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.