Projects per year
The climate of the past millennium was marked by substantial decadal and centennial scale variability in the Northern Hemisphere(1). Low solar activity has been linked to cooling during the Little Ice Age (AD 1450-1850; ref. 1) and there may have been solar forcing of regional warmth during the Medieval Climate Anomaly(2-5) (AD 950-1250; ref. 1). The amplitude of the associated changes is, however, poorly constrained(5,6), with estimates of solar forcing spanning almost an order of magnitude(7-9). Numerical simulations tentatively indicate that a small amplitude best agrees with available temperature reconstructions(10-13). Here we compare the climatic fingerprints of high and low solar forcing derived from model simulations with an ensemble of surface air temperature reconstructions(14) for the past millennium. Our methodology(15) also accounts for internal climate variability and other external drivers such as volcanic eruptions, as well as uncertainties in the proxy reconstructions and model output. We find that neither a high magnitude of solar forcing nor a strong climate effect of that forcing agree with the temperature reconstructions. We instead conclude that solar forcing probably had a minor effect on Northern Hemisphere climate over the past 1,000 years, while, volcanic eruptions and changes in greenhouse gas concentrations seem to be the most important influence over this period.