Forests Might Be Better Climate Fighters Than We Knew

By Emma Bryce

Reporter, Audubon Magazine

Published March 12, 2013

Tropical forest in Costa Rica. Photo by Frank Kehren / CC BY-NC-ND 2.0

As the polar bear has become a poster-child for melting ice, so have tropical rainforests become a symbol of the projected impacts brought by climate change. Previous studies have predicted that with a warming climate, rainforests will shed biomass, plant cover, in a decrease brought by elevated carbon dioxide levels and warming that will dry the climate and kill off trees. But now, a study published in Nature Geoscience suggests that our assumptions are wrong: Rainforests appear to be far hardier than we’ve ever believed.

To predict the effects of warmer conditions brought about by business-as-usual CO2 emissions, the researchers used 22 climate models to simulate climate impact on tropical forests in Africa, the Americas, and Asia. Their aim was to measure how much carbon these simulations predicted future forests might retain, in warmer temperatures with higher atmospheric CO2. Because living trees naturally store carbon—creating a so-called “carbon stock”—measuring how much stock there would be gave a sense of whether there would be more or less tree cover in future climes.

The expectation has long been that high CO2 levels would cause forest die-off, and consequently, would be indicated by a reduction in carbon stock as trees died. However, the researchers’ simulations showed something new: in the year 2100, carbon stocks were still stable. “In all but one simulation,” Nature reports, “rainforests across the three regions retained their carbon stocks even as atmospheric carbon dioxide concentration increased throughout the century.”

Lead author Chris Huntingford from the UK’s Centre for Ecology and Hydrology told Nature that the study offers “robust evidence for the resilience of tropical rainforests.” One explanation may be the in “the fertilizing effect of carbon dioxide, which boosts plant growth,” says Nature, meaning that the increased uptake of CO2 by new plants, if that uptake is large enough, might make up for the otherwise damaging release of carbon dioxide that occurs when trees die.

Of course, with projections this startling, the researchers are cautious about the findings, highlighting that their work is based only on simulations of how things may turn out. They emphasize that the research doesn’t suggest that tropical forests will necessarily survive the threats of climate change unhurt.

Co-author David Galbraith from the University of Leeds said in a news release:

…While these new results suggest that tropical forests may be quite resilient to warming, it is important also to remember that other factors not included in this study, such as fire and deforestation, will also affect the carbon stored in tropical forests. Their impacts are also difficult to simulate. It is therefore critical that modelling studies are accompanied by further comprehensive forest observations.

The projections model the effects of a warming climate—but not the more localized impacts of fires and deforestation that cause forest die-off and so reduce its carbon-storing capabilities. Neither can it account for forest biodiversity and how it might be affected by climate change in the future. The researchers also caution that the only simulation that didn’t predict forest resilience is the one behind some hallmark research that shows how a warmer future could leave large swathes of Amazon forest dried out. The impact of that study shouldn’t be dismissed.

Yet, the new data comes from advanced models that could be more dependable. Certainly, the new predictions are turning the debate: “This has been a big issue in science for many years,” says forest ecologist Daniel Nepstad, who directs the Amazon Environmental Research Institute in San Francisco, to Nature, “and the emerging view is that there is less sensitivity in tropical forests for climate-driven dieback.”