when a new satellite was launched, because we required a period of overlap for precise intercalibration. (Only two satellites are operational at a given time).

The temperature of the global atmosphere is shown for the lower troposphere and lower stratosphere in Figure 2 (courtesy R. Spencer). Since we live in the lower troposphere, that time series has received the most attention. You will notice that there are large variations, both month-to-month and year-to-year. Because these variations are independently observed by two satellites, we know they are real. The trend in the time series is slightly downward (0.05°C/decade or -0.09°F/decade). It is this relatively flat trend when compared to surface data (which show warming trends since 1979 of +0.09°C to +0.19°C/decade, depending on which data set is cited) that has attracted attention to the Spencer-Christy MSU data set.

Though the MSU temperature record has demonstrated high precision, there is also an element of ambiguity in the measurement. The layers measured by the MSU are several kilometers deep. Any intra-layer variability, therefore, would be masked by the vertical average. For example, a warming trend at low levels and a cooling trend at upper levels of one layer would be seen as no trend in the MSU vertical average.

One of the reasons the surface thermometer data have shown greater warming in the past 17 years is due to the fact that in continental regions the surface temperature responds with greater variation than the deep layer of air above. Over oceans (and in the global average), the opposite occurs. In the past 17 years there has been a tendency for the atmosphere over land areas to show warming (which is greater in the surface air response) while the atmosphere over oceans has exhibited cooling (greater effect in the MSU record). This pattern is thought to be due to natural variations. The net effect in the global average is a relative difference in the trends between surface air and the deep atmosphere. Thus, the uneven warming/cooling distribution of the past 17 years accounts for part of the difference.

Other differences are due to areas poorly sampled or not sampled at all by the surface network, as well as to some urban warming or land-use changes around many of the thermometers. It is a monumental achievement to construct a record of surface air temperatures, and most of these data sets have been subjected to many careful corrections to account for these non-natural temperature impacts.

Because of its precision and true global coverage, we believe that the MSU data set is the most robust measurement we have of the Earth's bulk atmospheric temperature. At the same time, it is still a relatively short data set for climate studies. As indicated in Figure 2, the data contain both long and short period fluctuations. To be useful in the global warming debate one must understand and carefully account for fluctuations in the data that may be masking or dominating the anticipated enhanced greenhouse signal.

In a recent study, Dr. Richard McNider, also of the University of Alabama in Huntsville, and I looked for the causes of these fluctuations. We found that by accounting for the influence of tropical Pacific Ocean temperatures (El Niño) and the cooling effect of volcanoes, we could explain over 60% of the monthly variations (Fig. 3). These natural, shorter-term fluctuations indicate to us how much the global temperature responds to specific causes. Once calculated and removed, we see that without El Niños and volcanoes, the temperature trend of the past 17 years is upward (+0.09°C/decade or +0.16°F/decade, Fig. 3, bottom). What is causing this upward trend? We do not know for sure. It may be the enhanced greenhouse effect. At the same time there could still be a longer term trend in the data due to variations in aerosols, water vapor, or other unknown factors that are masking the true magnitude of the greenhouse effect. The latest results from global climate models, which include the cooling effects of air pollution, indicate warming rates for the Earth of +0.08°C to +0.30°C/decade for the latter part of the 20th century. These are about half of the warming rates predicted a few years ago, when only increases in greenhouse gases were modeled. The present warming rate of +0.09°C/decade observed in the MSU data is barely within this model range, and yet is not inconsistent with fully natural variations on decadal time scales. Therefore, uncertainty remains as to the cause(s) of the trend the MSU has measured.

Why is there a discrepancy between the models' estimate of global warming and what the MSU data have shown? One must remember that temperature is essentially a response parameter. The MSU data in Figure 2 show us what has been happening to the climate but not why. A key goal of efforts to study the planet from space is to provide heretofore unmeasured data that can provide an understanding of why the Earth system behaves as it does. I believe that new observables such as aerosols, rain structures, water vapor distributions and surface characteristics, when used in conjunction with the MSU data set will provide answers to these questions. Our work demonstrates that satellites can be used to monitor the Earth on decadal time scales and that the vantage point of space offers the only truly global view of the Earth system that can give robust measures of key variables.

The Spencer-Christy MSU data set has been used by some as evidence that global warming is not important, which then undercuts the need and urgency of programs such as MTPE. I strongly disagree with this interpretation. By showing that the Earth's rate of warming is slower than predicted by earlier models or surface data sets, it does, perhaps, remove the sense of urgency to enact greenhouse gas controls or to shut off scientific debate. But most importantly, the slower warming rate in the last two decades in effect gives us the security of time for data from near-future missions to be used within the debate.

I believe that honest and open scientific debate with precise data is the key to making sound societal decisions. The cultivation of diversity of scientific thought is critical to vigorous debate. The MSU data set would not have been developed without the competitiveness and entrepreneurial spirit fostered by having separate NASA science centers and a broad university research program. Industry should recognize that good science and good data are their allies, whether in debates on acid rain or global warming. It is now more critical than ever that we study the planet's health with new diagnostic devices. Any delays in doing so may mean that the length of data records available to scientists is reduced and cannot be used in the societal debates.

The disagreement between models and the MSU simply illustrates how little we understand about the complexities and factors that control the Earth's climate. Every month Roy Spencer and I process the newly arrived data and eagerly look at the month's temperature to see what is happening to the Earth. If we knew everything we needed to know about the Earth's system, we would not be as anxious about the results. I look forward to the time when new data from planned satellite sensors, coupled with an understanding of the Earth's climate system developed under research programs emphasizing global change, make surprises in the MSU global temperature as rare as being surprised by landfalling hurricanes in this era of weather satellites.

5. The temperature of the lower stratosphere

The record of the lower stratosphere is fascinating in its own right. Clearly, here is an example of global change on the scale of years to decades (Figure 2). The two conspicuous warming events were due to explosive volcanic eruptions El Chichon (1982) and Mt. Pinatubo (1991). The aerosols injected by these explosions high into the stratosphere caused the warming through radiative interactions. Notice, however, that once the aerosols settled out, the global stratospheric temperature fell to levels below those observed at pre-eruption. It is widely thought that the loss of stratospheric ozone, both naturally from volcanic events and from human-generated chemicals, has caused this overall cooling. The increase in greenhouse gases, which will cause stratospheric cooling, is probably a factor as well.

The 1995 annual stratospheric temperature was the lowest annual value ever measured by satellite, and January 1996, was the coldest single month on record. Something is changing in the lower stratosphere -- the temperature tells us that much, but cannot specifically indicate the cause. (Others have much more experience here.) The extent of the stratospheric cooling trend points to the need to fully understand its cause.

Continued monitoring of global temperature through the Spencer-Christy method is expected as long as our good fortune holds and the two orbiting instruments do not fail (which almost happened recently). Thus, we should

continue to provide the scientific community with precise temperatures for deep atmospheric layers.

In any weather variable, e.g. temperature, rainfall, etc., it is the shorter-term fluctuations (week-to-week) that cause the greatest impact on human productivity. One valuable benefit of a program of escalating Earth observations is the resulting improvement in weather forecasts -- particularly out to two to three weeks and even to seasonal averages. The potential economic impact of improved long-range forecasts would be enormous. Virtually every sector of our economy is sensitive to weather, especially those related to energy production and consumption, agriculture, transportation, insurance and recreation. Improved knowledge of coming weather situations would be used to add value to the products and services generated by these industries.

A strong and continuing program in space-based atmospheric research has this more subtle benefit as well. There will be extreme climate events in the near future because that is the nature of weather and climate. Without a continuing program of research that places climate variations in proper perspective and reports with improving confidence on their causes, we will be vulnerable to calls for knee-jerk remedies to combat "climate change," which likely will be unproductive and economically damaging. We can protect ourselves from such pitfalls by improving our ability to measure what the climate is doing and determine the causes for its variations.

In simple terms, the "Global Climate" is our patient. We have taken its temperature in a few places and have seen just enough change to cause concern. Before prescribing any powerful medicine though, the patient should be given a complete physical as soon as possible, so we may then make the proper diagnosis and chart a correct course of action for the benefit of all.

References:

Christy, J.R., 1995: Temperature above the surface layer. Climatic Change, 31, 455-474. Christy, J.R. and J.D. Goodridge, 1995: Precision global temperatures from satellites and urban warming effects of non-satellite data. Atmospheric Environment, 29, 1957-1995. (Fig. 1 (Top) of testimony taken from this article.)

Christy, J.R. and R.T. McNider, 1994: Satellite greenhouse signal, Nature, 367, 325 (27 January 1994). (Fig. 3 of testimony, updated, taken from this article.)

Spencer, R.W. and J.R. Christy, 1990: Precise monitoring of global temperature trends from satellites. Science, 247, 1558-1562 (30 March 1990).

Spencer, R.W. and J.R. Christy, 1992: Precision and radiosonde validation of satellite gridpoint temperature anomalies. Part I: MSU channel 2. Journal of Climate, 5, 847-857. (Fig. 1 (Bottom) of testimony taken from this article.)

Two different satellites can observe the earth with independent MSU instruments. This graph shows the "signal" of temperature variation (upper curve) as the average daily temperature anomaly measured by two satellites, NOAA-10 and NOAA-11, over a two year period. The "noise" of the measurement is the difference in temperatures determined by the satellites (lower curve). Notice that the temperature varies substantially from day-to-day and week-to-week, but that the difference is almost nonexistent. The shows that MSU instruments are very precise.

In places on the earth where balloon measurements are taken, a direct comparison can be made between the balloon temperatures (dashed) and the temperature the satellite measures (solid) at that location. As shown for St. Cloud MN, the agreement is phenomenal (correlation of +0.98).