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Abstract
Future flood and drought risks have been predicted to transition from moderate to high levels at global warmings of 1.5 °C and 2.0 °C above pre-industrial levels, respectively. However, these results were obtained by approximating the equilibrium climate using transient simulations with steadily warming. This approach was recently criticised due to the warmer global land temperature and higher mean precipitation intensities of the transient climate in comparison with the equilibrium climate. Therefore, it is unclear whether floods and droughts projected under a transient climate can be systematically substituted for those occurring in an equilibrated climate. Here, by employing a large ensemble of global hydrological models (HMs) forced by global climate models, we assess the validity of estimating flood and drought characteristics under equilibrium climates from transient simulations. Differences in flood characteristics under transient and equilibrium climates could be largely ascribed to natural variability, indicating that the floods derived from a transient climate reasonably approximate the floods expected in an equally warm, equilibrated climate. By contrast, significant differences in drought intensity between transient and equilibrium climates were detected over a larger global land area than expected from natural variability. Despite the large differences among HMs in representing the low streamflow regime, we found that the drought intensities occurring under a transient climate may not validly represent the intensities in an equally warm equilibrated climate for approximately 6.7% of the global land area.
Highlights
Human activities and the associated greenhouse gas emissions have already caused an increase in global mean temperature (GMT) of ∼1.0 ◦C (0.8 ◦C– 1.2 ◦C) relative to the pre-industrial era
Field significance of flood and drought characteristics In the large ensemble, significant differences in the duration, intensity, and volume of floods under transient and quasi-equilibrium climates were found, on average (95th confidence interval across the ensemble indicated in bracket), over 0.9% (0%–3.4%), 1.1% (0%–4.9%), and 0.9% (0%–3.5%) of the global land area, respectively (figures 2(a)–(c))
Employing control simulations, we revealed the contribution of natural variability to the detection of significant differences in flood and drought characteristics
Summary
Human activities and the associated greenhouse gas emissions have already caused an increase in global mean temperature (GMT) of ∼1.0 ◦C (0.8 ◦C– 1.2 ◦C) relative to the pre-industrial era. The GMT is projected to increase by 1.5 ◦C relative to pre-industrial levels between 2030 and 2052. The special report Global Warming of 1.5 ◦C (Masson-Delmotte et al 2018) produced by the Intergovernmental Panel on Climate Change (IPCC) suggests that higher levels of GMT are associated with higher impacts on organisms, ecosystems, and human societies (Seneviratne et al 2018, Shindell et al 2018). The climate scenarios used in the previous studies, investigating the potential impacts of climate change for relatively low levels of global warming either stabilise greenhouse gas concentrations over the 21st century or limit end-of-century radiative forcing to a specific level (Rogelj et al 2019). By relying only on steadily warming climate (transient; 2015–2100 in figure 1), the condition of the climate system being in equilibrium (see 2100–2200 in figure 1), was ignored despite the IPCC’s suggestion to distinguish between the two climate response types (Masson-Delmotte et al 2018)
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