National Oceanography Centre, Southampton - News

Critical Review: Channel 4 TV programme �The Great Global Warming Swindle� � 23/3/07

Professor John Shepherd FRS, Tyndall Centre for Climate Change Research, National Oceanography Centre, University of Southampton 21 March 2007

Summary

This programme was extremely biased, and raised a large number of issues, many of which have been extensively discussed elsewhere, and largely dismissed: see for example www.realclimate.org. It presented a very confused and confusing message, mixing true (and uncontroversial) material, with many false or misleading implications drawn from it. The main points made were that

climate changes naturally (true)�

� and that recent warming is therefore also natural (false)

CO2 in the atmosphere is partly natural, and varies over time (true)�

� so increased CO2 in the atmosphere cannot be the cause of global warming (false)

variations in solar activity and its effect on clouds (via cosmic rays) provide a better explanation for climate change than the greenhouse effect (false)

Discussion

Some of the more important assertions or implications made by the programme are listed below in italics, and followed by brief comments on them.

Climate has changed quite a lot, due to natural causes, in the past�

This is true, and is not contested by anyone. Indeed, past natural changes, including the glacial-interglacial cycles of the last few million years, mostly seem to have occurred in response to remarkably small external forcing factors. They should therefore serve as a warning that the climate may actually be more responsive to anthropogenic forcing than might otherwise be expected, rather than taken as a reason for complacency.

� and recent warming is therefore also natural

Just because climate has changed naturally in the past does not automatically mean that present day changes are also due to natural causes. The possible causes need to be carefully examined and evaluated, which the Intergovernmental Panel on Climate Change (IPCC) does at some length in its analyses of �Attribution�, which conclude that the contribution of naturally occurring factors is minor [see the recent Summary for Policymakers: IPCC SPM (2007)] . Note that the precise magnitude of past changes such as the Little Ice Age and the Mediaeval Warm Period is still somewhat uncertain, and their causes are not yet firmly established, but this does not affect the conclusion.

Current climate models do not explain the cooling between ca 1940-1970

The slight cooling during this time was exaggerated by using an out-of-date graph: see Steve Connor�s article in �The Independent� detailing the use of old and/or misleading data by the programme � it is available at http://news.independent.co.uk/environment/climate_change/article2355956.ece. An up-to-date figure of the best current estimate can be found in the IPCC SPM (2007). Most of the levelling off in the rising trend of temperature during this period has been satisfactorily explained for some time now by the cooling due to anthropogenic aerosols (which has since then been much reduced). Note that the temperature for a few years around 1940 is still a little warmer than expected, and the cause of this is not known.

Carbon dioxide is only present in small quantities in the atmosphere�

True, and not contested, but irrelevant. The natural (pre-industrial) concentration was about 280 ppm (parts per million, by volume) or about 0.03%. It has increased by about 35% due to anthropogenic emissions and deforestation and is now about 380 ppm. During the last ice age it was quite a bit lower, at about 180 ppm.

� and therefore it cannot possibly cause significant warming

This is false. The major constituents of the atmosphere are nitrogen (as N2) and oxygen (as O2) molecules, which are almost transparent to infra-red radiation and so cause no greenhouse effect. Carbon dioxide, water vapour, methane and nitrous oxide do cause the greenhouse effect because they are all molecules with three or more atoms, which have different modes of vibration and do strongly absorb infra-red radiation. Even tiny quantities (parts per billion) of very strongly absorbing molecules like methane and nitrous oxide may have a significant greenhouse effect.

Most of the CO2 is in the ocean�

True, and not contested, but irrelevant. The oceans contain about 95% of the natural CO2 in the ocean/atmosphere/biosphere system, and about 90% of the man-made CO2 will also end up in the oceans eventually (i.e. in about one thousand years time). [Note: Carl Wunsch explained this correctly in the programme, but is very unhappy about the context in which his contribution was presented, see http://comment.independent.co.uk/commentators/article2359057.ece. The atmosphere normally contains only about 2% of the total CO2 (the rest is in the terrestrial vegetation and soils).

� so the (small) amounts released by burning fossil fuels are irrelevant

False. The amount released has been sufficient to increase atmospheric concentrations by about 35% (see above) even though about half of it has actually already been absorbed by the oceans and the terrestrial biosphere. It is CO2 left in the atmosphere that really matters, because that is where it causes the greenhouse effect, and whether or not there is also a large amount in the ocean is irrelevant.

The oceans release CO2 as they get warmer�

True, and not contested. The solubility of CO2 in water decreases by about 4% per �C, so the atmospheric CO2 concentration in equilibrium with seawater would increase by the same amount, per �C. [Note: Carl Wunsch also explained this correctly in the programme, but it was also misleadingly placed out of context, see the web-link above]. This process leads to a fairly small positive feedback (and hence amplification) of any global warming processes.

� and this is sufficient to explain the natural glacial-interglacial variations of atmospheric CO2 observed in the ice-core record>

False. Correlation does not necessarily imply causation (although it may provide support for a quantitative and plausible mechanism of the causal linkage). However, the glacial-interglacial temperature difference is estimated to have been only about 5 �C, so this effect would only explain a 20% increase (5 times 4%) in CO2 during warm interglacial periods like the present one. The observed increase from the pre-industrial level was actually about 55% (280 ppm compared to 180 ppm) so this simple temperature effect is only about one third of the size needed to provide an adequate explanation of the increase.

In fact, we still do not have a full quantitative explanation of the very high correlation between CO2 and temperature during glacial-interglacial cycles. The estimated greenhouse effect due to the change of CO2 (i.e. assuming that this alone might have caused the warming) is also only about half of that required to explain the temperature change. Thus although there are known effects by which temperature raises CO2 and vice versa, in a mutual closed loop relationship, the estimated magnitudes of the effects are too small (by a factor of two or three in both directions) for this to be the whole story. This is an outstanding �greenhouse puzzle� and the subject of intensive current research.

The time-lag between the rise of CO2 and temperature during the last de-glaciation shows that increased CO2 is a consequence, not the cause of increased temperature

Debatable, and probably false. As explained above, variations of CO2 and temperature are expected to be mutually re-inforcing, in a positive feedback loop (together with some as yet unknown additional processes). The timing (phasing) of changes in such a system may provide important information about how the system works. However, the timing of changes in CO2 (which is measured in the gas bubbles) and of temperature (measured in the ice) in ice core data must be treated with caution, because there is an offset in their ages. This arises because the air bubbles do not close off immediately after deposition, but only as the snow becomes sufficiently compacted. A timescale correction for this effect (which causes the air to be �younger� than the ice) needs to be applied, and this has to be estimated from models, and is not precisely known. Any error in this correction may cause the timing information to be unreliable.

Water vapour is by far the most important greenhouse gas�

True, and not contested, but irrelevant. The natural greenhouse warming due to the atmosphere of the Earth (about 33 �C, corresponding to a major radiative forcing of about 150 Wm-2) is mainly due to water vapour. Because the water vapour content increases with temperature, this also causes a positive feedback, which enhances the effect of any extra radiative forcing by about 35%.

� which is ignored by climate models�

False. The effects of water vapour (including the positive feedback effect) are included in all serious climate models, but are often �buried� deep inside the radiation codes which calculate the effects of greenhouse gases (where they belong) and are thus not immediately apparent to the casual observer. The forthcoming report of IPCC WG1 will contain a detailed analysis of water vapour effects and feedbacks in current models. [Note: in simpler models the effects are also often dealt with implicitly within the formulae used for out-going long-wave radiation (OLWR) calculations, and thus are similarly not easily visible].

� and the effects of any additional greenhouse gases must be trivial

False. The greenhouse effect is not a simple one-off shielding process, because any outgoing radiation absorbed also warms the air, and some is therefore re-emitted (potentially to be re-absorbed and re-emitted many times). When this is taken into account (see e.g. Houghton 1997) one finds that any additional absorption leads to additional warming. The effect never �saturates�, and the warming effect increases progressively as the greenhouse gas concentration is increased. Thus the presence of a large naturally occurring effect does not preclude an additional anthropogenic effect.

The mid-troposphere has not warmed as much as the Earth�s surface, in conflict with model expectations for the greenhouse effect.

This argument was in the past based on satellite observations, and has been extensively discussed � see e.g. http://www.realclimate.org/index.php/archives/2005/11/more-satellite-stuff/. It is now known that the satellite observations were systematically biased, and they have now been corrected, eliminating the discrepancy. The discussion now seems to have been resurrected in terms of radio-sonde data. However, it is well-known that radio-sonde sensors are not high-precision instruments, and are not ideal for long-term monitoring, and the data from them need to be handled very carefully indeed. The most recent analyses by the IPCC show no discrepancies greater than the observational errors.

There is no evidence that CO2 causes climate change�

Correlation does not necessarily imply causation, so it is true that there can never be absolute proof that it does. However, in this case there is a quantitative and plausible mechanism to support the interpretation of the correlation as cause and effect. Indeed, if we were deliberately injecting CO2 into the atmosphere as an experiment, there is little doubt that we would by now have concluded that it was causing warming (and we would probably have called off the experiment as being too dangerous).

� but solar forcing is a credible explanation for past and present changes

The assertion by Piers Corbyn & others that solar forcing is the dominant factor controlling climate is based on very dubious sources and is not supported by any reliable observations. The issue has been discussed at length on realclimate.org � see http://www.realclimate.org/index.php/archives/2006/10/taking-cosmic-rays-for-a-spin/ but in summary

� The assertion by Corbyn that solar forcing is a good predictor of climate (and weather) has not been published in any form and cannot therefore be evaluated by anyone else. It remains an unsubstantiated assertion.

� The magnitude of the variation of solar radiation over the solar cycle is known and is small (it is about 0.1%, which is about ten times smaller than the forcing due to doubling CO2). This is much too small to cause significant variation of Earth�s surface temperature directly, or by any known or plausible mechanism.

� Solar activity has not increased since 1950, and so cannot explain the exceptional warming trend of the last two decades. http://www.realclimate.org/index.php/archives/2005/08/did-the-sun-hit-record-highs-over-the-last-few-decades/

� The Friis-Christensen correlation is between the length (duration) of the solar cycle and temperature. Correlation does not necessarily imply causation, there is no known or plausible mechanism relating cycle duration to climate, and the predictive ability of the correlation is untested. See http://www.realclimate.org/index.php/archives/2005/07/the-lure-of-solar-forcing/#more-171

� The solar activity/cosmic ray/cloud condensation mechanism proposed by Svensmark & others, and the possible role of galactic cosmic rays is strongly disputed � see http://www.realclimate.org/index.php?p=153

� The possible role of solar forcing has been considered in detail by the IPCC, which has concluded that it is probably less than one tenth of that due to the combined effect of CO2 and aerosols (WG1 Summary for Policymakers)

The scientific community has colluded with Margaret Thatcher, George W. Bush, Al Gore (and even renegade ex-Stasi officers) to promote a global warming swindle

This claim is so fatuous and far-fetched that no further comment is necessary. Its level of credibility may however perhaps serve as a guide to the soundness to be expected of the arguments made elsewhere in the programme.

References and additional resources

The realclimate site is an excellent source of well-informed comment on climate change issues. It now has an extended discussion thread on the C4 programme, see http://www.realclimate.org/index.php/archives/2007/03/swindled/#more-414

The Intergovernmental Panel on Climate Change (IPCC)

IPCC (2007) 4th Assessment Report, WG1 Summary for Policymakers

The Royal Society �Guide to facts and fictions about climate change�

For more general scientific background:

Houghton, J. T. (1997). The Physics of Atmospheres, second edition, CUP.

Maslin, M. (2004). Global warming: a very short introduction. Oxford, OUP.

see also some basic Climate Dynamics lectures (Nos. 1 to 4) at www.jgshepherd.com/lectures/index.htm