Russian Roulette

“Put starkly, current emissions pathways risk playing Russian roulette with the world’s largest forest. We can’t just wait and see because there is no going back. We won’t know we have passed the point where the Amazon turns from a sink to a source until afterwards, when it will be too late.”

Simon Lewis, 2011

I came across this quote while browsing the excellent blog http://www.redd-monitor.org/ which tracks REDD+ in the news. It raises the worrying point that with rising global temperatures a climate “tipping point” will be reached when forests will switch from being a CO₂ sink to become a CO₂ source. This is described by Phillips et al (2008) who used long-term forest plots to monitor changes in tree growth. They found that forest dynamics (growth rate, recruitment rate, death rate etc) have accelerated through the 80s and 90s, most likely driven by increased atmospheric CO_2. They highlight the Amazon’s sensitivity to atmospheric changes and warn that in the future temperature increases will increase photorespiration and tree mortality which both release carbon into the atmosphere. There may also be compositional shifts towards faster-growing species which tend to have lower wood density and therefore contain less carbon.

 Graphic from Lewis et al 2011 showing the extent of the 2005 and 2010 Amazon droughts

These changes may already be happening. The Amazon experienced two severe droughts in 2005 and 2010 (illustrated above) which caused widespread tree mortality and carbon losses. Droughts also increase the risk of forest fires. The 2005 and 2010 droughts have been linked to anomalous elevated sea surface temperatures (SSTs) in the North Atlantic (Lewis et al 2011) and under future warming scenarios such droughts are predicted to become more frequent. Elsewhere, a short study by Lindroth et al (1998) observed Swedish boreal forest acting as a net source of CO₂. Worrying indeed.

This potential transition from forest friend to forest foe brings me to another excellent blog, The Global Mirror, written by Cathy Granneman, and in particular her recent post on forest albedo. To put it bluntly reading it made me feel slightly how I felt when I finished reading And Then There Were None or The Hollow Man for that matter: mind = blown. To summarise, forests are generally dark and have lower albedo than other biomes which mean they reflect less insolation. This warming effect, although compensated to some extent by evaporative cooling, has the potential to offset any benefits from carbon sequestration, particularly in boreal regions. Research by Gibbard et al (2005) simulated global land cover change with unconstrained SSTs and found that if all current vegetation across the globe was replaced by trees there would be a global mean warming of 1.6°C! This is clearly an unrealistic scenario but highlights that a lot more research is required before blindly relying on forest carbon storage to mitigate global warming.

It is undoubtedly a tricky one. Unlike in a detective novel we can’t flick to the end and see how it all works out. The future is far from certain but we should use the evidence available to us and try our damned hardest not to get shot in the head.