Plenty of Surprises as World Warms

By Tim Radford, Climate News NetworkThis piece first appeared at Climate News Network.

LONDON — Nobody knows for certain what climate change will bring but on the basis of the latest research by plant ecologists, one thing has been established: there will be surprises.

Milena Holmgren of Wageningen University in the Netherlands and colleagues report in Nature Climate Change that they used satellite data to look at the patterns of change in tree cover in three tropical belts: Africa, Australia and South America. The chief conclusion is that changes in rainfall patterns from year to year were linked to lower tree cover in the rainforests of all three continents.

In the dry tropics, however, the picture changed in puzzling ways. In South America, for instance, the higher overall variation in rainfall between years turned out to be a good thing, encouraging tree growth in the semi-arid regions. That confirmed other studies that had suggested that those unexpected episodes of heavy rainfall in normally arid areas provided a happy window of opportunity for trees to get a better hold.

But in Australia, although extremes of rainfall in the desert and dry plains certainly had some effect on tree growth and regeneration, this was usually overwhelmed by the negative effects of extremely dry years.

Meanwhile in the baking, dusty regions of tropical Africa, the procession of up-and-down rainfall levels seemed to make no great difference.

“During extremely rainy years, there is massive tree germination and if these young seedlings grow fast enough to escape from herbivores, then woodlands can expand,” says Dr Holmgren.

“With our analysis of satellite data, we can assess how general this response it. We found the positive effects of extremely rainy years is localised, and can be offset by certain conditions, as in Australia, by negative effects of extremely dry years.”

Ecosystem changes

This is not climate science as such, but rather another exploration of how the world works – and what, if anything, climate change will do for tropical tree cover.

The scientists say their work is relevant to global climate change as warming could increase the frequency of extreme events: so as floods and droughts and heat waves blight the tropics, ecosystems could change in ways that might present problems for humanity – or a bonus.

For instance, if trees do take hold in semi-arid grasslands, is that going to be a good thing for grazing livestock or wild herbivores? On the other hand, in those places where the woodland has perished – leaving eroded, dusty plains and valleys – sudden colossal downpours might just give trees a chance to regenerate, put down roots, provide some hospitality for the local biodiversity and perhaps even sequester a bit more carbon and lock it away in an increasingly fertile soil.

But these remain possibilities, not predictions. “The overall effects of climate variability are puzzling,” says Dr Holmgren.