Key words :
climate-carbon cycle feedback,
Carbon Balance and Management
15 May, 2008 12:38 pm
Current understanding of processes relevant for climate supports the view that the warming of the last decades is attributed basically to anthropogenic CO2 emissions.
According to the estimates published in the last Intergovernmental Panel on Climate Change (IPCC), these emissions amounted 6.0-6.4 GtC/a (25-28 GtCO2) in 1990s. Additional 0.5-2.7 GtC/a (2-10 GtCO2) were supplied by land use. As a result of these emissions, atmospheric CO2 concentration increased from the value 280 ppmv in late preindustrial period to 379 ppmv in 2005. Global surface air temperature grown by 0.74 oC during the last 100 years. Moreover, warming rate for the last 50 years is about twice larger than for the last century as a whole, and for the last 25 years this warming rate is tripled in comparison to the century-averaged value. This acceleration agrees with a drastic rise in anthropogenic emissions of greenhouse gases.
These emissions are expected to continue and even rise in the 21st century with an associated enhancement of global warming.
Not all anthropogenically emitted CO2 is stored in the atmosphere. Currently, from 1/2 to 2/3 of emissions are taken up by the ocean and terrestrial ecosystems. However, these uptakes are very sensitive to the climate state. In particular, climate warming is expected to suppress these fluxes with an associated enhancement of the airborne fraction of emitted CO2. This effect now is commonly referred as a climate-carbon cycle feedback.
As a result, currently a lot of efforts is devoted to couple the state-of-the-art climate models with carbon cycle modules. The most comprehensive intercomparison of such models was performed in a framework of the Coupled Climate-Carbon Cycle Model Intercomparison Project (C4MIP). It was shown that, basically, climate-carbon cycle becomes stronger during the 21st century if the models are forced by the realistic, albeit rather aggressive emission scenario.
In the A.M.Obukhov Institute of Atmospheric Physics of the Russian Academy of Sciences (IAP RAS, Moscow), similar simulations were performed by I.I. Mokhov and A.V. Eliseev with the IAP RAS climate model of intermediate complexity. This model includes carbon cycle and realistically simulates current and past climates. Idealised simulations with the IAP RAS CM were performed forced by emissions growing in time. In contrast to the C4MIP simulations, the IAP RAS CM runs were not stopped at year 2100 but continued by another several centuries.
It was shown in these simulations that climate-carbon cycle feedback strengthens initially, but eventually weakens and saturates. It was suggested the term eventual transient saturation. Physical reason for this eventual transient saturation is due to dependence of atmospheric absorption capacity for CO2 on its concentration, qCO2, which becomes weaker for larger qCO2. As a result, the larger airborne CO2 storage, the smaller radiative effect of CO2. For realistic values of the governing parameters of simulations, initial period with strengthening of the climate-carbon cycle feedback continues for a few centuries. After 7-8 centuries, when atmospheric CO2 concentration becomes large enough, about 3000 ppmv, this feedback gradually saturates. Similar atmospheric CO2 concentration value was estimated for the early Mesozoic (about 200-250 Myr before present).
An eventual transient saturation does not mean that anthropogenic emissions of CO2 are irrelevant for the future climate state. It only means that, starting from some time, an interaction between the climate and carbon cycle becomes one-sided. It may be described as a sum of 1) a carbon cycle forced by the anthropogenic CO2 emissions, and 2) a climate forced by the atmospheric concentration of carbon dioxide which is an output from the carbon cycle compartment. This is distinct from the earlier evolution of the coupled system when climate and carbon cycle experience a two-way interaction between each other. However, for both one- and two-way interaction between climate and carbon cycle, if the atmospheric CO2 concentration would grow, so does global temperature.
Igor I. Mokhov and Alexey V. Eliseev, "Explaining the eventual transient saturation of climate-carbon cycle feedback", Carbon Balance and Management 2008. Abstract available here .