Deforestation a greater threat to the Amazon than global warming
Rhett A. Butler, mongabay.com
February 25, 2008
Some researchers doubt the Amazon is on the brink of a major die-off due to climate change. Human disturbance is a greater threat, they say.
Examining charcoal and fossil records from across the Amazon basin, Francis E. Mayle and Mitchell J. Power of the University of Edinburgh report that Amazon forests appear to have been "remarkably resilient to climatic conditions significantly drier than those of today, despite widespread evidence of forest burning" during the Early-Mid- Holocene, a period 4000 to 8000 years ago. The conclusion challenges other research suggesting that the Amazon is on the brink of a dramatic die-back due to the interaction of accelerating deforestation, increasing incidence and severity of forest fires, and the effects of climate change.
Mayle and Power say the impact of drier conditions 4000 to 8000 years ago was variable across Amazonia but that the area that was most affected was that where the dry season is longest and most severe: the southern and eastern Amazon. Mayle and Power write that pollen records from the areas suggest a change from closed-canopy forest to a open forest and savanna during dry periods. Meanwhile in the western Amazon, along Andean flank of the basin, dry conditions triggered the replacement of cloud forest plants by lowland rainforest vegetation. In other words, rainforest species migrated to higher elevations as rainfall declined. However other sites showed little change in species composition. These may have served as "refugia" for future recolonization when wetter conditions returned.
The impact of fire
WORLDCLIM bioclimatic variables (Hijmans et al. 2005) of (a) annual precipitation and (b) precipitation of the driest quarter (driest three months) characterize present-day climatic variability across the Amazon. From Francis E. Mayle and Mitchell J. Power (2008)
"Humans, rather than climate, may have been the key agents of disturbance of Holocene forests in many parts of the basin, especially if 'pre-Conquest' Amazonia was much more densely populated than previously thought," they write. "However, a drier climate would have had an important influence by making forests more combustible. Anthropogenic burning would therefore have been a more effective tool for forest clearance and, through more frequent fire leakage, would have led to an increase in large wildfires as occurs today during particularly severe droughts."
"Our analysis shows that, notwithstanding floristic changes, the forest biome in most parts of Amazonia appears to have been remarkably resilient to climatic conditions significantly drier than those of today, despite widespread evidence of forest burning," the authors continue. "Although the effects of continually rising CO2, and different climate change scenarios, upon Amazonia's forests over the twenty-first century remain uncertain... our insights from the distant past suggest that the Amazon forest 'dieback' scenario simulated by Cox et al. (2000) and Betts et al. (2004) is unlikely."
"A projected temperature increase of 3-8C over the twenty-first century... in combination with drying and forest fragmentation, would be expected to increase water stress and vulnerability to dieback, although this may be offset by higher CO2 concentrations. Of much greater cause for concern should be the unprecedented rates of deforestation, forest fragmentation... and uncontrolled burning, which are much more serious and immediate threats than climate change."
Francis E. Mayle and Mitchell J. Power (2008). Impact of a drier Early-Mid-Holocene climate upon Amazonian forests. Phil. Trans. R. Soc. B, DOI: 10.1098/rstb.2007.0026