Present‐day irrigation mitigates heat extremes

Abstract

Irrigation is an essential practice for sustaining global food production and many regional economies. Emerging scientific evidence indicates that irrigation substantially affects mean climate conditions in different regions of the world. Yet how this practice influences climate extremes is currently unknown. Here we use ensemble simulations with the Community Earth System Model to assess the impacts of irrigation on climate extremes. An evaluation of the model performance reveals that irrigation has a small yet overall beneficial effect on the representation of present‐day near‐surface climate. While the influence of irrigation on annual mean temperatures is limited, we find a large impact on temperature extremes, with a particularly strong cooling during the hottest day of the year (−0.78 K averaged over irrigated land). The strong influence on extremes stems from the timing of irrigation and its influence on land‐atmosphere coupling strength. Together these effects result in asymmetric temperature responses, with a more pronounced cooling during hot and/or dry periods. The influence of irrigation is even more pronounced when considering subgrid‐scale model output, suggesting that local effects of land management are far more important than previously thought. Our results underline that irrigation has substantially reduced our exposure to hot temperature extremes in the past and highlight the need to account for irrigation in future climate projections.

Regional warming of hot extremes accelerated by surface energy fluxes

“…What seems increasingly apparent though is to capture these regional changes, and associated extreme events such as heat waves, climate models will have to utilize far higher spatial resolution [Palmer, 2014;Marotzke et al, 2017]. Irrigation was not considered in the CMIP5 ensemble but could induce a substantial cooling in simulated hot extremes in some regions if of sufficient scale [Thiery et al, 2017]. These, in turn, depend on the availability of soil moisture on the days when the synoptic conditions are conducive to extreme temperatures [Miralles et al, 2012;Quesada et al, 2012].…”

Uncertainties in modelling the climate impact of irrigation

“…Furthermore, in reality irrigation is often not limited by the water demand but rather by the water availability, which is not accounted for in most models. For simulations that deal with the present day climate impact of irrigation, a suitable strategy may be to limit simulated to observed irrigation rates (Thiery et al 2017). However, when observations are not available, e.g.…”

“…Studies using a variety of models have investigated how this redistribution affects the availability of freshwater (Döll and Siebert 2002;Tiwari et al 2009;Liu and Yang 2010;Wada et al 2012Wada et al , 2013Yoshikawa et al 2013) and climate. The majority of the studies that investigate irrigation's climate impact is focused on regional climate (Douglas et al 2006;Kueppers et al 2007;Douglas et al 2009;Lobell et al 2009;Saeed et al 2009;Lawston et al 2015), but also larger scales have been addressed in recent studies (Boucher et al 2004;Sacks et al 2009;Wei et al 2013;de Vrese et al 2016a;Krakauer et al 2016;Thiery et al 2017). The impact of irrigation has been compared to that of other anthropogenic influences such as deforestation (Gordon et al 2005;Lobell et al 2006b), and it has been investigated under changing climate conditions (Lobell et al 2006a;Puma and Cook 2010;Cook et al 2011Cook et al , 2014.…”

Potential of global land water recycling to mitigate local temperature extremes

“…There is a number of studies investigating the impact of irrigation 10 on climate with global climate models (Sacks et al, 2009;Cook et al, 2011;Guimberteau et al, 2012;Krakauer et al, 2016; de Vrese et al, 2016;Hirsch et al, 2017;Thiery et al, 2017). For mean temperatures, most studies report a small cooling effect.…”

“…LWR is able to overcome this drying, especially when allowing for storage 30 of water in a reservoir, causing the strong cooling in these regions. In historical irrigation simulations (as in Thiery et al, 2017) these regions do not receive as much water and therefore do not stand out as regions with a very strong cooling.…”