TCS Daily

Malthusian Warming

By Roy Spencer - March 23, 2005 12:00 AM

Two science studies (1, 2) were published last week that advanced the view that, even if we stop producing greenhouse gases immediately, global temperatures will continue to increase for decades to come. This effect is attributed to the long time necessary for the Earth's climate, especially the deep ocean, to readjust to the radiative forcing from greenhouse gases we have already put into the atmosphere. The fundamental concept is not new. It has long been recognized that if the Earth allows a radiative imbalance to persist (an important distinction) that it will take decades for much of the resulting temperature change to be realized by the deep oceans.


Radiative balance is the most important paradigm in climate change research. Sunlight is essentially the only energy source for the climate system, and must be exactly balanced by the loss of infrared (heat) radiation to space in order for the temperature of the system to remain constant. Anthropogenically produced greenhouse gases, particularly carbon dioxide from burning of fossil fuels and biomass, slightly enhance the Earth's natural greenhouse effect, which is dominated by water vapor. While it is widely agreed that this leads to a warming "tendency" in the lower layers of the atmosphere, how the climate system will respond to this tendency is more widely disputed.

While the new studies have led to a heightened sense of urgency about restricting the use of fossil fuels, it is at the same time accepted that little that can be done about global warming, no matter how much you believe it will be, without widespread abandonment of fossil fuels. At least in the near-term, this would require a substantial re-emergence of nuclear power. Small (say 10%) reductions in fossil fuel use will likely have unmeasurable effects on global warming, but will do widespread harm to the economies of the developed world. In contrast, something more like 50% reductions on a global basis are required to have a large impact on whatever future warming we believe will happen.

I would like to step back and examine global warming theory in the context of modern environmental predictions. I believe this can help us better assess not only our faith in predictions based upon our current understanding (the "science"), it will also give a better perspective on the resulting policy debate over what should be "done" about global warming.

Historically, science has a notorious habit of taking short term changes and relationships and extrapolating them into the future. An infamous example is the global cooling trend from the 1940's through the 1970's, which brought scientific predictions that a new ice age was just around the corner. (Another ice age is still widely expected...just not anytime soon). While the scientist might defensively respond, "well, we have to base decisions on science understanding at the time", I would retort, "OK, how well has that strategy worked in the past?"

Thomas Malthus in 1798 provided one of the most famous early examples of scientific thought and the failure of long term predictions in his "Essay on the Principle of Population". Malthus' central theme was that, since population increases faster than our ability to grow food, a lack of food would limit population growth, with mass starvation a real possibility. In modern times, Paul R. Ehrlich ("The Population Bomb") resurrected this basic theme, predicting global calamities by the 1990's that were ultimately tied to overpopulation. History has shown, however, the opposite. While population has indeed grown exponentially, per capita food production has grown even faster. Global per capita food and raw materials costs have fallen. High birth rates in poverty-stricken countries are falling as these countries' economies strengthen, and global population growth is slowing, and predicted to stagnate even further in the next century.

One of the central reasons for the failure of long-term predictions lies in the fact that metrics like population and food production are part of a nonlinear dynamical system with myriad interconnections. The whole system readjusts based upon meeting society's needs with human ingenuity. This has certain similarities to global warming theory.

Malthusian Science Predictions

I see a strong parallel with these failed past predictions and the current predictions of future global warming.

On the science side, it should be mentioned that the direct warming due to a doubling of carbon dioxide content of the atmosphere, by itself, would lead to only about 1 degree F warming. This small amount of warming would likely be easily adapted to by life on Earth. The much larger predictions of warming by the year 2100, as much as 10 degrees F, depend upon the positive feedbacks that are believed to exist in the climate system. These are changes in water vapor, cloud cover, etc., that amplify the warming tendency that the extra carbon dioxide initiates.

Probably the biggest positive feedback in climate model simulations of global warming is water vapor, the Earth's dominant greenhouse gas. It is commonly stated that surface warming will lead to more evaporation, thus an amplified greenhouse effect. But this is not necessarily true. The average amount of water vapor in the atmosphere is not governed by evaporation, but by the balance between evaporation (which supplies water vapor) and precipitation systems (which deplete water vapor). While water vapor has always had the potential for driving the climate system into a runaway greenhouse effect, it never has. This is because precipitation systems put a brake on warming, stabilizing the system. The scientific problem is that we don't have a good understanding of how precipitation systems will change with warming. We do suspect, however, that warmer systems (for example those we find in the tropics) are more efficient at depleting water vapor, and this could provide a basis for a significant negative feedback that is not currently contained in climate models.

A more famous uncertainty among climate modelers is that associated with clouds. Like the vapor-depleting precipitation systems, clouds also have a net cooling effect on the climate system by reflecting sunlight. Yet the predicted change in the frequency of different types of clouds in many climate models also leads to a positive feedback, amplifying any warming tendency. Finally, all of the positive feedbacks have a way of amplifying each other in these models, leading to still greater warming.

So what does the current state of global warming science have to do with the failed predictions of Malthus and Ehrlich? Those authors applied early scientific observations and principles to the world, and tried to predict the future. They failed partly because they did not understand the dynamic changes in the system that stabilize the system.

I believe a similar situation exists with global climate models. We know that clouds, on average, cool the climate. Do we really believe they will react to a small warming perturbation by amplifying that warming? We know that precipitation systems keep the Earth's natural greenhouse effect in check, greatly limiting surface warming (about 75% of the surface warming potential of the Earth's natural greenhouse effect is never realized -- evaporation and rainfall prevent it from occurring). Do we really believe that precipitation systems will permit a small warming tendency to cause water vapor to increase, amplifying that warming? Many researchers will answer "yes" to both these questions. I suspect, however, that the climate system is more stable than is widely believed, and we simply do not have the understanding of these complex components of the system to be able to predict its future state.

Malthusian Policy Predictions

Even if the popular version of global warming theory is right, once again the failure to expect technological advances can lead to a massive failure of predictions. Malthus and Ehrlich failed to anticipate human ingenuity in producing new disease-resistant high-yield crop strains and improved efficiencies in farming techniques. Similarly, there are a wide variety of advances that could, in the next 50 years or so, make current global warming predictions a moot point. Geological sequestration of carbon dioxide during the burning of coal, relatively safe nuclear technologies to produce electricity which then is used to produce hydrogen fuel for automobiles, and advances we can not even comprehend now, all provide hope for the future.

This rosier-than-normal view of the future is unpopular in today's risk-adverse culture. We fear the latest environmental scare without understanding that risk is unavoidable in life. Technological advances have brought much greater benefits to humanity than harm. Usually, the fear is unfounded, or at least greatly exaggerated, as is described in great detail in Bjorn Lomborg's "The Skeptical Environmentalist".

Obviously I have a certain level of faith when I anticipate further refinements in global warming theory that will eventually reduce predicted global warming, and the technological solutions to the energy problem that will eventually occur. (And I haven't even addressed the net benefits of some amount of carbon dioxide increases and warming.) But the historical evidence is on my side. Malthusian predictions have a notoriously poor track record.

We need to ask ourselves, if only the wealthy countries of the world can afford ongoing research into new energy technologies, do we really want to shoot ourselves in the foot by imposing economically-damaging restrictions on the production of carbon dioxide? While the new studies by Wigley and Meehl et al. will probably lead to a new round of calls for limits on greenhouse gas production, we must be mindful of not only the risks of such policy changes, but the historical track record of long-term predictions from science.


1. Wigley, T.M.L., 2005: The Climate Change Commitment. Science, Vol. 307, pp 

2. Meehl, G. A., W. M. Washington, W. D. Collins, J. M. Arblaster, A. Hu, L. E. Buja, 
    W. G. Strand, and H. Teng, 2005: How Much More Global Warming and Sea 
    Level Rise? Science, Vol. 307, pp 1769-1772.


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