April 14, 2004

Observations Not Models

Urging caution regarding “implausible” and “unduly pessimistic” IPCC climate scenarios, NASA’s Hansen opts for observations to guide his forecasts of a 0.75ºC temperature rise by the year 2050.

NASA’s James Hansen, who is widely credited as being the “father of global warming” recently wrote that the climate change scenarios put forth in the Intergovernmental Panel on Climate Change’s (IPCC) 2001 Third Assessment Report (TAR) “may be unduly pessimistic,” and that the IPCC extreme scenarios are “implausible.” In fact, he argues, the observed trends in atmospheric carbon dioxide and methane concentrations for the past several years fall below all IPCC scenarios, so consequently future temperature rise will most like be about 0.75ºC during the next 50 years.

Hansen makes these claims in articles including “Defusing the Global Warming Time Bomb,” which appeared in the March Scientific American. He bases his conclusions on simple empirical evidence that he considers more precise and reliable than model results “because it includes all the processes operating in the real world, even those we have not yet been smart enough to include in the models.”

That is precisely the same justification given by another prominent global climate scientist, the University of Virginia’s Patrick Michaels, thought by many to be to represent the opposing pole of scientific opinion from Dr. Hansen. Michaels, too, concludes that the IPCC scenarios in large part overestimate the potential temperature rise in the coming century. Like Hansen, Michaels relies on actual observations to gain insight into future climate behavior. In his 2002 Climate Research paper entitled “Revised 21st century temperature projections,” Michaels writes that “[Observations] are the perfect integrators of all processes that are currently active” and thus avoid the “varying degrees of uncertainties surrounding every aspect of the models.”

Using observations of the rate of the observed buildup of carbon dioxide, along with observations of the global temperature change during the past 25 years or so, Michaels determines that “our adjustments of the projected temperature trends for the 21st century all produce warming trends that cluster in the lower portion of the IPCC TAR range” and concludes that warming during the next 50 years will be somewhere near 0.75ºC—the exact same number given by Hansen years later.

That however, is where most of the agreement between these two climate researchers ends. Hansen says it is imperative that we undertake concerted and organized efforts to lower this warming rate even further to avoid what he describes as “dangerous human interference” with the climate system (echoing the words of the 1989 United Nations Framework Convention on Climate Change, a.k.a. the Rio Treaty). He declares that the “emphasis should be on mitigating the changes rather than just adapting to them.”

Michaels, on the other hand, argues that the likely modest temperature rise is one that can be readily adapted to, and even used to our ultimate advantage—longer growing seasons, reduced heating costs, enhanced global vegetation, and so forth. Since the rate of climate change is manageable, he explains, artificially forced changes to the global energy structure need not be undertaken. Instead, he advocates allowing market forces to dictate change. After all, fossil fuels are a limited resource, so as mankind progresses into the future, alternative energy sources will be developed.

Michaels bases his idea upon the collection of empirical evidence—the same basis that Hansen uses in developing his estimates of the future course of climate. Yet Hansen seems to ignore that evidence when it comes to developing his estimates of the impacts of future climate changes.

For instance, Hansen expresses that his biggest concern is the potential for large sea-level rise. Yet the empirical evidence shows that the rate of sea-level rise over the course of the 20th century (during which there was about 0.75ºC of warming) was about 1.8mm per year, resulting in a total rise during the past 100 years of only about 7 inches. Doubling that rate in the future, the rate that would be implied by a continued steady temperature rise, will not, in most places, present problems that cannot be controlled or adapted to.

There are other lines of evidence that show demonstrably positive impacts. For instance, research by Ramakrishna Nemani and colleagues, who have studied variations in global vegetation patterns based upon data collected from satellites, shows a remarkable enhancement of the growth of global vegetation. They attribute this enhanced growth to changes in the climate, as well as the atmospheric enhancement of the carbon dioxide—a plant fertilizer—that has taken place over the past two decades.

The litany of dire consequences that may potentially result from global warming generally scale with the magnitude of the warming. In the IPCC TAR, the range of potential warming manifest by the year 2100 is given as 1.4ºC to 5.8ºC, with no indication as to which value may be more likely. Thus, many future claims of drastic consequences are made based upon the high end of the IPCC range. However, as actual evidence (rather than modeled responses) grows, more researchers such as Hansen and Michaels are pointing out that the low end of the range is much better supported by the observed behavior of the system. The impacts associated with a warming at the low end of the IPCC TAR range are far less, and infinitely more manageable, than those that would accompany high-end warming. It is about time to dispense with the notion that future warming will be catastrophic and begin focusing on the implications of a modest warming whose benefits are likely to outweigh its costs.


Hansen, J.E., 2004. Defusing the global warming time bomb. Scientific American, March 2004, 69-77, http://www.sciam.com/media/pdf/hansen.pdf.

Hansen, J. E., Sato, M., 2002. Trends of measured climate forcing agents. Proceedings of the National Academy of Sciences, 26, 14778–14783.

Intergovernmental Panel on Climate Change, 2001. Climate Change 2001: The Scientific Basis. Houghton, J.T., et al. (eds.), Cambridge University Press, Cambridge, U.K., 881pp.

Michaels, P.J., et al., 2001.Revised 21st-century temperature projections. Climate Research, 23, 1–9.

Nemani, R.R., et al., 2003. Climate-driven increases in global terrestrial net primary production from 1982 to 1999. Science, 300, 1560–1563.

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