The Geological Perspective On Global Warming: A Debate

  • Date: 14/02/13
  • Dr Colin P. Summerhayes, Professor Robert Carter and Professor Vincent Courtillot

Dr Colin P.  Summerhayes, Vice-President of the Geological Society of London

Dear Dr Peiser,

In the interest of contributing to the evidence-based debate on climate change I thought it would be constructive to draw to your attention the geological evidence regarding climate change, and what it means for the future. This evidence was published in November 2010 by the Geological Society of London in a document entitled “Climate Change: Evidence from the Geological Record”, which can be found on the Society’s web page.

A variety of techniques is now available to document past levels of CO2 in the atmosphere, past global temperatures, past sea levels, and past levels of acidity in the ocean. What the record shows is this. The Earth’s climate has been cooling for the past 50 million years from 6-7°C above today’s global average temperatures to what we see now. That cooling led to the formation of ice caps on Antarctica 34 million years ago and in the northern hemisphere around 2.6 million years ago. The cooling was directly associated with a decline in the amount of CO2 in the atmosphere. In effect we moved from a warm “greenhouse climate” when CO2, temperature and sea level were high, and there were no ice caps, to an “icehouse climate” in which CO2, temperature and sea level are low, and there are ice caps. The driver of that change is the balance between the emission of CO2 into the atmosphere from volcanoes, and the mopping up of CO2 from the atmosphere by the weathering of rocks, especially in mountains. There was more volcanic activity in the past and there are more mountains now.

Superimposed on this broad decline in CO2 and temperature are certain events. Around 55 million years ago there was a massive additional input of carbon into the atmosphere – about 4 times what humans have put there. It caused temperatures to rise by a further 6°C globally and 10°C at the poles. Sea level rose by some 15 metres. Deep ocean bottom waters became acid enough to dissolve carbonate sediments and kill off calcareous bottom dwelling organisms. It took over 100,000 years for the Earth to recover from this event. More recently, during the Pliocene, around 3 million years ago, CO2 rose to levels a little higher than today’s, global temperature rose to 2-3°C above today’s level, Antarctica’s Ross Ice Shelf melted, and sea level rose by 10-25 metres.

The icehouse climate that characterised the past 2.6 million years averaged 9°C colder in the polar regions and 5°C colder globally. It was punctuated by short warm interglacial periods. We are living in one of these warm periods now – the Holocene – which started around 11,000 years ago. The glacial to interglacial variations are responses to slight changes in solar energy meeting the Earth’s surface with changes in: our planet’s orbit from circular to elliptical and back; the position of the Earth relative to the sun around the Earth’s orbit; and the tilt of the Earth’s axis. These changes recur on time scales of tens to hundreds of thousands of years. CO2 plays a key role in these changes. As the Earth begins to warm after a cold period, sea ice melts allowing CO2 to emerge from the ocean into the atmosphere. There it acts to further warm the planet through a process known as positive feedback. The same goes for another greenhouse gas, methane, which is given off from wetlands that grow as the world warms. As a result the Earth moves much more rapidly from cold to warm than it does from warm to cold. We are currently in a cooling phase of this cycle, so the Earth should be cooling slightly. Evidently it is not.

The Geological Society deduced that by adding CO2 to the atmosphere as we are now doing, we would be likely to replicate the conditions of those past times when natural emissions of CO2 warmed the world, melted ice in the polar regions, and caused sea level to rise and the oceans to become more acid. The numerical models of the climate system that are used by the meteorological community to predict the future give much the same result by considering modern climate variation alone. Thus we arrive at the same solution by two entirely independent methods. Under the circumstances the Society concluded that “emitting further large amounts of CO2 into the atmosphere over time is likely to be unwise, uncomfortable though that fact may be.”

Yours Sincerely,

Dr Colin P.  Summerhayes

Vice-President Geological Society of London and Emeritus Associate Scott Polar Research Institute, Cambridge.

8 February 2013



Professor Robert Carter and Professor Vincent Courtillot respond:

Dear Dr Peiser,

Thank you for your invitation on behalf of the Foundation to reply to Dr Summerhayes’ letter about geological evidence in relation to the hypothesis of dangerous anthropogenic global warming (DAGW) that is favoured by the Intergovernmental Panel on Climate Change (IPCC).

We are in agreement with many of Dr Summerhayes’ preliminary remarks about the geological context of climate change. This reflects that a large measure of scientific agreement and shared interpretation exists amongst most scientists who consider the global warming issue.

Points of commonality in the climate discussion include:

* that climate has always changed and always will,

* that Earth has often been warmer than it is today, and that its present climatic condition is that of a warm interglacial during a punctuated icehouse world,

* that carbon dioxide is a greenhouse gas and warms the lower atmosphere (though debate remains as to the magnitude and timescale of the warming),

* that a portion of human emissions are accumulating in the atmosphere,

* that a global warming of around 0.5°C occurred in the 20th century, but that there has been no global temperature rise over the last 16 years.

The first two points are rooted in geological evidence (as discussed in more detail by Dr Summerhayes), the third is based upon physical principle and the last three are mostly matters of instrumental measurement (i.e. observation). Despite the disparate scientific disciplines involved, all these points are relevant to achieving a quantitative understanding of climate change, together with several other disputed scientific matters such as those that we discuss below.

One of the disputed scientific matters is represented by Dr Summerhayes’ assertion that cooling over the last 34 million years “was directly associated with a decline in the amount of CO2 in the atmosphere”.

The word “associated” is ambiguous. It may simply mean that temperature and CO2 were correlated, in the sense that their trends were parallel. But as everyone knows correlation is not causality and whether one drives the other, or the two are driven by a third forcing factor, or the correlation is the result of chance, requires careful analysis and argument.  Though it may be true that a broad correlation exists between atmospheric CO2 content and global temperature, at least on some timescales, it remains unclear whether the primary effect is one of increasing CO2 causing warming (via the greenhouse effect) or of warming causing CO2 increase (via outgassing from the ocean). We are familiar with the argument that the currently decreasing carbon isotope ratio in the atmosphere is consistent with a fossil fuel source for incremental CO2 increases, and therefore with the first of these two possibilities, but do not find it compelling because other natural sources (soil carbon, vegetation) also contribute isotopically negative carbon to the atmosphere.

A second area of uncertainty, related to the point just discussed, is the rate, scope and direction of the various feedbacks that apply during a natural glacial-interglacial climatic cycle. Dr Summerhayes provides a confident, and perhaps plausible, account as to how changing insolation (controlled by orbital change), melting sea-ice and increasing CO2 and CH4 jointly drive the asymmetrical glacial-interglacial cycles that have characterised recent planetary history. However, our knowledge of the climate system and its history currently remains incomplete; some of the forcing mechanisms and feedbacks may not be known accurately, or even at all. For example, we do not yet know whether clouds exert a net warming or cooling effect on the climate. Similarly, variations in ultraviolet radiation and high-energy particle emission from the Sun, in atmospheric electricity and in galactic cosmic rays may all play larger roles in controlling climate change than is currently assumed, yet these effects are absent from most of the current generation of deterministic computer models of the future climate. The temperature projections made by these models may well be affected by our ignorance of the magnitude, the sign, or even the existence of some of the forcings and feedbacks that are actually involved.

Thirdly, Dr Summerhayes also briefly discusses the issue of sea level change. He quotes an estimated increase of 15 m in sea level associated with a temperature increase of 6–10°C 55 million years ago. He then quotes a range of 10–25 m rise for a 2–3°C warming 3 million years ago. To this we might add the further examples of the 125 m sea level rise that has accompanied the 6°C temperature rise since the last glacial maximum, and the 0.2-m rise associated with the ~0.5°C 20th century warming. It appears from these examples that a 1°C temperature rise can be associated with a sea level rise of as little as 0.4 m or as much as 8 m, and all values in between! This indicates an uncertainty in our understanding of the temperature/CO2/sea-level connection that surely lessens its value for contributing to policy formulation.

Figure 1. Temperature curve reconstructed from oxygen isotope measurements in a Greenland ice core over the last 10,000 years (Lappi 2010 after Alley,2000).

Fourth, and last, Dr Summerhayes says that because orbitally-forced climate periodicity is currently in a cooling phase “the Earth should be cooling slightly. Evidently it is not”. The statement is tendentious, because whether Earth is seen to be cooling or warming depends upon the length of climate record that is considered. Trends over 1, 10, 100 or 1000 years are not the same thing, and their differences must be taken into account carefully. We reproduce two figures that may be used to demonstrate that Earth is currently not warming on either the longer-term millennial timescale (Figure 1) or the short-term decadal/meteorological timescale (Figure 2). We note also that on the intermediate centennial timescale (1850–2010) the temperature trend has been one of a slight (0.5°C) rise. In assessing which of these timescales is the “proper” one to consider in formulating climate policy, we observe that the results conveyed in Figure 2 have little scientific (and therefore policy) meaning unless they are assessed in the context of the data in Figure 1.

Figure 2. Mean temperature of lower atmosphere: HadCRUT4 annual means 1997-2011

We acknowledge that the data in Figure 1, which are drawn from a Greenland ice core, represent regional rather than global climate. But a similar pattern of Holocene long-term cooling is seen in many other records from around the world, including from Antarctic ice cores. Also, evidence for a millenial solar cycle has been accumulating over the past years, and, representing that rhythm, the Medieval Warming (also called Medieval Climatic Optimum) appears to have been both global and also warmer than today’s climate.

Regarding Figure 2, the data demonstrate that no warming has occurred since 1997. In response, some leading IPCC scientists have already acknowledged that should the temperature plateau continue, or turn into a statistically significant cooling trend, then the mainstream IPCC view will need revision. It is noteworthy, too, that over the 16 years during which global temperature has remained unchanged (1997-2012), atmospheric carbon dioxide levels have increased by 8%, from 364 ppm to c.394 ppm. Given a mixing time for the atmosphere of about 1 year, these data would invalidate the hypothesis that human-related carbon dioxide emissions are causing dangerous global warming. In any case, observed global temperatures are currently more remote than ever from the most recent predictions set out in IPCC AR4.

The areas of uncertainty in the prevailing argument over DAGW are therefore not only geological but also instrumental and physical. Current debate, which needs to be resolved before climate policy is set, centres on the following three issues:

* whether any definite evidence exists for dangerous warming of human causation over the last 50 years,

* the amount of net warming that is, or will be, produced by human-related emissions (the climate sensitivity issue), and

* whether the IPCC’s computer models can provide accurate climate predictions 100 years into the future.

In assessing these issues, our null hypothesis is that the global climate changes that have occurred over the last 150 years (and continue to occur today) are mainly natural in origin. As summarised in the reports of the Nongovernmental International Panel on Climate Change (NIPCC), literally thousands of papers published in refereed journals contain facts or writings consistent with this null hypothesis, and plausible natural explanations exist for all the post-1850 global climatic changes that have been described so far. In contrast, no direct evidence exists, and nor does the Geological Society point to any, that a measurable part of the mild late 20th century warming was definitely caused by human-related carbon dioxide emissions.

The possibility of human-caused global warming nonetheless remains, because carbon dioxide is indubitably a greenhouse gas. The major unknown is the actual value of climate sensitivity, i.e. the amount of temperature increase that would result from doubling the atmospheric concentration of CO2 compared to pre-industrial levels. IPCC models estimate that water vapour increases the 1°C effect that would be seen in a dry atmosphere to 2.5-4.5°C, whereas widely cited papers by Lindzen & Choi (2011) and Spencer & Braswell (2010) both describe empirical data that is consistent with negative feedback, i.e. sensitivity less than 1°C. The conclusion that climate sensitivity is significantly less than argued by the IPCC is also supported by a range of other empirical or semi-empirical studies (e.g., Forster & Gregory, 2006; Aldrin et al., 2012; Ring et al., 2012).


Gathering these various thoughts together, we conclude that the risk of occurrence of damaging human-caused global warming is but a small one within the much greater and proven risks of dangerous natural climate-related events (not to mention earthquakes, volcanic eruptions, tsunamis and landslides, since we are dealing here with geological topics). Moreover, the property damage and loss of life that occurred in the floods in the UK in 2007; in the 2005 Katrina and 2012 Sandy storms in the USA; and in deadly bushfires in Australia in 2009 and 2013 all attest that even wealthy and technologically sophisticated nations are often inadequately prepared to deal with climate-related hazard.

The appropriate response to climate hazard is to treat it in the same way as other geological hazards. Which is to say that national policies are needed that are based on preparing for and adapting to all climate events as and when they happen, and irrespective of their presumed cause. Every country needs to develop its own understanding of, and plans to cope with, the unique combination of climate hazards that apply within its own boundaries. The planned responses should be based upon adaptation, with mitigation where appropriate to cushion citizens who are affected in an undesirable way.

The idea that there can be a one-size-fits-all global solution to deal with just one possible aspect of future climate hazard, as recommended by the IPCC, and apparently supported by Dr Summerhayes on behalf of the Geological Society, fails to deal with the real climate and climate-related hazards to which all parts of the world are episodically exposed.

Yours sincerely,

Professor Robert (Bob) Carter
Professor Vincent Courtillot

14 February 2013 


Aldrin, M. et al. 2012. Bayesian estimation of climate sensitivity based on a simple climate model fitted to observations on hemispheric temperature and global ocean heat content. Environmetrics, doi:10.1002/env.2140.

Alley, R.B. 2000. The Younger Dryas cold interval as viewed from central Greenland. Quaternary Science Reviews 19: 213–226

Forster, P.M. & Gregory, J.M. 2006. The climate sensitivity and its components diagnosed from Earth radiation budget data. Journal of Climate 19, 39-52.

Lappi, D. 2010. 65 million years of cooling

Lindzen, R.S. & Choi, Y-S. 2011. On the observational determination of climate sensitivity and its implications. Asia-Pacific Journal of Atmospheric Sciences 47, 377-390.

Ring, M.J. et al. 2012. Causes of the global warming observed since the 19th century. Atmospheric and Climate Sciences 2, 401-415.

Spencer R. W. & Braswell, W.D. 2010. On the diagnosis of radiative feedback in the presence of unknown radiative forcing. Journal of Geophysical Research 115, D16109.