 |
| One extreme after another: Long periods of drought, such as that shown here in Greece, have the effect that the ecosystem absorbs considerably less carbon than under normal climate conditions. © Marcel van Oijen |
When the carbon dioxide content of the atmosphere rises, the Earth not only heats up, but extreme weather events, such as lengthy droughts, heat waves, heavy rain and violent storms, may become more frequent. Whether these extreme climate events result in the release of more CO2from terrestrial ecosystems and thus reinforce climate change has been one of the major unanswered questions in climate research. It has now been addressed by an international team of researchers working with Markus Reichstein, Director at the Max Planck Institute for Biogeochemistry in Jena. They have discovered that terrestrial ecosystems absorb approximately 11 billion tons less carbon dioxide every year as the result of the extreme climate events than they could if the events did not occur. That is equivalent to approximately a third of global CO2 emissions per year.
In 2003, Central and Southern Europe sweltered in a heat wave 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). 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.
The indications that the part played by extreme weather events in the carbon balance had been underestimated prompted scientists from eight countries to launch the CARBO-Extreme Project. 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.
Satellites and recording stations document extreme events
.jpg) |
The many and varied consequences of extreme weather: droughts, heat waves, storms, heavy rain and extreme frost affect the carbon balance of forests, grasslands and agricultural land in different ways. The arrows pointing upward represent additional carbon dioxide in the atmosphere. The arrows pointing downward indicate that carbon dioxide is removed more slowly from the atmosphere. Orange arrows stand for short-term and purple arrows for long-term effects.
© Nature, issue from 15.8.2013, doi 10.1038/nature12350
|
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 tonnes of carbon dioxide than it would in a climate that does not experience extremes. "That is roughly equivalent to the amount of carbon sequestered in terrestrial environments every year," says Markus Reichstein. "It is therefore by no means negligible."
Droughts hit vegetation particularly hard
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 Markus Reichstein. He and his colleagues expect extreme weather events to have particularly pronounced, varied and long-term effects on forest ecosystems. Drought can not 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.
.jpg) |
| Drought hits the global ecosystems particularly hard: The map shows the regions where drought (blue), heat (red), both (purple) and other (grey) extreme events prevent plant uptake of carbon dioxide and to what extent. Light colours represent a weak effect, darker colours a strong effect. © Nature, issue from 15.8.2013, doi 10.1038/nature12350 |
The investigations of the current study, however, show that the consequences of weather extremes can be far-reaching. "As extreme climate events reduce the amount of carbon that the terrestrial ecosystems absorb and the carbon dioxide in the atmosphere therefore continues to increase, more extreme weather could result," explains Markus Reichstein. "It would be a self-reinforcing effect."