Temperature predictions from some climate models assuming the SRES A2 emissions scenario. (Photo credit: Wikipedia)
Guest post by Christopher Monckton of Brenchley
Abstract
Global CO2 emissions per unit increase in atmospheric CO2 concentration provide an independent constraint on climate sensitivity over the timescale of the available data (1960-2008), suggesting that, in the short term and perhaps also in the long, climate sensitivity may lie below the values found in the general-circulation models relied upon by the IPCC.
Introduction
The Intergovernmental Panel on Climate Change (IPCC, 2001, p. 358, Table 6.2), citing Myhre et al. (1998), takes the CO2 forcing ΔF as 5.35 times the logarithm of a proportionate change Cb/Ca in CO2 concentration, where Cais the unperturbed value. Warming ΔT is simply ΔF multiplied by some climate sensitivity parameter λ.
Projected 21st-century anthropogenic warming, as the mean of values on all six IPCC emissions scenarios, is 2.8 K (IPCC, 2007, table SPM.3: Annex, Table 0). Of this, 0.6 K is stated to be in the pipeline. Of the remaining 2.2 K, some 0.65 K is attributable to non-CO2 forcings, since the CO2 fraction of anthropogenic warming is 71% (the Annex explains the derivation). Thus the IPCC’s current implicit central estimate of the warming by 2100 that will be attributable solely to the CO2 we emit this century is only 1.56 K.
Projected CO2 concentration C2100 in 2100, the mean of the values on all six IPCC emissions scenarios, is 713 ppmv (Annex, Table 3), 345 ppmv above the 368 ppmv measured in 2000 (Conway & Tans, 2011). Therefore, the IPCC’s implicit climate-sensitivity parameter for the 21st century is 1.56 / [5.35 ln(713/368)], or 0.44 K W–1 m2. This value, adopted in (1), is half of the IPCC’s implicit equilibrium value 0.88 K W–1 m2 (derived in the Annex).
Global warming from 1960-2008
The IPCC’s implicit central estimate of CO2-driven warming from 1960-2008 is at (1):
The CO2 forcing coefficient 5.35 was given in Myhre et al. (1998). Initial and final CO2 concentrations were 316.9 and 385.6 ppmv respectively (Tans, 2012). Since the 0.46 K warming driven by the CO2 fraction is 71% of anthropogenic warming, use of the IPCC’s methods implies that, as a central estimate, all of the 0.66 K observed warming from 1960-2008 (taken as the linear trend on the data over the period in HadCRUt3, 2011) was anthropogenic. However, attribution between Man and nature remains problematic: an independent approach to constraining climate sensitivity produces a very different result.
An independent constraint on climate sensitivity
Since few non-linearities will obtrude at sub-centennial time-scales, to warm the Earth’s surface by 1 K the CO2 concentration in the atmosphere must increase by 345/1.56 = 223 ppmv K–1. From 1960-2008, the trend in the ratios of annual global CO2 emissions to annual increases in atmospheric CO2 concentrations does not differ significantly from zero (Fig. 1). The mean emissions/concentration-growth ratio over the period was 15.5 Gt CO2 ppmv–1, which, multiplied by 223 ppmv K–1, gives 3450 GTe CO2 K–1, the quantum of CO2 emissions necessary to raise global temperature by 1 K.
Figure 1. Near-zero trend in annual emissions/concentration-growth ratios, 1960-2008. Data and methods are described in the Annex. Spikes caused by volcanic eruptions are visible. Excluding effects of major eruptions makes little difference to the outcome.
Total global CO2 emissions from 1960-2008 were 975 Gte CO2 (Boden et al., 2011). Accordingly, CO2-driven warming expected over the period, by the present method, was 975 divided by 3450, or 0.28 K. Allowing for the non-CO2 fraction, some 0.40 K warming over the period, equivalent to 61% of observed warming, was anthropogenic, not inconsistent with the estimate in IPCC, 2007 that at least 50% of observed warming from 1950-2005 was anthropogenic. However, inconsistently with (1), this method yields a CO2-driven warming that is only 61% of the central estimate derived from the IPCC’s general-circulation models.
Implications
On the assumption that the coefficient in the CO2 forcing function, cut from 6.3 to 5.35 in Myhre et al. (1998), is now correct, one implication of the present result is that the climate-sensitivity parameter λ appropriate to a 50-year period is not 0.44 K W–1 m2, as the models suggest, but as little as 0.27 K W–1 m2. Since the value of the instantaneous or Planck sensitivity parameter λ0 is 0.31 KW–1 m2 (IPCC, 2007, p. 631 fn.), temperature feedbacks operating during the period of study may have been somewhat net-negative, rather than appreciably net-positive as implied by (1).
If feedbacks operating over the short to medium term are indeed net-negative, there is no warming in the pipeline from past emissions; in the rest of this century CO2-driven warming may be little more than 1 K; anthropogenic warming from all sources may be less than 1.5 K; and supra-centennial-scale warming may also be significantly less than currently projected. If so, all attempts at mitigation will prove cost-ineffective, and the cost of adaptation to future warming will be well below current estimates.
References
Boden, T., G. Marland, and R. Andres, 2011, Global CO2 Emissions from Fossil-Fuel Fossil-Fuel Burning, Cement Manufacture, and Gas Flaring: 1751-2008, available from http://cdiac.ornl.gov/ftp/ndp030/global.1751_2008.ems
Conway, T., & P. Tans, 2011, Recent trends in globally-averaged CO2 concentration, ww2.esrl.noaa.gov/gmd/ccgg/trends/global.html#global.
Garnaut, R., 2008, The Garnaut Climate Change Review: Final Report. Cambridge University Press, Port Melbourne, Australia, 680 pp, ISBN 9780521744447.
IPCC, 2001, Climate Change 2001: The Scientific Basis: Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change [Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell and C.A. Johnson (eds.)]. Cambridge University Press, Cambridge, United Kingdom, and New York, NY, USA.
IPCC, 2007, Climate Change 2007: the Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007 [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Avery, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom, and New York, NY, USA.
Myhre et al., 1998, New estimates of radiative forcing due to well mixed greenhouse gases. Geophysical Research Letters25:14, 2715–2718, doi:10.1029/98GL01908.
Ramanathan, V., R. Cicerone, H. Singh and J. Kiehl, 1985, Trace gas trends and their potential role in climate change, J. Geophys. Res.90: 5547-5566.
Solomon, S., G.-K. Plattner, and P. Friedlingstein, 2009, Irreversible climate change due to carbon dioxide emissions, PNAS 106:6, 1704-1709, doi:10.1073/pnas.0812721106.
Tans, P., 2012, Atmospheric CO2 concentrations (ppmv) at Mauna Loa, Hawaii, 1958-2008, at ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/co2_annmean_mlo.txt.
Acknowledgements
The author is grateful to Dr. Patrick Michaels for having drawn his attention to the near-zero-trend in the annual CO2 emissions/concentration-growth ratios that is confirmed here.
Annex: supplementary material
Values of the climate sensitivity parameter λ
If net temperature feedbacks exceed zero, the climate sensitivity parameter λ is not constant: as longer- and longer-acting feedbacks begin to act, it will tend to increase between the time of a forcing to the time when equilibrium is restored to the climate 1000-3000 years after the forcing that perturbed it (Solomon et al., 2009). Illustrative values of λ are given below.
The sensitivity parameter derived from the present result and applicable to the 49 years 1960-2008 is 0.27 K W–1 m2.
Where temperature feedbacks sum to zero, the instantaneous value λ0 is 0.31 K W–1 m2 (derived from IPCC (2007, p. 631 fn.: see also Soden & Held, 2006).
Garnaut (2008) talks of keeping greenhouse-gas rises to 450 ppmv CO2-equivalent above the 280 ppmv prevalent in 1750, so as to hold 21st-century global warming since then to 2 K, implying λ262 = 2 / [5.35 ln{(280 + 450) / 280}] = 0.39 K W–1 m2.
As explained in the text, the IPCC’s implicit climate-sensitivity parameter for the 21st century is λ100 = 1.56 / [5.35 ln(713/368)] = 0.44 K W–1 m2.
On each emissions scenario, the IPCC’s estimate of the bicentennial-scale transient-sensitivity parameter λ200 is 0.49 K W–1 m2 (derived in Table 0), a value supported by IPCC (2001, p. 354, citing Ramanathan, 1985).
The implicit value of the equilibrium-sensitivity parameter λequ is the warming currently predicted in response to a CO2 doubling, i.e. 3.26 K (IPCC, 2007, p. 798, Box 10.2), divided by the forcing of 5.35 ln 2 = 3.71 W m–2 at that doubling. Thus, λequ = 0.88 K W–1 m2.
Additional tables in the annex (which cannot reproduce properly here in blog format) are in the PDF file for this paper:
