Shining light on the cold winter conundrum?

Richard Black is right to caution early in his BBC News article today that variations in ultraviolet (UV) light from the Sun have no impact on global warming. That is indeed so, but I would go further and advise you not to read much into research published yesterday which shows that changes in UV radiation may be connected with recent cold winters in northern Europe, and warmer than usual winter weather elsewhere. Correlation does not imply causation, and in this case the cause-effect chain is highly complex.

Reporting their findings in Nature Geoscience, Met Office scientist Sarah Ineson and others show that variations in solar UV irradiance may be larger than previously thought. This has implications for climate modelling, and especially for periods around the minimum in the 11-year solar cycle. Changes in surface heating are reflected in atmospheric circulation patterns, and in particular the negative phase of the North Atlantic Oscillation, which brings cold and snowy conditions to northern Europe and the US, but not the highest latitudes or equatorial zone.

Ultraviolet light is absorbed in the upper atmosphere by ozone molecules. When there is less UV radiation, as at solar minimum, the stratosphere is relatively cool. This percolates down through the atmosphere, but the rate of diffusion is sensitive to a number of factors, and to pin them down requires a very detailed knowledge of the prevailing atmospheric chemistry and dynamics.

This latest research on UV variations is very interesting, but the results are provisional, and I’m not so sure that it will lead to a more accurate prediction of regional weather and climate.

Further reading

Ineson et al., “Solar forcing of winter climate variability in the Northern Hemisphere”, Nature Geoscience (2011)

Katja Matthes, “Solar cycle and climate predictions”, Nature Geoscience (2011)