Global warming and the role human activities play in climate change are often in the news. It is well established that Earth's average temperature has risen about one degree Fahrenheit since the early 1900s, a century in which atmospheric CO₂ concentrations also rose. This trend is startling to many climatologists. Any role humans may be playing in any alteration of a global system would be unprecedented and could carry unpredictable consequences. However, determining the extent of the relationship between human activities and recent climate change presents a difficult challenge -- certainly one requiring more than the 100 years of written data that are available to us.

To assess the significance of the past century's data, scientists are exploring records of Earth's climate that extend much further back in time than any written histories. This evidence comes from a variety of sources, including tree rings, deep-sea and lake sediments, and perhaps most importantly, ice cores.

Scientists obtain ice cores by boring into glaciers and polar ice sheets using hollow drill bits. The largest such operations have produced a series of short cylinders of ice with a total length of more than 3,000 meters (9,840 feet). Cores taken from ice sheets in Antarctica and Greenland provide a record of Earth's climate extending nearly 800,000 years into the past.

Ice is particularly important in climate research because it encapsulates components of the atmosphere that existed when the ice was formed. Snow falling on top of a glacier traps dust particles, chemicals, and the air itself within and between its crystals. As layers are added, the snow is compressed into ice and the evidence is locked inside. Scientists unlock this evidence when they remove the ice cores. For example, scientists can determine temperature by analyzing the ratio of different forms, or isotopes, of oxygen atoms in the core. By analyzing tiny air bubbles trapped in a layer of ice, they can also determine the composition of the atmosphere when the layer was formed.

What seems clear from ice core analyses is that Earth's climate has varied significantly over time. Over the course of the past 800,000 years, our planet has seen eight 90,000-year ice ages interspersed with 10,000-year warm periods. In addition, scientists have found a direct correlation between global temperatures and greenhouse gas concentrations, lending support to the observations made in the last century. What's more, in 2004, scientists studying an ice core taken from Antarctica concluded that current CO₂ concentrations are higher than they have been in 440,000 years, an observation with significant implications for understanding future climate change.

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