Session Topic:
CO2 and Climate change: past – present – future
Speaker:
Yasuhiro Yamanaka, Hokkaido University
Title: Stabilizing the Atmospheric CO2 Concentration & Ocean Acidification
1. Introduction
The atmospheric level of carbon dioxide (CO2)has increased from 280 ppmv (= 0.028% of total airvolume) in the preindustrial period (until the mid 18th century) to 380 ppmv at present (e.g., IPCC, 2001). This is due to CO2 releases associated with human activities (the anthropogenic CO2 release). Around half of the CO2 released is absorbed by the ocean and terrestrialvegetation, and the rest remains in the atmosphere. As well known, the increase in atmospheric CO2 causes global warming through changes in heat and water balances and atmospheric and oceanic circulations. In the ocean, the increased carbon concentration decreases the pH of the seawater;which we call “ocean acidification”. In this session, I will talk about the global carbon cycle focusing on ocean acidification.
2. Basics of carbon chemistry in the ocean
Pristine seawater has pH 8 to 8.3, i.e., the ocean is naturally somewhat alkaline. Carbon dioxide dissolved in the water works as a weak acid. Dissolution of carbon dioxide in seawater is nothing short of a reaction neutralizing an alkali with an acid. We describe this reaction as follows:
(1)
(2)
These equations mean that for each mole of carbon dioxide dissolved in seawater, one mole of carbonate ion is consumed, together with one mole of water. This is a fundamental chemical reaction taught in senior high schools.
The pH of modern surface seawater is about 0.1 lower than it was in the preindustrial period, and it will fall an additional 0.3 by the end of this century, as the atmospheric CO2level continues to rise (Orr et al., 2005). This decrease of 0.4 in the pH of seawater will reduce the carbonate ion concentration by 50%. Surface seawaters in the Southern Ocean and part of the North Pacific will become under-saturated for calcium carbonate (CaCO3). This will make it difficult for certain organisms (cold-water corals and pteropods, a group of zooplankton) to grow shells of aragonite, a mineral form of calcium carbonate. Once the atmospheric CO2 level reaches about 600 ppmv, cold polar surface waters will start to becomeunder-saturated. Basic chemistry tells us that the uncertainties of ocean acidification are small, unlike for climate predictions.
3. The reduction of the anthropogenic release to stabilize the atmospheric CO2 level
The CO2 uptake by the ocean is regulated by the slow exchange between the surface and deep waters taking around 1000 years, although the atmosphere and ocean surface water reach equilibrium (with respect to CO2 concentration) within one year. If the anthropogenic release were stopped at the present, after several thousand years, 80% or more of the total released CO2 would be absorbed by the ocean, and the atmospheric CO2 concentration would return to below 300 ppmv. Although the oceanic uptake increases with increasing anthropogenic release, this does not compensate completely. In the 1990s, the oceanic uptake was about 2 PgC/yr (= 1015g of carbon per year) while the anthropogenic release was about 8 PgC/yr (= 6 Pg/yr from burning of fossil fuels plus 2 Pg/yr from deforestation).
To stabilize the atmospheric CO2 level, which is the ultimate objective of the United Nations Framework Convention on Climate Change (UNFCCC) produced at the Earth Summit in Rio de Janeiro in 1992, the anthropogenic CO2 release must balance with the uptake by the ocean and terrestrial vegetation. Because terrestrial vegetation will reach its equilibrium state within a hundred years or less under the new climate conditions, its CO2 uptake will quickly decrease at the end of this century. Therefore, in the long-term carbon balance, the anthropogenic CO2 release can only be balanced by oceanic uptake, which will remain around 2 PgC/yr during the next thousand years.
4. Conclusion
Earth science tells us that,to stabilize the atmospheric concentration of carbon dioxide (e.g., at 450 ppm), the anthropogenic CO2 release must be immediately reduced by about 4 Pg/yr from the emission level in 1990 (i.e., 8 Pg/yr) by around 2050. This reduction is 20 times larger than the reduction of about 0.2 Pg/yr, promised in the Kyoto Protocol (including the reduction by the U.S.A.). That is, the Kyoto Protocol was one giant leap politically, but one small step for stopping global warming.
To achieve this reduction, we have not only to introduce renewable energy (e.g., biofuels, wind and solar energies) and to conserve energy, but also to earnestly approach the problem: 1) technically (e.g., carbon capture and storage in the deep underground and ocean, and hydrogen technology), 2) politically, both internationally and domestically (e.g., activities such as the Kyoto Protocol and introducing carbon taxes), and 3) educationally (e.g., sustainable life style).
References
IPCC: Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, 881pp (2001) CambridgeUniversity Press.
Orr, J., V. J. Fabry, O. Aumont, L. Bopp, S. C. Doney, R. M. Feely, A. Gnanadesikan, N. Gruber, A. Ishida, F. Joos, R. M. Key, K. Lindsay, E. Maier-Reimer, R. Matear, P. Monfray, A. Mouchet, R. G. Najjar, G.-K. Plattner, K. B. Rodgers, C. L. Sabine, J. L. Sarmiento, R. Schlitzer, R. D. Slater, I. J. Totterdell, M.-F. Weirig, Y. Yamanaka, and A. Yool:Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature, 437, 681-686, doi:10.1038/nature04095, 2005.