Is climate change turning continental carbon sinks into sources?


Climate change is already having a dramatic impact on the ability of Europe’s terrestrial ecosystems to absorb carbon dioxide (CO2) from the atmosphere, the CarboEurope project has revealed.

The aim of the project is to understand and quantify the terrestrial carbon balance of Europe; in short, how much CO2 does the continent take up, and how much does it emit?

Terrestrial and oceanic ecosystems mop up just over half of all CO2 emissions, effectively slowing down climate change. The fact that these vital carbon sinks are becoming less efficient means that it will be even harder for us to stabilise CO2 levels and so ward off the most severe impacts of climate change.


Where does the carbon go?

Plants and other organisms take up carbon dioxide from the atmosphere by photosynthesis, during which CO2 and water are converted into oxygen and sugars. This process is a key part of the global carbon cycle. Recent research involving CarboEurope scientists revealed that land and sea plants absorb 55 % of all CO2 emitted. Around 30 % is taken up by land ecosystems, and 25 % by oceanic ecosystems. The remaining 45 % accumulates in the atmosphere.

CarboEurope is building on work by previous EU-funded projects to shed new light on how carbon sinks work. Longer term observations are needed to reveal how the picture is changing, and to find out how forest and farmland can be managed to absorb the most CO2.

An experiment on a continental scale

CarboEurope brings together 75 partners from 16 EU Member States plus Switzerland, with the aim of gathering the data necessary to create an accurate, high resolution picture of the carbon flows of an entire continent.

The project has three aims: to determine the carbon balance of Europe and understand how it changes over time; to investigate the mechanisms which influence the carbon balance; and to develop an observation system which will be able to detect changes in carbon sinks and sources related to the EU’s commitments under the Kyoto Protocol.

The Kyoto Protocol sets out countries’ commitments to reduce their greenhouse gas emissions. CarboEurope’s results will help the EU quantify its contribution to this important international effort.

What makes the project stand out is its application of a single, comprehensive experimental strategy. This means that the same techniques and experimental designs – ground based and atmospheric measurements – are being used right across the continent to measure CO2 fluxes and atmospheric CO2 concentrations on the local, regional and European scales. As a result, data can be easily compared, integrated and analysed.

A network of sites across Europe is gathering information on the carbon flows and carbon balance of the continent’s main landscape types, and through the project’s Atmospheric Observing System, atmospheric CO2 levels are being measured by ground-based instruments, tall towers and aircraft.

Early results from CarboEurope highlight the devastating impact that climate change is already having on the ability of Europe’s ecosystems to absorb CO2 from the atmosphere. If this trend continues, it will seriously jeopardise our efforts to tackle climate change.

Summer 2003: when Europe switched from sink to source

In the summer of 2003, large parts of Europe were hit by a heat wave and a severe drought. Thousands of people died, crops failed and fires raged across large areas. However, results from the CarboEurope project published in the journal Nature in September 2005 reveal an additional, equally serious impact of that exceptionally hot summer: Europe’s terrestrial ecosystems turned from a carbon sink into a carbon source.

The combination of extreme heat and low rainfall reduced plant growth by a third, and so less CO2 than usual was taken up from the atmosphere. The CarboEurope team estimates that during July and August 2003, 500 million tonnes of carbon were released into the atmosphere by Europe’s fields and forests, around twice as much as the emissions from fossil-fuel burning in Europe during the same period.

The researchers warn that more frequent droughts in the future could turn temperate ecosystems from carbon sinks into carbon sources, thereby accelerating climate change.

Warmer autumns causing carbon release

It is known that rising temperatures in spring lead to increased carbon absorption at the start of the growing season, and many wondered if the same would be true in autumn. Research involving the CarboEurope project, published in the journal Nature Geoscience in 2008, revealed that both photosynthesis (which removes carbon from the atmosphere) and respiration (which releases it) increase during warmer autumns.

However, the increase in respiration is greater than the rise in photosynthesis, which is limited by lower light levels in autumn. The result is a net release of carbon in warmer autumns. What’s more, the CO2 released in warmer autumns almost entirely cancels out the extra carbon absorbed during warmer springs.

The global context – dwindling sinks contributing to rising CO2 levels

CarboEurope’s findings feed into an international initiative called the Global Carbon Project. In this global context, it becomes clear that the alarming situation in Europe is far from unique. Droughts around the world have led to the weakening of other terrestrial carbon sinks. Meanwhile out at sea, oceanic carbon sinks are also becoming less efficient.

According to the Global Carbon Project, the declining efficiency of the world’s carbon sinks is one of the main reasons why atmospheric CO2 levels are rising faster than expected.

The implications of this are clear: if we do not act now to reduce CO2 emissions, the carbon sinks will continue to weaken and it will be even harder for us to avoid catastrophic climate change. The information on continental carbon sinks obtained by the CarboEurope project will be crucial if we are to tackle this problem head on.