|MadSci Network: Earth Sciences|
The level of CO2 in the atmosphere rises and falls naturally in response to seasonal changes each year. This has probably occurred for several tens of millions of years. What is more serious is an overall, very rapid increase in the "background" level of CO2 that we can trace back to the beginnings of the industrial revolution. The amount of carbon dioxide in the atmosphere is a balance between the things producing it (the "sources:" breathing, burning, decomposition) and the things removing it from the air (the "sinks:" plant growth, mineral formation, dissolving in water). Why does this concentration change seasonally? Because when spring arrives, all the plants begin growing again, and this process uses up CO2 -- so during fall & winter carbon dioxide builds up, and during spring and summer this supply is drawn down. But if it is spring in the Northern Hemisphere, then it is fall in the Southern Hemisphere -- doesn't this all just balance out? No, for two reasons. First, if you look at a map of the whole Earth, you will notice that there is a lot more land in the Northern Hemisphere than in the Southern Hemisphere. This means there are a lot more plants, so the variation in the Northern Hemisphere is much greater. Second, the air in the Northern Hemisphere doesn't mix very quickly with the air in the Southern Hemisphere -- it takes longer than a year to mix thoroughly, so differences between the hemispheres aren't "lost" with all the stirring. The following figure shows the concentration of CO2 (the vertical axis) as a function of time (left to right) and as a function of latitude (in and out of the page). Notice how much bigger the waves in CO2 concentration are in the Northern Hemisphere compared to the Southern Hemisphere. Note also that when it is high in the North, it is low in the South. The final thing to notice is that the whole pattern is climbing as we go to the right--this is the "background" level of CO2 that is rising, on top of which the seasonal ripples go up and down. What about the oceans? What role do they play? Carbon dioxide dissolves in water, so if we increase the concentration of CO2 in the air, it will slowly seep into the oceans. However, the oceans are so huge--and mix so slowly--that this increase in CO2 in the atmosphere has affected only the top few hundred meters of the ocean. Why has it probably been this way for millions of years? The fact that there is more land in the Northern Hemisphere than the Southern is due to plate tectonics--the relative movement of the continents on the surface of the Earth. Now that's a really slow process--the fastest plates move about as fast as you fingernails grow, so the pattern of land and water does not change very fast at all. So why is an increase in CO2 serious? An increase in carbon dioxide has both good and bad effects, but we are not sure which are more serious. It is good in that certain kinds of plants (some of which we use for food) grow better with more CO2 so we might be able to feed more people. It is also bad because CO2 is a "green house gas" that traps heat from the sun, so that the average temperature on the surface would get hotter, making for more intense weather patterns. Basically, we think more CO2 will mean it will get hotter where it is already hot, colder where it is already cold, drier where it is dry, and wetter where it is wet. Bigger deserts, bigger floods, more strong hurricanes. But we will also have more usable cropland in Alaska, Canada and Siberia. We might be able to grow more crops, but they might also get wiped out more often! Other sources of info: EPA Global Warming Web Site: Climate System, Emissions, Impacts and Actions Common Questions about Climate Change: A good overview from the World Meteorological Organization GCMD Learning Center - Q&A: Climate Change, Global Warming, Greenhouse Gases, Sea Level Rise, El Niño ARM Education Site: Global Warming Climate Change: State of Knowledge U.S. National Assessment - The Potential Consequences of Climate Variability and Change What are the numbers behind all this? Can you get more technical? (The material below is based on material taken from the US Carbon Cycle Science Plan) Throughout the climate extremes of the past 40,000 years, during which there were four major glacial cycles, atmospheric CO2 concentrations varied by no more than twenty percent from a mean of about 240 parts per million (ppm). Concentrations of CO2 are now higher by more than 30% than the previous highest levels. This very rapid increase has occurred since the industrial revolution. The rapid increase in atmospheric concentrations of CO2 over the past 150 years, reaching current concentrations of about 370 ppm, corresponds with combustion of fossil fuels since the beginning of the industrial age. Conversion of forested land to agricultural use has also redistributed carbon from plants and soils to the atmosphere. There has been growing concern in recent years that these high levels of carbon dioxide not only may lead to changes in the Earth's climate system but may also alter ecological balances through physiological effects on vegetation. Only about half of the CO2 released into the atmosphere by human activity (''anthropogenic'' CO2 from combustion of fossil and biomass fuels and from land use changes) currently resides in the atmosphere. Over the last 10-20 years, more than half of the CO2 released by burning fossil fuels has been absorbed on land and in the oceans. These uptake and storage processes are called 'sinks' for CO2 although the period over which the carbon will be sequestered is unclear. The efficiency of global sinks has been observed to change from year to year and decade to decade, due to a variety of mechanisms, only partly understood. The understanding of carbon sources and sinks has advanced enormously in the last decade. There is now clear evidence that global uptake of anthropogenic CO2 occurs by both land plants and by the ocean. The magnitude of the oceanic sink, previously inferred from models and observations of chemical tracers such as oceanic radiocarbon and tritium distributions, has recently been confirmed by direct observation of the increase in dissolved inorganic carbon. Analysis of new tracers such as chlorofluorocarbons provide further refinements to our understanding of carbon uptake by the oceans. The importance of the sink due to the terrestrial biosphere has emerged from analysis of the global carbon budget, including improved estimates of the ocean carbon uptake, as well as data on 13CO2/12CO2 isotopic ratios and from changes in the abundance of O2 relative to N2. lsotopes can give information on the terrestrial sink, for example, since plants preferentially select certain isotopes during photosynthesis and leave a global signature in the isotopic ratio of carbon dioxide in the atmosphere. Forest inventories and remote sensing of vegetation appear to confirm a significant land sink in the Northern Hemisphere and provide insight into the underlying mechanisms. However, we cannot yet quantitatively define the global effects of human activities such as agriculture and forestry or the influence of climate variations such as El Niño. Studies to determine these effects have emerged as critical for understanding long-term changes in atmospheric concentration in the past, and will help to dramatically enhance understanding of how the Earth's climate will evolve in the future. Carbon cycle research relating to climate change fundamentally concerns three large scientific issues. The first is the natural partitioning of carbon among the "mobile" reservoirs-the ocean, atmosphere, and soil and terrestrial biosphere-partitioning that is influenced itself by climate change. The second issue is the redistribution of fossil fuel CO2 within those same three reservoirs, and assessments of proposals to prevent emissions or sequester carbon through new technologies. The third issue concerns transfers between the terrestrial biosphere and the atmosphere induced by other human activities such as forest clearing and regrowth, the management of agricultural soils, and the feedback potential of anthropogenically driven changes in atmospheric chemistry and climate to alter these transfer rates. Historical changes in the quasi-steady state of the carbon system are clearly reflected in ice core and isotopic records, which also record the unprecedented changes caused by anthropogenic CO2 emissions (Raynaud et al. 1993). The globally averaged atmospheric CO2 mole fraction is now over 365 micromole/mol (or parts per million, ppm)-higher than it has been for hundreds of thousands of years. Atmospheric concentrations of CO2 had remained between 270 and 290 ppm during the last several thousand years, but rose suddenly to the present level during the second half of the 20th century. This increase Is coincident with the rapid rise of fossil fuel burning. The modern rate of atmospheric CO2 increase is accurately known from contemporary direct atmospheric measurements as well as from air stored in ice and firn (Battle et al. 1996, Etheridge et al. 1996). The Intergovernmental Panel on Climate Change (IPCC) assessment has summarized the state of scientific knowledge in this area (Schimel et al. 1995). During the decade of the 1980s,the rate of increase in the atmosphere was 3.3±0.2 billions of metric tons (also called gigatons) of carbon per year (Gt C/yr). The next best-known piece of the puzzle is the rate of fossil fuel emissions, which was 5.5±0.5 Gt C/yr during the 1980s (Marland et al. 1994). Thus, it is clear that on average a large fraction of the emitted CO2 did not remain in the atmosphere. Conservation of mass implies that the missing emissions must have entered the ocean, the terrestrial biosphere, or both. The ocean carbon sink is the best known of these two remaining components of the carbon budget. According to the IPCC, the ocean absorbed 2.0±0.8 Gt C/yr. This finding suggests that the global net terrestrial biosphere sink was only 0.2±1.0 Gt C/yr. However, the estimated loss of carbon from the terrestrial biosphere due to deforestation is estimated to have been 1.6±1.0 Gt C/yr. Thus, there must have been a compensatory storage of carbon in the terrestrial biosphere of 1.8±1.6 Gt C/yr. The latter figure largely compensates for the land use source, so that the global net storage in terrestrial ecosystems was close to zero. It has not been easy to arrive at these estimates. Regarding ocean uptake, if the CO2 entering the ocean were mixed homogeneously, the cumulative increase in the total carbon content after 10 years at the rate of 2 Gt C/yr would only be 1.2 micromole/kg. This equals the detection limit of the currently best analytical techniques. Most of the CO2 has been added to the upper few hundred meters, however. The signal is therefore detectable, but it must be extracted from large natural variations of up to about one to two hundred micromole C/kg. For this reason, until very recently, ocean uptake has been estimated using models whose parameters are calibrated against the penetration into the ocean of other tracers such as 14C and tritium from nuclear tests, and chlorofluorocarbons. Estimates of terrestrial carbon loss have been based on surveys of land use and changes in land use, above- and below-ground average carbon densities for ecosystem classes, and models of the development of carbon storage after disturbance, under human management, and during succession (e.g., Houghton et al. 1987). These assessments have varied greatly, as reflected in the range for tropical biomass destruction assessed by the IPCC to be 1.6±1.0 Gt C/yr. Part of the problem in this evaluation is the great heterogeneity of carbon content on small spatial scales. But also, surveys have been difficult to compare because of incompatible definitions, different counting methods, the treatment of secondary forest growth, and similar reasons. Estimates of terrestrial carbon uptake have been obtained from mass balance considerations such as those described above, which give a total uptake of 1.8±1.6 Gt C/yr. Other estimates come from direct observations of forest carbon inventory changes (0.9 Gt C/yr in the Northern Hemisphere), models of NOx fertilization (0.6 to 0.9 Gt C/yr), and CO2 fertilization (0.5 to 2.0 Gt C/yr) (Schimel et al. 1995). There are strong indications that there is a large uptake in the Northern Hemisphere, but the location, magnitude, and mechanisms of this uptake are poorly understood. Battle, M., M. Bender, T. Sowers, P. P. Tans, J. H. Butler, J.W. Elkins, J.T. Conway, N. Zhang, P. Lang and A. D. Clarke, Atmospheric gas concentrations over the past century measured in air from firn at the South Pole, Nature, 383, 232-235, 1996. Etheridge, D. M. , L. P. Steele, R. L. Langenfelds and R.J. Francey, Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn. J. Geophys. Res., 101, 4115-4128, 1996. Houghton, R.A., R. D. Boone, J. R. Fruci, J. E. Hobbie, J. M. Melillo, C.A. Palm, B.J. Peterson, G. R. Shaver, G. M. Woodwell, B. Moore, D. L. Skole and N. Myers, The flux of carbon from terrestrial ecosystems to the atmosphere in 1980 due to changes in land use: geographic distribution of the global flux, Tellus, 39B, 122-139, 1987. Marland, G., R.J. Andres, and T.A. Boden, Global, regional and national CO2 emissions, In: Trends '93, T.A. Boden, D. P. Kaiser, R.J. Sepanski, and R.W. Stoss, (Eds.), publication No. ORNL/CDIAC-65, Carbon Dioxide Information Center, Oak Ridge National Laboratory, Oak Ridge, TN, pp. 505-581, 1994. Raynaud, D., J. Jouzel, J. M. Barnola, J. Chappellaz, R. J. Delmas and C. Lorius, The ice record of greenhouse gases, Science, 259, 926-934, 1993. Schimel, D., I. G. Enting, M. Heimann, T. M. L. Wigley, D. Raynaud, D. Alves and U. Siegenthaler, CO2 and the Carbon Cycle, in Climate Change 1994, vol. edited by J.T. Houghton, L. G. M. Filho, J. Brace, H. Lee, B.A. Callander, E. Haites, N. Harris and K. Maskell, 35-71 , Cambridge University Press, Cambridge, 1995.
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