One hardly knows what to make of the current debate regarding global warming. On one side, we hear that Earth's temperature is rising precipitously, ensuring global environmental calamities. On the other hand, skeptics tell us that global warming is "junk science", that satellite data show the Earth is actually cooling, and that the buildup of carbon dioxide in the atmosphere from human activities will fortuitously forestall the imminent arrival of the next Ice Age. Proponents of governmental controls on greenhouse-gas emissions have been portrayed as anti- technology extremists who would prefer to see us all live in grass huts and eat vegetables, while contrarians are depicted as the paid tools of industrial polluters, selling out humanity for the sake of their greedy benefactors.
One fact that emerges from the noise of this debate is that over the last 138 years, mean global temperature has risen by about 1 degree Fahrenheit. This is where the consensus ends. Environmentalists and some scientists claim that at least part of this warming is due to human activity, and that it is a mere harbinger of a more precipitous warming which is sure to follow. However, we might ask the following difficult question: one degree warmer than what? Measurements of temperature sufficient in quantity and quality to reconstruct global climatic conditions extend back only to the year 1860. As a result, it is not clear if the modest warming trend of the last 138 years represents a substantial warming above long-term climatic conditions, or is instead a
recovery from a period of unusually cold temperatures known as the "Little Ice Age", which persisted from about 1450 to 1850 A.D. The onset of the Little Ice Age ended the Viking's settlements in Greenland, and it was the Little Ice Age which was responsible for the bitter winters George Washington and his troops endured at Valley Forge.
We recently learned that 1997 was the warmest year of record since 1880. But how would 1997 compare if we extended our climatic record back farther into the past? Geologists have long suspected that climate over much of the last 10,000 years was warmer than it is today. However, the thermometer was not invented until the year 1714. It is therefore difficult to know precisely what temperatures were in the distant past and understand the context of the modest temperature rise we have observed over the past 100 years. Geologists who study ancient climates have to rely upon indirect methods. These include the thickness of tree rings, and historical records of the dates at which certain bodies of water froze. For example, we have records dating back to 1444 which inform us of the calendar date at which Lake Suwa in Tokyo first froze. The problem is that the interpretation of such records is ambiguous. If a lake froze early in the winter season, it must be interpreted as an indication of a winter which was colder than average. However, the latter part of that winter may well have been warmer than average. Similarly, a thin tree ring is usually interpreted as an indicator of colder conditions. However tree growth also depends on precipitation, and a slow period of growth may have been due to less rainfall, not colder temperatures.
In the absence of a working time machine it would appear to be impossible to constrain precisely what mean global temperatures were in the distant past, and thus understand the context of the temperature rise we have observed over the last 138 years. However, the record of past temperature changes at the Earth's surface is recorded in underground temperatures. When temperature rose at the end of the last ice age 10,000 years ago, this warming penetrated the solid Earth and started propagating downward. At depths of about 2000 to 3000 feet, the temperature today is still changing from this ancient warming. Measurements of temperatures in boreholes can thus be used to reconstruct climatic conditions at the surface for the past several thousand years. In a landmark study published in the scientific journal Geophysical Research Letters in August of 1997, Professor Henry Pollack of the University of Michigan and his colleagues estimated mean global temperatures for the last 10,000 years from geothermal measurements in more than six thousand boreholes from around the world. This work is significant, because it is the first reconstruction of ancient climatic conditions based upon actual temperature measurements instead of ambiguous proxies. The University of Michigan scientists found that the modest one degree temperature rise recorded by meteorological instruments over the last 138 years is present in the borehole measurements. However, the borehole data also showed that present day climatic conditions are in fact colder than average when compared to climatic conditions that prevailed over the rise of human civilization. Most of the warming observed by meteorologists appears to be a recovery from the Little Ice Age, which may be one of the coldest periods of the last 10,000 years.
So, how hot is it? Perhaps not so hot as one might think from perusing articles in the popular and advocacy press. When compared to the period of time over which human civilization rose, present day temperatures are colder than average. Even if mean global temperature were to rise another degree, it would still be colder than it has for much of the last 10,000 years.
David Deming is an associate professor of Geology & Geophysics, School of Geology & Geophysics, University of Oklahoma.
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