Forests Fighting Climate

The rise of populations and technological and social development has driven up global demands for bio fuels and grain as feed for animals for meat. This creates powerful incentives for agro-industries to expand into forest regions, notably the Amazon rainforest, causing dramatic and often irreversible change to the environment (Nepstad et al. 2008). Forest fires, drought and logging increase susceptibility to further burning while deforestation and smoke can inhibit rainfall, exacerbating fire risk in this positive feedback loop. If sea surface temperature anomalies and associated droughts to continue, approximately 55% of the forests of the Amazon will be cleared, logged, damaged by drought or burned over the next 20 years, as shown in Figure 1 (Nepstad et al. 2008). The trees of the Amazon contain 90–140 billion tons of carbon, equivalent to approximately 9–14 decades of current global human-induced carbon emissions each year (Canadell et al. 2007). A lot of this is released back to the atmosphere, partly by a reduction in carbon uptake by the trees, but also in burning and soil process changes. Lenton et al. (2008) predict a timescale of 50 years for the Amazon to switch to an alternative state with severely decreased biodiversity and rainfall. A large fraction of precipitation in the Amazon basin is recycled, and reductions in precipitation lead to  lengthening of the dry season, and increases in summer temperatures that make it forest re-establishment difficult, and suggest the system may exhibit bi-stability, with two stable states (Lenton et al. 2008).

Figure 1 - Amazon forest degradation map (Nepstad et al., 2008)

It is also proposed that human induced climate change is impacting boreal forests, as shown by a study in the western United States that links forest “greenness” to fluctuating year-to-year snow-pack. This study showed that mid-elevation - those between approximately 6,500 to 8,000 feet - mountain ecosystems are most sensitive to rising temperatures and changes in precipitation and snow-melt (Trujillo et al, 2012). The study by University of Colorado, funded by NASA, used satellite and ground data to identify the threshold where mid-elevation forests sustained primarily by moisture shift into higher-elevation forests sustained primarily by sunlight and temperature. They found that  mid-elevation forests are very sensitive to snow that fell the previous winter, with about half of the mid-elevation forest greenness attributed to the previous winter’s snow accumulation (Trujillo et al, 2012). Climate studies indicate that snow-pack in mid-elevation forests in the Western United States and in similar forests around the world has been decreasing in the past 50 years due to regional warming (Trujillo et al., 2012), producing a feedback system that will continue to increase warming due to decreased albedo and reduced carbon sequestration. Lenton et al. (2008) predict that the decline of boreal forest would cause a biome switch on a scale of about 50 years, transitioning to open woodlands or grasslands. Under climate change the complex interaction between tree physiology, permafrost, and fire would experience increased water stress, increased peak summer heat stress causing increased mortality, vulnerability to disease and subsequent fire, as well as decreased reproduction rates (Lenton et al., 2008)

Nepstad et al (2008) conclude that trends in Amazon economies, forests and climate may lead to the replacement or severe degradation of more than half of the Amazon basin forests by 2030. They suggest that recent success in changing landholder behaviour, as well as the designation of protected areas and practical techniques for concentrating livestock production on smaller areas of land that could reduce the likelihood of severe environmental change.

Canadell, J. G. et al. (2007) “Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks”. Proc. Nat. Acad. Sci. USA 104, 18 866–18 870.

Lenton, T., M., H. Held, E. Kriegler, J. W. Hall, W. Lucht, S. Rahmstorf, and H. J. Schellnhuber, (2008) “Tipping elements in the Earth’s climate system” PNAS, 105, 6, 1786–1793

Nepstad, D., C., C. M. Stickler, B. Soares-Filho, and F. Merry. (2008) Interactions among Amazon land use, forests and climate: prospects for a near-term forest tipping point. Phil. Trans. R. Soc. B 363, 1737–1746

Trujillo, E., N. P. Molotch, M. L. Goulden, A. E. Kelly and R.C. Bales (2012) “Elevation-dependent influence of snow accumulation on forest greening” Nature Geoscience, 5, 705–709