TCS Daily

Global Warming: The Smoking Gun?

By Roy Spencer - May 3, 2005 12:00 AM

Last week's publication of a new climate modeling study (1) investigating the evidence for man-made climate change is destined to have more than the average impact on the global warming debate. The study's lead author, Dr. James Hansen, has been a central figure in global warming research and helped bring the issue into the public's consciousness with his congressional testimony in 1988.

The new study uses the Goddard Institute for Space Studies (GISS) computer climate model to simulate global climate over the last 120 years when forced with estimates of greenhouse gases, aerosols (both man-made and volcanic), and ozone depletion. The current formulation of the model, which has evolved over the years, produces a global average temperature history that looks remarkably like the thermometer measurements. The authors specifically emphasize the agreement between a slight warming of the oceans over the last ten years or so, and the radiative forcing that the model produces when the anthropogenic gases and aerosols are included.

Taking this agreement as evidence that the model has substantially correct physics, they further note that the radiative imbalance in the model as of the end of their simulation period (2003) is still substantial (0.85 watts per square meter). This suggests that, even if we could stop producing greenhouse gases right now, there is still significant warming in our future, which they estimate to be about 1 degree F. The warming would be necessary to eventually restore the ocean-atmosphere system to a state of radiative balance. While the atmosphere could, by itself, adjust in a matter of months to such an imbalance, it is the immense heat sink of the ocean that ends up taking many years to fully respond with warming.

Last week's TCS article on this new study noted that, even taking the study at face value, the amount of global warming the GISS model produces is now considerably less than what it used to be. Thus, one unstated conclusion of the new work could have been "global warming won't be as bad as previously thought". Instead, Dr. Hansen emphasizes the need to keep future warming below a relatively modest level, appealing to some research that has suggested that the climate system is unstable if pushed beyond a relatively small amount of warming.

A five-page write-up companion to the research paper was distributed to the climate research community that describes what Dr. Hansen believes are the implications of the new study. He makes it clear that the model's radiative imbalance constitutes "smoking gun" evidence of anthropogenic climate change. But instead of a "smoking gun", I would like to suggest that the new study really just presents one, internally consistent interpretation of what might be going on in the climate system.

The authors' interpretation of their model is indeed a possible one, but there are others as well. First of all, the model has been repeatedly "tuned" with various forcings in an attempt to explain the global temperature record of the last century. There is nothing inherently wrong with this strategy, but it must be kept in mind that how we think the climate system works is guided by the historical temperature record, and what we think has influenced it. Since we really don't understand how natural climate fluctuations (except for volcanic eruptions) influence that record, we are restricted to what we do understand: mankind's production of greenhouse gases and aerosol pollutants. The real climate signal we are interested in, a gradual warming from increasing greenhouse gas concentrations, is dominated by a single mode -- an upward trend. We also know that carbon dioxide concentrations have similarly increased. Does this prove cause and effect? The assumption has been that there have been no significant natural long-term independent changes in clouds, water vapor, precipitation efficiency, deep ocean heat storage, or a variety of other known (or unknown?) processes that could affect global temperatures in a similar manner.

For instance, what if recent warming of the oceans is more due to a slight decrease in low clouds than to increased trapping of infrared radiation by greenhouse gases? Or what if a temporary change in the rate of heat exchange with the deep ocean has caused the recent warming of the upper layers of the ocean? There are surely other possibilities as well.

Dr. Hansen's prediction that we will soon experience a year with globally averaged warmth greater than that of 1998 will be an interesting test of his current view of the human influence on climate.

So, what would constitute "smoking gun" evidence for human-induced global warming? I would like to suggest the following as an example of such evidence. We would have needed 50 to 100 years of accurate satellite observations (not model-based inferences) of the radiative components of the Earth's radiation budget: at least the amount of absorbed sunlight and the amount of emitted infrared radiation. It has been exceedingly difficult from the limited record of satellite measurements to extract a signal of radiation imbalance in the climate system (let alone diagnose what has caused it). Over the period of a sufficiently accurate satellite record, we would need to be able to measure an increasing imbalance between absorbed sunlight and Earth-emitted infrared radiation. Then, through additional global-scale measurements, we would need to be able to rule out natural changes in clouds, water vapor, deep ocean heat storage, and any other process that could explain some or the entire radiation imbalance.

Some believe that the recent global warmth is greater that it has been in the last 1,000 years, which by itself would be evidence for man-caused global warming. I personally don't have much faith in the indirect estimates of global temperatures before the twentieth century, but I agree that the current warmth is unlikely to be entirely natural in origin. The big question is, how much of it?

Of course, it will be a long time before we ever reach the state of knowledge that would meet my above test as smoking-gun evidence, and so policy decisions will have to be made (or not be made) without all of the answers. Thus it is imperative that all of the uncertainties are laid out on the table before decisions are made.

Of course, if reducing greenhouse gas emissions was easy or cost effective, it would have already been done. So until we develop energy technologies that either greatly reduce or eliminate the production of carbon dioxide, the stark fact is that there will be a human toll to mandated reductions. A few percent reduction from using more solar- and wind-generated power might make us feel good about our efforts, but it will have almost no effect on future global temperatures. Ultimately, the problem will be solved through energy technology research, which necessarily requires strong economies that can afford to fund that research, which in turn requires access to affordable energy now.

In summary, the recent study by Hansen et al. indeed presents one possible interpretation of the available evidence, but it is a very human-centric one that assumes that natural decadal- to century-scale climate fluctuations are not to blame for at least some part of what has been recently observed. Or, using the "smoking gun" metaphor, it isn't yet clear whether mankind is the perpetrator, Mother Nature is an accomplice, or vice versa.

Nevertheless, the study represents the kind of synthesizing science that is needed in order to better understand the extent to which mankind is influencing the climate system.

1. Hansen, J., et al., 2005: Earth's Energy Imbalance: Confirmation and Implications. Science Magazine, 28 April.


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