Future hydrological extremes: the uncertainty from multiple global climate and global hydrological models

172

136

Abstract. There is growing evidence that climate change will alter water availability in Europe. Here, we investigate how hydrological low flows are affected under different levels of future global warming (i.e. 1.5, 2, and 3 K with respect to the pre-industrial period) in rivers with a contributing area of more than 1000 km 2 . The analysis is based on a multi-model ensemble of 45 hydrological simulations based on three representative concentration pathways (RCP2.6, RCP6.0, RCP8.5), five Coupled Model Intercomparison Project Phase 5 (CMIP5) general circulation models (GCMs: GFDL-ESM2M, HadGEM2-ES, IPSL-CM5A-LR, MIROC-ESM-CHEM, NorESM1-M) and three state-of-theart hydrological models (HMs: mHM, Noah-MP, and PCR-GLOBWB). High-resolution model results are available at a spatial resolution of 5 km across the pan-European domain at a daily temporal resolution. Low river flow is described as the percentile of daily streamflow that is exceeded 90 % of the time. It is determined separately for each GCM/HM combination and warming scenario. The results show that the low-flow change signal amplifies with increasing warming levels. Low flows decrease in the Mediterranean region, while they increase in the Alpine and Northern regions. In the Mediterranean, the level of warming amplifies the signal from −12 % under 1.5 K, compared to the baseline period 1971-2000, to −35 % under global warming of 3 K, largely due to the projected decreases in annual precipitation. In contrast, the signal is amplified from +22 (1.5 K) to +45 % (3 K) in the Alpine region due to changes in snow accumulation. The changes in low flows are significant for regions with relatively large change signals and under higher levels of warming. However, it is not possible to distinguish climate-induced differences in low flows between 1.5 and 2 K warming because of (1) the large inter-annual variability which prevents distinguishing statistical estimates of periodaveraged changes for a given GCM/HM combination, and (2) the uncertainty in the multi-model ensemble expressed by the signal-to-noise ratio. The contribution by the GCMs to the uncertainty in the model results is generally higher than the one by the HMs. However, the uncertainty due to HMs cannot be neglected. In the Alpine, Northern, and Mediterranean regions, the uncertainty contribution by the HMs is partly higher than those by the GCMs due to different representations of processes such as snow, soil moisture and evapotranspiration. Based on the analysis results, it is recommended (1) to use multiple HMs in climate impact studies and (2) to embrace uncertainty information on the multimodel ensemble as well as its single members in the adaptation process.

Section: Low-flow Indicator Used Uncertainty Metrics and Spatial Agmentioning

confidence: 99%

“…6a and b). It is estimated as the ensemble median divided by the ensemble inter-quartile range (Giuntoli et al, 2015). Using the inter-quartile range partly accounts for outliers in the ensemble simulations.…”

Section: Uncertainty Contributions From Gcms and Hmsmentioning

confidence: 99%

Climate change alters low flows in Europe under global warming of 1.5, 2, and 3 °C

Marx

Kumar

Thober

et al. 2018

172

136

“…Although the selections of GCM and emission scenario are more likely to be the largest sources of uncertainty in hydro-climatic modeling (Kay et al, 2009;Wilby and Harris, 2006;Duan and Mei, 2014b), the other sources may also affect the results to different extents. The roles of uncertainties from different sources can be particularly equivocal when investigating seasonal/monthly variability and extreme events (Bosshard et al, 2013;Giuntoli et al, 2015;Bae et al, 2011;Kay et al, 2009). Second, we focused on the independent effects of potential climate changes, while assuming that the interrelationship among the meteorological variables and waterbalance components remains the same as in historical periods.…”

Section: Uncertainties and Caveatsmentioning

confidence: 99%

Future shift of the relative roles of precipitation and temperature in controlling annual runoff in the conterminous United States

Duan

Sun

McNulty

et al. 2017

Abstract. This study examines the relative roles of climatic variables in altering annual runoff in the conterminous United States (CONUS) in the 21st century, using a monthly ecohydrological model (the Water Supply Stress Index model, WaSSI) driven with historical records and future scenarios constructed from 20 Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models. The results suggest that precipitation has been the primary control of runoff variation during the latest decades, but the role of temperature will outweigh that of precipitation in most regions if future climate change follows the projections of climate models instead of the historical tendencies. Besides these two key factors, increasing air humidity is projected to partially offset the additional evaporative demand caused by warming and consequently enhance runoff. Overall, the projections from 20 climate models suggest a high degree of consistency on the increasing trends in temperature, precipitation, and humidity, which will be the major climatic driving factors accounting for 43-50, 20-24, and 16-23 % of the runoff change, respectively. Spatially, while temperature rise is recognized as the largest contributor that suppresses runoff in most areas, precipitation is expected to be the dominant factor driving runoff to increase across the Pacific coast and the southwest. The combined effects of increasing humidity and precipitation may also surpass the detrimental effects of warming and result in a hydrologically wetter future in the east. However, severe runoff depletion is more likely to occur in the central CONUS as temperature effect prevails.

“…Although changes in high and low extremes of streamflow may not be directly interpreted as changes in flood and drought events, since the thresholds for flood and drought damage vary according to factors such as mean climate, the magnitude of water demand, and engineering works for water storage and transport, such changes affect the likelihood of occurrence of those events and can be considered a reasonable indicator of climate impacts on large-scale flood and drought hazard, respectively (Vörösmarty et al, 2000). Accurate simulation of weather fields such as precipitation, as well as simulation of the diverse hydrological processes that lead to streamflow generation, is a major source of uncertainty in streamflow simulation (Giuntoli et al, 2015;Hagemann et al, 2013;Schewe et al, 2013). Some earlier adoptions of climate model projections for flooding studies utilized single global hydrological models (GHMs) for flow routing and streamflow simulation under the GCM-simulated climate (Hirabayashi et al, 2013;Koirala et al, 2014).…”

Section: Introductionmentioning

confidence: 99%

Global change in streamflow extremes under climate change over the 21st century

Asadieh

Krakauer

2017

115

Abstract. Global warming is expected to intensify the Earth's hydrological cycle and increase flood and drought risks. Changes over the 21st century under two warming scenarios in different percentiles of the probability distribution of streamflow, and particularly of high and low streamflow extremes (95th and 5th percentiles), are analyzed using an ensemble of bias-corrected global climate model (GCM) fields fed into different global hydrological models (GHMs) provided by the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP) to understand the changes in streamflow distribution and simultaneous vulnerability to different types of hydrological risk in different regions. In the multimodel mean under the Representative Concentration Pathway 8.5 (RCP8.5) scenario, 37 % of global land areas experience an increase in magnitude of extremely high streamflow (with an average increase of 24.5 %), potentially increasing the chance of flooding in those regions. On the other hand, 43 % of global land areas show a decrease in the magnitude of extremely low streamflow (average decrease of 51.5 %), potentially increasing the chance of drought in those regions. About 10 % of the global land area is projected to face simultaneously increasing high extreme streamflow and decreasing low extreme streamflow, reflecting the potentially worsening hazard of both flood and drought; further, these regions tend to be highly populated parts of the globe, currently holding around 30 % of the world's population (over 2.1 billion people). In a world more than 4 • warmer by the end of the 21st century compared to the pre-industrial era (RCP8.5 scenario), changes in magnitude of streamflow extremes are projected to be about twice as large as in a 2 • warmer world (RCP2.6 scenario). Results also show that inter-GHM uncertainty in streamflow changes, due to representation of terrestrial hydrology, is greater than the inter-GCM uncertainty due to simulation of climate change. Under both forcing scenarios, there is high model agreement for increases in streamflow of the regions near and above the Arctic Circle, and consequent increases in the freshwater inflow to the Arctic Ocean, while subtropical arid areas experience a reduction in streamflow.