You can look at the warming of Lake Tanganyika as a geographical and scientific curiosity; but you're probably wiser to look at it with a considerable sense of foreboding.

Africa may well be the region where global warming hits hardest in the coming century, a possibility clearly spelled out in the last report of the UN's Intergovernmental Panel on Climate Change (IPCC) published in 2007.

"New studies confirm that Africa is one of the most vulnerable continents to climate variability and change because of multiple stresses and low adaptive capacity," the report said. "Some adaptation to current climate variability is taking place; however, this may be insufficient for future changes in climate."

The "multiple stresses" include increased water shortages, severely compromised agricultural production and more widespread disease; and also, as the report noted presciently, "local food supplies are projected to be negatively affected by decreasing fisheries resources in large lakes due to rising water temperatures, which may be exacerbated by continued overfishing."

For more than 20 years the world's attention has been largely focused, as far as global warming is concerned, on predictions: forecasts by supercomputer models of what the world's climate is likely to do, where, and when.

Now the predictions are starting to be replaced by happenings: melting ice; trees coming into leaf three weeks earlier than in the 1950s; birds laying their eggs a week to 10 days earlier than they used to do. The warming of Lake Tanganyika was a prediction; now it is a happening, and a signal of many more happenings to come.