Scientists can reconstruct a record of temperatures that occurred in the past by analyzing the ratio of oxygen isotopes trapped within air bubbles in the ice. The graph below shows how the temperature has changed over the past 165,000 years. The graph shows temperature anomalies—variations in the temperature above or below Vostok’s present day average temperature of -56ºC.

  !   Examine the graph carefully to interpret how time is represented along the bottom axis. Look for places on the graph that represent a warming trend, when the temperatures increased over time, reaching a peak at some maximum temperature before a cooling trend began.

1. Over the past 165,000 years, how much has the temperature varied above and below the current average temperature?

  !   Move your cursor over the peak that occurred around 133,000 years ago to highlight that part of the graph.

Follow the graph from right to left, examining the large-scale temperature changes. (Ignore the smaller peaks and dips.) Around 145,000 years ago, the temperature began to increase rapidly, reaching a peak about 133,000 years ago. Then the average temperature decreased fairly steadily, reaching its lowest point around 107,000 years ago.

  !   Move your cursor over the low temperature dip that occurred around 64,000 years ago to highlight that part of the graph.

Beginning from a peak around 82,000 years ago, the average temperature started dropping, and it reached a minimum at 64,000 years. Following this, the temperature increased until it peaked again at around 52,000 years.