Publié le par Scorpio

Global Warming Consensus?

The phrase “scientific consensus” suggests something approaching unanimous agreement among scientists. However even a rudimentary survey of scientific literature reveals there to be very little agreement on the subject of climate change.

The unfortunate and inaccurate characterization of consensus is used as a rhetorical bludgeon of skeptics and is the basis of a push for industrialized nations to “do something” to reduce the atmosphere’s greenhouse gas concentration.

Dr. Richard Lindzen of the Massachusetts Institute of Technology offered an explanation of the phenomenon during a recent National Press Club briefing “Climate Alarm — Where does it come from?”:

With respect to science, consensus is often simply a sop to scientific illiteracy. After all, if what you are told is alleged to be supported by all scientists, then why do you have to bother to understand it? You can simply go back to treating it as a matter of religious belief, and you never have to defend this belief except to claim that you are supported by all scientists except for a handful of corrupted heretics.


The theme of scientific consensus creeps into documents such as the Arctic Climate Impact Assessment Overview Report (2004) and the UN Intergovernmental Panel on Climate Change’s (IPCC) Third Assessment Report (2001). A good example of how this plays out was demonstrated in the December 3, 2004 edition of Science in which University of California-San Diego’s Naomi Oreskes warns readers, “[O]ur grandchildren will surely blame us if they find that we understood the reality of anthropogenic climate change and failed to do anything about it.”

We understood the reality? What’s the basis of her claim?
The IPCC “states unequivocally that the consensus of scientific opinion is that Earth is being affected by human activities,” she writes. She goes on to specify that the IPCC concludes, “[M]ost of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.” Those statements suffer a serious shortcoming; our grandchildren will need only study the scientific literature available to their grandparents to discover that, in fact, neither the IPCC nor a particular scientist has convincingly demonstrated how human-caused contributions to atmospheric greenhouse gases are responsible for ‘most’ of the observed warming over the past fifty years.

To highlight the difficulty of reaching true scientific consensus, one need only consider the infamous 1,000-year temperature history prominently featured in the 2001 IPCC Third Assessment Report (TAR). It re-appeared in the 2004 Arctic Climate Impact Assessment (ACIA) Overview Report and is a perfect example of the complex challenge of achieving quantitative understanding of the earth’s climate system.


The ‘hockey stick curve’ was almost unanimously hailed as a scientific consensus that strongly suggests the exceptional nature of temperature change in the last fifty years. But it wasn’t long before independent-minded scientists and researchers discovered holes in the theory. That fact alone invalidates the claim of consensus.

One of them, Professor Hans von Storch, who directs the Institute of Coastal Research at Geesthacht, Germany, recently remarked on “the ethical implications of the premature acceptance of the ‘hockey stick’ by the scientific community.” He writes:


When we recently established that the method behind the so-called “hockey stick” curve of Northern Hemisphere temperature is flawed, this result was not so much attacked as scientifically flawed but was seen both in private conversations and public discourse as outright dangerous, because it could be instrumentalized and undermine the success of the IPCC process. [our added emphasis]


We note that Von Storch and his co-authors followed with an aside that “the concept of anthropogenic climate change is compelling even if the hockey-stick curve is false.” That kind of statement further confirms the dangerous implications of the constant invocation of scientific consensus.

One thing about which there is undeniable scientific consensus is the difficulty even the most sophisticated climate models face in deciding on the impact of clouds under the familiar scenario of doubling the atmospheric carbon dioxide concentration. Figure 1 makes a key point about models producing contradictory results.

The discredited U. S. National Assessment suffered from such chicanery (see concerning regional precipitation projections, by way of example).

Depending on which climate model you examine, any one of three combinations of increased, decreased, or little or no change in future cloud cover is assumed. This places modelers in a very awkward position if they suggest an ability to predict future climate. If nearly any tendency is possible, the models are incapable of making a reliable prediction.

Such a devastating realization should — scientifically speaking — stop cold all the what-if scenarios commonly used to justify the need for immediate, drastic cuts in CO2 emissions.


Figure 1. Variable cloud coverage is predicted under a scenario of doubling atmospheric CO2. The result depends on which model is selected. Thus, there should be ‘scientific consensus’ that no prediction is possible concerning the amount of low clouds.

The difficulty and ambiguity of determining what could happen to the cloud field under an enhanced greenhouse effect easily can be be appreciated in this latest review of the state-of-the-art modeling of Arctic clouds:

Correct radiation at the surface requires a correct cloud field, which is far from trivial. ... A cursory look at the liquid water path reveals that the models mostly do reasonably well, except during winter when the observations reveal substantial liquid water while models almost entirely have ice-only clouds. …


On average, the modeled bias in net radiation is about -10 [Watts per meter squared], however, the net errors in the individual models range from -44 to 5 [Watts per meter squared].


More serious is the fact that the net errors in the different models come from different components of the radiation budget. In the some models, errors in incoming and outgoing radiation compensate, but in others they reinforce. It is quite possible that the balance between the terms in the net radiation budget shifts in a changed climate. Thus using these formulations in a climate model, the net-radiation errors may shift in a quite unpredictable manner. [our emphasis]


The Arctic researchers (Tjernstrom et al., 2004) explained how, according to Arctic cloud observations utilizing both radar and lidar during the so-called SHEBA (Surface Heat Budget of the Arctic Ocean project) campaign, liquid water was detected for clouds as high as 6.5 km and at temperatures 34ºC below the freezing point for water. They further note, “Moreover, none of the [six] models consider Arctic haze or indirect effects of aerosol particles.”

Another emergent scientific consensus concerns lack of understanding of the climatic impacts of both anthropogenic and natural aerosols. Early on, Gary Russell and colleagues (2000) of NASA’s Goddard Institute for Space Studies cautioned “[o]ne danger of adding aerosols of unknown strength and location is that they can be tuned to give more accurate comparisons with current observations but cover up model deficiencies.”

More recently, Theodore Anderson and colleagues (2003) issued several strong cautions in Science:


[W]e argue that the magnitude and uncertainty of aerosol forcing may affect the magnitude and uncertainty of total forcing [i.e., “the global mean sum of all industrial-era forcings”] to a degree that has not been adequately considered in climate studies to date. Inferences about the causes of surface warming over the industrial period and about climate sensitivity may therefore be in error. ...


Unfortunately, virtually all climate model studies that have included anthropogenic aerosol forcing as a driver of climate change (diagnosis, attribution, and projection studies; denoted “applications” in the figure) have used only aerosol forcing values that are consistent with the inverse approach. If such studies were conducted with the larger range of aerosol forcings determined from the forward calculations, the results would differ greatly.


The forward calculations raise the possibility that total forcing from preindustrial times to the present ... has been small or even negative. If this is correct, it would imply that climate sensitivity and/or natural variability (that is, variability not forced by anthropogenic emissions) is much larger than climate models currently indicate. ...


In addressing the critical question of how the climate system will respond to this [anthropogenic greenhouse gas] positive forcing, researchers must seek to resolve the present disparity between forward and inverse calculations. Until this is achieved, the possibility that most of the warming to date is due to natural variability, as well as the possibility of high climate sensitivity, must be kept open. [our emphasis]


For those involved in impact assessments of future climate change, James Hansen (2002) of NASA Goddard Institute for Space Studies warns of clear exaggeration of the CO2 emission scenarios adopted by the IPCC:


One problem with IPCC reports is that each report produces new (and more numerous) greenhouse gas scenarios with little attempt to discuss what went wrong with the previous ones. As a result, dramatic changes that have occurred since the 1980s in prospects for future climate forcings receive inadequate attention. ... CO2 scenarios are the most critical. We note that growth rate of CO2 (fossil fuel) emissions has declined from about 4% per year to 1% per year in recent decades.


It is noteworthy that the current IPCC (2001) scenarios have a growth rate in the 1990s that is almost double the observed rate of 0.8%/year ... but it is consistent with their failure to emphasize data. [our emphasis]


In other words, the IPCC emission-scenario team fails to account for the relatively lower and slower rates of emission trends. They do not account for the observed tendencies.

On these varied bases, there should be clear scientific consensus that the IPCC or ACIA CO2 emission scenarios for the future are biased, exaggerated and/or uncertain. Instead, Naomi Oreskes and others cite the ‘consensus of scientists’ the IPCC or ACIA supposedly represents. In Dick Lindzen’s words, that’s a sop to scientific illiteracy.

January 14, 2005

Anderson et al., 2003. Climate forcing by aerosols—a hazy picture. Science, 300, 1103-1104. (see also exchanges later in Crutzen et al., 2003, Science, 303, 1679-1681)

Arctic Climate Impact Assessment (ACIA) Overview Report, 2004. Impacts of a Warming Arctic: Arctic Climate Impact Assessment (Cambridge University Press). [available online at]

Hansen, 2002. A brighter future. Climatic Change, 52, 435-440.

UN Intergovernmental Panel on Climate Change Third Assessment Report, 2001. The Scientific Basis (Cambridge University Press). [available online at]

Lindzen, 2004. Climate Alarm-Where does it come from? Presentation at the National Press Club, December 1 (the lecture is available from email request to

Oreskes, 2004. The scientific consensus on climate change. Science, 306, 1686.

Russell et al., 2000, Comparison of model and observed regional temperature changes during the past 40 years. Journal of Geophysical Research, 105, 14891-14898.

Tjernstrom et al., 2004. Modeling the Arctic Boundary Layer: An evaluation of six ARCMIP regional-scale models with data from the SHEBA project. Boundary-Layer Meteorology, in press.

von Storch et al., 2004. Sustainability and the issue of climate change. Essay draft dated 11/30/2004 and available from



Publié dans climate change

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