Question 2

Are humanity’s emissions of carbon dioxide and other ‘greenhouse gases’ (GHG) having a dangerous impact on global climate?

1st expert response by Richard S Lindzen

Dr Lindzen is the Alfred P Sloan Professor of Meteorology, Department of Earth, Atmospheric and Planetary Sciences, at the Massachusetts Institute of Technology.
Prof. Lindzen is a recipient of the AMS’s Meisinger, and Charney Awards, the AGU’s Macelwane Medal, and the Leo Huss Walin Prize. He is a member of the National Academy of Sciences, and the Norwegian Academy of Sciences and Letters, and a fellow of the American Academy of Arts and Sciences, the American Association for the Advancement of Sciences, the American Geophysical Union and the American Meteorological Society. He is a corresponding member of the NAS Committee on Human Rights, and has been a member of the NRC Board on Atmospheric Sciences and climate and the Council of the AMS.   He has also been a consultant to the Global Modeling and Simulation Group at NASA’s Goddard Space Flight Center, and a Distinguished Visiting Scientist at California Institute of Technology’s Jet Propulsion Laboratory. (Ph.D., ’64, S.M., ’61, A.B., ’60, Harvard University). Dr Lindzen’s response below first appeared in the Wall Street Journal on 30 November 2009.

A gigantic step backward in the science of climate.

Is there a reason to be alarmed by the prospect of global warming? Consider that the measurement used, the globally averaged temperature anomaly (GATA), is always changing. Sometimes it goes up, sometimes down, and occasionally—such as for the last dozen years or so—it does little that can be discerned.

Claims that climate Change is accelerating are bizarre. There is general support for the assertion that GATA has increased about 1.5 degrees Fahrenheit since the middle of the 19th century. The quality of the data is poor, though, and because the changes are small, it is easy to nudge such data a few tenths of a degree in any direction. Several of the emails from the University of East Anglia’s climate Research Unit (CRU) that have caused such a public ruckus dealt with how to do this so as to maximize apparent changes.

The general support for warming is based not so much on the quality of the data, but rather on the fact that there was a little ice age from about the 15th to the 19th century. Thus it is not surprising that temperatures should increase as we emerged from this episode. At the same time that we were emerging from the little ice age, the industrial era began, and this was accompanied by increasing emissions of greenhouse gases such as CO2, methane and nitrous oxide. CO2 is the most prominent of these, and it is again generally accepted that it has increased by about 30%.

The defining characteristic of a greenhouse gas is that it is relatively transparent to visible light from the sun but can absorb portions of thermal radiation. In general, the earth balances the incoming solar radiation by emitting thermal radiation, and the presence of greenhouse substances inhibits cooling by thermal radiation and leads to some warming.

That said, the main greenhouse substances in the earth’s atmosphere are water vapor and high clouds. Let’s refer to these as major greenhouse substances to distinguish them from the anthropogenic minor substances. Even a doubling of CO2 would only upset the original balance between incoming and outgoing radiation by about 2%. This is essentially what is called “climate forcing.”

There is general agreement on the above findings. At this point there is no basis for alarm regardless of whether any relation between the observed warming and the observed increase in minor greenhouse gases can be established. Nevertheless, the most publicized claims of the U.N.’s Intergovernmental Panel on climate Change (IPCC) deal exactly with whether any relation can be discerned. The failure of the attempts to link the two over the past 20 years bespeaks the weakness of any case for concern.

The IPCC’s Scientific Assessments generally consist of about 1,000 pages of text. The Summary for Policymakers is 20 pages. It is, of course, impossible to accurately summarize the 1,000-page assessment in just 20 pages; at the very least, nuances and caveats have to be omitted. However, it has been my experience that even the summary is hardly ever looked at. Rather, the whole report tends to be characterized by a single iconic claim.

The main statement publicized after the last IPCC Scientific Assessment two years ago was that it was likely that most of the warming since 1957 (a point of anomalous cold) was due to man. This claim was based on the weak argument that the current models used by the IPCC couldn’t reproduce the warming from about 1978 to 1998 without some forcing, and that the only forcing that they could think of was man. Even this argument assumes that these models adequately deal with natural internal variability—that is, such naturally occurring cycles as El Nino, the Pacific Decadal Oscillation, the Atlantic Multidecadal Oscillation, etc.

Yet articles from major modeling centers acknowledged that the failure of these models to anticipate the absence of warming for the past dozen years was due to the failure of these models to account for this natural internal variability. Thus even the basis for the weak IPCC argument for anthropogenic climate Change was shown to be false.

Of course, none of the articles stressed this. Rather they emphasized that according to models modified to account for the natural internal variability, warming would resume—in 2009, 2013 and 2030, respectively.

But even if the IPCC’s iconic statement were correct, it still would not be cause for alarm. After all we are still talking about tenths of a degree for over 75% of the climate forcing associated with a doubling of CO2. The potential (and only the potential) for alarm enters with the issue of climate sensitivity—which refers to the change that a doubling of CO2 will produce in GATA. It is generally accepted that a doubling of CO2 will only produce a change of about two degrees Fahrenheit if all else is held constant. This is unlikely to be much to worry about.

Yet current climate models predict much higher sensitivities. They do so because in these models, the main greenhouse substances (water vapor and clouds) act to amplify anything that CO2 does. This is referred to as positive feedback. But as the IPCC notes, clouds continue to be a source of major uncertainty in current models. Since clouds and water vapor are intimately related, the IPCC claim that they are more confident about water vapor is quite implausible.

There is some evidence of a positive feedback effect for water vapor in cloud-free regions, but a major part of any water-vapor feedback would have to acknowledge that cloud-free areas are always changing, and this remains an unknown. At this point, few scientists would argue that the science is settled. In particular, the question remains as to whether water vapor and clouds have positive or negative feedbacks.

The notion that the earth’s climate is dominated by positive feedbacks is intuitively implausible, and the history of the earth’s climate offers some guidance on this matter. About 2.5 billion years ago, the sun was 20%-30% less bright than now (compare this with the 2% perturbation that a doubling of CO2 would produce), and yet the evidence is that the oceans were unfrozen at the time, and that temperatures might not have been very different from today’s. Carl Sagan in the 1970s referred to this as the “Early Faint Sun Paradox.”

For more than 30 years there have been attempts to resolve the paradox with greenhouse gases. Some have suggested CO2—but the amount needed was thousands of times greater than present levels and incompatible with geological evidence. Methane also proved unlikely. It turns out that increased thin cirrus cloud coverage in the tropics readily resolves the paradox—but only if the clouds constitute a negative feedback. In present terms this means that they would diminish rather than enhance the impact of CO2.

There are quite a few papers in the literature that also point to the absence of positive feedbacks. The implied low sensitivity is entirely compatible with the small warming that has been observed. So how do models with high sensitivity manage to simulate the currently small response to a forcing that is almost as large as a doubling of CO2? Jeff Kiehl notes in a 2007 article from the National Center for Atmospheric Research, the models use another quantity that the IPCC lists as poorly known (namely aerosols) to arbitrarily cancel as much greenhouse warming as needed to match the data, with each model choosing a different degree of cancellation according to the sensitivity of that model.

What does all this have to do with climate catastrophe? The answer brings us to a scandal that is, in my opinion, considerably greater than that implied in the hacked emails from the climate Research Unit (though perhaps not as bad as their destruction of raw data): namely the suggestion that the very existence of warming or of the greenhouse effect is tantamount to catastrophe. This is the grossest of “bait and switch” scams. It is only such a scam that lends importance to the machinations in the emails designed to nudge temperatures a few tenths of a degree.

The notion that complex climate “catastrophes” are simply a matter of the response of a single number, GATA, to a single forcing, CO2 (or solar forcing for that matter), represents a gigantic step backward in the science of climate. Many disasters associated with warming are simply normal occurrences whose existence is falsely claimed to be evidence of warming. And all these examples involve phenomena that are dependent on the confluence of many factors.

Our perceptions of nature are similarly dragged back centuries so that the normal occasional occurrences of open water in summer over the North Pole, droughts, floods, hurricanes, sea-level variations, etc. are all taken as omens, portending doom due to our sinful ways (as epitomized by our carbon footprint). All of these phenomena depend on the confluence of multiple factors as well.

Consider the following example. Suppose that I leave a box on the floor, and my wife trips on it, falling against my son, who is carrying a carton of eggs, which then fall and break. Our present approach to emissions would be analogous to deciding that the best way to prevent the breakage of eggs would be to outlaw leaving boxes on the floor. The chief difference is that in the case of atmospheric CO2 and climate catastrophe, the chain of inference is longer and less plausible than in my example.

2nd expert response by Dr David Evans

Dr. David Evans is a mathematician and engineer who did carbon accounting for the Australian Government, who has described his conversion to a skeptic. “I devoted six years to carbon accounting, building models for the Australian government to estimate carbon emissions from land use change and forestry. When I started that job in 1999 the evidence that carbon emissions caused global warming seemed pretty conclusive, but since then new evidence has weakened the case that carbon emissions are the main cause. I am now skeptical.”

A Simple Proof that Global Warming Is Not Man-Made


Figure 1: Carbon emissions by humans. Source: Carbon Dioxide Information Analysis Center. The vast bulk of human emissions occurred after 1945, during post-WWII industrialization. Half of all human consumption of fossil fuels and cement production has occurred since the mid 1970s.

Now that ClimateGate has buried the fraudulent hockey stick for good, it is easy to prove that global warming is not man-made: just compare the timing of our carbon dioxide emissions with the timing of global warming.
Human Emissions of Carbon Dioxide
Emissions of carbon dioxide by humans are easy to estimate from our consumption of coal, oil, and natural gas, and production of cement:

Global temperature proxies (sediments, boreholes, pollen, oxygen-18, stalagmites, magnesium to calcium ratios, algae, cave formation, etc. over a wide geographical range) show a warming trend starting around 1700, with warming  and cooling periods about the trend. :


Figure 2: Mean global temperature reconstruction based on 18 non-tree-ring proxies, to 1935. Only 11 proxies cover the period after 1935, dotted line. Sources 1, 2, 3, 4: Dr Craig Loehle, National Council for Air and Stream Improvement. Global thermometer records are more reliable and precise, but only go back to 1880. They confirm that the warming trend extends back to at least 1880, and show warming and cooling periods of about thirty years in each direction:


Figure 3: The global instrumental temperature record to 2000, in the yellow box. Simply draw a trend line through the data. In 2009 we are where the green arrow points.  Source: Dr Syun Akasofu, International Arctic Research Center, University of Alaska Fairbanks.

Compare the Timing

The timing is all wrong for the theory of man-made global warming:
* Temperature increases started in 1700, and the underlying rate of increase has been roughly steady (though there have been warming and cooling fluctuations around the trend).
* Human emissions of carbon dioxide were negligible before 1850, and really only took off after 1945.
If human emissions of carbon dioxide caused global warming, then there would be massive and accelerating global warming after 1945 and almost no global warming before 1945. Obviously this is not the case.


1. There is almost no relationship between human emissions and global temperature, so global warming is not mainly due to human emissions of carbon dioxide.
2. Something other than human emissions caused the global warming prior to 1850.
3. The steadiness of the underlying temperature trend since 1700 suggests that whatever caused the warming prior to 1850 is still causing warming, and that the effect of human emissions of carbon dioxide is relatively insignificant.


1. This only proves that the recent global warming was not mainly due to human emissions of carbon dioxide. It does not rule out all possible man-made influences, but since the popular debate is overwhelmingly about the role of our carbon dioxide emissions this simplification is justifiable in this context.
2. Obviously the proxy global temperature in Figure 2 is deadly to the idea of man-made global warming. The alarmists tried to replace Figure 2 with their hockey stick graph, which shows global temperatures falling slightly since 1000 AD then suddenly increasing from 1910. The hockey stick graph does not show the little ice age or the medieval warm period—it reckons that the world was about 0.8°C cooler than present in the medieval period. The hockey stick graph is now firmly established as a fraud:
* Last week’s ClimateGate leaks (see here, here, here, and here) include computer code that shows blatant fudging to create the hockey stick shape (see here). (By the way, the leaked documents and emails were carefully selected, which would have taken a  considerable time. Hackers quickly grab what they can before being detected, so it probably wasn’t a hack.)
* Steve McIntyre showed that the original hockey stick graph created by Michael Mann was invalid, based on cherry picked data and biased statistical processing (if you feed stock price data into the software used to create the graph, a hockey stick usually emerges).  The US Congress appointed a committee led by Edward Wegman to investigate, and it concluded “Overall, our committee believes that Mann’s assessments  that the decade of the 1990s was the hottest decade of the millennium and that 1998 was the hottest year of the millennium cannot be supported by his analysis.”
* Another hockey stick graph was constructed from tree ring data by Briffa in 1995. When his data was finally divulged in late 2009 after years of denied requests, it was found that his graph essentially relied on one freakish tree in the Yamal Peninsula of northern Russia. See here, here, and here.
* The medieval warm period is real, according to  768 individual scientists from 454 separate research institutions in 42 different countries, and hundreds of peer-reviewed papers (see here). Here (on page 9) is a map of the world showing how much hotter than today it was in many various parts of the world, according to these studies—there are many warmer results (and a very few cooler results), in every continent except Australia (which had no studies).
* It is easily verifiable that it was a lot colder in the 1700s. Example 1: During the 1700s the Thames River in London would regularly freeze over, and people would hold fairs on the ice—the last time the Thames froze over was 1804. Example 2: There are many reports of it being so cold in Europe in the 1700s that animals in barns would die of cold—which never happens any more. The little ice age is also real.
* The IPCC prominently displayed the hockey stick in six diagrams in their Third Assessment Report (2001), and the IPCC adopted the hockey stick graph in their logo. Then it was revealed as a fraud by McIntyre and Wegman. The Fourth Assessment Report (2007) omits any reference to the hockey stick graph, and the IPCC dropped the hockey stick graph from their logo.
Now that ClimateGate has proven the hockey stick is a fraud beyond any credibility, perhaps the alarmists will finally have the decency to admit that Figure 2 is as good a picture of the past as we have. The conclusions of the proof above are then obvious and undeniable: human emissions of carbon dioxide are not the main cause of global warming.
To quote the hockey stick graph—and thereby deny the temperature reconstruction in Figure 2, the medieval warm period, and the little ice age—is anti-science fraud. But that’s what the alarmists had to do to prevent the obvious truth of the proof above.
3. Even without Figure 2, the global thermometer record in Figure 3 is sufficient to cast very considerable doubt on the idea that human emissions of carbon dioxide cause global warming: the temperature rise from 1880 to 1950 is roughly the same as that from 1950 to the present, but the human emissions were very different. In a similar vein, there have been a 29% increase in carbon dioxide emissions since 2000 but satellites show that the global temperature has fallen since then.
4. Tree rings are easy to use as a temperature proxy, but are unreliable because tree growth varies strongly in response to many factors other than temperature (such as water, carbon dioxide, fertilizer, tree age), because the width of tree ring to temperature is not linear, and because trees adapt genetically to climate changes and change basic properties like size and root-to-shoot ratio. See here (p. 1050). The temperature reconstruction in Figure 2 simply combines all the best non-tree-ring data that is available.

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