When a drought strikes, these regions see the sharpest drop in photosynthesis and the steepest rise in respiration.

By contrast, Mediterranean woodlands expect short summer dry spells. They cope better until dryness arrives earlier in spring or drags on longer into autumn.

Fields in France’s Central Massif already appear to have crossed from sink to source. Long‑term drying means they now release more carbon than they lock away. Other lowland farms and even some mixed forests may follow if groundwater drops or irrigation fails.

Mountain zones such as the Alps and the Apennines still gain carbon because winters warm just enough to spur extra spring growth, but their soils are also drying. The cushion is thinning everywhere.

Protecting natural carbon sinks

Protecting the region’s natural sink starts with water. Land managers can slow runoff by planting ground cover, restoring wetlands, and shading soils to cut evaporation. Adding organic mulch helps soils hold moisture far into summer.

Moreover, farmers can switch to drought‑tolerant crop varieties and trim deep ploughing that exposes moist earth to hot air. Finally, forest stewards can thin overcrowded stands to reduce water stress and leave coarse wood on the ground to shade roots.

Rapid monitoring matters too. The study shows that satellite‑based fluorescence – a faint glow released by chlorophyll – drops as soon as leaves shut down.

Agencies can use that signal to flag stressed regions days or weeks before carbon losses peak. Early warnings allow water‑saving rules and firefighting plans to kick in sooner.

Importance of emissions targets

Policy makers should also revise emissions targets with smaller natural offsets in mind. The assumption that forests will always pick up humanity’s slack no longer holds in a warming, drying climate.

Cutting fossil‑fuel use remains the safest path, while adaptation keeps ecosystems functioning as long as possible.

The lesson for the wider world is clear. Southwest Europe still stores more carbon than it emits most years, but the buffer is razor‑thin.

As heat waves and droughts grow more common, that buffer can disappear in one bad summer. Keeping soils damp, canopies green, and emissions low gives nature its best shot at staying on the sink side of the carbon ledger.

The study is published in the journal Global Biogeochemical Cycles.

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