Editing IPCC:AR6/WGI/Chapter-3 (section)

==== 3.3.2.4 Streamflow ====

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Streamflow is to-date the only variable of the terrestrial water cycle with enough in-situ observations to allow for detection and attribution analysis at continental to global scales. Based on evidence from a few formal detection and attribution studies, particularly on the timing of peak streamflow, and the qualitative evaluation of studies reporting on observed and simulated trends, AR5 concluded that there is ''medium confidence'' that anthropogenic influence on climate has affected streamflow in some middle and high latitude regions ([[#Bindoff--2013|Bindoff et al., 2013]]). The AR5 also noted that observational uncertainties are large and that often only a limited number of models were considered.

([[IPCC:Wg1:Chapter:Chapter-2#2.3.1.3.6|Section 2.3.1.3.6]] assesses that there have not been significant trends in global average streamflow over the last century, though regional trends have been observed, driven in part by internal variability. Only a limited number of studies have systematically compared observed streamflow trends at continental to global scales with changes simulated by global circulation models in a detection and attribution setting. H. [[#Yang--2017|]] [[#Yang--2017|Yang et al. (2017)]] did not find a significant correlation between observed runoff changes and changes simulated in CMIP5 models in most grid cells, consistent with the assessment that observed changes are dominated by internal variability. In a pan-European assessment, [[#Gudmundsson--2017|Gudmundsson et al. (2017)]] attributed the spatio-temporal pattern of decreasing streamflow in southern Europe and increasing streamflow in northern Europe to anthropogenic climate change, but also concluded that additional effects of human water withdrawals could not be excluded. Focussing on continental runoff between 1958 and 2004, [[#Alkama--2013|Alkama et al. (2013)]] found a significant change only when using reconstructed data over all rivers, and a large uncertainty in the estimate of the global streamflow trend due to opposing changes over different continents. [[#Gedney--2014|Gedney et al. (2014)]] detected the influence of aerosols on streamflow in North America and Europe, with aerosols having driven an increase in streamflow due to reduced evaporation (see Section 8.3.1.5 for details on processes). There is also evidence for a detectable anthropogenic contribution toward earlier winter-spring streamflows in the north-central US ([[#Kam--2018|Kam et al., 2018]]) and in western Canada ([[#Najafi--2017|Najafi et al., 2017]]). From a model evaluation perspective, [[#Sheffield--2013|Sheffield et al. (2013)]] reported that CMIP5 models reproduce spatial variations in runoff in North America well, though they tend to underestimate it.

Recently, [[#Gudmundsson--2021|Gudmundsson et al. (2021)]] performed a global detection and attribution study on streamflow and found that some regions are drying and others are wetting. Moreover, the simulated streamflow trends are consistent with observations only if externally forced climate change is considered, and the simulated effects of water and land management cannot reproduce the observed trends. The effects of volcanic eruptions in driving reduced streamflow have also been detected in the wet tropics ([[#Iles--2015|Iles and Hegerl, 2015]] ; [[#Zuo--2019|Zuo et al., 2019]]).

In summary, there is ''medium confidence'' that anthropogenic climate change has altered local and regional streamflow in various parts of the world and that the associated global-scale trend pattern is inconsistent with internal variability. Moreover, human interventions and water withdrawals, while affecting streamflow, cannot explain the observed spatio-temporal trends (''medium confidence'').

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