When ice ages held Europe in their grip, Africa also felt the pinch - though in a different way.

It has long been suspected that there is a connection between the west African monsoon and climate at higher latitudes - especially over geological timescales, says David Lea at the University of California, Santa Barbara. "But until now, there hasn't been enough supporting evidence." Now Lea, with team leader Syee Weldeab and colleagues, has reconstructed the most detailed history of the monsoon yet, spanning 155,000 years and two ice ages.

The team analysed the amount of barium in plankton shells found in an ocean sediment core drilled beneath the Gulf of Guinea. Barium is found in freshwater run-off from the river Niger, says Lea, and is a gauge of past run-off levels and monsoon intensities. When the northern latitudes were frozen over, monsoon rains were much weaker, only gaining strength again when the temperatures in the north increased, the team found.

They also discovered big swings in monsoon activity over timescales as small as 100 years, linked to rapid climate change caused by changes in ice sheet size (Science, vol 316, p 1303). "Something that happens right up in the poles can have a dramatic effect on the climate in the tropics," says Lea.