Contributed by Corinne Le Quéré, University of East Anglia.
This question keeps coming back, although we know the answer very well: all of the recent CO2 increase in the atmosphere is due to human activities, in spite of the fact that both the oceans and the land biosphere respond to global warming. There is a lot of evidence to support this statement which has been explained in a previous posting here and in a letter in Physics Today . However, the most convincing arguments for scientists (based on isotopes and oxygen decreases in the atmosphere) may be hard to understand for the general public because they require a high level of scientific knowledge. I present simpler evidence of the same statement based on ocean observations, and I explain how we know that not only part of the atmospheric CO2 increase is due to human activities, but all of it.
On time-scales of ~100 years, there are only two reservoirs that can naturally exchange large quantities of CO2 with the atmosphere: the oceans and the land biosphere (forests and soils). The mass of carbon (carbon is the “C” in CO2) must be conserved. If the atmospheric CO2 increase was caused, even in part, by carbon emitted from the oceans or the land, we would measure a carbon decrease in these two reservoirs.
Number of observations of carbon decreasing in the global oceans: zero.
Number of observations of carbon increasing in the global oceans: more than 20 published studies using 6 independent methods.
The methods are:
(1) direct observations of the partial pressure of CO2 at the ocean surface (Takahashi et al. 2002),
(2) observations of the spatial distribution of atmospheric CO2 which show how much carbon goes in and out of the different oceanic regions (Bousquet et al. 2000),
(3) observations of carbon, oxygen, nutrients and CFCs combined to remove the mean imprint of biological processes (Sabine et al. 2004),
(4) observations of carbon and alkalinity for two time-periods combined with an estimate of water age based on CFCs (McNeil et al. 2002), and the simultaneous observations of atmospheric CO2 increase and the decrease in (5) oxygen (Keeling et al. 1996), and (6) carbon 13 (Ciais et al. 1995) in the atmosphere.
The principle of the last two methods is that both fossil fuel burning and biospheric respiration consume oxygen and reduce carbon 13 as they produce CO2, but the exchange of CO2 with the oceans has only a small impact on atmospheric oxygen and carbon 13. The measure of atmospheric CO2 increase together with oxygen or carbon 13 decrease gives the distribution between the different reservoirs.
All the estimates show that the carbon content of the oceans is increasing by ~ 2±1 PgC every year (current burning of fossil fuel is ~7 PgC per year). One method is able to go back in time and shows that the carbon content of the oceans has increased by 118±19 PgC in the last 200 years. There is some uncertainty about the exact amount that the oceans have taken up, but not about the direction of the change. The oceans cannot be a source of carbon to the atmosphere, because we observe them to be a sink of carbon from the atmosphere.
What about the land biosphere? We know that deforestation has contributed to the increase in atmospheric CO2. Yet because carbon needs to be conserved, observations of the carbon increase in the atmosphere and the oceans combined with estimates of fossil fuel burning tell us that deforestation has been largely compensated by enhanced growth by the land biosphere. For example, during 1980 to 1999, fossil fuel burning was 117±5 PgC, and the carbon increase in the atmosphere and the oceans were 65±1 and 37±8 PgC, respectively. Thus that leaves 15±9 PgC that has been taken up by the land. This 15±9 PgC includes deforestation (and other land-use changes) which reduced the land biosphere by 24±12 PgC, and an additional land uptake of 39±18 PgC in response to elevated CO2 and climate changes (Sabine et al. 2004). Here also there is some uncertainty about the exact amount, but there is no uncertainty that the land biosphere has taken up a quantity of CO2 that is roughly equivalent to the deforestation.
Why are the ocean and land taking up carbon, when we know that warming of the oceans reduces the solubility of CO2 and warming of the land accelerates bacterial degradation of the soils? The answer is that warming is not the only process that influences the oceans and land biosphere. The dominant process in the oceans is the response to increasing atmospheric CO2 itself. If the oceans had not warmed, they might have taken up even more carbon, although we cannot say for sure because warming may have other impacts, for example on marine biota. On land, bacterial degradation of the soils may have increased in response to warming, but for the moment this effect is smaller than the land response to other processes (for example fertilization by CO2 and nitrogen, changes in precipitation, etc).
Is this consistent with what we know of the glaciations? Yes. During glaciations, the balance of processes was very different. Cooling and other climate changes occurred first. The response of the oceans and land biosphere to climate caused the atmospheric CO2 to decrease, which caused more cooling (more on the feedbacks between temperature and CO2 can be found here). During glaciations, there were no external changes in atmospheric CO2 and the oceans and land biosphere responded primarily to climate change. In the last 200 years, there have been large changes in atmospheric CO2 as a result of human activities, and the oceans and land biosphere respond primarily to rising CO2.
In summary, we know that the rise in atmospheric CO2 is entirely caused by fossil fuel burning and deforestation because many independent observations show that the carbon content has also increased in both the oceans and the land biosphere (after deforestation). If the oceans or land had contributed to the rise in atmospheric CO2, they would hold less carbon. Their response to warming may be real, but it is less than their response to increasing CO2 and other climate changes for the moment.
More on the carbon budget can be found in the last IPCC report here, which includes budgets and uncertainties for different time periods and additional numbers for the small contribution of volcanoes and other geological reservoirs.
Bousquet et al. (2000), Regional changes of CO2 fluxes over land and oceans since 1980, Science, Vol 290, 1342-1346.
Ciais et al. (1995), A Large Northern Hemisphere Terrestrial CO2 Sink Indicated by the 13C/12C Ratio of atmospheric CO2, Science, Vol 269, pp. 1098-1102.
Keeling, Piper and Heimann (1996), Global and hemispheric CO2 sinks deduced from changes in atmospheric O2 concentration, Nature, Vol 381, 218-221.
McNeil et al. (2003), Anthropogenic CO2 uptake by the ocean based on the global chlorofluorocarbon data set, Science, Vol 299, 235-239.
Takahashi et al. (2002), Global sea-air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects, Deep Sea Research, Vol 49, 1601-1622.