Monday, October 3, 2011

Risk of severe climate change impact on the terrestrial biosphere – Heyder et al. (2011)

Risk of severe climate change impact on the terrestrial biosphere – Heyder et al. (2011) [PDF]“The functioning of many ecosystems and their associated resilience could become severely compromised by climate change over the 21st century. We present a global risk analysis of terrestrial ecosystem changes based on an aggregate metric of joint changes in macroscopic ecosystem features including vegetation structure as well as carbon and water fluxes and stores. We apply this metric to global ecosystem simulations with a dynamic global vegetation model (LPJmL) under 58 WCRP CMIP3 climate change projections. Given the current knowledge of ecosystem processes and projected climate change patterns, we find that severe ecosystem changes cannot be excluded on any continent. They are likely to occur (in > 90% of the climate projections) in the boreal–temperate ecotone where heat and drought stress might lead to large-scale forest die-back, along boreal and mountainous tree lines where the temperature limitation will be alleviated, and in water-limited ecosystems where elevated atmospheric CO2 concentration will lead to increased water use efficiency of photosynthesis. Considerable ecosystem changes can be expected above 3 K local temperature change in cold and tropical climates and above 4 K in the temperate zone. Sensitivity to temperature change increases with decreasing precipitation in tropical and temperate ecosystems. In summary, there is a risk of substantial restructuring of the global land biosphere on current trajectories of climate change.”

AGWObserver's weekly summary of new research is up -- and, as is usually the case, there's some amazing papers, including this one. Can I just say, and not for the first time, how awesome AGWObserver is? You not only catch many important papers that I for one would never know to look for, but you also get a window into what an active area of research this is, the different approaches used, and the variety of results -- some reassuring, some positively frightening, and many just plain interesting. 

Heyder & friends explain the rationale for their paper:

Most studies on biospheric climate change impacts focus on one or a few isolated ecosystem processes only (Cramer et al 2001, Friedlingstein 2006, Gerten et al 2007). However, given the many potential responses of an ecosystem to external pressure and the possibility of homoeostasis in key ecosystem functions through compensatory mechanisms (Ernest and Brown 2011), a comprehensive risk assessment of ecosystem changes should comprise several aspects of ecosystem functioning e.g. Scholze et al (2006).
This is a key concept.  Scientists are not to blame for focusing their research on a key variable of interest; looking at one thing at a time, where possible, is basic to the design of an effective experiment. If you don't do that, you'll never untangle what changed what to what extent and why.

But at some point we do have to reintegrate the insights from focused experiments into the broader ecological framework, which is irreducibly complex. Water, warmth, disease, competitive species, and so on. This research echoes the concern among many scientists about the possibility of a profound forest die-off that could accelerate global warming:

Boreal forests change at their leading edge due to alleviated temperature limits and at their trailing edge due to increasing heat and drought stress. Such heat- and drought-related forest die-back (which might be amplified by insect outbreaks) has already been observed over considerable areas in North America (Berg et al 2006, Allen et al 2010).
Increased carbon uptake outside the forests may compensate, but to what extent is unknown:
Dryland ecosystems are projected to experience increased vegetation productivity.    This is in line with the known beneficial effect of elevated atmospheric CO2 concentration on the water use efficiency of plants (Ainsworth and Long 2005), which will—together with warming—lead to increasing NPP and vegetation carbon stocks in many regions. In this study a major factor is not included: NPP could be constrained by nitrogen supply and other nutrients (Norby et al 2010), so that the increased NPP due to enhanced CO2 availability as implied in our results should be interpreted as a maximum effect.

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