What everyone has been saying about the earth going in cycles with regard to global warming/cooling is completly true (it is called the carbon cycle). In the past the earth has responded to rises in temperature/CO2 levels through a variety of methods (such as absorption into the oceans), and continued the cycle (the orbit of the earth also has a role in this cycle). However, these methods only can do so much at a time, and if the CO2 emmission levels become too high these natural methods will not work well enough. So the CO2 will be dumped into the atmospehere, and global heat levels will rise. Eventually the global system will probably be able to pull all the CO2 out of the atmosphere, but it will take a long time and likely poison much of the earth (especially the oceans). I think these two charts may illustrate my points the best.
This first chart shows the CO2 levels over the past 400 thousand years. You can see the cycles working, but the recent industrial revolution has caused a hugh increase in CO2 past any previous high.
Here is the corresponding heat graph. Again you can see a rather large jump at the end.
Here are the data sources for the first graph incase anyone doubts their factuality.
1. (blue) Vostok ice core: Fischer, H., M. Wahlen, J. Smith, D. Mastroianni, and B. Deck (1999). "Ice core records of Atmospheric CO2 around the last three glacial terminations". Science 283: 1712-1714.
2. (green) EPICA ice core: Monnin, E., E.J. Steig, U. Siegenthaler, K. Kawamura, J. Schwander, B. Stauffer, T.F. Stocker, D.L. Morse, J.-M. Barnola, B. Bellier, D. Raynaud, and H. Fischer (2004). "Evidence for substantial accumulation rate variability in Antarctica during the Holocene, through synchronization of CO2 in the Taylor Dome, Dome C and DML ice cores". Earth and Planetary Science Letters 224: 45-54. DOI:10.1016/j.epsl.2004.05.007
3. (red) Law Dome ice core: D.M. Etheridge, L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola and V.I. Morgan (1998) "Historical CO2 records from the Law Dome DE08, DE08-2, and DSS ice cores" in Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.
4. (cyan) Siple Dome ice core: Neftel, A., H. Friedli, E. Moor, H. Lötscher, H. Oeschger, U. Siegenthaler, and B. Stauffer (1994) "Historical CO2 record from the Siple Station ice core" in Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.
5. (black) Mauna Loa Observatory, Hawaii: Keeling, C.D. and T.P. Whorf (2004) "Atmospheric CO2 records from sites in the SIO air sampling network" in Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.
and for the second:
1. (dark blue 1000-1991): P.D. Jones, K.R. Briffa, T.P. Barnett, and S.F.B. Tett (1998). "High-resolution Palaeoclimatic Records for the last Millennium: Interpretation, Integration and Comparison with General Circulation Model Control-run Temperatures". The Holocene 8: 455-471.
2. (blue 1000-1980): M.E. Mann, R.S. Bradley, and M.K. Hughes (1999). "Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations". Geophysical Research Letters 26 (6): 759-762.
3. (light blue 1000-1965): Crowley and Lowery (2000). "Northern Hemisphere Temperature Reconstruction". Ambio 29: 51-54. Modified as published in Crowley (2000). "Causes of Climate Change Over the Past 1000 Years". Science 289: 270-277.
4. (lightest blue 1402-1960): K.R. Briffa, T.J. Osborn, F.H. Schweingruber, I.C. Harris, P.D. Jones, S.G. Shiyatov, S.G. and E.A. Vaganov (2001). "Low-frequency temperature variations from a northern tree-ring density network". J. Geophys. Res. 106: 2929-2941.
5. (light green 831-1992): J. Esper, E.R. Cook, and F.H. Schweingruber (2002). "Low-Frequency Signals in Long Tree-Ring Chronologies for Reconstructing Past Temperature Variability". Science 295 (5563): 2250-2253.
6. (yellow 200-1980): M.E. Mann and P.D. Jones (2003). "Global Surface Temperatures over the Past Two Millennia". Geophysical Research Letters 30 (15): 1820. DOI:10.1029/2003GL017814 .
7. (orange 200-1995): P.D. Jones and M.E. Mann (2004). "Climate Over Past Millennia". Reviews of Geophysics 42: RG2002. DOI:10.1029/2003RG000143
8. (red-orange 1500-1980): S. Huang (2004). "Merging Information from Different Resources for New Insights into Climate Change in the Past and Future". Geophys. Res Lett. 31: L13205. DOI:10.1029/2004GL019781
9. (red 1-1979): A. Moberg, D.M. Sonechkin, K. Holmgren, N.M. Datsenko and W. Karlén (2005). "Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data". Nature 443: 613-617. DOI:10.1038/nature03265
10. (dark red 1600-1990): J.H. Oerlemans (2005). "Extracting a Climate Signal from 169 Glacier Records". Science 308: 675-677. DOI:10.1126/science.1107046
(black 1856-2004): Instrumental data was jointly compiled by the Climatic Research Unit and the UK Meteorological Office Hadley Centre. Global Annual Average data set TaveGL2v [2] was used.