Vegetation piles on the carbs

Human activty is putting nearly ten billion tons of additional CO2 into the atmosphere each year. These emissions derive from burning of fossil fuels and changes in land use. Only half of this additional CO2 stays in the atmosphere, however. The rest is absorbed by oceans and by land (vegetation + soils). Climate researchers investigate how these complex carbon reservoirs (atmosphere, ocean and land) operate under future climate change scenarios.

So far vegetation have responded positively to increased CO2, a fact that is sometimes called “carbon fertilisation”. The data suggest that terrestrial ecosystems have absorbed more than 90 Gt (Gt = 1 billion metric tons) of additional CO2 since 1959. 90 GtC equivalent is a lot of vegetation. For comparison, all the world’s tropical rainforests = 200GtC equivalent. Even without any greenhouse effect, anthropogenic CO2 emissions have already had a large impact on terrestrial ecosystems.

CO2 record

Atmospheric CO2 has been routinely recorded at Mauna Loa since 1958 and is now recorded at many other locations worldwide. Monthly average data from Mauna Loa are available from the Scripps Institute. The Carbon Dioxide Information and Analysis Center (CDIAC), Oak Ridge National Laboratory, provide historical fossil fuel emissions data, and emissions associated with landuse change. CDIAC also have atmospheric CO2 data obtained from ice core studies, for example from Law Ice Dome in the Antarctic.

The “Global Carbon Budget” from 1959 is summarized by Le Quéré at the University of East Anglia. The fraction of CO2 emissions which remain in the atmosphere (called the Airborne Fraction) is shown in the top chart of the figure below. The mean value of the Airborne Fraction is 43%. The linear fit suggests that a slightly greater proportion of CO2 is remaining in the atmosphere now than in the past.

uptake

Le Quéré also gives results of a global ocean calculation of annual CO2 absorbed by the oceans, based on observations.  In this estimate, oceans absorb about 29%  of emissions on average. Whatever is left over (27%) must equal the amount absorbed by terrestrial sinks. In the ocean model, there has been a decrease in the ability of the cold Southern oceans to absorb CO2 (centre chart). This means there is a downward trend in the ocean uptake fraction. This explains the upward trend in the Airborne Fraction. The remaining terrestrial fraction (bottom chart) is highly variable, but has no significant trend.

Two more things are worth noting from the above figure.

(1) the terrestrial uptake is similar in magnitude to the ocean uptake even though only 25% of the surface area of the earth is covered by vegetation.

(2) the terrestrial uptake is much more volatile than the ocean uptake. In some years, vegetation was a source of CO2.

Why is the variability of the terrestrial carbon fluxes so large? The answer is that this reflects vulnerability of vegetation to climatic variability, particularly droughts and fires. The negative effects of the 1987/8 and 1998 El Nino events are obvious on the bottom chart.  The 1991 eruption of Mount Pinatubo apparently lead to increased CO2 uptake.[2]

Accumulation of Carbon by Vegetation

The net cumulative uptake of CO2 by vegetation since 1959 is shown on the left hand plot below. Again this is derived from one specific model of the ocean uptake.

cumulative

The right-hand plot shows the relation between terrestrial accumulation and cumulative emissions since 1959. This is accurately linear. In some climate models, future global warming causes terrestrial ecosystems to degrade and eventually become net sources of CO2. If this starts to occur, the right hand plot would begin to flatten out.

Where is the extra Vegetation?

For a long time it was believed that the primary terrestrial carbon sink was in growing Northern forests. However more recent work suggests that about 1 Gt of additional CO2 is absorbed per year by mature Tropical Rainforest, as well as 1GtC per year in Northern forests.[3] Of course, deforestation of tropical forests is also the major source of  “land use change” emissions.

It is remarkable that so much uncertainty surrounds such a basic issue. The arrival of new CO2 sensing satellites may improve this situation in the near future.

References

[1]Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change, Le Quéré et al http://www.sciencemag.org/cgi/content/abstract/1136188

[2] Anthropogenic and biophysical contributions to increasing atmospheric CO2 growth rate and airborne fraction, M. R. Raupach et al http://www.biogeosciences-discuss.net/5/2867/2008/bgd-5-2867-2008.pdf

[3] Missing carbon mystery: Case solved? Nature Report Climate Change Jane Burgermeister