7/8/2013 6:49:37 PM
The Cold Truth
Ice Cores Tell Us We Are Changing the Atmospheric Composition
Atmospheric carbon dioxide (CO2) levels reached an unsettling 400 parts-per-million (ppm) in late May 2013 at the Mauna Loa Observatory in Hawaii. The rise in atmospheric CO2 is a concern because it can cause the Earth to warm and ice to melt.
The atmospheric CO2 amount can be pictured by first imagining a box with one million air molecules scooped up from the atmosphere. That box would contain 400 molecules of CO2; the remaining molecules would consist primarily of oxygen, nitrogen, and water vapor. The amount of CO2 in the atmosphere is slightly lower this month (July 2013) as vast areas of vegetation in the Northern Hemisphere take-up a few ppm in summer, only to release it again in winter.
The Mauna Loa Observatory has measured atmospheric CO2 since 1958, and the average CO2 has risen steadily each year. Scientists know from studying air bubbles trapped in the ice sheets of Antarctica for the past 800,000 years that prior to the 20th century, CO2 only just touched 300 ppm. To obtain this data, they drilled over 3,000 meters (nearly two miles!) to extract an ice core from Antarctica at Dome C, one of the highest points on the continent.
The same ice cores that trap air bubbles can also be used to learn about past temperatures. The ice core records show us that CO2 levels varied from about 200 to 300 ppm in large swings that took about 100,000 years. Scientists also infer the temperature over the whole Earth on average also swung by about 5° Celsius (or 9° Fahrenheit).
Scientists don’t think that the CO2 swings drove the temperature swings, but rather that the CO2 amplified the temperature swings, which were driven by changes in Earth’s orbit around the sun. It takes many thousands of years for orbital changes to add up to an observable warming or cooling. In contrast, the warming Earth has undergone in the last century is much faster, as the CO2 has risen from about 300 to 400 ppm.
The important point is that Earth has not experienced such high CO2 in the past 800,000 years and when temperatures did swing high in the past, the rise in atmospheric CO2 made the temperature swing higher still.
Ice cores provide a critical view of the climate and atmospheric composition of the past. From ice cores and other proxies of Earth’s climate, scientists know that the changes we’ve measured this century with thermometers and other instruments are almost certainly unprecedented in their character. In this blog post I will describe roughly how scientists make measurements with ice cores and how they interpret them.
How do scientists know that CO2 is higher now than at anytime for at least the last 800,000 years? Because the vast domes of ice engulfing Antarctica today contain ice from snow that fell over the last several million years. When snow accumulates to great depths on the ice sheet, it compacts under the weight of the snow load above. The arms of snowflakes begin to break off and eventually the snow appears like tiny grains of wheat. The grains are pressed together so hard that they freeze together while trapping bubbles of air. It may take hundreds of years for the air bubbles to close off from the air above, so each bubble contains a mixture of air from a few centuries in the past.
Scientist sawed the 3,000-meter-long ice core extracted from Dome C into chunks of 55 centimeters (1.5 feet) in length. A tiny piece of each chunk is then ground up to release the air in the bubbles. This air is passed through an instrument where a laser is used to measure the composition. The process is tedious, but the precious information is worth the effort.
The way temperature can be inferred from ice cores is trickier to grasp. Scientists make use of two types of water molecules; both are H2O but the oxygen has either an atomic weight of 16 or 18. These two types of water molecules are called isotopes of water. Of the two, H2O with oxygen-16 (or H216O) is much more common. (The figure below attempts to illustrate processing of isotopes in the water cycle. It may help you visualize what I am describing.) When water evaporates from the ocean, the H216O molecules escape in greater relative numbers than the H218O molecules simply because the H216O molecules are lighter. Some of the water vapor condenses and forms droplets that make up clouds. The heavier molecules are slightly more likely to condense. When a cloud precipitates, the droplets with heavier molecules are more likely to fall.
The basic idea is that the proportion of heavier and lighter molecules varies in each component of the water cycle because heavier molecules behave differently than lighter ones. Specifically, heavier molecules don't evaporate as easily but condense and fall as rain or snow more easily. The last and most important detail is that the ease at which heavier versus lighter molecules change their form (that is, from ocean water to vapor to droplet to rain or snow) depends on the temperature. The isotopic proportions from the past are determined by literally measuring the proportion of H218O to H216O molecules in the intervals of the ice core.