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

Tipping the biosphere

My previous posts have described how critical transitions lead to state shifts, causing abrupt changes and unanticipated effects. Although humans appear to dominate Earth, we have a huge dependence on the biosphere and ecosystem functioning for resource capture, primary production, and decomposition and recycling of nutrients, as well as potentially ecosystem stability (Cardinale et al, 2012). If the relationships mentioned by Cardinale et al (2012) transpose to a planetary scale, the implication is that global biodiversity and species richness positively correlate with the resilience and functioning of the biosphere (Lenton et al. 2013). For this reason, there has been an almost compulsory growth in interest in forecasting biological responses on all temporal and spatial scales (Barnosky et al, 2012).

But how do these changes occur?

Barnosky et al. (2012) describe biological states as neither steady nor in equilibrium, and say critical thresholds may be crossed by a ‘threshold’ effect in incremental values or a ‘sledgehammer’ effect from a large event, such as forest clearance. Localized ecological systems are known to shift abruptly and irreversibly across critical thresholds to new mean conditions outside the range of fluctuation of the previous state (Barnosky et al, 2012). Tipping points in the terrestrial biosphere can also cross continents if vegetation and atmosphere are tightly coupled, (Lenton et al. 2013), potentially becoming global if there are interrelated drivers acting on a global biological or ecological threshold, causing all locations to ‘tip’ simultaneously (Brook et al. 2013). Brook et al (2013) think this is unlikely given the heterogeneity of climate change and ecosystems. Jefferies et al. (2006) show that intercontinental biotic connectivity and coupled regime shifts have been demonstrated by intensive agriculture in western USA, causing dramatic losses of Arctic ecosystem structure and biogeochemical cycling due to increased populations of migrating snow geese, promoted by agricultural crop as increased food source. Similarly, coral reef ecosystems appear to have disappeared globally and suddenly at the Triassic–Jurassic transition, driven by global increase in CO2 causing increased ocean acidity and temperature (Brook et al. 2013).

What are the consequences?

Several extinction events have been linked to oceanic anoxic events, crossing the tipping point in which the onset of anoxia on shelf seas triggered is phosphorus recycling from sediments, fuelling a spread of anoxia, and Lenton et al. (2013) state that the effects on biodiversity were a consequence rather than an intrinsic part of the tipping mechanism. As well as this may be, feedback loops often mean that a biological forcing applied on one scale can cause a critical transition to occur on another scale, for example, anthropogenic selection for younger maturation of individual cod as a result of heavy fishing pressure; and cascades of ecological changes triggered by the removal of top predators (Barnosky et al. 2012). Lenton et al (2013) suggest that species richness is a poor and misleading indicator of Earth-system function, with minimal basis in ecological theory for identifying a number of unique species required to maintain the general health of the biosphere. They also point out the distinction between tipping points in climate or biogeochemical dynamics and subsequent ecological responses to them (Lenton et al. 2013).

To summarize, the terrestrial biosphere, in isolation, is not the right place to be looking for a planetary-scale tipping point; the complex coupled dynamics of the Earth system as a whole need to be assessed (Lenton et al, 2013). Many of the feedbacks, and their consequences for other systems and scales, in the face of changing global climate are as yet unknown. However, planetary scale critical transitions have occurred previously in the biosphere, and evidence suggests that humans are now forcing another such transition, potentially transforming Earth into an irreversible state unknown in human history (Barnosky et al. 2012). As Hobbs et al. (2006) suggest, ‘we should perhaps move away from the one-dimensional dichotomy between natural and human dominated to a more effective depiction of how human beings interact with nature’.

Barnosky et al. (2012) “Approaching a state shift in Earth’s Biosphere”, Nature, 486, 52-58
Brook, B.W. et al. (2013) "Does the terrestrial biosphere have planetary tipping points?" Trends in Ecology & Evolution, 28, 396–401.

Cardinale, B.J. et al. (2012) "Biodiversity loss and its impact on humanity". Nature 486, 59–67
Hobbs, R.J. et al. (2006) "Novel ecosystems: theoretical and management aspects of the new ecological world order". Global Ecology and Biogeography. 15, 1–7

Jefferies, R.L. et al. (2006) "A biotic agent promotes large-scale catastrophic change in the coastal marshes of Hudson Bay". Journal of Ecology. 94, 234–242

Lenton, T., M., and H. T. P. Williams (2013) “On the origin of planetary-scale tipping points, Trends in Ecology & Evolution, 28, 7, 380-382