Testimony of John R. Christy Before the Senate Committee on Environment and Public Works

Senate Committee on Environment and Public Works

2 May 2001

Written Testimony

John R. Christy

University of Alabama in Huntsville

Mr. Chairman and Committee Members, I am pleased to accept your invitation to speak to you again about climate change. I am John Christy, Professor of Atmospheric Science and Director of the Earth System Science Center at the University of Alabama in Huntsville. I am also Alabama’s State Climatologist and recently served as one of the Lead Authors of the IPCC.


The concentration of carbon dioxide (CO2) is increasing in the atmosphere due primarily to the combustion of fossil fuels. Fortunately (because we produce so much of it) CO2 is not a pollutant. In simple terms, CO2 is the lifeblood of the biosphere. The green world we see around us would disappear if not for atmospheric CO2. These plants largely evolved at a time when the atmospheric CO2 concentration was many times what it is today. Indeed, numerous studies indicate the present biosphere is being invigorated by the human-induced rise of CO2. In and of itself, therefore, the increasing concentration of CO2 does not pose a toxic risk to the planet. It is the secondary impact of CO2 that may present challenges to human life in the future. It has been proposed that CO2 increases could cause climate change of a magnitude beyond what naturally occurs that would force costly adaptation or significant ecological stress. For example, enhanced sea level rise and/or reduced rainfall would be two possible effects likely to be costly to those regions so affected. Data from the past and projections from climate models are employed to provide insight on these concerns.


Climate models attempt to describe the ocean/atmospheric system with equations which approximate the processes of nature. No model is perfect because the system is incredibly complex. One modest goal of model simulations is to describe and predict the evolution of the ocean/atmospheric system in a way that is useful to discover possible environmental hazards which lie ahead. The goal is not to achieve a perfect forecast for every type of weather in every unique geographic region, but to provide information on changes in large-scale features. If in testing models for current large-scale features one finds conflict with observations, this suggests that at least some fundamental process, for example heat transfer, are not adequately described in the models.


A common feature of climate model projections of global average temperature changes due to enhanced greenhouse gasses is a rise in the temperature of the atmosphere from the surface to 30,000 feet – the true bulk of the atmosphere. This temperature rise itself is projected to be significant at the surface, with increasing magnitude as one rises through this layer called the troposphere. Most people use the term Global Warming to describe this possible human-induced temperature rise.

Over the past 22-years various calculations of surface temperature do indeed show a rise between +0.52 and +0.63 °F (0.29 and 0.35 °C depending on which estimate is used.) This represents about half of the total surface warming since the 19th century. In the troposphere, however, the values, which include the satellite data Dr. Roy Spencer of NASA and I produce, show only a very slight warming between +0.00 and +0.15 °F (+0.00 and +0.08 °C) – a rate less than a third that observed at the surface (Fig. 1). New evidence shown in Figs. 2 and 3 continues to show the remarkable consistency between independent measurements of these upper air temperatures.

Since the last time I testified before this committee, 1998 was above the long term average, but 1999 and 2000 were below. So, rather than seeing a warming over time that increases with altitude as climate models project, we see that in the real world the warming decrease substantially with altitude.

It is critically important in my view to correctly model tropospheric temperature changes because this is where much of the global atmospheric heat is stored, moved about and eventually expelled to space. This layer also has a strong influence on surface temperature through radiation processes. It is conceivable that a model which retains too much heat in the troposphere, may also retain too much at the surface when integrated over long time periods.

It is certainly possible that the inability of the present generation of climate models to reproduce the reality of the past 22+ years may only reflect the fact that the climate experiences large natural variations in the vertical temperature structure over such time periods. By recognizing this however, any attention drawn to the surface temperature rise over the past two decades must also acknowledge the fact that the bulk of the atmospheric mass has not similarly warmed.


This disparity between observations and model results is a curious and unexplained issue regarding the global average vertical temperature structure. But we do not live 30,000 feet in the atmosphere, and we do not live in a global average surface temperature. We live in specific places. Local and regional projections of surface climate are very difficult and challenging. An example from Alabama’s past is useful here only to illustrate the difficulty of providing local predictions with a high level of confidence.

In Fig. 4 you will see several climate model runs which attempt to reproduce Alabama’s temperature from 1860 to the present, and one that attempts to predict its temperature out to 2100. These complex models incorporate solar changes, increasing CO2, aerosol cooling (a highly uncertain hypothesis) and so on. It is clear that the model runs did not do especially well over the time period of observations, with none predicting the actual cooling we have seen in Alabama over the last century. If in trying to reproduce the past we see such errors, one must assume that predicting the future of regional climate will be at least as difficult.

The models may have done fairly well in the global average, and may have done acceptably well in many geographic locations, but these results do not give me the confidence to understand how the weather will be different in the coming century. (Please note that every century is different from its predecessor because of natural variations.) If in trying to reproduce the past we see such model errors, one must assume that predicting the future would produce similar opportunities for errors on a regional basis.


I want to encourage the committee to be suspicious of media reports in which weather extremes are given as proof of human-induced climate change. Weather extremes occur somewhere all the time. For example, in the 48 conterminous states, the U.S. experienced the coldest combined November and December in 106 years, yet that does not prove U.S. or global cooling.

Has hot weather occurred before in the US? In my region of Alabama, the 19 hottest summers of the past 108 years occurred prior to 1955. In the midwest, of the 10 worst heatwaves, only two have occurred since 1970, and they placed 7th and 8th. Hot weather has happened before and will happen again.

Similar findings appear from an examination of destructive weather events. The intensity and frequency of hurricanes have not increased. The intensity and frequency of tornadoes have not increased. The same is true for thunderstorms and hail. (Let me quickly add that we now have more people and much more wealth in the paths of these destructive events so that the losses have certainly risen, but that is not due to climate change.) Droughts and wet spells have not statistically increased or decreased (Fig.5). Last summer’s drought in Texas was not the worst that state has seen. In fact, temperature trends for summers in Texas are actually slightly downward.

One century is a relatively short time in climate scales. When looking at proxy records of the last 2000 years for drought in the Southwest, the record suggests the worst droughts occurred prior to 1600 (Fig. 6). The dust bowl of the 1930’s appears as a minor event on such a time scale. This should be a warning that with or without any human influence on climate we should be prepared for a significant, multi-year drought. (Low cost energy would help mitigate the costs of transporting water to the stricken areas.)

When considering information such as indicated above, one finds it difficult to conclude the climate change is occurring in the US and that it is exceedingly difficult to conclude that part of that change might have been caused by human factors.

In the past 150 years, sea level has risen at a rate of 6 in. ± 4 in. (15 cm ± 10 cm) per century and is apparently not accelerating. Sea level also rose in the 17th and 18th centuries, obviously due to natural causes, but not as much. Sea level has been rising naturally for thousands of years (about 2 in. per century in the past 6,000 years). If we look at ice volumes of past interglacial periods and realize how slow ice responds to climate, we know that in the current interglacial period (which began about 11,000 years ago) there is still more land ice available for melting, implying continued sea level rise.

One of my duties in the office of the State Climatologist is to inform developers and industries of the potential climate risks and rewards in Alabama. I am very frank in pointing out the dangers of beach front property along the Gulf Coast. A sea level rise of 6 in. over 100 years, or even 50 years is minuscule compared with the storm surge of a powerful hurricane like Fredrick or Camille. Coastal areas threatened today will be threatened in the future. The sea level rise, which will continue, will be very slow and thus give decades of opportunity for adaptation, if one is able to survive the storms.


Regional climate change, including that part that might be human related, is essentially impossible to predict at this point. Will there be an increase in 3-year droughts or a decrease? No one knows. I can say with a high degree of certainty that some regions will see an increase and some a decrease, because the climate is always changing.

I am decidedly an optimist. As Fig. 7 shows we in the U.S. will continue to produce more and more of what the world wants (its food, products, technology, defense, medical advances, and so on) with less and less energy. I remember as a college student at the first Earth Day being told it was a certainty that by the year 2000, the world would be starving and out of energy. Such doomsday prophesies grabbed headlines, but have proven to be completely false. Similar pronouncements today about catastrophes due to human-induced climate change sound all too familiar and all too exaggerated to me as someone who actually produces and analyzes climate information.