MadSci Network: Earth Sciences

Re: what is the signifigance of the increase and decrease of atmospheric CO2

Date: Wed Apr 11 16:57:58 2001
Posted By: Paul Filmer, Staff, Geosciences Directorate, National Science Foundation
Area of science: Earth Sciences
ID: 985916036.Es

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 

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 

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, 

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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