Abstract
We present one of the first climate change impact assessments on river runoff that utilises an ensemble of global hydrological models (Glob-HMs) and an ensemble of catchment-scale hydrological models (Cat-HMs), across multiple catchments: the upper Amazon, Darling, Ganges, Lena, upper Mississippi, upper Niger, Rhine and Tagus. Relative changes in simulated mean annual runoff (MAR) and four indicators of high and low extreme flows are compared between the two ensembles. The ensemble median values of changes in runoff with three different scenarios of global-mean warming (1, 2 and 3 °C above pre-industrial levels) are generally similar between the two ensembles, although the ensemble spread is often larger for the Glob-HM ensemble. In addition the ensemble spread is normally larger than the difference between the two ensemble medians. Whilst we find compelling evidence for projected runoff changes for the Rhine (decrease), Tagus (decrease) and Lena (increase) with global warming, the sign and magnitude of change for the other catchments is unclear. Our model results highlight that for these three catchments in particular, global climate change mitigation, which limits global-mean temperature rise to below 2 °C above preindustrial levels, could avoid some of the hydrological hazards that could be seen with higher magnitudes of global warming.
Highlights
Article 2 of the 2015 United Nations Framework Convention on Climate Change (UNFCCC) Paris Agreement includes an action to limit any future increase in global-mean temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit to 1.5 °C, recognising that this would significantly reduce risks and impacts of climate change (UNFCCC 2015)
Our model results highlight that for these three catchments in particular, global climate change mitigation, which limits global-mean temperature rise to below 2 °C above preindustrial levels, could avoid some of the hydrological hazards that could be seen with higher magnitudes of global warming
The approach allows us to explore the extent to which current runoff will be affected if the UNFCCC 2 °C target is met, and what the impacts might be if it is missed and global-mean warming rises to 3 °C. In this context the application of a Glob-HM ensemble and catchment-scale hydrological models (Cat-HMs) ensemble to assess global warming impacts across several catchments is a significant advancement because with a companion study that we conducted in parallel (Hattermann et al 2016) it is the first time that impacts across two large multi-model ensembles of Glob-HMs and Cat-HMs have been compared
Summary
Article 2 of the 2015 United Nations Framework Convention on Climate Change (UNFCCC) Paris Agreement includes an action to limit any future increase in global-mean temperature to well below 2 °C above pre-industrial levels and to pursue efforts to limit to 1.5 °C, recognising that this would significantly reduce risks and impacts of climate change (UNFCCC 2015). If the latest Government climate action pledges from 185 countries were implemented global-mean warming would still reach 2.7 °C (CAT 2015) Whilst this is almost 1 °C lower than an alternative future in which only existing policies remain enacted (CAT 2015), it misses the UNFCCC targets. Whilst the UNFCCC target is framed in terms of global-mean temperature, the impacts will be felt heterogeneously across the world (Arnell et al 2016) and one of the key impacts of global warming will be on water resources (Arnell and Gosling 2013) Within this context, we present an assessment of the impact of different levels of global warming on river runoff, focusing on eight major river catchments across the world.
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