Uncertainty in Projected Changes in Precipitation Minus Evaporation: Dominant Role of Dynamic Circulation Changes and Weak Role for Thermodynamic Changes

Geophysical Research Letters

Chadwick

Saint-Lu

et al. 2023

The latest generation of coupled ocean-atmosphere global climate models project a global increase in annual mean precipitation of 1%-3% for every 1°C of warming . This increase is determined by a robust response to global mean surface air temperature (2%-3% per 1°C) that is partly offset by fast adjustments to atmospheric radiative heating by greenhouse gases and aerosols (Allan et al., 2020;Fläschner et al., 2016). More intense but less frequent precipitation events have been observed across many regions (Donat et al., 2019;Giorgi et al., 2011), with projections of an increased incidence of extreme precipitation events coupled with longer dry spells (Sillmann et al., 2013;Thackeray et al., 2018). Yet the projections of regional precipitation remain highly uncertain, and their total variance is still dominated by model uncertainty rather than emission scenarios or internal climate variability Lehner et al., 2020).Beyond the fast adjustment due to its radiative effect, increased atmospheric carbon dioxide (CO 2 ) may also trigger a fast vegetation adjustment that is now accounted for in most Earth System Models. In response to high ambient CO 2 concentrations, many plants reduce their stomatal conductance and transpiration in order to minimize water loss, thereby increasing their plant water-use efficiency. This decrease in transpiration, however, may be partly offset by a CO 2 fertilization effect that favors photosynthesis and vegetation density. The net physiological CO 2 effect can affect the amount and variability of soil moisture and evapotranspiration, which in turn can influence the frequency, intensity, and duration of precipitation events. It may thus contribute to model uncertainty, especially in forested areas (

Section: Discussionsupporting

confidence: 92%

Drivers of Dry Day Sensitivity to Increased CO₂

Geophysical Research Letters

Chadwick

Saint-Lu

et al. 2023

“…panels b-d in Fig 1). Model versions within an acceptable range of climate sensitivity may poorly represent regional water cycle changes compared with models exhibiting climate sensitivity that is deemed too high or too low, and there is increasing evidence that narrowing climate sensitivity will not help constrain future hydroclimate changes in most regions [16]. Therefore, the AR6 advances in constraining global temperature projections have not so far resulted in more robust water cycle projections.…”

Section: Model Uncertainty In Global Water Cycle Projectionsmentioning

confidence: 99%

Water remains a blind spot in climate change policies

Allan²,

Arias³

et al. 2022

PLOS Water

For the first time in the latest Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), water has been the focus of dedicated chapters in both Working Group 1 (Chapter 8) and 2 (Chapter 4). Nevertheless, we argue here that water has not yet received the full attention it deserves from both scientists and policymakers for several reasons. Firstly, the historical focus on temperature change has been further increased with the use of global warming levels motivated by an aim to be consistent with current policy framings. Secondly, an increasing attention paid to extreme weather has sometimes overshadowed longer time-scale changes such as the aridification of an increasing fraction of arable land and the increasing variability of the water cycle from month to month, season to season, and year to year that also yield cascading impacts on all water use sectors. Thirdly, a stronger focus is needed on understanding the effectiveness of current and future adaptation strategies in reducing water-related climate risks. Finally, the role of water has not been adequately recognized in the assessment of mitigation strategies although the compliance with the Paris Agreement and the current pledges all require a massive deployment of land-based strategies whose feasibility and efficiency heavily depend on water resources. It is thus essential to develop a more integrated approach to water and climate change, that would allow scientists and policymakers to “close the loop” between mitigation options, water cycle changes, hydrological impacts and adaptation.

“…3D). This land-sea contrast is intriguing because it is sometimes assumed that surface evaporation will increase proportionally to the control evaporation under warmer conditions ( 6 ). However, possible changes in both soil moisture and plant stomatal conductance can invalidate this hypothesis over land.…”

Section: Discussionmentioning

confidence: 99%

“…Chapter 8 pointed out that water cycle changes result not only from changes in precipitation patterns but also from an increase in the atmospheric evaporative demand because of greater warming over land than over the ocean and, consequently, a reduction in continental near-surface RH (2)(3)(4). However, both observed and simulated changes in evapotranspiration, and in precipitation minus evaporation (P − E), are still very uncertain, even when globally averaged over land (2,5,6). Structural and parametric model uncertainties still dominate the overall uncertainty in climate projections (2,7), and the gap between understanding and predicting water cycle changes has not been bridged from AR5 to AR6.…”

Section: Introductionmentioning

confidence: 99%

A drier than expected future, supported by near-surface relative humidity observations

Willett

2023

Sci. Adv.

Despite continuous progress in climate modeling, global projections of the terrestrial water cycle remain highly model dependent. Here, we use quality-controlled gridded observations of temperature and humidity to constrain projected changes in continental near-surface relative humidity across the 21st century. Results show that the projections are poorly constrained when using surface temperature observations only and argue for mitigation policies that are not only rooted in global warming levels. Projections constrained with both near-surface temperature and relative humidity observations show an inevitable continental drying, especially in the northern midlatitudes where anthropogenic aerosols have, however, countered this long-term response until the late 1980s. A “strong drying” storyline is then used to highlight the urgent need for careful adaptation strategies and to suggest a possible contribution of land surface processes to model uncertainties.