To stop making the problem worse, we need to stabilize CO2 levels in the atmosphere — we need to stop putting out more CO2 than can be absorbed naturally. Global warming is driven by the cumulative total amount of CO2 we have put into the atmosphere less cumulative natural absorption. We have dumped approximately 2,000 billion metric tons of CO2 (GtCO2) into the atmosphere since the dawn of large scale fossil fuel consumption in the industrial area. Of that total, approximately 1,200 GtCO2 has been absorbed by the oceans and other carbon sinks, leaving 800 extra GtCO2 in the atmosphere. The atmosphere is vast but finite. That 800 extra GtCO2 amounts to 100 parts per million in the atmosphere (this is a measured fact; it also one that one can compute using high school physics and geometry). That increment from the pre-industrial base has led to approximately 1 degree C of average warming. Note that even if we immediately stopped increasing the concentration of carbon dioxide in the atmosphere, there would still be some updrift of temperature by a few tenths of a degree as the climate responds to current concentrations. See page 13 of the recent policy maker summary from working group 1.
To truly stabilize in a thousand-year-plus perspective, we will essentially have to stop emitting carbon (or find ways to artificially sink it). There is no evidence of any permanent natural process that would sink more than a tiny fraction of our current carbon production (known candidates might absorb less then 1 GtCO2 per year as compared to our current production of over 25 GtC02 per year). See the third International Panel on Climate Change, Working Group I Report (“TAR/WG1”), section 3.7.3.4 on this point. See TAR/WG1, Chapter 3, executive summary, for an overview of the carbon cycle.
In the imaginable future of two or three centuries, reduced but continuing fossil fuel emissions are consistent with stabilized atmospheric concentrations of carbon dioxide. This is because the oceans can and will absorb a huge amount of carbon dioxide. The problem is that they will do it very slowly. Surface water saturates relatively quickly, so continued absorption depends heavily on bringing unsaturated water to the surface through slow moving vertical ocean circulation. See TAR/WG1, section 3.2.3.2. The rate of absorption is also function of the positive difference between atmospheric concentrations and surface water concentrations. So, if atmospheric CO2 concentrations are higher, the absorption rates are higher (allowing higher continuing emissions without increasing concentrations), but will decline as more and more of the ocean water becomes saturated.
The TAR/WG1 addresses this dynamic with a set of model curves that, according to section 3.7.3.4, suggest that “CO2stabilisation at 450, 650 or 1,000ppm would require global anthropogenic CO2 emissions to drop below 1990 levels within a few decades, about a century, or about two centuries, respectively.” This doesn’t sound completely unmanageable, however, the levels must continue to drop — for example, to less than half of 1990 levels by the end of the century to stabilize at the 450ppm level. Therecently released report of IPCC’s fourth assessment working group 1 suggests that the next generation models indicate that carbon cycle feedbacks (ways in which climate changes slow net carbon absorption) will require lower stabilization levels, see summary for policy makers at page 16. Compare the discussion of this issue in the Stern report, at page 197, especially note 4, which reads the TAR/WG1 model curves as requiring an 80% reduction below “current” levels by the second half of the next century (2150 onwards), apparently to achieve stabilization at the 450ppm or 550ppm level.
400 parts per million (including carbon equivalents of other greenhouse gases) would be likely to keep us in a safe temperature range — not much beyond 2 (1.0 to 3.1) degrees above pre-industrial levels. Unfortunately, we are already at the 430 level and rising at 2.3ppm per year when all GHG’s are included. 450ppm might keep temperatures at the 2 degrees above industrial level, and would create a limited risk of going beyond 3 degrees of increase from pre-industrial levels.
Because of the uncertainties involved, emission levels are correlated with a range of possible temperature outcomes in most presentations. A good inventory of estimates by level of GHG’s appears at page 11 of the Stern Report; for another compilation using probability of exceeding different temperatures, see page 195 of the Stern Report; see also the 2007 update of the Stern Report. The Hadley Center projects a range of 490 to 670 ppm of CO2 equivalents to put likely temperature at 2 degrees above present day — roughly consistent.
The 2007 IPCC’s new policy maker summary from working group 1, also presents roughly comparable estimates at page 13. However, the comparison to the ranges in the Stern and Hadley Center reports requires some analysis, because the projections from the working group are based on emissions scenarios in which atmospheric levels are a derived quantity.
Comparing the new IPCC working group report to Stern: The table on page 13 of the working group report presents temperature outcomes for emissions scenarios which are described in previously issued special report on emissions scenarios and summarized in footnote 14 on page 12. The scenarios contemplate different political-economic futures for the world over the next century. None of the scenarios explicitly assume coordinated action to control carbon emissions. What is relevant is the total amount of carbon dioxide and other greenhouse gases expressed as carbon dioxide equivalents in each scenario. In the lowest emission scenario, GHGs stabilize at approximately 600ppm of CO2 equivalents. (Note again that stabilization level is estimated by modeling from assumed emissions less carbon absorption; climate change based on stabilization level is another facet of the modeling). The 600PPM CO2 equivalent level, according to the climate change modeling in the new from working group 1 analysis leads to an increase of 1.8 degrees centigrade from current levels — enough to create significant problems. The likely range is between 1.1 and 2.9 degrees, indicating a 2/3 probability that the global average temperature would increase within that range, with equal 1/6 probabilities that it would fall above or below that range. Of course, this scenario includes CO2 equivalents as well, the modeled level of carbon alone appears to be approximately 550ppm — the underlying scenario analysis offers a range of carbon and specific estimates for other gases for each scenario and then chooses a total for radiative forcing due to all gases. See pages 807-823 in the appendices of the third assessment report for detail on carbon and other GHG levels. For the temperature to CO2 concentration analysis from the third IPCC assessment, see section 9.3.3.1.