Elements of Change 1995: Session IISchneider

The carbon dioxide content of the atmosphere has increased by 25% and methane has increased 150% since the industrial revolution. It is not controversial that this is the result of human activities. World population and methane concentration trends are closely correlated up to the present (but this need not necessarily always be the case). The existence of the greenhouse effect is a well established scientific fact; how much it will be enhanced by human activities is what is at issue. In attempting to predict how the climate system will respond to this enhancement, the net effect of all the positive and negative feedbacks is the greatest scientific unknown.

Global temperature change scenarios to 2100 AD range by over a factor of ten because they confound uncertainties in feedbacks and in population/affluence/technology scenarios. Scientists have only recently begun to allow themselves to voice subjective probabilities based on their expert knowledge. Such scientific estimates of how much the climate will change are not based primarily upon extrapolations from the past, but rather on an understanding of climate system dynamics and how it might change based on future conditions which may never have occurred in the past, at least not at the accelerated rates we observe today.

Globally, the vast majority of mountain glaciers are receding. This fact is not intended to be diagnostic of global warming. Scientists are not using this as political "evidence," but rather as part of their observation of overall Earth system response. But such facts are often viewed as "evidence" in the media and political fora. Ice core data show that CO₂ concentration and temperature are highly correlated over the past 160,000 years. There has been a relatively high degree of stability in global average temperature over the past 10,000 years. Sustained average rates of natural surface temperature change are about one degree per thousand years. Projected human-induced changes are on the order of degrees per century instead of per millennium. The concern is not so much over the magnitude of change but rather over the rate of change.

How do we gain a perspective on something as complex as the climate system and tap into the expert judgments of knowledgeable scientists? Scientists' usual mode of "truth seeking" can be very different than that of other groups. Advocacy is not an acceptable motive for scientists, but motive is certainly a significant factor in weighing evidence in the political process.

Projected human-induced changes are on the order of degrees per century instead of per millennium. The concern is not so much over the magnitude of change but rather over the rate of change.

An increasing fraction of the knowledgeable scientific community believes that the evidence for global climate change is becoming convincing, but regionally, we have less certainty regarding what may happen. For example, aerosols are regional while greenhouse gas effects are global. Even if there were to be no net global average climate change, there could be enormous regional climate change.

Scientists, like all people, have value systems, and it is better to have them be explicit than implicit. Statements of probabilities and consequences are needed from scientists. And scientists must not consciously distort what they believe to be the probabilities of the case or leave out any important potential out comes. Scientists should more consistently identify outliers potential outcomes with low probabilities but large consequences. There is a need to define consensus broadly. It is no longer unrespectable to be involved in assessment and even advocacy. On the subject of advocacy, Schneider says the contrarian side is better funded and they are out to "win" for their "clients." They represent a small portion of scientists but unfortunately are given equal scientific status by the media.

Schneider says the popular press provides a poor information base on global change issues. For example, Greg Easterbrook's book A Moment on the Earth is laden with scientific inaccuracies, some of which are detailed in a paper by the Environmental Defense Fund. Much of the public, as well as elected officials, get the majority of their information from sources such as this and popular television programs and magazines.

How do we gain a perspective on something as complex as the climate system and tap into the expert judgments of knowledgeable scientists?

In 1990, the Intergovernmental Panel on Climate Change (IPCC) assessment predicted that a doubling of CO₂ would produce an equilibrium temperature rise of 1.5° to 4.5°C with a "best guess" of 2.5°C. The current draft of the summary for policymakers in the IPCC's second scientific assessment says that research since 1990 has served to confirm many of the basic conclusions of that report and its updates in 1992 and 1994. Furthermore, it points out that important progress has been made in predicting transient climate change using coupled atmosphere-ocean models, quantifying the role of aerosols in climate change, characterizing the trends and relative roles of greenhouse gases, and attributing climate change to its various causes.

There are several key new conclusions in the current draft IPCC report. One of these is that the probability that the observed half degree warming trend since last century is part of the natural variability is low, probably less than 1 in 10. More importantly, there is evidence of an emerging pattern of climate response to forcings by greenhouse gases and sulfate aerosols in the observed climate record. This is evident in the geographical, seasonal, and vertical patterns of temperature change in the atmosphere. Taken together, these results provide growing confidence that a significant part of the observed climate warming can be attributed to human activities.

It is clear that the system is highly nonlinear and difficult to credibly predict at the regional level. Since 1990, there is more confidence in global predictions but perhaps less confidence in regional predictions. The decreases in growth rates of CO₂ and methane in the early 1990s have largely disappeared, not affecting the long term trends in a significant way. Increased CO₂ and sulfate aerosols affect the pattern of warming/cooling. When aerosols are factored into the models, they reproduce reality far more closely, with the correlation increasing over time. Interhemispheric asymmetry also shows up well in the models forced with both CO₂ and sulfate aerosols. This is a case where the evidence is still circumstantial but growing.

David Keith and Granger Morgan of the Carnegie-Mellon cross-disciplinary group conducted extensive (4 hour) interviews with 16 scientists considered experts on the climate issue. While the results reveal a diversity of expert opinion, Schneider believes that they show more agreement than is portrayed in the media. Figure 15.1 shows the answers given to the question of how much the equilibrium temperature change will be under doubled CO₂ conditions. The experts polled responded with subjective probability assessments. Schneider also points out that the basic opinion of the knowledgeable community is pretty close to a consensus that the 20th century warming trend is unlikely to be wholly natural. Important questions in such studies include how to select which experts to ask, and on what questions are they truly expert.

There is growing confidence that a significant part of the observed climate warming can be attributed to human activities.

Another example of eliciting expert opinion comes from William Nordhaus' research on the economic value of climate change. Figure 15.2 shows what a group of experts thought the economic impact of a doubling of atmospheric CO₂ resulting in 3°C of warming by 2090 AD would be as a percentage loss of gross world product. Overall, it was clear that the economists generally believed there would be less impact while ecologists believed there would be a much greater impact on the economy. Still, the best guesses of most of the experts were tightly clustered, supporting Schneiders' view that there is more agreement among experts than is portrayed in the media. In Nordhaus' study, it was left to each respondent to determine the value placed on various categories. In his response, Schneider placed a significant fraction of the total loss value in non-standard accounts because factors such as biodiversity may be valued more highly in 100 years than they are now.

There is more agreement among experts than is portrayed in the media ... and there is an extreme disconnect between what scientists' believe and what the public knows.

The question remains as to how to translate this kind of information into responsible sound bites and hearing testimony so that the public and policymakers can learn what a representative cross section of experts actually believe. One problem is that our educational process teaches that the answers come from the front of the room and that scientists give us "right" answers rather than subjective probabilities. When attempting to forecast future scenarios of something as complex as Earth's climate, the most we can expect is the subjective probability estimates of experts. And, Schneider stresses, there is an extreme disconnect between what scientists' believe and what the public knows.