How do we study "Global Warming?"
There are several ways that scientists study how the Earth's temperature is changing. Although these methods have some uncertainties, each suggests a similar story - that the Earth has warmed dramatically over the last 140 years and that the Earth is now warmer than it has been in the last 1000 years.
Some scientists look to satellites to reveal something about the Earth's changing climate. Although the satellite record is very short (ca. 20 years) and hard to interpret due to changes in instruments and orbits, the latest satellite studies confirm the same story - the globe is warming.
The record of instrumental temperature measurements, which extends back to the 19th century, provides one clear indication that the modern earth is warming: that the mean annual surface air temperatures of the earth have risen approximately 0.5°C (0.9°F) since 1860.
Paleoclimatic data, generated from the study of things like tree rings, corals, fossils, sediment cores, pollens, and ice cores, and cave stalactites, both provide an independent confirmation of this recent warming, and place the 19th to 20th century (1860 to present day) warming in a long term context. The paleoclimatic record not only allows us to look at global temperature fluctuations over the last several centuries, it also permits scientists to examine past climate even further back in time over the course of millennia and longer. This perspective is an important capability in our quest to understand the possible causes of the 20th century global warming. We can look at hypothesized warm periods in the distant past (e.g., 1,000; 6,000; 125,000; and even 165,000,000 years ago) to see if they provide clues for natural processes that could be causing the global warming we are now experiencing. So far, paleoclimatologists have been unable to find any natural climatic explanations for our present-day warming.
National Climatic Data Center
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Asheville NC 28801-5001