A new study shows that during the 2003 heatwave, European plants produced
more carbon dioxide than they absorbed from the atmosphere.
They produced nearly a tenth as much as fossil fuel burning globally.
The study shows that ecosystems which currently absorb CO2 from the atmosphere
may in future produce it, adding to the greenhouse effect.
The 2003 European summer was abnormally hot; but other studies show that these
temperatures could become commonplace.
In some parts of Europe, 2003 saw temperatures soaring six degrees Celsius above
normal; hot enough that estimates of the deaths which it caused run into the
tens of thousands.
It was also significantly drier than usual; and these two factors appear to have
had a major impact on plant growth.
Up the tower
"The data we used mainly comes from a set of 18 flux towers which are set up
across Europe," said Andrew Friend from the Laboratoire des Sciences du Climat
et de l'Environnement (LSCE) in Gif-sur-Yvette near Paris, whose team published
their study in the scientific journal Nature.
The towers, managed through a project called CarboEurope, measure the flow of
carbon dioxide, water and energy between the atmosphere and the ground; most are
set up n forests.
"About half of the mass of a plant is carbon; so by measuring the flow of CO2
into the plants, we can see how well they're doing," Dr Friend told the BBC News
website.
The result coming from the 18 sites was that during 2003, plants took up less
CO2 from the air and grew more slowly - a finding corroborated by satellite
measurements of the area under leaf.
So much for natural ecosystems; but what about farmland?
Here, the researchers drew on data from the UN Food and Agriculture Organisation,
which showed a fall in European crop yields during the 2003 summer.
Putting all the data together, the headline figure is that overall, European
lands were 20% less productive than during an average year.
"We expect that many crops will be affected by high temperatures, especially
during critical phases of development such as flowering," said Professor Julia
Slingo from the Natural Environment Research Council's Centre for Global
Atmospheric Modelling in the UK, who recently organised a Royal Society seminar
on food crops in a changing climate.
"This study found that crops reaching maturity in August were particularly badly
affected; some of the fall-off could be related to water stress, but could also
have been related to high temperatures during flowering," she told the BBC News
website.
"The heatwave also led to higher levels of ozone at ground level, and that can
have damaging effects on plants."
Saint becomes sinner
The really surprising finding came with the calculation that during the heatwave,
European plants and their ecosystems were putting more carbon dioxide into the
air than they were absorbing.
"In the past we expected that climate change would benefit European ecosystems
because growth tends to be limited by the short growing season," said Andrew
Friend, "but this analysis hadn't taken into account the possibility of extreme
events.
"The conclusion of our study is that this extreme event meant a loss of carbon
across Europe - a loss which undoes many years of net uptake."
Plants can absorb and emit carbon dioxide and oxygen; the process of respiration
takes oxygen in and releases CO2, whereas in photosynthesis, the reverse
happens.
Other parts of the ecosystem such as soil bacteria can also contribute to the
overall flow of these gases to and from the atmosphere.
During an average year, the net effect is that European plants absorb around 125
million tonnes of carbon (MtC).
But in 2003, according to this analysis, they released 500 MtC to the
atmosphere.
By comparison, global emissions from burning fossil fuels amounts to about 7,000
MtC; by giving rather than taking, European plants were adding about 10% to the
global total.
"This shows that short-term climatic events such as the 2003 heatwave occurring
over regional areas like Europe can have major effects on the climate globally,"
commented Julia Slingo.
The heat to come
The wider context for all this is a study published last year suggesting that
summers as hot and dry as that of 2003 will become commonplace as the global
climate changes.
"We concluded that on a middle-of-the-road scenario for emissions - assuming we
don't do very much to combat climate change - temperature heatwaves as high as
the one in 2003 would be occurring every other year by middle of this century,"
said Dr Myles Allen of Oxford University.
"By the end of the century, 2003 would be a cool year."
Plants could of course adapt to the changing climate, meaning that the switch
from net absorption of CO2 to net production might not happen.
But, said Andrew Friend, this finding may be a sign of things to come.
"In the tropics, where it's already warm, higher temperatures are predicted to
increase the flux of carbon from plants to the atmosphere," he said.
"We have generally assumed that in northern systems, we would see increased
carbon uptake; but that might not be the case."