How neglecting plant physiology systematically overestimates drought projections

The impact of climate change on water availability and drought events is commonly
attributed to rising global temperatures and altered precipitation patterns. To study
future droughts, the scientific community relies on standardized indices, such as the
Standardized Precipitation Evapotranspiration Index (SPEI) or the Palmer Drought
Severity Index (PDSI). The Atmospheric Evaporative Demand (AED) necessary to
retrieve such indices is typically calculated using outputs from climate models
following approaches such as Thornthwaite, Hargreaves or Penman-Monteith (Milly
and Dunne, 2017). However, they often overlook a fundamental climatic and
biophysical mechanism: the direct effect of elevated atmospheric carbon dioxide (CO₂)
concentrations on plant physiology. This omission risks compromising the robustness
of global projections, leading to a systematic overestimation of AED and consequently
of the magnitude and impacts of future droughts (Swann et al., 2016). Increasing atmospheric CO₂ concentrations trigger two key vegetation responses: they
stimulate biomass production (the carbon fertilization effect) and induce a partial closure
of stomata. This process translates into a substantial decrease in transpiration rates and an
increase in water-use efficiency. Common methods to retrieve AED assume constant
stomatal resistance over time and hence tend to overestimate projected AED (Lemaitre-
Basset et al., 2022).
To quantify the extent of this methodological gap, we conducted a systematic literature
review of evapotranspiration-based drought projections. Results showed that nearly 90% of
studies relying on evapotranspiration-based drought indices omit the CO₂-driven
vegetation feedback. Even more critically, only 10% of the published studies
acknowledge this limitation within their discussion (Villani et al., 2026).
Neglecting the vegetation response to rising CO₂ creates a substantial systematic bias,
especially in drier climates and with high CO 2 concentration scenarios (Figure 1). Our
analyses indicate that in scenarios with CO₂ concentrations that exceed 900 ppm, the area
affected by drought is overestimated by up to 17.4 ± 10.6%. Furthermore, some studies
reviewed showed that the plant physiological effect is so pronounced that it can even
reverse the projected direction of change in some specific areas (Villani et al., 2026).