A history of climate is recorded in glacial ice. Glaciers form as layers of snow accumulate on top of each other. Over time, the buried snow compresses under the weight of the snow above it, forming ice. Particulates that were captured by falling snow become a part of the ice, as do bubbles of trapped air. Layers of ice accumulate over seasons and years, storing information about the climate conditions at the time of formation (including precipitation, temperature, chemical composition of the atmosphere, volcanic eruptions, and solar activity). Scientists can study cores of glacial ice to learn about Earth's climate history. For example, scientists can measure the concentration of atmospheric gases in the bubbles of air and infer average temperatures through analysis of the ratio of oxygen isotopes.
Ice cores are cylinders of ice that are extracted from glaciers and ice sheets using hollow drill bits. Layers in the ice correspond to seasons or years, with the oldest ice at the bottom of the core and the youngest ice at the top. The ice-core record provides a natural time line of climate history. The National Ice Core Laboratory (NICL), a joint facility operated by the U.S. Geological Survey and the National Science Foundation, is the primary repository of ice cores in the United States. Located in Lakewood, Colorado, it houses over 45,000 feet (14,000 meters) of ice cores taken from locations in Greenland, Antarctica, and North America. The samples of ice are stored in a carefully temperature-controlled environment to ensure the integrity of the cores for current and future scientific investigations.
Using ice cores, scientists have constructed a history of climate extending back about 800,000 years. Ice cores show evidence of long cold periods interspersed with shorter warm periods; these natural climate variations are likely related to changes in Earth's orbit around the Sun. However, Earth's average temperature has risen at an alarming rate of about 1.4 degrees Fahrenheit (0.8 degrees Celsius) over the past century. The ice-core record shows that Earth's climate varies naturally and that it has gone through a number of warming and cooling trends in the past; however, it also indicates that the current warming is human induced. Scientists have found that there is a positive correlation between rising levels of greenhouse gases (such as carbon dioxide and methane) and global temperatures. The magnitude and rate of the abrupt recent increase in greenhouse gas levels, as well as the geochemistry of the carbon in the ice-core record, suggest it is caused by human activity—in particular, the burning of fossil fuels, which has greatly increased since the Industrial Revolution. The rapid temperature increase over the past century could cause portions of Greenland's ice sheet to melt faster than they have during previous warming events.
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NARRATOR: At the National Ice Core Lab, in Lakewood, Colorado, a giant freezer stores over 45,000 feet of ice, drilled from 34 sites around the cryosphere. Dating back hundreds of thousands of years, these ice cores are time capsules that allow scientists, like Jim White, to peer deep into the history of ice.
JAMES WHITE (University of Colorado at Boulder): This piece of ice is interesting, 'cause it has a couple of things you can see, right away. One is there are bubbles throughout here. These bubbles are little packets of air. It's these bubbles we can take out and measure CO2 and methane and nitrous oxide. It's the only medium that really collects the atmosphere itself.
The other thing you can see in here, quite clearly, is you can see the layers, and the thickness is going to tell you how much snow fell that year, so you get a couple of pieces of climate information and a dating scale, just out of visually looking at this ice core.
NARRATOR: Most importantly, scientists have identified a direct historical link between increases in greenhouse gases, like carbon dioxide, and steep rises in global temperatures.
At every peak, big rises in sea level followed as Greenland's ice sheet shrank.
The ice core records also reveal a particularly telling moment in Greenland's history. Roughly 125,000 years ago, temperatures rose by about seven degrees Fahrenheit; the entire southern portion of the ice sheet melted, and global sea levels rose by over 10 feet.
It was caused by a change in the Earth's orbit around the Sun, which increased temperatures and released carbon dioxide from the oceans.
The more recent ice core record shows the potential for a similar meltdown. Right now, greenhouse gas levels in the atmosphere are even higher than they were 125,000 years ago, higher than they've ever been in the last half-million years. Temperatures are already following suit.
The only explanation is the burning of fossil fuels.
JAMES WHITE: What we see in this ice core is very solid evidence that what's happening today in the atmosphere is different. It's not a normal part of the climate cycle. It's something caused by human beings.
NARRATOR: Rising temperatures are once again pushing Greenland towards a major meltdown, but what the ice cores can't tell us is how long it will take. The last time Greenland lost a significant portion of its ice, White suspects, it happened over thousands of years. But this time, it could happen much faster.
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