This is an example of skepticism about the attribution of climate change [enhancement of global warming] to the continual rises in annual carbon dioxide concentration

Problems of associated with the attribution of global warming to increases of CO2 in the atmosphere.

The IPCC position as given in the FAR, 2007, Chapter 9 of the Physical Science Basis

Estimates of the climate sensitivity are now better constrained by observations. Estimates based on observational constraints indicate that it is very likely that the equilibrium climate sensitivity is larger than 1.5°C with a most likely value between 2°C and 3°C. The upper 95% limit remains difficult to constrain from observations. This supports the overall assessment based on modelling and observational studies that the equilibrium climate sensitivity is likely 2°C to 4.5°C with a most likely value of approximately 3°C. The transient climate response, based on observational constraints, is very likely larger than 1°C and very unlikely to be greater than 3.5°C at the time of atmospheric CO2 doubling in response to a 1% yr–1 increase in CO2, supporting the overall assessment that the transient climate response is very unlikely greater than 3°C.

[ Hegerl, G.C., F. W. Zwiers, P. Braconnot, N.P. Gillett, Y. Luo, J.A. Marengo Orsini, N. Nicholls, J.E. Penner and P.A. Stott, 2007: Under­standing and Attributing Climate Change. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis,K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.]

The simple equation [from TAR, The Physical Basis, 2001, page 358] that allows an estimate of the radiative forcing caused by variations in the atmospheric concentration of CO2 is:

ΔF = 5.35 ln(C/C0)

ΔF is the forcing in W m-2, C is the upper value of the CO2 concentration and, strictly the symbol C0 refers to a lower standard value of the CO2 concentration sometimes equated to the value in pre-industrial times, but according to Myhre et al2 the equation is relevant only to the CO2 concentration range of 300-600 ppmv. The equation itself was derived empirically from the results from a GCM and is therefore subject to doubts about the parameterization of the physics used.

The equation allows, with a little leeway the estimation of the forcing caused by the increase to the present time in the atmospheric CO2 concentration since pre-industrial times, taken to be ~100 ppmv [385 - 285 ppmv]. This amounts to 5.35 ln (385/285) = 1.6 W m-2 and together with the relationship between forcing and surface temperature increase:

ΔT =  ΔF/2

This equation, with the IPCC value for the ‘sensitivity’ of 0.5°C (W m-2)-1 gives an estimate of 0.8°C for the expected temperature increase caused by the 100 ppmv increase in atmospheric CO2 concentration. Coincidentally or otherwise, that agrees with the observations of changes in the global mean temperature. This is one area of contention between the IPCC and sceptics.

One immediate reaction would be to wonder, if the increased CO2 were to be the cause of the temperature rise, what about other forcings and what about natural causes. A good paper appeared in Nature, 350, 324, (1991).

 

 Interdecadal oscillations and the warming trend in global temperature time series

M. Ghil & R. Vautard

The abstract follows.

 The ability to distinguish a warming trend from natural variability is critical for an understanding of the climatic response to increasing greenhouse-gas concentrations. Here we use singular spectrum analysis to analyse the time series of global surface air temperatures for the past 135 years, allowing a secular warming trend and a small number of oscillatory modes to be separated from the noise. The trend is flat until 1910, with an increase of 0.4°C since then. The oscillations exhibit interdecadal periods of 21 and 16 years, and interannual periods of 6 and 5 years. The interannual oscillations are probably related to global aspects of the El Niño-Southern Oscillation (ENSO) phenomenon. The interdecadal oscillations could be associated with changes in the extratropical ocean circulation. The oscillatory components have combined (peak-to-peak) amplitudes of >0.2 °C, and therefore limit our ability to predict whether the inferred secular warming trend of 0.005 °C yr-1 will continue. This could postpone incontrovertible detection of the greenhouse warming signal for one or two decades.

Nothing much has changed since 1991 and their conclusion is still valid today.

A somewhat controversial paper was given by William M. Gray at a recent Heartland Conference called Climate Change: Driven by the Ocean not Human Activity

The full paper is available at http://tropical.atmos.colostate.edu

The paper discusses how the variation in the global ocean’s Meridional Overturning Circulation (MOC) resulting from changes in the Atlantic Thermohaline Circulation (THC) and deep water Surrounding Antarctica Subsidence (SAS) can be the primary cause of climate change.

Without discussing the whole paper, a diagram of great interest is given and reproduced below.

 

 

 The background data are those from the terrestrial record and the superposed blue and red lines are the responsibility of the author. He makes the point that the full blue lines are the best straight lines through the limited ranges of data and these have been noted and discussed frequently in other papers and at conferences. The dotted lines are simply extrapolations of the limited linear trends. Gray points out quite rightly the dangers of such procedures and the diagram makes obvious the foolhardiness of drawing such lines. The dotted blue lines are those in which the future years are cooler than the observed data indicate and the dotted red lines are those in which the future years are warmer than expected from the extrapolations. The observed cooling from ~1940 to ~1976 is infamous since many of our present day climatologists used it to suggest that the next ice age was forthcoming. They, now incorporated in the IPCC, are showing confidence in the dotted red line from 2000 onwards, although recent observations are closer to the blue line. Gray, quite rightly pours cold water on such predictions, but he also goes a little too far in expecting another 25 years or so of cooling. That there is cooling over the last seven years is consistent with Gray’s ‘predictions’, but seven years is not a very long time in climate change and we shall have to wait a while before building up confidence in what the trend really is. For sure, those sceptics that are proposing that seven years of cooling deal a death blow to the underlying theory of anthropogenic global warming are possibly storing up the same kind of derisory reaction that they provide the IPCC with. A bit of caution is needed.

Gray’s main point, that climate change is primarily driven by various ‘oscillations’ and nothing connected with the rise in atmospheric greenhouse gas concentrations is extremely doubtful. The ‘jagged’ or zig-zag nature of the temperature plots imply that there are several cycles at work, including the major oscillations mentioned by Gray. Scrutiny of any of the temperature records show that the Earth’s mean global temperature can change by up to 0.5°C within the space of a fortnight. This corroborates the work of Ghil and Vautard described above and underlines the difficulty associated with trying to estimate a global trend. The doubt about Gray’s conclusion stems from the obvious statement that, although oscillations undoubtedly have impacts on global mean temperature by exerting their observed regional effects, they cannot be a prime cause of climate change. They have to draw their energy from somewhere and that somewhere is the warmed atmosphere/surface system that is warmed by solar radiation combined with the effects of the greenhouse gases.

The Gray paper shows the variations of global temperature on short and intermediate timescales. Gray’s main conclusion that oscillations affect the medium term temperature variations and that these are the drivers of climate change overlooks the most important observation in the above graph which is that underlying the variations discussed, there is an ever increasing trend. That is not mentioned, but a paper by Douglass and Christy [Energy & Environment, August, 2008] deals with a multivariate analysis of the restricted (1979 to date) set of satellite observations of the temperature of the lower troposphere. They factorize out the oscillatory effects and are left with a warming trend of 0.06 ± 0.01 K per decade that is associated with the increasing CO2 concentration. That is considerably lower than that expected by any of the IPCC ‘scenario’s’ and more in line with what several professional sceptics suggest.