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Uncertainty Looms Over Terrestrial Ecosystems

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Swiss Federal Institute for Forest, Snow and Landscape Research WSLBy Jutta Wolf | IDN-InDepth NewsAnalysis

BERLIN (IDN) - The doomsday clock has not yet struck zero hour but it is now beyond scientific doubt that atmospheric concentrations of carbon dioxide (CO2), one of the greenhouses gases responsible for climate change, have reached levels that are higher than any time during the past one million years.

As Markus Reichstein, Director at the Max Planck Institute for Biogeochemistry (MPI-BGC) in Jena points out, increasing atmospheric concentrations of greenhouse gases do not only lead to gradual ‘global warming’, but also to changed patterns of rain and snowfall (precipitation), more weather extremes such as heat waves, longer dry spells, variability of growing season length, recurrent heavy rainfall, and storms. And, there is general concern that climate change will have fundamental impacts on our natural environment, our economic activities and life.

An international team of researchers working with Reichstein, has discovered that as a result of extreme climate, terrestrial ecosystems are absorbing approximately 11 billion tons less carbon dioxide every year than they could if the events did not occur. "That is equivalent to approximately a third of global CO2 emissions per year," says the MPI-BGC director in a summary of the research paper posted to the Institute's website.

In fact, the fate of the European terrestrial biosphere, which is a net carbon sink – removing more carbon from the atmosphere than it emits – is highly uncertain. "There is evidence from observations, writes Reichstein, that there has been a change in the occurrence and frequency of some extremes, and we do know that climate variability and extremes will play an important role for the carbon cycle."

But climate variability and extreme weather events have not, until now, been sufficiently accounted for in modelling and experimental studies, leading to a critical knowledge gap about the future fate of the European carbon cycle.

Against this backdrop, scientists from several countries in Europe such as Belgium, Britain, France, Germany, Italy, Sweden and Switzerland launched the CARBO-Extreme Project, funded by the European Commission. For the first time, the consequences of various extreme climate events on forests, bogs, grass landscapes and arable areas throughout the world underwent systematic scrutiny.

Central and Southern Europe sweltered in a heat wave in 2003 that set alarm bells ringing for researchers. "It was one of the first large-scale extreme weather events which scientists were able to use to document in detail how heat and drought affected the carbon cycle (the exchange of carbon dioxide between the terrestrial ecosystems and the atmosphere)," explains the MPI-BGC director.

"Measurements indicated that the extreme weather events had a much greater impact on the carbon balance than had previously been assumed. It is possible that droughts, heat waves and storms weaken the buffer effect exerted by terrestrial ecosystems on the climate system. In the past 50 years, plants and the soil have absorbed up to 30% of the carbon dioxide that humans have set free, primarily from fossil fuels," he adds.

Satellites and recording stations

Reichstein points out that the researchers took different approaches to their study from the ecosystem perspective. Satellite images from 1982 to 2011 revealed how much light plants in an area absorb so that they can perform photosynthesis. From this, they were able to determine how much biomass the ecosystem in question accumulates during or after an extreme weather event.

The researchers also used data from a global network of 500 recording stations, some in operation for more than 15 years, which record carbon dioxide concentrations and air currents in the atmosphere a few meters above ground or in forest canopies. Calculations from these values indicate how much carbon an ecosystem absorbs and releases in the form of carbon dioxide.

The team then fed the various readings into complex computer models to calculate the global effect of extreme weather on the carbon balance. The models showed that the effect is indeed extreme: on average, vegetation absorbs 11 billion fewer tons of carbon dioxide than it would in a climate that does not experience extremes. "It is therefore by no means negligible," says Reichstein.

Droughts hit vegetation

Droughts, heat waves, storms and heavy rain have not yet become dramatically more frequent and pronounced as a consequence of anthropogenic climate change. However, many climate researchers expect that they will in the future. This would mean more carbon dioxide in the atmosphere as a result of extreme weather conditions.

Periods of extreme drought in particular reduce the amount of carbon absorbed by forests, meadows and agricultural land significantly. "We have found that it is not extremes of heat that cause the most problems for the carbon balance, but drought," explains Reichstein. He and his colleagues expect extreme weather events to have particularly pronounced, varied and long-term effects on forest ecosystems.

Drought cannot only cause immediate damage to trees; it can also make them less resistant to pests and fire. It is also the case that a forest recovers much more slowly from fire or storm damage than other ecosystems do; indeed, grasslands are completely unaffected by high winds.

The researchers also discovered that serious failures to absorb carbon are distributed according to a so-called power law, like avalanches, earthquakes and other catastrophic events. This means that a few major events dominate the global overall effect, while the more frequent smaller events occurring throughout the world play a much less significant part.

The researchers are planning more studies to improve their understanding of the consequences of extreme events. For example, they want to investigate the way the different ecosystems respond in laboratory and field experiments.

"These experiments have already been carried out, but mostly they only look at extreme events which occur once in a 100 years," explains Michael Bahn, a project partner from the German University of Innsbruck. "We should also take account of events which so far have only happened once in 1,000 or even 10,000 years, because they are likely to become much more frequent towards the end of this century." [IDN-InDepthNews – August 22, 2013]

Image credit: Swiss Federal Institute for Forest, Snow and Landscape Research WSL

2013 IDN-InDepthNews | Analysis That Matters

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