This is the first in a series of group posts by a few of us bloggers interested in the science of climate change. For our first “mob” post, Tamino at Open Mind, Eli at Rabbet Run and yours truly here at Maribo are all writing about the carbon cycle and atmospheric carbon dioxide.
Much of the discussion on Maribo centers around the science politics of setting a short- and long-term GHG or carbon emissions target in order to stabilize atmospheric concentrations and avoid ‘dangerous’ climate change.
The emissions targets depend on how much - and for how long - the carbon dioxide we emit actually remains in the atmosphere. We need to understand the ability of the planet to take carbon out of the atmosphere, and how that itself is sensitive to climate change. The figure (IPCC WG1, Fig. 7.4) shows the annual fraction of fossil fuel emissions that remained in the atmosphere (black line is a five year mean). I'll come back to this.
The atmosphere is often compared to a bathtub. The emissions of carbon dioxide – the flow into the bathtub – are currently greater than the uptake of carbon – the flow out the drain. So carbon dioxide is accumulating in the atmospheric tub.
Personally, I like to say emissions are currently faster than the planetary uptake. Over geological time, millions of years, carbon is removed from atmosphere by weathering of rock and by burial in marine sediments. Burning fossil fuels releases this ‘fossil’ carbon to the atmosphere; deforestation and biomass burning quickly releases carbon that was stored over decades or centuries in trees. We’ve effectively sped up the flow of carbon into the atmosphere.
The increase in atmospheric CO2 since the Mauna Loa record began in the 1950s is only about half (~55%) of fossil fuel emissions. The rest has been absorbed by the oceans and terrestrial ecosystems.
The ocean ‘sink’ is best understood and easiest to measure. It can be almost entirely explained by the dissolution of CO2 in sea water, the reason the pH of the oceans is declining. Since solubility of CO2 decreases with temperature, much of this uptake has occurred in cold waters of the Southern Ocean. Other potential, but currently negligible on a global scale, ocean sinks include increases in photosynthesis by plankton [and deep-water burial of the ‘fixed carbon’] and changes in ocean circulation.
So we know with good confidence that about 30% of fossil fuel emissions have been absorbed by the oceans and the remainder by terrestrial ecosystems. The remainder must be taken up by terrestrial ecosystems.
The land sink is more challenging to quantify. We know there has been a net uptake of carbon on land. The knowledge of anthropogenic emissions and good estimate of the ocean sink allow us to infer this total land uptake or land sink. So that means carbon uptake by photosynthesis by terrestrial ecosystems is greater than carbon emissions by those ecosystems, from respiration, but also from disturbances like fires and deforestation.
Notice that I did not include deforestation as a CO2 sources above – just fossil fuel emissions. Deforestation is responsible for about 20% of total anthropogenic CO2 emissions; fossil fuels and the like for the other 80%. But since I’m talking about the net exchange of carbon between land and the atmosphere, carbon emissions from deforestation is folded into the equation.
Anyhow, field observations, including forest inventories, satellite observations of terrestrial productivity, data from ‘flux’ towers at specific locations, and modeling point to a few key players:
- Re-growth of forests on abandoned farmland in the Northern Hemisphere has led to a net uptake of carbon (at least until the trees reach maturity)
- Higher concentration of atmospheric CO2 can increase rates of photosynthesis and hence carbon uptake (“CO2 fertilization”).
- Deposition of nitrogen, emitted by burning of fossil fuels and application of fertilizer, may also be unintentionally ‘fertilizing’ forests
Knowledge of the sinks lets us calculate how anthropogenic CO2 emissions translate into increases in atmospheric concentration. Eli’s post provides a model for doing some simple experiments.
Why does this matter? Our understanding of the modern-day carbon cycle underpins to all that stuff about climate policy that you read, see, hear and smell in the news. Right now, we emit about 8 Gt of C per year, and that translates to, as Tamino points out, an increase of about 2 ppm of CO2/year in atmosphere. But what if climate change alters that way the oceans and the land take up carbon? Then the model has to change.
This is one of the great challenges in climate change science AND climate change policy. To work out what percent reduction is necessary to hit a stabilization level, we need to understand carbon cycle feedbacks: how will climate change alter the fraction of emissions that remain in the atmosphere? Here are three (of many) possible feedback effects:
ii) Drying in the tropics: Reduced rainfall in the Amazon would reduce carbon uptake and increase carbon release through fires
iii) Ocean circulation: A slowing of ocean circulation could limiting productivity in the surface ocean and sinking of carbon (via increasing stratification – topic for another day)
One way to get at these questions is to examine the year-to-year variability in CO2 growth in the atmosphere. What you see in that IPCC figure at the top of the post is that the rate of uptake by the planet varies widely year to year, from less than 20% of emissions, to over 70% of emissions.
There are a few interesting features. The year-to-year variability mostly originates from tropical forest. For example, you can see high airborne fractions or high growth rates during El Nino events (e.g., 1997-1998, 1972-3, 1982-3) due to related droughts (less C uptake) and fires (more C release). That’s not too surprising. It does serve as a warning: future drying in the tropics, due to climate and/or deforestation, could reduce the carbon sink.
In the past, most of the general circulation or climate models used in the IPCC assessments did not included a complete carbon cycle. The atmospheric CO2 concentrations were imposed based on externally generated scenarios. With a complete representation of the carbon cycle, we could instead impose emission, and allow the model to simulate the change in concentrations and uptake by land and oceans.
The latest IPCC assessment includes a comparison of some ‘coupled’ climate-carbon cycle models. All the models predict a decrease in the sink or an increase in the fraction of emissions that remain in the atmosphere. But more on that next time.