Has the Northern Hemisphere Mid-Latitude Hydrological Cycle Responded to Twentieth Century Aerosol Forcing?

Abstract:

Changes in observed tropical mean precipitation and Northern
Hemisphere surface temperature show a clear, well-understood response
to increases in atmospheric aerosols during the second half of the
20th century. Based on simple theory and climate modelling, a robust
negative response to aerosols is also expected in Northern Hemisphere
mid-latitude (NHML) precipitation, but none is detectable in the
surface-based record. In fact, a weak positive response seems to
occur.

In this study, we explore possible reasons for this using
corresponding surface runoff observations and a new ensemble of land
surface models taken from the TRENDY project that simulate changes in
runoff in response to known physical processes when driven by observed
precipitation data and changes in land use. We find that observed and
modelled changes in runoff anomalies are similar in the second half of
the 20th century, but that all-century values differ markedly. Given
that observed values are much more consistent with the expected
response to aerosol forcing than modelled values, a possible
explanation is that the precipitation observations driving the TRENDY
models is in error. A range of other possible explanations for the
discrepancy are discussed.

In common with previous studies, it is recommended that pre-1950 NHML
precipitation observations are not used for climate model validation
work. An exception is Europe, where we find excellent correspondence
between precipitation and runoff observations and model simulations.