Abstract
Droughts are anticipated to intensify in many parts of the world due to climate change. However, the issue of drought definition, namely the diversity of drought indices, makes it difficult to compare drought assessments. This issue is widely known, but its relative importance has never been quantitatively evaluated in comparison to other sources of uncertainty. Here, encompassing three drought categories (meteorological, agricultural, and hydrological droughts) with four temporal scales of interest, we evaluated changes in the drought frequency using multi-model and multi-scenario simulations to identify areas where the definition issue could result in pronounced uncertainties and to what extent. We investigated the disagreement in the signs of changes between drought definitions and decomposed the variance into four main factors: drought definitions, greenhouse gas concentration scenarios, global climate models, and global water models, as well as their interactions. The results show that models were the primary sources of variance over 82% of the global land area. On the other hand, the drought definition was the dominant source of variance in the remaining 17%, especially in parts of northern high-latitudes. Our results highlight specific regions where differences in drought definitions result in a large spread among projections, including areas showing opposite signs of significant changes. At a global scale, 7% of the variance resulted independently from the definition issue, and that value increased to 44% when 1st and 2nd order interactions were considered. The quantitative results suggest that by clarifying hydrological processes or sectors of interest, one could avoid these uncertainties in drought assessments to obtain a clearer picture of future drought change.
Highlights
Droughts are projected to intensify under climate change in many parts of the world (Madakumbura et al 2019, Zhou et al 2019, Padrón et al 2020, Takeshima et al 2020)
Soil moisture and runoff drought showed a larger area with low member agreement than precipitation drought regardless of GHG concentration scenarios, which is consistent with preceding studies (Touma et al 2015, Berg et al 2017, Dai et al 2018, Ukkola et al 2018)
Using a multi-model and -scenario dataset, this study evaluated changes in the drought frequency of three drought categories by considering four accumulation temporal scales to investigate where and to what extent differences among drought definitions could result in pronounced variance compared with other sources of uncertainty
Summary
Droughts are projected to intensify under climate change in many parts of the world (Madakumbura et al 2019, Zhou et al 2019, Padrón et al 2020, Takeshima et al 2020). The confidence level of drought projections for regions experiencing substantial drought intensification is considered relatively high. The overall confidence of drought projections is low to medium as noted in a series of reports from the Intergovernmental Panel on Climate Change (IPCC) (IPCC 2012, 2014, 2018, 2019) because drought projections for regions experiencing insignificant changes entail insufficient agreement of projections of drought changes. As one of the reasons for the confidence level, the IPCC reports underline that the diversity of drought definitions employed in drought studies has made it difficult to understand changes in diverse drought conditions (IPCC 2012, 2014, 2018, 2019). Because each drought index considers specific hydroclimate processes, the intersubstitutability of drought indices is low (Wanders et al 2017). Many studies have been based on only one drought index or category, while it is essential to better understand broad responses of the full hydrological cycle to warming. The word ‘drought’ is often ambiguously used despite the multiple hydrological processes included
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