Future changes in vapor pressure deficit “stress waves”

Abstract:

Vapor pressure deficit (VPD) is one of the main drivers of ecosystem function, as increasing VPD results in stomatal closure that can limit carbon uptake and evapotranspiration. While climate change is expected to promote long-term increases in mean VPD over much of the planet, we know much less about shifting dynamics of VPD “stress waves”, which are conceptually similar to heat waves, and represent multiple successive days of VPD elevated well beyond expectation from long-term climate means, resulting in ecosystem stress. We define a stress wave as a positive VPD anomaly for a calendar day relative to the same calendar-day 90^th percentile estimated from historical conditions. These periods of elevated VPD can be a primary driver of flash-droughts and can occur even in the absence of associated changes in soil moisture content. Here, we use an ensemble of CMIP6 GCMs and Shared Socioeconomic Pathway 5-8.5 to examine the occurrence, magnitude, and length of VPD stress waves. We compare these metrics using the GCM ensemble mean for the mid-21^st century (2040-2069) and the late-21^st century (2070-2099) relative to a pre-industrial time period (1850-1879) for the extended summer season (May through September for the Northern Hemisphere; November through March for the Southern Hemisphere). For the mid-21^st century we find, based on the terrestrial global average, an extended summer increase of VPD stress days of 29 days, a magnitude of 0.13 kPa during stress days, and an average increase of 1.9 consecutive stress days. For the late-21^st century we find an extended summer increase of VPD stress days of 47 days, a magnitude of 0.28 kPa during stress days, and an average increase 4.4 consecutive stress days. The coupling between VPD and soil moisture during the stress events, and how that changes over time, is also explored. While increases are found globally, many regions are particularly sensitive, including western North and South America, southern Europe, northern Africa, western Asia, and western Australia. The results indicate that VPD will continue to increase into the future with implications for carbon uptake and plant water use.