Over the 20th and 21st centuries, both anthropogenic greenhouse gas
increases and changes in anthropogenic aerosols have affected rainfall
in the Sahel. Using multiple characteristics of Sahel precipitation, we
construct a multivariate fingerprint that allows us to distinguish
between the model-predicted responses to greenhouse gases and
anthropogenic aerosols. Models project the emergence of a detectable
signal of aerosol forcing in the middle of the twentieth century and a
detectable signal of greenhouse gas forcing at the beginning of the
twenty-first. However, the signals of both aerosol and greenhouse gas
forcing in observations emerge earlier and are stronger than in the
models, far stronger in the case of aerosols. The similarity between the
response to aerosol forcing and the leading mode of internal variability
makes it difficult to attribute this model-observation discrepancy to
errors in the forcing, errors in the forced response, model inability to
capture the amplitude of internal variability, or some combination of
these. For greenhouse gases, however, the forced response is distinct
from internal variability as estimated by models, and the observations
are largely commensurate with the model projections.