Leaves and plant canopies will warm faster than the surrounding air as climate change advances, according to research led by the University of Arizona. The study, published in Nature Communications, warns that many models use air temperature as a proxy for plant heat stress and may therefore underestimate the effects on ecosystems and the carbon cycle.
Projections indicate that by 2085-2099 the difference between canopy and air temperature will increase by about 0.11°C, a 16% expansion of that gap. The number appears small, but it directly affects temperature-sensitive processes including photosynthesis, respiration and transpiration. Plant surface temperature, rather than air temperature alone, determines when those mechanisms lose efficiency.
Leaves receive solar radiation and can become much hotter than their surroundings, just as an exposed surface warms above the air. Plants regulate some of that heat through transpiration by moving water to their leaves, where evaporation provides cooling. When the air is too hot and dry, water loss can exceed the supply that roots can replace from the soil.
Under that deficit, many plants close their stomata and reduce photosynthesis and transpiration to conserve water. This response limits carbon dioxide intake and removes evaporative cooling, causing the leaf to heat even more. At the same time, less water evaporates into the surrounding air, producing feedback among heat, dryness and lower plant activity.
The largest projected differences occur in regions expected to become drier, but tropical ecosystems are also a concern. Many tropical species evolved within relatively narrow temperature ranges and may be sensitive to modest changes. Reduced productivity or increased mortality would alter vegetation distribution, habitats and the capacity of forests to remove CO2 from the atmosphere.
The effect extends beyond carbon. Water vapor released by vegetation contributes to clouds and precipitation; if plants reduce transpiration, atmospheric moisture, rainfall patterns and the radiation reaching the surface can change. Lower photosynthesis leaves more CO2 in the air, while lower vapor release can alter regional climate, creating two pathways that may reinforce warming.
Researchers propose explicitly including canopy temperature in Earth system models and adaptation assessments. This will require better surface observations, remote sensors and measurements across different ecosystems. Adjusting models does not prevent the impact, but it can identify more accurately which forests, crops and regions will reach physiological heat limits first.
