Published online 27 August 1998 | Nature | doi:10.1038/news980827-1


A quest for fire

A long-term analysis of the record of wildfires in sub-Saharan Africa shows up a distinct anomaly: the past 10,000 years have been the most fiery in the past million. The timing is suspicious, as the period coincides with the beginnings of Homo sapiens as a creature given to a settled life, in which mankind slashed-and-burned its way through the forests to create farmland and pasture.

Forest fires caused by human activities have hit the news a lot lately: the memories of the pall of smoke that hung over south-east Asia last year, caused by forest fires in Indonesia, will be slow to fade. But forest fires happen all the time through natural causes, so just how important are human-caused forest fires to the global climate?

To understand that, we must find a way to set forest fires in context. This is why Michael I. Bird and J. A. Cali of the Australian National University in Canberra, Australia, have been looking at the record of fire in Africa stretching back a million years. They report their findings in the 20 August 1998 issue of Nature. This work gives a good idea of the 'baseline' rate of forest fires - the amount of fires one would expect were human beings absent - and a measure against which the collective human pyromania in context.

This quest for fire starts in a surprising place - the bottom of the ocean. For several million years, prevailing winds have blown westwards across sub-Saharan Africa, depositing dust and detritus in the eastern Atlantic Ocean. If that dust contains ash from fires, that will be deposited too, finding its way to the bottom of the sea.

Certain parts of the ocean bed are quite and stable, accumulating material from above in a steady, uninterrupted way for hundreds of thousands of years. This period has been climatically turbulent, with several major ice ages, or glacials, punctuated by warmer, interglacial intervals. Cores of finely-layered ocean-floor sediment reveal much about changes in the Earth's climate, and record these climatic changes in a range of ways.

But as Bird and Cali show, deep-sea cores also preserve the remnants of long-gone forest fires. A core drilled off West Africa comes from a part of the seabed downwind of a catchment that extends all over sub-Saharan Africa. The researchers assessed the changing amounts of amorphous carbon - a product of forest fires - at various points in the core, to come up with a time-chart of forest fires going back a million years.

The story that this carbon record tells is very revealing. Up until around 400,000 years ago, forest-fires were relatively rare. After that point, the record became more interesting. At that time, the cycle of glacials and interglacials was getting into full swing. In Africa, forest fires were common when the climate was cooling from an interglacial to a glacial. There were no ice-sheets in Africa: instead, glacials in the Northern Hemisphere were matched by dryness in Africa, interglacials with wetter periods. All the lush greenery built up during interglacials became a tinderbox as the climate dried out, leading to forest fires.

We are currently living in an interglacial that has already lasted for about 10,000 years. If history is a guide, the rate of forest fires at the beginning of this period should be relatively small. This is not the case - the present interglacial has been unprecedentedly fiery. It is very likely that human activity is the cause. The authors point to what some researchers think is the unusually high aridity of much of Africa - unusual, that is, for an interglacial - and wonder whether the presence of human beings could have resulted in regional modifications to vegetation patterns and climate.