The response of Northern Hemisphere mid-latitude precipitation to aerosol forcing

We analyse the relationship between 20th century precipitation, temperature and aerosol forcing in the best observed Northern Hemisphere mid-latitude region. Using CMIP5 model experiments, we find that observed temperature changes are consistent with the IPCC aerosol forcing best estimate of -1.2 Wm^-2, but that precipitation changes apparently show little evidence of an aerosol response. It is possible that this is linked to changes in precipitation measurement that increased observed precipitation totals at the same time that aerosol forcing was expected to reduce it. Our physical understanding of regional precipitation is relatively weak. Models and some theoretical considerations suggest that Northern Hemisphere mid-latitude precipitation should change in proportion to changes in the global mean hydrological cycle. Precipitation change is not just a function of temperature change, but also the direct effects of forcings agents driving temperature change. We would expect an evident decrease in precipitation following a significant strengthening in aerosol forcing. Model simulated responses for precipitation and temperature appear to reflect changes in the strength of aerosol forcing over the twentieth-century. Notably, there is no evidence of aerosol forcing in precipitation observations. Investigation of this inconsistency reveals that as little as a 3-4% increase in observed precipitation amounts in the early-twentieth-century would bring precipitation in line with temperature. Biases greater than this have been identified in countries within the Northern Hemisphere mid-latitude region previously, notably the former USSR. These observations are frequently used as a metric for the quality of model simulated precipitation and in wider scientific research. More homogeneity studies would be of huge benefit.