  Increasing greenhouse gases will change many aspects of the Earth's climate,
from its annual mean to the frequency of extremes such as heat waves and
droughts. Here we report that the current generation of climate models predicts
a delay in the seasonal cycle of global rainfall and ocean temperature in
response to increasing greenhouse gases, with important implications for the
regional monsoons. In particular, the rainy season of the semi-arid African
Sahel is projected to start later and become shorter: an undesirable change for
local rainfed agriculture and pastoralism. Previous work has highlighted the
uncertainty in this region's response to anthropogenic global warming: summer
rainfall is predicted either to decrease or increase by up to 30% depending
which model is used. The robust agreement across models on the seasonal
distribution of rainfall changes signifies that the onset date and length of
the rainy season should be more predictable than annual mean anomalies.
