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IPCC AR6: Run-off, Streamflow and Flooding, Unspun Edition

01 Dec 2021 | Science Notes

Continuing our look at what the latest IPCC report says about rivers and flooding, especially in light of dogmatic alarmist claims that the recent British Columbia floods are entirely due to greenhouse gases as opposed to, say, failure of local governments to react sensibly to warnings based on the long history of flooding in that region, we take a look at what the new IPCC report says on the subject. Herewith the AR6 Chapter 8 Section 8.3.1.5, as usual with reference lists replaced with (--) and a few details elided out, but otherwise verbatim, starting with its summary statement: “the assessment of observed trends in the magnitude of runoff, streamflow, and flooding remains challenging, due to the spatial heterogeneity of the signal and to multiple drivers. There is however high confidence that the amount and seasonality of peak flows have changed in snowmelt-driven rivers due to warming. There is also high confidence that land use change, water management and water withdrawals have altered the amount, seasonality, and variability of river discharge, especially in small and human-dominated catchments.”

AR5 reported low confidence in the assessment of trends in global river discharge during the 20th century. This is because many streamflow observations have been impacted by land use and dam construction, and the largest river basins worldwide differ in many characteristics, including geography and morphology. In regions with seasonal snow storage, AR5 WGII assessed that there is robust evidence and high agreement that warming has led to earlier spring discharge maxima and robust evidence of earlier breakup of Arctic river ice, as well as indications that warming has led to increased winter flows and decreased summer flows where streamflows are lower and that the observed increases in extreme precipitation led to greater probability of flooding at regional scales with medium confidence. [The Special Report on the Cryosphere] found robust evidence and high agreement that discharge due to melting glaciers has already reached its maximum point and has begun declining with smaller glaciers, but only low confidence that anthropogenic climate change has already affected the frequency and magnitude of floods at the global scale.

Significant trends in streamflow and continental runoff were observed in 55 out of 200 large river basins during 1948-2012, with an even distribution of increasing and decreasing trends (Dai, 2016) (see also Section 2.3.1.3.6). A global detection and attribution study shows that the simulation of spatially heterogeneous historical trends in streamflow is consistent with observed trends only if anthropogenic forcings are considered (--). Section 3.3.2.3 assesses with medium confidence that anthropogenic climate change has altered regional and local streamflows, although a significant trend has not been observed in the global average (--). Multiple human-induced and natural drivers have been shown to play an important but variable role in observed regional trends of streamflow for several different areas (--). For instance, decreasing runoff during the dry season has been observed over the Peruvian Amazon  since the 1980s (--). Up to 30–50% of the recent multi-decadal decline in streamflow across the Colorado River Basin can be attributed to anthropogenic warming and its impacts on snow and evapotranspiration (--). In the Upper Missouri River basin, (--) found that warming temperatures have contributed to streamflow reductions since at least the late 20th century. Cold regions in the Northern Hemisphere have experienced an earlier occurrence of snowmelt floods, an overall increase in water availability and streamflow during winter, and a decrease in water availability and streamflow during the warm season (--).

Some studies have suggested that dam construction and water withdrawals can be the dominant drivers in observed trends in streamflow amount (--). Regionally, land use and land cover changes have been identified as important factors for streamflow (--). The impact of surface dimming from aerosol emissions on evaporation was identified as a discernible influence in Northern Hemisphere streamflows (--). While changes in annual mean streamflow present a complicated picture, recent studies of changes in the timing of streamflow in snow-influenced basins continue to support a prominent influence from warming (--). Global land runoff variations correlate significantly with ENSO variability (--).

Observed changes in flooding are assessed in detail in Chapter 11 and are summarized as follows. For changes in the magnitude of peak flow, recent studies show strong spatial heterogeneity in the sign, size and significance of trends (Section 11.5.2.1). For changes in the frequency and magnitude of high flows, the conclusions remain limited by the large influence of water management (Section 11.5.2.2). For changes in timing of peak flows, recent studies further support observed changes in snowmelt-driven rivers (Section 11.5.2.3). Observed changes in runoff and flood magnitude cannot be explained by precipitation changes alone given the possible season- and region-dependent decreases in antecedent soil moisture and snowmelt which can partly offset the increase in precipitation intensity (--), or the expected effect of urbanization and deforestation which can on the contrary amplify the runoff response (--). Simulations of mean and extreme river flows are consistent with the observations only when anthropogenic radiative forcing is considered (--).

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