Zaitchik, B.F., Rodell, M., Biasutti, M. and Seneviratne, S.I., 2023. Wetting and drying trends under climate change. Nature Water, pp.1-12.
https://www.amazon.co.uk/Drought-Interdisciplinary-Perspecti...
Climate change is expected to alter global water regimes significantly, although there is disagreement on regional specifics and uncertainty in precipitation models (Trenberth et al., 2014). Precipitation and runoff modeling is complicated by incomplete historical field measurements, an inability to reproduce existing regional drying patterns, sensitivity to the choice of initial conditions for sea surface temperature, and poor aerosol forcing accuracy among other challenges (Dai, 2013).
While these and other uncertainties, such as the net effects of vegetation feedback and CO2 levels on soil moisture are acknowledged by the authors, the most recent climate model ensemble, CMIP6, forecasts further whole year drying in western North America, Central America, Europe and the Mediterranean, the Amazon, Southern Africa, Southeast Asia and Australia (Cook et al., 2020). This general aridification forecast is not universally supported, and other researchers believe that since the paleoclimate record demonstrates that warmer paleoclimates are also wetter, it is more likely that the vegetation response modeling in CMIP6 is decisively incorrect, and that it is more likely that overall precipitation will increase while intra-year seasonal variability also increases under climate change (Zaitchik et al., 2023).
The combination of increased temperatures, more variable soil moisture and higher CO2 levels make forecasting future crop yield responses under climate change challenging and response results from experimental simulation and statistical modeling vary widely (Jones et al., 2014). For example, in a well-controlled growth chamber study of wheat response under RCP8.5 temperature and CO2 levels, crop lifecycle shorted appreciably but yields were not affected and one of the study’s cultivars had approximately 25% higher yield under simulated climate change (Sabella et al. 2020). Conversely, a meta-analysis of 91 statistical studies of crop response showed a mildly negative response for most crops/climate combinations to climate, but the variance of results across the included studies was extremely high (Challinor et al. 2014).
While future water regimes are uncertain, there is high certainty about how current water regimes compare to historical averages. With some regional exceptions, the last hundred and fifty years has been a well-watered period compared to the millennial average. From 951CE to 1500CE, the North American continent experienced four major multi-decadal droughts, and the early 19th century was a period of recurring severe drought in the America West (Cook et al., 2007). During the last millennium, Europe experienced two exceptional periods of multi-decadal dryness, from 1400 to 1480CE and from 1770 to 1840CE (the Sporer and Dalton minima respectively) (Ionita et al., 2021).
More recently, Europe experienced significant agricultural drought in 2003, 2015 and 2018 and recent tree-ring research indicates that drought severity in 2015-2018 is anomalous compared to a baseline of 1600CE to present (Freund et al., 2023). Globally, drought frequency has increased in China and India; and recent droughts in the African Sahel are historically unprecedented in duration and frequency (Mishra and Singh, 2010). However, to date, the overall magnitude of the increase in drought severity, as measured by the Palmer Drought Severity Index (PDSI: range -10 to +10), has been small. In North America, for example, there has been a mean -0.3 decrease in the PDSI vs. a 1950-1970 baseline, compared to the historical overall range of approximately -4 to +4 (Trenberth et al., 2014).