The impact of varying ice sheets and greenhouse gases on the intensity and timing of boreal summer monsoons

Climate models, forced only with insolation, indicate that boreal summer monsoons respond to orbital forcing with a zero phase both at the precession and obliquity bands. Discrepancies exist among data with respect to the timing of the response. Some late Pleistocene monsoon records show small lags of 2e3 kyr, close to model results, while many others show considerably longer lags of 5e8 kyr. It has been hypothesized that such lags arise from factors that were, up till now, not included in the modelling experiments, namely variations in glacial-age boundary conditions. Here we address this issue using long, time-dependent climate simulations that do include varying ice sheets and greenhouse gas concentrations. Inclusion of these additional forcings introduces a small peak in the monsoon spectra at the 100 kyr period, while monsoon variance remains dominated by precession with a smaller contribution from obliquity. At the precession band orbital forcing remains the dominant control, with lags close to zero. At the obliquity band varying ice sheet and greenhouse gases explain most of the simulated African and Indian monsoon variance, with orbital forcing playing a minor role. For the East Asian monsoon orbital forcing remains dominant. As a result the simulated obliquity phase of different monsoon systems lies between summer insolation maxima and ice minima/greenhouse gas maxima, with a lag that varies with distance to the Eurasian ice sheet