The political and scientific debate over whether the Earth is warming due to human activities was stirred up earlier this month when a research paper, published in the journal Nature by physicists Frank Wentz and Matthias Schabel, claimed the satellite temperature data were flawed.
Satellite data are the only truly global temperature data scientists have. But contrary to surface readings, satellites have shown a slight cooling trend since readings began in 1979. Mr. Wentz and Mr. Schabel claimed that adjusting the data to account for gradual changes in the orbits of these satellites would result in a slight warming trend. As a result, newspaper headlines trumpeted "the satellite data finally support global warming." This is quite misleading.
Mr. Wentz and Mr. Schabel of Remote Sensing Systems, a California-based research firm, did convincingly establish an effect that we had failed to account for in processing the satellite data. The very slow fall of the Earth-orbiting satellites (called "orbital decay") changes the angle of the satellites' view of the Earth's surface, causing a very slight - and false - cooling in the global average temperature record. But even if Mr. Wentz and Mr. Schabel's adjustment was correct, their estimated temperature trend, an increase of 0.08 degrees Celsius per decade during 1979-1997 would still have been only one-third of the 0.24 degree Celsius increase per decade that computer climate models predict for the next century in the lower atmosphere.
Were it not for the standoff between the White House and Congress over ratification of the Kyoto Protocol, and the concern over recent record-high temperatures, this would be just another technical debate hashed out on the pages of the scientific journals. But for better or for worse, climate science has run headlong into politics and policy. Taxpayers, who have been footing the bill for all this climate research, deserve to kept informed.
The precision satellite monitoring method, which I developed with John Christy of the University of Alabama in Huntsville's Earth System Science Laboratory, began explicitly incorporating orbital decay (and other partially offsetting effects) into the data analysis in February. With those corrections made, our detailed review of the satellite data between 1979 and 1997 still shows a cooling but at a smaller rate - dropping at 0.01 degrees Celsius per decade. Given the measurement uncertainty, this is no temperature trend.
Also, though not mentioned in most news accounts, instruments aboard weather balloons provide an independent measure of global temperatures in the lower troposphere, the same layer where satellite readings are taken. Between 1979 and 1997, readings from thousands of weather balloons, and analyzed separately by teams of scientists in three countries - Great Britain, Russia and the United States - actually show a stronger global cooling.
One problem has already cropped up in the Wentz/Schabel research. It appears that our processed satellite data already had unintended corrections for orbital drift, both in height and in time of day. Proper adjustments for these effects must be done on the raw satellite measurements, not on the processed data sets we provide to the research community. Unfortunately, it will likely take more than a year for our publication of such a complex analysis. This is in contrast to science news journals, such as Nature, that promise quick publication, but at the expense of much needed detail.
With the many statements from politicians and some scientists expressing certainty about global warming, what the public needs to realize is the small disparity in temperature trends being debated here: a tenth of a degree Celsius per decade, or less! Moreover, it is extremely difficult to measure human-induced global warming when the climate system is perfectly capable of going through wild fluctuations on its own. Warming over the last century, suggested by surface thermometer readings, is about 0.6 degrees Celsius (about 1 degree Fahrenheit). This is so small no one would have noticed it without a painstaking effort to patch together a wide variety of disparate measurements that were never intended to detect such a small signal over such a long time.
The 1997-98 El Nino, its effects still lingering, has contributed to record warmth in recent months. January through July of this year have shown the highest readings in the 20-year satellite record, which now has a trend of +0.04 deg. C/ decade. The surface thermometer data suggest most of the last year has been the warmest period since reliable surface measurements have been kept, about 100 years or so. But both thermometer and satellite readings will very likely drop in the coming months as conditions return to normal, or a period of even cooler temperatures, the so-called La Nina, sets in. Has global warming contributed to this recent record warmth? The vast majority of climate scientists would put the blame on El Nino, and I would add they were blaming unusual weather on El Ninos long before it became fashionable to blame it on global warming.
It is curious that the thermometer data have not had to endure the level of intense scrutiny that the satellite data have undergone in recent years. Is this because the surface data support global warming?
The surface data are less than perfect, to put it mildly. Unlike the satellites, which orbit the Earth, each taking some 40,000 readings every day, thermometers cover less than half of the Earth's surface and are unevenly distributed, with more measurements being taken in the Northern Hemisphere than in the Southern Hemisphere. On land, temperature readings have to be corrected for the "heat island" effect, a local warming that occurs over time as cities spread outward. Then there's the difficulty in patching together records of measurements taken by different collection methods. Until the 1940s, ships would measure sea surface temperatures by dropping a thermometer into a bucket of sea water. Today, sea water temperatures are measured by thermometers affixed to buoys, or in the intake ports of ships.
Recently, the addition of ocean buoy measurements in the tropical East Pacific and their role in recording a possible false warming has come under investigation. There is also evidence that air temperatures taken just above the ocean surface have not risen nearly as fast as sea water temperatures, and it is sea water temperatures that have, up until now, been included in global temperature estimates. Finally, although land-based thermometer readings have had some correction for the "heat island" effect, there is reason to believe these corrections have not been sufficient. Even small towns and rural thermometer sites, which are uncorrected, have in general experienced population growth. In short, thermometer estimates of global warming are not "truth" either, and will likely be revised.
Bias is widespread in the global warming debate. Scientists are human too, and have their own pet theories, political and world views, and heartfelt beliefs. Nobel Laureates who expound on the threat of global warming typically have no training in the atmospheric sciences. And while a majority of the climate community probably agrees that some amount of global warming is likely in the next century, there is no consensus on how much warming will occur.There are still too many uncertainties about how the climate system will respond to the gradual increase in greenhouse gases, especially carbon dioxide. Ultimately, what the debate boils down to is whether scientists believe the Earth to be fragile or resilient.
Many scientists involved in the process feel the official U.N. Intergovernmental Panel on Climate Change's firm predictions of substantial warming were guided more by policy-makers and politicians than by scientists. To some extent, this can be excused since it is often difficult to pin ascientist down to a definite answer. The American public is clearly divided on the issue, with the balance of opinion often depending upon how survey questions are phrased. The public's confusion is justified, since nearly the same level of confusion exists in the climate science community.
Even though I am a global warming skeptic, if global warming is proven to be a dire threat, I hope I am the one who proves it. But in today's politically correct climate, I can guarantee you no one will ever receive a Nobel Prize for proving it was not a threat.
Roy W. Spencer, senior scientist for climate studies at the Marshall Space Flight Center of the National Aeronautics and Space Administration, analyzes global temperature data from weather satellites. The views expressed in this article are his own.
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