Select Committee on Economic Affairs Second Report

CHAPTER 2: The Uncertain Science of Climate change

The greenhouse effect

5.  Our dominant concern is with certain aspects of the economics of climate change, but clearly, any investigation must begin with the underlying science.

6.  The Earth's surface is warmed by the sun. This incoming solar radiation is fairly constant—it does not vary with time. The Earth's temperature is controlled by the balancing between this incoming short-wave radiation, which warms the Earth, and the loss of this energy as it is bounced back into space. The re-radiated energy cools the Earth. Energy-out balances energy-in, and the Earth maintains a constant global temperature. Without this balance, the Earth would become steadily hotter and life would cease. Of the incoming solar radiation, roughly 30% bounces back into space from clouds, atmospheric aerosols and bright, reflective areas of the Earth's surface, such as deserts. That leaves 70% of the incoming radiation to be absorbed, mostly by land areas and the oceans. But even this 70% cannot stay permanently absorbed, otherwise the Earth would again continually warm up and life would not be possible. It is re-emitted primarily as long-wave, infra-red radiation back into space. But some of this re-radiated energy is absorbed by water vapour and by "greenhouse gases" which exist in the atmosphere. The principal greenhouse gas is carbon dioxide, but the principal absorbing agent is overwhelmingly water vapour. The effect of this absorption of the re-radiated energy is to produce another round of re-radiation, this time back to the Earth's surface, where it is absorbed once again. This is the "greenhouse effect". This re-absorption process is natural: it is what maintains the Earth's average temperature at +15oC rather than at levels below freezing[4].


The greenhouse gases

The main greenhouse gases are carbon dioxide (CO2) which is emitted by the use of fossil fuels and by the burning of forests; methane (CH4) which comes from decaying degradable matter, e.g. in landfill sites, and from livestock; nitrous oxides (N2O) from fertilisers, industrial processes, and fossil fuel burning; and a group of other gases, such as perfluoromethane (CF4) and perfluoroethane (C2F6) used in aluminium production, and sulphur hexafluoride (SF6) from dielectric fluids. Other gases, such as carbon monoxide (CO) and nitrogen oxides (NOX), have indirect effects on greenhouse warming through various chemical reactions.

The power of the main greenhouse gases to "force" temperature rises varies substantially. The conventional way of expressing these forcings is the "Global Warming Potential" (GWP). The GWP for carbon dioxide is set equal to 1. Then the other forcings are as follows:

Carbon dioxide   = 1

Methane     = 23

Nitrous oxide     = 296

Hydroflurocarbons  = 12 to 12000 depending on the gas

Perfluorocarbons  = 5000 to 12000

Sulphur hexafluoride  = 22200

However, CO2 remains the most important gas because of the quantities in which it is emitted.

7.  It is not this natural greenhouse effect that gives rise to concern. It is the fact that the relatively short period in the world's history since the Industrial Revolution has seen significant increases in the emissions of the greenhouse gases, especially carbon dioxide and methane. These greenhouse gases add to the concentrations already in the atmosphere. Moreover, they accumulate and stay in the atmosphere for decades (their "atmospheric residence time"). While they get generally mixed in the atmosphere, it is common in pictorial terms to show these increased concentrations as a "blanket" that traps the outgoing long-wave radiation and returns it to Earth. It is this accelerated or enhanced greenhouse effect that causes the concern, since the effect is to warm the Earth's surface even more than the level achieved naturally. In effect, what is happening is that the greenhouse gases are upsetting the natural energy balance in such a way that "something has to give" to restore the balance, and it is surface warming that is bringing about the adjustment.

8.  Evidence from Antarctic ice cores suggests that atmospheric concentrations of CO2 were fairly constant over 1000 years until the Industrial Revolution[5]. In the year 1000 (measured by ice core samples), concentrations were 280 ppm (parts per million), and the concentrations were the same around 1800, whereas today they are some 375 ppm. Currently, concentrations are growing at some 1.5 ppm every year, as recorded by the Mauna Loa observatory in Hawaii, which has been monitoring concentrations since 1959. A similar picture, but with more variability over time, emerges for N2O, at around 270 ppb (parts per billion) between 1000 and 1700, rising to 310 ppb in 2000. Methane, CH4, is also fairly constant between 1000 and 1750 at 750 ppb, rising to over 1600 ppb in 2000[6]. Longer historical records from ice cores also suggest that carbon dioxide and methane concentrations are now at their highest levels for the past 400,000 years[7].

Negative forcing

9.  Not all greenhouse gases—gases that contribute in some way to the enhanced greenhouse effect—create "positive forcing", i.e. warm the atmosphere[8]. Some have a cooling effect. Aerosols—tiny particles of liquid or dust in the atmosphere, such as soot, volcanic ash and dust—give rise to cooling effects. Clouds can have a cooling effect as well, reflecting radiation back into space. The level of understanding of the behaviour of clouds and aerosols is unfortunately far less than the level of understanding for the main warming greenhouse gases. An important cooling aerosol is sulphate which comes from sulphur dioxide (when mixed with oxygen), which in turn comes from sulphur-bearing fossil fuels such as coal. These sulphate aerosols reflect sunlight and hence produce a cooling or "dimming" effect. In the rich world, substantial controls exist over sulphur emissions because of damage caused by local air pollution and transboundary acid rain. As a result, sulphur emissions are declining. But in the poorer world, there are still considerable pressures to burn fuels such as coal and lignite, and sulphur emissions are rising. The scenarios of future warming therefore depend in part on what happens to this balance of sulphur emissions.


The basic linkages in climate change

Emissions of greenhouse gases (GHGs) cumulate in the atmosphere because the rate at which they diffuse in the atmosphere exceeds the rate at which they decay naturally, allowing also for the fact that they reside for various "lifetimes" in the atmosphere. Thus atmospheric concentrations of GHGs have risen steadily over time. In turn, because of the accelerated greenhouse effects, the increasing concentrations translate into radiative forcing which raises the mean surface temperature of the Earth. The exact relationships between emissions, concentrations, forcing and temperature change are not known with certainty. The diagrams below show a stylised picture of a situation in which global annual CO2 emissions (the main GHG) stay constant at current levels for the next 300 years (the dashed lines), and an alternative scenario in which emissions grow from now until 2050 and then decline dramatically (the continuous line). Panel (a) shows the emissions trajectories. Panel (b) shows the resulting atmospheric concentrations, and panel (c) shows the resulting temperature profiles. While the dashed line is unrealistic—it assumes immediate cessation of the growth in CO2 emissions—it serves to show that such a cessation would still result in steadily rising temperatures over the next few hundred years, illustrating the time lags and non-linearities in the climate system. The continuous line is consistent with radical action now, but emissions would nonetheless continue to rise for around 50 years, after which actions taken now and in the near future would dramatically cut emissions. The radical scenario achieves a 550 ppm concentration target by around 2100, a target that is widely being regarded as the long-term goal that might realistically be achieved. Again because of the lags in the system, stabilisation at this level in 2100 still results in rising temperatures thereafter, but temperature is stabilised at around +2.5oC in 2300. In practice, CO2 emissions are still rising, although they are currently rising at a decreasing rate. The dashed line is therefore increasing rather than staying constant, underlining what many climate scientists regard as the urgency of early action.

Temperature change

10.  Box 3 shows that the growth of emissions of greenhouse gases is linked to global temperature changes after some considerable time-lags. Since greenhouse gas emissions, especially carbon dioxide, increased with the onset of the Industrial Revolution, current temperatures should have responded to these past emissions. The recent historical record of global temperature change is not disputed. Instrumental records (using thermometers) for temperatures in the Earth's Northern Hemisphere do not really begin until around 1860[9]. "Global" (i.e. Northern Hemisphere) average temperatures show marked fluctuations around a rising trend[10]. The (approximate) observed cycles are upwards for 1860 to 1875, downwards to 1890, up to 1900, down to 1915, up to 1942, down to 1970 and upwards since then. A more "smoothed" series would suggest a reasonably constant temperature to 1920, upwards to around 1940, downwards to 1970 and upwards since then. Since 1860, the mean temperature change has been around 0.6oC.

11.  Any test of the link between temperature change and greenhouse gases must therefore account for these cycles. Mathematical models that try to explain temperature change are known as general circulation models (GCMs). These models attempt to mimic the forces at work that change the Earth's climate. If they can "explain the past", then they can be used to predict the future, assuming we have a reasonable idea of how the various determining factors (e.g. the greenhouse gases themselves) will behave in the future. GCMs tend to be very complex and have to run on powerful computers.

Scientific consensus and scientific doubt

12.  Testing the validity of climate models is obviously difficult. In so far as the models predict climate change, the predictions can easily be in error and only the passage of time can validate the predictions. But if the science of climate change as embodied in IPCC reports is correct, the option of "waiting and seeing" may be risky because of the manner in which current emissions of greenhouse gases add to the stock of gases in the atmosphere. Once cumulated, the decay processes are very long term and hence the gases cannot be "decumulated" in short periods of time. Other tests are therefore needed. These tend to comprise (a) ensuring the internal consistency of the models—i.e. the extent to which they are consistent with received theory, and (b) the extent to which they "predict the past". In case (b) two historical tests are used. The first looks at the detailed temperature record since the mid-19th century, when instrumental records become widespread, and the second looks at the extremely long run record embodied in ice cores, tree rings and other "proxy" data going back hundreds of thousands of years.


The main IPCC publications
YearWorking Group 1 Working Group 2 Working Group 3 Other
1991Volume 1 The IPCC Climate Assessment Volume 2 The IPCC Impacts Assessment Volume 3 The IPCC Response Assessment Emission Scenarios
1992Supplementary Report to the Scientific Assessment Supplementary Report to the Impacts Assessment Climate Change: The IPCC 1990 and 1992 Assessments
1995 Climate Change 1994 - Radiative Forcing of Climate Change and the Evaluation of the IS92 Emission Scenarios
1996Climate change 1995 - The Science of Climate Change Climate Change 1995 - Impacts, Adaptations and Mitigation of Climate Change Climate Change 1995 - Economic and Social Dimensions of Climate Change Climate Change 1995 - IPCC Second Assessment Synthesis of Scientific-Technical Information
2000 Emission Scenarios - IPCC Special Report
2002Climate Change - The Scientific Basis Climate Change 2001 - Impacts, Adaptation and Vulnerability Climate Change 2001 - Mitigation Climate Change 2001 - Synthesis Report

13.  The Committee heard from several scientific witnesses on the theory. No one disputes the fact of temperature rise in the last 100 years or so. No one disputes that carbon dioxide is a greenhouse gas and few dispute that it has an enhanced "greenhouse effect". What is disputed, albeit by a minority of scientists, is the scale of this effect. In the view of Professor Richard Lindzen of MIT, current climate models would have predicted a substantially greater increase in the past temperature than has been observed in the past 150 years, perhaps +3oC compared to the +0.6oC we have witnessed. In his view, this suggests that the models are biased upwards and that, while warming will occur, it is the lower end of the IPCC spectrum that is relevant, not the upper limits, which he regarded as "alarmist"[11]. Our understanding of the scientific response to this apparent anomaly is that (a) cooling effects, including those from sulphates, have masked the expected rise in warming, and (b) only climate models that combine natural variability and anthropogenic forcings "fit" the past data[12], as outlined in paragraph 15.

14.  We recognise that there is a strong majority view on climate change. Majorities do not necessarily embody the truth, but we note that major associations of scientists have adopted similar positions. The IPCC tends to be the focus of the majority view which has been confirmed by the Royal Society[13], and by the US National Academy of Sciences, the American Meteorological Society, the American Geophysical Union and the American Association for the Advancement of Science. Despite this, it is a concern that the IPCC has not always sought to ensure that dissenting voices are given a full hearing. We document these concerns later in the Report.

15.  As far as the recent temperature record is concerned, as noted above, the temperature record is not one of consistent warming. Indeed, there was a distinct "cooling period" in the 1960s and 1970s—see Box 5. The conventional explanation of this phenomenon is, first, that this period was associated with substantial sulphur emissions in North America and Europe, with sulphates having a cooling effect. As sulphur emissions came to be controlled, the underlying upward trend in warming resumed. Second, there was a natural variation in temperature in this cooling period due to changed sunspot activity. The IPCC is clear that GCMs that contain only anthropogenic temperature forcing predict more temperature change than has been observed in the 20th century. It claims that GCMs that embody only natural variation understate the temperature rise of the past 30 years or so. Only when anthropogenic and natural forcings are combined is the temperature record accurately simulated[14].


Recent temperature change

It is customary to show temperature in terms of deviations from the average of 1961-1990 temperatures. "Decomposing" the chart into approximate time periods, there is roughly a 0.6oC increase from 1860 to 2000. There appears to be no trend increase or decrease, i.e. temperature is fairly constant, in the period 1860 to 1920. There is continuous warming from 1920 to 1945, followed by a period of "cooling" from 1945 to 1965, in turn followed by continuous warming from 1965 to the present.

16.  In his evidence to us, Sir David King drew attention to recent research which, it is claimed, shows that changes in ocean temperatures have been accurately predicted by the GCMs, further validating the models[15].

17.  Apart from the issue of explaining the divergence between actual and expected recent past warming, we heard doubts expressed about other features of the accepted science. These include:

  • concerns that changes in ice-core record CO2 concentrations might have followed temperature rise rather than the other way round;
  • the poor nature of the data used to compute the long run historical record, or alleged misinterpretation of the long-run historical temperature record;
  • the GCMs fail to "reconstruct" the long term historical record;
  • the view of some that the relative importance of the natural factors affecting climate variability, e.g. variation in solar output, is underplayed in the IPCC assessments;
  • apparent divergences between land-based temperature records and satellite-based measurements, the latter showing some cooling rather than warming in recent years;
  • the manner in which the GCMs are adjusted until they align with the observed data;
  • the uncertain role of cloud cover. Professor Lindzen argued that clouds generate a negative feedback effect (cooling) rather than the positive feedback effect assumed in the GCMs; and that
  • the models fail to predict sudden weather events.

18.  We do not propose to evaluate these doubts, nor are we qualified to do so. We are also aware that climate scientists who adhere to the human-induced warming hypothesis have responses to most of these sources of doubt[16]. But the science of climate change remains debatable. We heard from witnesses who seemed in no doubt at all about the science, while others expressed one or more of the above concerns. That makes it clear that the scientific context is one of uncertainty, although as the science progresses these uncertainties might be expected to diminish and be resolved, one way or the other. Hence it is important that the Government continues to take a leading role in supporting climate science, and encourages a dispassionate evidence-based approach to debate and decision making.

19.  In terms of policy on climate mitigation and adaptation, the issue becomes one of how to behave in the face of uncertainty. Given this uncertainty, the effective irreversibility of climate change, and the potential for large-scale damage, a precautionary approach is called for. But precaution cannot be the right option at any cost. We return to this issue later.

A note on the "hockey stick" debate

20.  While we have not entered into the scientific debate in any detail, we received a significant amount of evidence on the so-called "hockey stick" debate and hence feel we should comment on this issue.

21.  The hockey stick refers to the shape of the long-run time series curve of temperature change—see Box 6. This appears to show gently declining global (actually Northern Hemisphere) temperature from at least 1000 until about 1700, with a rise from then until the present. Most importantly, the recent past shows a sharp upturn such that the later part of the 20th century is warmer than any previous period. Thus the series resembles a hockey stick with the blade facing upwards—see Box 6. We noted earlier that a similar graph is suggested for carbon dioxide concentrations. The importance of the hockey stick shape is that the upturns in both temperature and CO2 coincide and both are relatively recent phenomena, i.e. in the last 150 years or so. The hockey stick thus appears to be persuasive visual evidence that the recent temperature change is human-induced.


The "hockey stick"

According to the hockey stick literature, the time-profile of temperature in the Northern Hemisphere has the shape shown in the stylised diagram below. Temperatures before the mid-19th century tend to be inferred from ice-core and tree-ring data ("proxy" data). The implication is that natural climate variability has not generated temperature variations that compare with the temperature change witnessed since the onset of industrial times. Hence the temperature change of the last few centuries must be due to human-induced factors. The historical evidence is debated. Several studies have found fairly long periods in the last millennium in which variations of up to +1oC may have occurred[17]. In their evidence to us, the Royal Society drew attention to these papers but argued that natural variation alone cannot explain recent warming[18]. In a separate critique, Dr McIntyre and Professor McKitrick of Canada argue that one of the prominent hockey stick series is consistent with marked increases in temperatures between 1400 and 1500.

Source: P. Jones and M. Mann, Climate over past millennia. Reviews of Geophysics, 2004.
42: 1-42.

22.  Some critics argue that the experience of the last few hundred years is too short a period for the climate models to determine the balance of natural and man-made factors in temperature change. This is why considerable attention has been paid to the longer run temperatures and the "hockey stick". One attempt to reconstruct a long-term temperature record is that of Professor Michael Mann of the University of Virginia[19]. The picture that emerged for the period 1000-1980 is very much the hockey stick shape. The Mann hockey stick appeared in the IPCC Climate Change Assessment of 2001, thus achieving, as one journalist put it, "iconic status"[20]. In an analysis of Mann's et al data, Dr Stephen McIntyre and Professor Ross McKitrick of the University of Guelph in Canada claim that the analysis involves "collation errors, unjustifiable truncation or extrapolation of source data, obsolete data, geographical location errors" and other defects[21]. Their reconstructed series shows close correlation with Mann's series from 1550 to 1980 but shows temperatures higher between 1400 and 1500 than any of the 20th century temperatures. If correct, the late 20th century is no longer historically unprecedented and the "hockey stick" does not exist. We sought evidence that refuted the claims of McIntyre and McKitrick, but have not come across any detailed rebuttal. One curious feature of the debate over Professor Mann's time series is that the critics appear to ignore other studies which secure similar hockey stick pictures[22].

23.  We are in no position to determine who is right and who is wrong in the growing debate on the hockey stick. If there are historical periods of marked temperature increase, it seems to us it is important to know why these occurred. Overall, we can only urge that the issue is pursued in the next IPCC Assessment.

On past scares

24.  Some of our witnesses drew attention to previous environmental and resource exhaustion scares. The implication is that since these scares did not materialise, neither might accelerated global warming. While forecasters do seem to indulge periodically in "end of the world" stories, there is no guarantee that if they were wrong before they will be wrong again. More importantly, the science of global warming has advanced following substantial expenditures on research. Previous alarms, such as the 1970's Limits to Growth debates (which have not, in any event, gone away), earlier fears of global cooling (rather than warming), and even the fear in the 19th century over exhaustion of coal supplies, were based on more limited scientific investigation. We do not believe that today's scientists are "crying wolf": they may turn out to have been wrong in some respects, but the arguments on which they base their case are better researched than in earlier cases. That said, this Chapter has sought to highlight some pressing issues which we believe deserve a further response from the scientific community in order to enhance understanding and resolve current controversies.

4   The effect is actually to warm the Earth by around 35oC, i.e. to +15oC rather than the approximate -20oC that would otherwise prevail. Back

5   Sir John Houghton, Global Warming: the Complete Briefing. Cambridge University Press. 3rd Edition. 2005, p32. Ice cores can also be used to construct temperature and CO2 records going back over 400,000 years (and, most recently, cores have been extracted that go back 900,000 years). As snow fell, the air in the snow became trapped in the ice that subsequently formed, so that greenhouse gas concentrations in the trapped air bubbles can be measured. This gives the CO2 record for the whole period. Examination of the oxygen and hydrogen isotopes in the ice core also permits temperature readings. The two time-series-temperature and CO2-appear to show a very close correlation, suggesting that the two are closely linked. Ice ages had low levels of CO2 (about 210 ppm) and warm periods had high levels of CO2 (around 270 ppm). See J. Petit et al. Climate and atmospheric history in the past 420,000 years from the Vostok ice core in Antarctica. Nature, 399, June 3, 429-436, 1999. While correlation is not causation, and there remains some dispute over the nature of the linkage, there is also evidence that CO2 concentrations "lead" temperature rather than the other way round. See M. Maslin, Global Warming: A Very Short Introduction. Oxford University Press, 2004, p.60. Back

6   R. Watson et al., op.cit. p.47.  Back

7   J. Weier, Global Warming, Earth Observatory, NASA, Washington DC. 8 April 2002. Back

8   "Radiative forcing" refers to the amount of energy trapped by the atmosphere and is measured in watts per metre squared (Wm-2). Back

9   A unique series exists for Central England from 1659 and can be accessed at .  Back

10   It is important to understand how temperature changes are computed and portrayed. Temperature can obviously be measured daily and even hourly, so there are huge numbers of observations from the instrumental record. These are made more manageable by a process of averaging through time. A "moving average" of, say, 5 years, would take the average over the first 5 years 1 to 5, then the average of the 5 year period from years 2 to 6, and so on. The larger the averaging period, say 50 years instead of 5 years, the "smoother" the resulting trend line becomes. Turning points in this moving average therefore tend to change with the averaging period. In the climate science literature the difference between this moving average trend line and the actual temperature is known as an "anomaly". To test whether temperature and a greenhouse gas like CO2 are correlated, it is the anomalies in temperature that are compared to CO2 concentrations. This allows the correlation not to be unduly influenced by the time trends in the series. Back

11   Evidence from R. Lindzen (Vol II, pp 44-55) Back

12   On (a) see Sir John Houghton, Global Warming: The Complete Briefing. Cambridge University Press, 2005. p.103. On (b) see R. Watson et al., op.cit., p.198. Back

13   Evidence from the Royal Society (Vol II, pp 293-306) Back

14   R. Watson et al., op.cit. p.198 Back

15   Evidence from Sir D. King (Vol II, pp 96-106) Back

16   An excellent description of most of these debates is to be found in M. Maslin, Global Warming: A Very Short Introduction. Oxford: Oxford University Press, 2004. Back

17   H. von Storch et al. Reconstructing past climate from noisy data. Science. 2004.306:679-682; A. Moberg et al. Highly variable Northern Hemisphere temperatures reconstructed from low-and-high resolution proxy data. Nature. 2005. 433: 613-7.  Back

18   Evidence from the Royal Society (Vol II, pp 293-306). Back

19   M. Mann, R. Bradley and M. Hughes. Global-scale temperature patterns and climate forcing over the past six centuries. Nature. 392, 1998. 779-787. 1999. M. Mann, R. Bradley and M. Hughes. Northern hemisphere temperatures during the past millennium: inferences, uncertainties and limitations. Geophysical Research Letters. 26. 1999. 759-762. M. Mann, R. Bradley and M. Hughes. Global-scale temperature patterns and climate forcing over the past six centuries: Corrigendum. Nature. 430. 2004. 105. The 1998 paper by Mann et al. is for the period 1400-1980. The 1999 paper expands the historical coverage back to 1000. Back

20   D. Appell. Behind the hockey stick. Scientific American. March 2005. Back

21   Evidence from R. McKitrick (Vol II, pp 262-266). See also S. McIntyre and R. McKitrick. Corrections to the Manne et al. (1998) proxy data base and Northern Hemisphere average temperature series. Energy and Environment. 14. 6.2003. 751-771. S. McIntyre and R. McKitrick. The IPCC, the Hockey Stick Curve and the Illusion of Experience. Washington DC: The George C Marshall Institute. S. McIntyre and R. McKitrick. Verification of multi-proxy paleoclimatic studies: a case study. Accepted Abstract. American Geophysical Union Meetings, Paper PP53A-1580, December 2004.  Back

22   K. Briffa et al. Low frequency temperature variations from a northern tree ring density network. Journal of Geophysical Research, 106, (D3), 2001, 2929-41. Back

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