The evolution and vulnerable nature of carbon sinks

While it is predicted that the biosphere will continue sequestering carbon in the first half of this century, uncertainty remains over the likely impact of climate variability on the carbon cycle. We spoke to Dr Markus Reichstein about his work assessing soil carbon vulnerability.

Europe's terrestrial ecosystems currently soak up between seven to twelve per cent of the continent's overall fossil fuel emissions, but how does climate variability and extreme weather affect Europe's terrestrial carbon sinks? CARBO-Extreme, a project funded by European Community's Seventh Framework Programme, wants to overcome this knowledge gap.

"We aim to better understand the effect of climate variability and extremes on the carbon balance of European ecosystems. In future there are likely to be more extreme events like the European heatwave of 2003. Such hotter and drier summers can change ecosystems and their thermo- and carbon-dynamics quite dramatically, with a strong influence on the overall carbon balance," says Dr Markus Reichstein, the project's overall coordinator. Based at the Max Planck Institute for Biogeochemistry, Germany, Dr Reichstein says the nature and scale of modern environmental problems demands international collaboration. In CARBO-Extreme, twenty-five partner institutions from across the continent are collaborating on observing, modelling and assessing soil carbon vulnerability.

"There are a lot of different aspects to the question of the carbon balance and what affects it, and so we have to combine very different data streams and methodological approaches to get an answer," he points out. "This includes flux measurements, where we directly measure the exchange of carbon and water between the ecosystems and the atmosphere. We also look at tree rings and data that has accumulated over long timescales, this allows us to look at extreme events and climate variability in general over the last century."

To be able to understand our current and future climate, particularly the carbon cycle, it's crucial to understand the past environmental context of our planet. Biogeochemical and biophysical models have already been developed, but they don't yet include the relevant processes with regard to climate variability and extreme events, something of which Dr Reichstein is well aware. "The challenge is to improve existing models with the respective observations - on the one hand to include more of the important processes in the models, and on the other to constrain the models with the observations," he explains. With models based on physical mechanisms analysis, the project focuses in particular on analysing carbon sink vulnerability with respect to water availability and temperatures.

"These two are actually interlinked issues, and that makes the overall problem critical, because quite often what we call regional feedbacks affect the carbon cycle. When it gets drier there is less moisture in the soil and so less evaporation. That means there is less cooling, because if water evaporates it gets cooler. Similarly if it gets drier there is also feedback in that it gets hotter and drier; it's actually been shown that this kind of feedback exists on the regional scale," says Dr Reichstein. "Our goal is to simulate realistic regional scenarios with squares of 25 kms x 25 kms based on mean changes in temperature and precipitation. The results will be presented as maps for countries and some regions, as well as for different sectors like forestry and agriculture."

"In the regional scenarios the year-to-year variability - including extreme climate fluctuations - are simulated, from which we can then see the effect of strong variations such as extremely dry conditions over longer periods or extremely low or high temperatures that might cause tree mortality, and then go on to have a long-term effect on the carbon cycle that gradual change might not have had."

Recent studies show that climate variability and extreme weather events can undo the effects of several years of an ecosystem acting as a carbon sink - a real concern, in light of the EU's stated climate protection goal of stabilising CO2 greenhouse gas concentrations. This involves not only reducing emissions of greenhouse gases, but also improving our understanding of key terrestrial ecosystems. "Forests tend to be carbon sinks at the moment, while agricultural areas are neutral. What we are looking at is how vulnerable these sinks are. If droughts and other stresses on the climate become more frequent then these sinks might disappear, or even turn into sources, just through the mortality of forests. In fact this has already happened as a result of the 2003 heatwave," warns Dr Reichstein.

Further extreme weather events occurred as recently as 2006 and 2007, and research into their effects on the carbon cycle presents complex results. "In 2006 it was also very hot in certain areas but it wasn't as dry as 2003, which is why there weren't such strong effects on the carbon cycle," explains Dr Reichstein. "It was very different in 2007, which was a warm year as the temperature was consistently above average - but not a hot one. The particularly warm autumn and winter caused vegetation to develop and flower very early in 2007. That had quite a positive effect on the carbon cycle as a lot of carbon was sequestered. So extreme weather doesn't automatically have a negative impact on the carbon cycle, extreme droughts are the biggest problem."

The findings are politically relevant in terms of environmental policy and EU wide adaptation and mitigation strategies; and scientifically relevant in regards to climate modelling. Until now much of the global attention has been focused on reducing greenhouse gas emissions, and while this is clearly crucial to stabilising the CO2 concentration of the atmosphere, extreme events and their impacts on terrestrial ecosystems also have a key role to play. "We are continually trying to improve the models by incorporating more observed data. This will then form the basis for model simulations, and hence predictions based on those observations" explains Dr Reichstein. "As new mechanisms involved in the carbon cycle were only identified relatively recently, there's still a lot of work to do, and the results will definitely be of interest to the global climate modelling community and stakeholders addressing the topic of climate change."

For more information on the project, contact Dr Markus Reichstein at markus.reichstein@bgc-jena.mpg.de 

Published: Monday, 8th March 2010 by Tom Freeman

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