Solar dimming and river flows - background to new paper

A new Met Office-led paper has just been published in the journal Nature Geoscience, entitled “Detection of solar dimming and brightening effects on Northern Hemisphere river flow” by Nic Gedney et al.

Chris Huntingford, who is a co-author, writes:

“In any debate surrounding large-scale environmental change, invariably there is discussion as to the effect of increasing levels of atmospheric greenhouse gases. This is particularly in terms of temperature change, and much attention has been placed on fusing together models and data to identify whether there is a human influence on the climate system. Such analysis (often called “detection and attribution”) does suggest that to a high level of confidence, fossil fuel burning has altered the climatic state by increasing atmospheric concentrations of gases such as carbon dioxide. However these analyses of temperature measurements also confirm that an atmospheric aerosol signal is present. The raised level of such particulates, regionally at least, has a partial cooling offset of global warming. Additionally higher aerosol concentrations are known to reduce the amount of the Sun’s light and energy reaching the Earth’s surface.

In this study we accept that both raised greenhouse gas concentrations and aerosols alter meteorological conditions (via temperature levels and indirectly rainfall), which in turn will influence river flows. However to confirm the presence of any aerosol signal, here we instead focus specifically on the impact of changes in sunlight and energy reaching the land surface, and on implications for observed river flows in multiple basins in Northern Hemisphere industrialised areas. This is through the so-called “dimming” and subsequent “brightening” period, the latter a consequence of clean-air acts in many regions.

In our study, we find that when we use a river runoff model, forced with and without estimates of aerosol-induced changes in surface shortwave radiation (i.e. surface energy), then we can only fully explain the river flow data once the aerosol effect is included. Lower surface energy tends to enhance river flows to levels higher than they might otherwise have been. This is because this suppresses water loss by evaporation across the land surface. We can see this effect over Central Europe, in the periods of major solar dimming.

A major focus on climate change will always remain as characterising changes in surface temperature. However to additionally observe and explain anthropogenic influences in a key impact such as river flows does raise confidence in ability to model our changing environment. Such confidence is required, in order to then be able to rely on future computer-based projections and the role they may play in underpinning future policy on the global environment.”

The paper authors are from the Met Office, Centre for Ecology & Hydrology, University of Reading, Laboratoire de Météorologie Dynamique in France, and the University of Exeter.

Chris Huntingford is a Climate Modeller at the Centre for Ecology & Hydrology.

Additional information

Air pollution increases river flows - CEH news story

Paper details:

Detection of solar dimming and brightening effects on Northern Hemisphere river flow.
Gedney, N., Huntingford, C., Weedon, G.P., Bellouin, N., Boucher, O. and Cox, P.M. (2014), Nature Geoscience, doi: 10.1038/NGEO2263

Staff page of Dr Chris Huntingford, CEH