Particulate Air Pollution Weakens Plant Water-Use Efficiency by Suppressing Photosynthesis

Plants are widely regarded as important allies in a warming world because rising atmospheric carbon dioxide can allow them to absorb more carbon while using water more efficiently. But a new global analysis suggests that this familiar climate narrative is missing a major piece of the puzzle: fine particulate air pollution. Tiny airborne particles known as PM2.5 appear to undermine plant water-use efficiency across a wide range of ecosystems, weakening the connection between carbon uptake and water conservation at a time when both are becoming increasingly important.

The study, published in Nature Climate Change, combines evidence from three major observation systems: tree-ring isotope records, eddy-covariance measurements of ecosystem carbon and water exchange, and satellite observations of vegetation. Together, these datasets allowed researchers to examine how plants respond to PM2.5 across different spatial and temporal scales. Their central finding is strikingly consistent: although the strength of the effect varies from place to place, PM2.5 generally reduces water-use efficiency rather than improving it or leaving it unchanged.

Water-use efficiency, or WUE, describes the amount of carbon a plant gains through photosynthesis for the water it loses to the atmosphere. At the leaf level, plants take in carbon dioxide through microscopic openings called stomata. At the same time, water vapor escapes through these openings in a process known as transpiration. A plant with high WUE assimilates relatively more carbon for each unit of water lost. This balance is essential for forests, crops, grasslands and other vegetation exposed to drought, heat and increasing atmospheric demand for water.

The researchers found that PM2.5 primarily damages this carbon–water balance by suppressing photosynthesis, not by causing a comparable reduction in evapotranspiration. Photosynthesis is the process through which plants use light energy to convert carbon dioxide and water into sugars and oxygen. When carbon uptake falls while water loss remains comparatively stable, the amount of carbon gained per unit of water declines. In practical terms, polluted vegetation may continue releasing substantial amounts of water into the atmosphere while producing less carbon, making its water use less efficient.

One of the main mechanisms identified is the reduction of photosynthetically active radiation, the portion of sunlight that plants can use to drive photosynthesis. PM2.5 particles scatter and absorb incoming radiation, changing both the quantity and quality of light reaching leaves. This effect can reduce the energy available for carbon fixation. Although diffuse light can sometimes penetrate plant canopies more effectively than direct sunlight, the study indicates that the overall influence of PM2.5 on WUE is predominantly negative at the global scale.

The analysis also points to a decline in carboxylation capacity, another key limitation on photosynthesis. Carboxylation is the chemical step in which the enzyme Rubisco incorporates atmospheric carbon dioxide into organic molecules. If the biochemical machinery responsible for this process becomes less effective, plants cannot convert available carbon dioxide into sugars as efficiently. The combination of weaker light conditions and reduced carboxylation capacity provides a biological explanation for why PM2.5 lowers carbon uptake without necessarily producing an equivalent change in evapotranspiration.

The pollution signal was not identical everywhere. Its magnitude changed with geography and was shaped by interactions among vegetation characteristics, PM2.5 concentration and climate. Forests and non-forest ecosystems displayed distinct patterns, reflecting differences in canopy structure, leaf traits, rooting systems and responses to environmental stress. A dense forest canopy, for example, may alter how particles are intercepted and how light is distributed among leaves, while grasslands and croplands may respond differently because their vegetation is shorter and more directly exposed to near-surface conditions.

Climate also influences how strongly plants respond to fine-particle pollution. Temperature, humidity, radiation and soil moisture all affect stomatal behavior, photosynthetic activity and atmospheric water demand. These factors can either amplify or modify the pollution effect. A plant already experiencing heat or drought may have limited physiological capacity to compensate for reduced light or impaired carbon fixation. Conversely, local environmental conditions may change the way PM2.5 interacts with leaves and canopies, helping explain why the observed relationship varies across regions rather than following a single universal pattern.

The study’s use of multiple independent observation approaches is important because WUE can be estimated in different ways. Tree-ring isotopes preserve long-term information about how trees regulate carbon uptake and water loss. Eddy-covariance towers measure exchanges of carbon dioxide and water vapor between ecosystems and the atmosphere, providing direct information about ecosystem-scale functioning. Satellites offer broad spatial coverage by tracking vegetation properties and activity from orbit. The agreement among these perspectives strengthens the conclusion that PM2.5 is linked to a widespread weakening of plant water-use efficiency.

The findings also expose a potential blind spot in current ecosystem models. Many models simulate the effects of carbon dioxide, temperature, precipitation and other climate drivers but do not explicitly represent aerosol pollution and its effects on radiation and photosynthetic machinery. According to the study, models that omit aerosol processes fail to reproduce the observed PM2.5–WUE relationship. This mismatch suggests that changes in climate alone cannot fully explain the decline in plant carbon–water coupling seen in polluted environments. If aerosol effects remain unaccounted for, projections of future carbon storage, drought resilience and ecosystem productivity may be systematically incomplete.

The implications extend beyond plant physiology. Vegetation helps regulate the climate by storing carbon, cooling the land surface and moving water through the atmosphere. If PM2.5 reduces carbon uptake while leaving water loss relatively less affected, polluted ecosystems could become less effective carbon sinks and less efficient participants in regional water cycles. The consequences may be especially important in areas where air pollution overlaps with water scarcity, intensive agriculture or climate-driven increases in evaporative demand.

The research does not suggest that every particle-rich atmosphere will affect every plant in precisely the same way, nor does it eliminate the well-known complexity of aerosol–vegetation interactions. Particle composition, concentration, atmospheric persistence and local meteorology can all matter. Nevertheless, the global pattern identified in the study gives PM2.5 a new significance in discussions of climate change. Air pollution is not only a threat to human health and visibility; it may also be quietly weakening the ability of plants to turn water and sunlight into carbon.

As nations work to reduce greenhouse-gas emissions and adapt to a warmer, more water-limited world, the study argues for treating clean air and ecosystem resilience as connected goals. Reducing PM2.5 could deliver immediate benefits for human health while also restoring some of the photosynthetic capacity that ecosystems need to maintain carbon uptake. The message is simple but consequential: the future performance of the world’s vegetation will depend not only on how much carbon dioxide is in the atmosphere, but also on what else is suspended in the air.