Top Skeptic, Prominent Scientist Agree on Likely Global Warming Scenario by 2050

NASA’s James Hansen widely is credited as “the father” of the global warming issue because of his 1988 congressional testimony concerning his detection of a human influence on world climate. His work with a General Circulation Model developed at the Goddard Institute for Space Studies led him to that conclusion, just as GCMs subsequently led others to the offer up “scary scenarios” of our climate future. It is remarkable, then, when Hansen writes in the March 2004 edition of Scientific American (PDF) that the climate change scenarios put forth in the Intergovernmental Panel on Climate Change’s (IPCC) 2001 Third Assessment Report “may be unduly pessimistic” and that the IPCC extreme scenarios are “implausible.”

In “Defusing the Global Warming Time Bomb” Hansen argues that the observed trends in atmospheric carbon dioxide and methane concentrations for the past several years fall below all IPCC scenarios. As a consequence, he concludes, future temperature rise will most likely be about 0.75ºC over the next fifty years.

In reaching this conclusion, Hansen relies on simple empirical evidence 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.” As a consequence, he and the University of Virginia’s Patrick Michaels, a climatologist characterized by many as representing the opposite pole of scientific opinion on this issue, find themselves in agreement.

Michaels, like Hansen, believes the IPCC scenarios in large part overestimate the potential temperature rise in the coming century. And like Hansen, Michaels relies on observations for his insight into future climate behavior. In his 2002 Climate Research paper “Revised 21st century temperature projections,” Michaels writes, “[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.”

Michaels used observations of the rate of the observed buildup of carbon dioxide in the atmosphere, along with observations of the global temperature change during the past twenty-five years or so, to determine, “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.” Michaels concludes the warming during the next fifty years will be somewhere near 0.75ºC — precisely the conclusion at which Hansen arrived two years later. Sadly, this is where most of the agreement between the two climate researchers ends. Note from the poster: this is double the rate of warming in the 20th century.

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

Michaels argues such a likely and modest temperature rise is one to which earth and its inhabitants can readily adapt. He contends it may even offer great advantages — longer growing seasons, reduced heating costs, enhanced global vegetation, and so forth. Because the rate of climate change is manageable, Michaels believes, it isn’t necessary to induce changes in the global energy structure. He advocates allowing market forces to dictate change because fossil fuels are finite and mankind will have to develop alternative energy sources.

How can two scientists who base their conclusion upon a collection of empirical evidence end up so at odds?

Hansen says his biggest concern is the potential for a large rise in sea-level. Yet 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 1.8 mm per year. Therefore the cumulative rise over the past 100 years has been about 7 inches. Double that rate, as implied by a continued steady rise in temperature, and in most places there are no problems that cannot be controlled or adapted to. **see note below in comment

Other lines of evidence demonstrate the potential for positive impacts. Research by Ramakrishna Nemani and colleagues (who studied variations in global vegetation patterns based upon data collected from satellites) shows a remarkable enhancement of the growth of global vegetation. They attribute the growth to two decades of change in the climate and to the increased atmospheric concentration of carbon dioxide, which acts as a plant fertilizer.

The litany of dire consequences that could result from global warming generally is in step with the magnitude of the warming. In the IPCC’s Third Assessment Report the range of potential warming manifest by 2100 is presented as being between 1.4ºC and 5.8ºC. The IPCC does not indicate which value is more likely. However claims of drastic future consequences are based upon the possibility of temperatures ending up on high end of the IPCC range (for instance see the claims made by the environmental organization The Bluewater Network which are reportedly behind some recent climate actions by Senators John McCain (R-AZ) and Ernest Hollings (D-SC), http://www.co2andclimate.org/wca/2004/wca_14c).

If one looks at actual evidence (rather than modeled responses), as do Hansen and Michaels, it is the lower end of the range that is more likely. The impacts associated with warming at the low end of the IPCC TAR range are far less dramatic and infinitely more manageable than those that accompany high-end warming.

It seems about time to dispense with the notion that future warming will be catastrophic and begin to focus on the implications of a modest warming where benefits are likely to outweigh costs.

I've now read not only the article but Hanson's original paper. It is more elaborate than a correlation but not by much, and relies on some poor reasoning to fix climate sensitivity too fast. Without paying any attention to the physical limits on temperature response to a given level of forcing.

First he estimates long ice age forcing as 6.6 watts below now. This is based entirely on changes in atmosphere composition and large surface factors (albedo, in effect). Which we can know for these remote periods from things like ice core data.

Problem - we do not remotely know that the 6.6 watts we can ID are actually all the power sources that changed between an ice age and now. To infer a 0.75C per watt sensitivity from this is not warranted. Other than triggering atmosphere and ice changes, he ascribes nothing to changed solar forcing, ice ages to now.

The physics of it say 6.6 watts can't do that. There is far too much power coursing through the system just to maintain the baseline 285-290 K temperature. A 5 C change requires a 7% power term change. If 6.6 watts were 7% of the real total, the total power would be 94 watts. Direct sunlight at the edge of space is 342 watts. This is wrong, empirically.

Physics does not say you won't get 6.6 watts from changing atmosphere and ice sheet stuff. That sounds entirely plausible. But in the old data, temperatures take off well before the atmosphere responds. This 6.6 watt measure is almost certainly the positive feedback, not the entire signal that causes ice ages.

It might be 20% of the signal, roughly split between 10% land-ice-albedo and 10% air-CO2etc-greenhouse. You'd have a 25 W driver, and these 3 W feedbacks piling on. That would have the driver 35 watts, fitting an estimate of 500 W to maintain temperature now (sunlight plus greenhouse etc).

Maybe it is as high as a third of the signal, 20 W overall, with albedo reducing sunlight etc. But that is a lower bound (it is just sunlight times a .83 albedo - the earth could have that much power with no atmosphere at all). Then the picture of ice ages would be, you get a 13 W driver from the sun, and you tack on 25% feedback sizes from albedo and atmosphere feedbacks.

Those ranges suggest climate sensitivities, reduced to one coefficient, in the range 0.25 to 0.15 C per watt. It is critical to understand that the only thing he has, up to this point in the argument, supporting the idea that instead it is 0.75 C per watt, is the notion that the 6.6 W he can ID from ice core data are the entire forcing for ice ages. This is the old, one would have thought by now discredited, idea, that ice ages are caused purely by atmosphere (and ice sheet) changes. It is just now dressed up as empiricism - since I can measure these factors 100,000 years ago, they must be the driving factors in ice ages, and the same sensitivity should be expected today.

But we also just know this isn't the real ice age story. Because these 6.6 W he can ID, show up after the warming starts. Not only is the amount too small on physical considerations, but it comes at the wrong time to set off the escalating temperature run (low level during, big only after them). Clearly the real power driver comes before them and it is just plain missing from his budget.

Why does this matter? If the same measures are used now, won't the relation be the same? No. If the other factors are follow on feedbacks to the ID-able 6.6 Ws, they might be the same now as then. But if they are solar, they aren't feedbacks at all, aren't driven but driving, and CO2 changes today are not going to set them off.

Then - 13 to 25 W from unmeasured sources, touching off 6.6 W from measured sources. Now - less than 6.6 W from the same measured sources, and no reason to believe the bigger unmeasured one is operating at all.

So, first understand, there is good physical reason to believe the power needed to drive ice ages is higher than his budgeted 6.6 W, more like 20-35W. Which imply climate sensitivities on the order of 0.15 to 0.25 C per W, not 0.75.

Now turn to the modern data. He gives his estimate of forcing factors from roughly 1850 to 2000. He gets 1.6 plus or minus 1W. If you look at the individual error bars of the internals, then going half way high yields 2.9W, and all at their 1 SD lines gives 4.2W. Thus -

His centerpoint - 1.6 W
Within his error bar - 2.6 W
Within 1/2 on all - 2.9 W
Within 1 on all - 4.2 W

Why does this range matter? Because we have an observed temperature change to set against the forcing estimates. It is 0.75C - although some say it is more like 0.6C, the former is the figure he gives. Well, that means the implied sensitivity coefficients run from 0.47, to 0.29, to 0.26, to 0.18 down to 0.14 if you use 0.6C as well. But none of them support 0.75. 0.15 to 0.25 is physically believable territory, not needing large unknown power terms. I'd have no difficulty allow small ones, bumping the range up to 0.3 to 0.375. My range estimate, certainly wide, would be 0.15 to 0.375.

But his own data for this modern period do not support the 0.75 he got from the ice ages - his own centerpoint of 1.6W forcing gives 0.47 on his high estimate of the temperature change. Yet all the later model run comparisons use 0.75. Which is twice to five times what makes physical sense and lies within his own error bars.

Understand just what that +/- 1W contains. It means a range from 0.6 to 2.6 - a factor of 4.33 times. We've also got maybe a 0.25 range factor in the temperature measurement (0.6C to 0.75 C). Put them together, and the estimate of the climate sensitivity is varying over a factor of 5, within the error bars. With only the low end of it compatible with physical considerations (absent giant unseen and unnamed additional power terms).

When we come to projected future changes he sees 1.5 W additional forcing. He tosses in an extra 0.5 W for no strong reason, about an unknown and to get closer to IPCC. Call it 1.5 to 2.0 W. That means 0.47 (his own data) say 0.7C to 0.95C, while his ice age induced 0.75 gives 1.1 to 1.5C.

My range and the same forcing predicts 0.23C to 0.75C. Physical considerations alone say 0.23C to 0.5C - the higher possibility reflects an implicit allowance for as yet unseen feedbacks, but smaller ones than the signal. For his 0.85W forcing optimistic emissions controlled case, I'd give 0.13 to 0.32C with 0.13 to .21 most likely.

Personally I doubt forcing will be that small and do not see reason to seek it with draconian regulation. He thinks for ice sheet and sea level management reasons the temperature change from today should be held to under 1C. But his own estimate of 1850 to now forcing would do that (0.47 sensitivity, with 1.5 to 2 W future forcing). I think the sensivity is half of that and a third of his ice age figure. And I'm well within his error bars for 1850 to 2000 believing it. I'm also compatible with the power needed to maintain a higher temperature.

The epicycles are falling away, but he is still clinging to the ice age theory to justify the 0.75 sensitivity figure. Which is physical nonsense if meant to describe all the power sources needed. Ice age nonsense in that it acscribes the entire change to factors we know lag the actual temperature changes in ice ages.

There is simply no good reason to think the temperature response is so high, and plenty of reason to think it is appreciably lower. His own 1850 to now estimates give a figure 37% lower; his error bars there are huge and accomodate much lower still; physics alone says 65-80% lower.

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