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They can measure exactly how much carbon dioxide has been in the Earth’s atmosphere historically. Climatologists (scientists specializing in climate science) have drilled deep — as deep as 2 miles (3 kilometers) — into ancient ice in places such as Antarctica and Greenland. They’ve pulled up ice cores — long, thin samples of many layers of ice that has been packed down over thousands of years, which look like really (really) long pool noodles. (Figure 3-2 shows two scientists drilling for an ice core sample.) When the layers are clearly visible, the ice core looks like a pool noodle with horizontal stripes.

National Oceanic and Atmospheric Administration

FIGURE 3-2: Drilling for an ice core sample.

Scientists can date an ice core by counting the layers of ice — just like you can tell the age of a tree by counting its rings. The layers of ice tell them exactly when the ice was formed. Each layer of ice includes little pockets of trapped air. These frozen air bubbles are like time capsules of the ancient atmosphere. They’re full of gas, including carbon dioxide, that has been trapped for hundreds of thousands of years. Each layer of ice in the ice core also contains deuterium, a hydrogen isotope that enables scientists to determine what the temperature was when that ice layer was formed.

An atmospheric temperature change of just 1.8 degree F (1 degree Celsius) leads to a change of 9 parts per million (ppm) in the amount of deuterium stored in the ice. By contrasting the ancient temperatures revealed through the analysis of the layer’s deuterium and carbon dioxide, scientists can glimpse the relationship between historical levels of carbon dioxide and temperature. The two run side-by-side almost like the lanes of a race track.

Scientists still don’t know the exact cause and effect relationship between GHGs and temperature throughout the planet’s history. The cause of the last ice age, for instance, probably wasn’t a drop in atmospheric carbon dioxide, but a result of the Earth tilting away from the sun in a phase in the planet’s Milankovitch cycle (which we discuss in the section “Considering Causes of Global Warming Other Than GHGs,” earlier in this chapter). This cooling then spurred the atmosphere’s carbon dioxide to drop, and the two events in tandem brought about the ice age. Ultimately, scientists still aren’t sure whether temperature affects carbon dioxide, or whether carbon dioxide affects temperature — it’s a question of which came first, the chicken or the egg.

What scientists do know for certain is that a distinct pattern and relationship between carbon dioxide and temperature exists; when one is high, so is the other, and when one is low, the other plunges, too. Scientists also know that the Milankovitch cycle has little to do with climate change over the past 200 or 300 years. In that time, human-produced carbon dioxide levels have skyrocketed, and temperature is starting to follow. As a result, scientists are certain that human-produced GHGs are currently warming the Earth. This close relationship between GHG concentrations and temperature suggests these higher levels of carbon dioxide will cause temperatures to continue rising.

Figure 3-3 shows the historic connection between carbon dioxide concentrations and fluctuations in temperature, as captured in ice-core deuterium levels.