Australasian Science: Australia's authority on science since 1938

Calculating Carbon

By Pep Canadell

Research supports a new approach to counting net CO2 emissions.

The world’s ecosystems, mainly forests and oceans, remove around 54% of carbon dioxide (CO2) emitted by deforestation and fossil fuel combustion each year. The concentration of atmospheric CO2 would be increasing more than twice as fast as observed if it were not for these natural sinks. Yet the conservation and management of these ecosystems are rarely part of any domestic and international climate change policies, beyond penalising forest destruction.

In a recent paper published in Nature Climate Change we have suggested a new carbon accounting approach by which national climate change policies could consider the contribution of each emitting region to the increase in atmospheric CO2 and place a value on regional ecosystems or sinks that sequester the CO2.

Developed countries are responsible for more than 80% of atmospheric CO2 from human activities since 1850, but countries and their CO2 emissions also contribute to the creation of CO2 sinks through new plant growth owing to the fertilisation effect of atmospheric CO2 and changes in climate, particularly warming in high latitudes.

Our study analysed the correlations between the year-to-year atmospheric CO2 variations and the changes in temperature and rainfall during El Niño years (when temperatures increase in tropical regions and rainfall decreases) and during large volcanic eruptions (when global temperatures decrease due to volcanic aerosols in the atmosphere).

For instance, about one-third of current fossil fuel emissions in Australia are offset by our land sinks, and more than half of the emissions from African and Latin American countries since 1850 have been offset by their own land carbon sinks.

However, not only are each nation’s own carbon sinks important. Emissions from developed countries since 1850 have created additional sinks in tropical regions equivalent to 13 years worth of their own emissions at current levels. The maintenance of these carbon-accumulating tropical forests constitutes a massive sink service for developed nations from tropical developing nations.

Carbon accounting systems are human constructs, offering different approaches to attribute responsibility for the growth of atmospheric CO2 and the level of intervention necessary for a given climate stabilisation scenario. The broader questions of who is responsible for what, and who owes what to whom, are judgments beyond science, although they are informed by the science.

Adding the land and ocean carbon sinks, in addition to carbon emissions, alters the picture by accounting for the “sink service” provided by regions that are large sequesters of CO2. An approach like this ensures that climate policies provide the incentives to maintain and enhance carbon sinks, particularly those in forests for which we have the tools and knowledge to manage.

The need for such an approach has become clear in the light of a second report in Nature Climate Change. This found that European forests sinks are approaching their saturation state, with rates of carbon accumulation slowing down due to forests reaching maturity, and increased deforestation and disturbance. Managing forests for carbon sinks has consequences for other important forest functions, primarily biodiversity conservation, while there are other competing demands on forested lands to provide food production and energy crops.

Nevertheless, the European example sends a strong warning on the real possible long-term loss of forests sinks in some parts of the world and how that will affect the excess anthropogenic CO2 left in the atmosphere, and therefore contribute to climate change.

The role of CO2 sinks provided by terrestrial ecosystems and the ocean thus deserves more attention for determining the potential for the success of CO2 mitigation strategies.

Dr Pep Canadell is a CSIRO scientist and Executive Director of the Global Carbon Project.