A general framework for understanding the response of the water cycle to global warming over land and ocean

Abstract

Abstract. Climate models project increases in globally averaged atmospheric specific humidity that are close to the Clausius-Clapeyron (CC) value of around 7 % K −1 whilst projections for mean annual global precipitation (P ) and evaporation (E) are somewhat muted at around 2 % K −1 . Such global projections are useful summaries but do not provide guidance at local (grid box) scales where impacts occur. To bridge that gap in spatial scale, previous research has shown that the "wet get wetter and dry get drier" relation, (P − E) ∝ P − E, follows CC scaling when the projected changes are averaged over latitudinal zones. Much of the research on projected climate impacts has been based on an implicit assumption that this CC relation also holds at local (grid box) scales but this has not previously been examined. In this paper we find that the simple latitudinal average CC scaling relation does not hold at local (grid box) scales over either ocean or land. This means that in terms of P − E, the climate models do not project that the "wet get wetter and dry get drier" at the local scales that are relevant for agricultural, ecological and hydrologic impacts. In an attempt to develop a simple framework for local-scale analysis we found that the climate model output shows a remarkably close relation to the long-standing Budyko framework of catchment hydrology. We subsequently use the Budyko curve and find that the local-scale changes in P − E projected by climate models are dominated by changes in P while the changes in net irradiance at the surface due to greenhouse forcing are small and only play a minor role in changing the mean annual P − E in the climate model projections. To further understand the apparently small changes in net irradiance we also examine projections of key surface energy balance terms. In terms of global averages, we find that the climate model projections are dominated by changes in only three terms of the surface energy balance: (1) an increase in the incoming long-wave irradiance, and the respective responses (2) in outgoing longwave irradiance and (3) in the evaporative flux, with the latter change being much smaller than the former two terms and mostly restricted to the oceans. The small fraction of the realised surface forcing that is partitioned into E explains why the hydrologic sensitivity (2 % K −1 ) is so much smaller than CC scaling (7 % K −1 ). Much public and scientific perception about changes in the water cycle has been based on the notion that temperature enhances E. That notion is partly true but has proved an unfortunate starting point because it has led to misleading conclusions about the impacts of climate change on the water cycle. A better general understanding of the potential impacts of climate change on water availability that are projected by climate models will surely be gained by starting with the notion that the greater the enhancement of E, the less the surface temperature increase (and vice versa). That latter notion is based on the conservation of energy and is an ...

Decadal climate variability and the global energy balance

“…Interannual coupling of GPCP precipitation with HadCRUT Ts over this period is 3.0±0.7 %/K (r=0.37), consistent with estimates of temperature dependent precipitation sensitivity (Andrews et al, 2010;Myhre et al, 2017). Global precipitation increases with global Ts primarily due to the enhanced radiative loss for higher surface and atmospheric temperatures, set by the thermodynamics of the coupled system (Roderick et al 2014;Myhre et al, 2017) although this is tempered by the additional absorption of sunlight by higher water vapor loadings (Allan 2009) and modified by sensible heat flux changes. However, apparent short-term increases of 2-3% in global precipitation during warm El Niño events (e.g.…”

Estimates of the climatological land surface energy and water balance derived from maximum convective power

“…In recent work, we used such an analytical approach to evaluate the sensitivity of the global hydrologic cycle to surface warming (Kleidon and Renner, 2013b), were able to reproduce the mean response of global climate models, but also identified different roles of solar vs. terrestrial radiation in driving the surface energy balance. A similar type of analysis could be performed with the extensions presented here to evaluate the sensitivity of the terrestrial energy and water balance to surface warming (similar to the recent work of Roderick et al, 2014), but also to other forms of global change (e.g., land cover change). By doing so, we can compare this analytical response to the behavior of much more complex climate models and thereby identify the most important processes that govern the change.…”

Projections of aridity and its regional variability over China in the mid‐21st century

“…However, most studies on future ET o have been conducted at local area or basin, thus the regional differences and pattern of future changes in evapotranspiration have been poorly understood over China. Undertaking regional variability assessments is important to reveal detailed impacts of climate change, particularly on agriculture crop production (Izaurralde et al, 2003;Eitzinger et al, 2013) and hydrological cycle (Held and Soden, 2006;Roderick et al, 2014). This highlights the need to investigate the regional variability of projected evapotranspiration and aridity changes over China, as it is characterized by various climate zones from cold temperate in the north to tropical in the south, and from humid in the east to arid in the west.…”