The Past 13 Centuries
Since the cave men did not have thermometers thousands of years ago, scientists today have to rely on indirect methods to determine what the earth’s climate once was. Scientists need to work more like crime scene detectives to discover what the earth was like in the past. Today, scientists carefully search for obscure pieces of evidence left buried in the snows of Alaska, among centuries-old debris on the ocean fl oor, or within multiple layers of coral. As part of this forensic effort, scientists carefully compare the different types of atoms that may have been present in a geologic sample thousands of years old.
What they are finding is that the most recent 50-year period in the northern hemisphere is warmer than it has been for the previous 1000 years. What is happening currently is statistically unusual and cannot be considered simply part of the normal ebb and flow of natural cycles. The earth’s temperature is off the charts.
This chart takes the form of a “hockey stick,” which in statistics is typically associated with an abrupt change or a breakout pattern. The last century is that part of the hockey stick that contacts the puck, and the previous 1200 years represent the stick.
The darker line on the right hand side of the graph is the instrumental record, which includes calibrated thermometers and, more recently, satellite measurements. Reliable direct temperature measurements can only take us back a little more than a century and a half. The instrumental record inherently has a greater degree of precision than the indirect (paleontologic) records.
The multiple lines on the left show data derived from a variety of techniques used in this thermal detective work, including tree-ring patterns, ice-core sample composition, and coral reef growth-band analysis. (We will discuss how this is done a little later in this chapter). To compare apples with apples more easily, the temperature anomalies rather than the actual temperatures are shown. A baseline is established for all the data for the time period 1961–1990 (where the anomaly is defined as zero). This graph then shows how much greater or lesser the temperatures for a particular year are compared with the baseline period.
