Most analyses of the data show that since that time, a statistically significant increase in temperature has occurred. This trend has accounted for a roughly 1 degree Fahrenheit increase in global temperature, about half what would be predicted on the basis of the concentration of greenhouse gases. A closer look at the data reveals that temperatures were relatively stable between 1860 and 1920, when an upward trend developed which lasted until the late 1940's. A downward trend prevailed until the mid 1970's, when a warming trend continued into the early 1990's. The question arises as to the place of these trends in historical context.
Archeological analyses reveal that the earth's temperature has varied greatly over time.
It was about 18 degrees F warmer than at present about 100 million years ago when the dinosaurs roamed the earth. It is estimated that at the other extreme, the earth was more than 10 degrees F cooler than at present during the last ice age, which ended about 10 thousand years ago. Most climate models predict that if global warming occurs, it will not produce globally uniform effects. Most places will become warmer, but some will actually cool down. Global precipitation will increase due to increased evaporation from the oceans, but some areas will receive substantially less rainfall than today.