The identification of a solar signal in climate records of the last 500 years using proxy and model-based analysis and the implications for natural climate variability
There has been confirmation in the last two decades, through instrumental measurements onboard satellites, that the 'solar constant' does, as has long been hypothesized, vary. While there is no consensus as to the best method for estimating past variations in solar output, it seems likely that over the last 500 years, the sun has played a role in the changing climate. However, there is little evidence to suggest that changes in irradiance are having a large impact on the current warming trend.
A complementary approach of empirical and model-based analysis is used to determine if the climate effects of an estimated change in solar irradiance were significant in the pre-industrial era and what climate patterns emerge in response to reduced solar forcing at that time. Also investigated is the modification of solar-induced climate patterns by a hitherto underrepresented forcing - changes in Earth's orbit - and how solar and orbital forcing compare to that of increasing atmospheric greenhouse gas concentration. Finally a brief analysis of the effects of a Maunder Minimum-like solar irradiance on a climate forced by doubled CO2 is undertaken.
Clear evidence is established herein for the ability of relatively small changes in solar irradiance to impact the global climate. Both at the century scale and at shorter periodicities, the variability of the solar 'constant' is influential in defining the global mean climate and more importantly, the regional characterization of that climate. Influencing the preferred mode of decadal variability, such as North Atlantic Oscillation, solar variability alters the mean climate for northern Europe and the North Atlantic region. The global response for temperature is found to be near-linear, while precipitation is more complex. Excitation of important feedbacks, such as sea-ice, plays an important role in determining the resulting pattern of response and ensures that even a much smaller forcing (solar variability) can exert a similar fingerprint to that of a larger forcing (greenhouse gases). Orbital forcing, typically excluded from model experiments for the decadal-to-centennial scale, is found to provide important modification of regional response and may be critical for determining a more accurate 'forecast' for future climate.