The Indian Monsoon rainfall (IMR) variability in the C20C simulations is analyzed on interannual and decadal timescales. Potential and actual predictabilities of IMR are assessed. Whereas on interannual timescale the potential and actual predictabilities are modest, there is a large predictability on decadal timescale. It is shown that the IMR exhibited a distinct decadal variability, largely forced by tropical sea surface temperatures (SSTs). In particular, there has been a decrease from the 1950s to the 1990s that corresponds to a general warming of tropical SSTs. Furthermore, using a selection of control integrations from the Intergovernmental Panel on Climate Change 4th assessment report (IPCC AR4), it is shown that the increase of greenhouse gases (GHG) in the 20th century has not significantly contributed to the observed decadal variability.