Ground Data and Satellite Data Discrepancy

Introduction

Measurements taken over time can be very useful in revealing changes. This is certainly true for the atmosphere that surrounds Earth. Temperature measurements of the atmosphere at Earth's surface have been taken for more than 150 years. Whereas surface temperatures will vary from place to place due to variations in the local geography and land use, data from such a long time period have been useful in revealing air temperature changes.

Since the late 1950s, atmospheric measurements of the lower layers of the atmosphere—the regions in which weather occurs and the influences of surface activities will be felt—have been obtained using instruments sent aloft in weather balloons. Beginning in 1978, satellite measurements of the atmosphere have been possible.

Comparison of the data obtained nearer the ground with the satellite data have not been consistent. Although the ground data indicated the gradual warming of the atmosphere, as would be expected in global warming, satellite data have instead revealed that little if any temperature change has occurred in the troposphere.

Is the atmosphere warming or not? Is global warming real? Scientists have been working hard to find explanations for the discrepancy between the ground and satellite data, which are crucial to making climate models that can accurately predict atmospheric changes and the role of human activities.

Historical Background and Scientific Foundations

People have been measuring surface temperatures using thermometers for more than 150 years. Although measurements from any single point over time can provide a meaningful assessment of climate change (at least the climate very close to the ground), trying to piece together the countless measurements taken all over the world to assess the state of Earth's surface since the 1850s has been very complicated.

However, the task is not impossible. For scientists who have toiled to piece together the millions of ground data measurements, the global patterns obtained from various studies have been similar and have revealed that since the 1960s the nighttime ground temperatures have been increasing at approximately twice the rate as daytime temperatures. Because the sunlit daytime air tends to mix more than nighttime air, the greater warming measured at night supports the idea that the ground temperature is warming more quickly than the rest of the atmosphere.

A big criticism of ground data is that the temperature readings tend to be collected in urban areas, which are warmer than rural areas. Thus, the increasing ground temperatures could just reflect the growth of cities during the last half of the twentieth century. However, data collected over the ocean and in rural areas have also revealed a temperature increase, although not as great as that obtained in cities. Overall, the ground data indicate a warming of approximately 2.7°F (1.5°C) per decade. This is consistent with the predictions of atmospheric warming from climate models of global warming.

The discrepancy in this ground data and data obtained from weather balloons began around 1979. The balloon measurements have indicated that the temperature of the troposphere (a layer of the atmosphere in which most weather occurs) has been fluctuating a bit, but has not been increasing decade by decade.

The difference has become even more significant with the coming of the satellite era. Atmospheric measurements that have been obtained with orbiting satellites beginning in 1979 agree with the measurements obtained from weather balloons.

WORDS TO KNOW

ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.

CLIMATE MODELS: Mathematical representations of climate processes. Climate models are computer programs that describe the structure of Earth's land, ocean, atmospheric, and biological systems and the laws of nature that govern the behavior of those systems. Detail and accuracy of models are limited by scientific understanding of the climate system and by computer power. Climate models are essential to understanding paleo-climate, present-day climate, and future climate.

KYOTO PROTOCOL: Extension in 1997 of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by almost all countries with the goal of mitigating climate change. The United States, as of early 2008, was the only industrialized country to have not ratified the Kyoto Protocol, which is due to be replaced by an improved and updated agreement starting in 2012.

TROPOSPHERE: The lowest layer of Earth's atmosphere, ranging to an altitude of about 9 mi (15 km) above Earth's surface.

WAVELENGTHS: Distances between the crests or troughs of repeating waves. Sound waves, water waves, and electromagnetic waves (e.g., light) all have characteristic wavelengths. The greenhouse effect depends on the fact that some gases, such as carbon dioxide, are transparent to some wavelengths of light (those radiated by the sun) but not to others (those radiated by warm substances such as land, sea, and air). Energy from the sun arrives at wavelengths that are allowed to enter Earth's climate system, but once at the surface or near the surface is transformed into heat and re-radiated as energy that cannot so easily leave.

For climate scientists, the discrepancy between ground data and satellite/balloon data is serious, because it goes to the heart of the global warming debate. Climate models of global warming predict that the troposphere should be increasing in temperature more than the atmosphere very near the ground. If it is not, then either the models of global warming are incorrect, or global warming itself is a myth rather than a reality.

However, a number of studies that have been published since 2005 indicate that the discrepancy may not be real, that it is simply a result of how the balloon and satellite measurements were made.

Upper atmosphere temperature measurements are not direct recordings of temperature. Rather, they measure differences in the release of energy at different wavelengths, with the data being mathematically treated to provide an indication of temperature. The detection of different wavelengths uses different channels of the instrumentation, essentially windows that let in certain wavelengths while excluding others. The discrepancy with ground data is mostly due to the data obtained using one of the windows. Re-analysis of the balloon/satellite data has demonstrated a good match between these data and the ground data. As of 2007, the discrepancy appears to have been resolved.

Impacts and Issues

The debate that has arisen over the discrepancy between ground data and satellite/balloon data has been contentious. Some scientists have argued that only ground data from developed countries such as the United States can be trusted. Other scientists point out that this excludes large regions of the globe and could indicate that global warming is not occurring when it really is. Arguing that climate science was not complete, the U.S. government under President George W. Bush opted out of the Kyoto Protocol.

The re-analysis of the balloon satellite data now supports the idea that the atmosphere is indeed warming at a rate predicted by models of global warming. Although a majority of climate scientists have accepted the re-analysis, a small minority of scientists continue to argue that the atmosphere is not warming due to anthropogenic (human-caused) activities. Scientists continue to collect data from satellites in an effort to learn more.

See Also Satellite Measurements.

BIBLIOGRAPHY

Books

DiMento, Joseph F. C., and Pamela M. Doughman. Climate Change: What It Means for Us, Our Children, and Our Grandchildren. Boston: MIT Press, 2007.

Gore, Al. An Inconvenient Truth: The Planetary Emergency of Global Warming and What We Can Do About It. New York: Rodale Books, 2006.

Seinfeld, John H., and Spyros N. Pandis. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change. New York: Wiley Interscience, 2006.

Periodicals

Fu, Qiang, Celeste M. Johanson, Stephen G. Warren, and Dian J. Seidel. “Contribution of Stratospheric Cooling to Satellite-Inferred Tropospheric Temperature Trends.” Nature 429, no. 6987 (May 6, 2004): 55–58.