For the last several million years, the climate has been in an ice age that exhibits a discernible pattern. There are roughly 100,000-year glacial periods relieved by brief 10,000-year-long respites of warmth. During the glacial periods, the average global temperature is lower by approximately 8 degrees C than in the interglacials. The recent warm interglacial period -- called the Holocene -- is ending.
The pattern of alternating glacial and interglacial conditions likely arises from the regular changes in the earth's spin on its axis and geometric changes in the earth's orbit around the sun. But how the extreme cold is triggered, sending the massive ice sheet from the north polar regions to shroud Canada and much of the northern United States for 100,000 years - and, oppositely, how the ice retreats -- is not understood.
With the information from ice core records, scientists had noticed a correlation between the air's temperature and carbon dioxide levels during the major advances and retreats of the great ice sheets - the air's carbon dioxide content is low during a glacial period, and high during the interglacial.
Al Gore rhetorically asked in Earth in the Balance, "[D]oes that mean that the dramatic changes in CO2 now under way... will lead to rapid changes in temperature on the warm side of a magnitude that on the cold side produced the ice ages?"(p. 95)
In other words, does the observed correlation between the air's carbon dioxide content and air temperature seen in the ice cores support the notion that recent human actions that increase the carbon dioxide concentration in the air will cause severe global warming?
Gore's rhetorical question is a good starting point for a scientific study. But Gore drops science when he continues, "Given the apparent close relationship between CO2 and temperatures in the past, it hardly seems reasonable - or even ethical - to assume that it is probably all right to keep driving up CO2 levels" (p. 96)
On the contrary, the ethics of science means that one does not presume how nature works, but finds out. And new ice core measurements undercut the idea that carbon dioxide levels in the air tidily explain the fluctuating climates of the ice age. To see why, let's look at the allure of the icy records, which owes to the scientists' ability to decode their chemical signatures, specifically, for air temperature.
A recent article in The New Yorker magazine ("Ice Memory") recounts the difficult daily life of the drilling in Greenland for ice cores so precious to climate researchers. Now one thing Greenland and Antarctica have is plenty of ice. What would be so valuable about vertical plumes of it, drilled from the hardscrabble cold piled up for more than one tenth of a million years?
It is precisely the layering of the ages of ice that makes these ice cores a treasure. Antarctica and Greenland's surface ice has been accumulating for more than 100,000 years. As part of each year's snowfall refuses to melt, it forms a base for the following year's layer. An ice core two miles deep can now be retrieved and analyzed to reveal changes in the air content. In Greenland the record extends over 120,000 years, and in Antarctica, ice cores have gone back much farther - to 420,000 years.
Each year's layer carries information on the content of the air. For example, a volcano might erupt a cloud of shivering particles around which snowflakes form. Carrying the unmistakable chemical debris of a volcano, that year's snow layer would mark the eruption.
Further, air bubbles trapped in a snow layer bear information on the air's carbon dioxide concentration, while the water in the snow contains a chemical signature indicating the air temperature.
The great experimental physicist Ernest Rutherford (1871 - 1937) helped fracture the secret of the ice cores, because Rutherford cracked the atom. In the absence of definitive evidence, the atom had been envisioned for millennia as a hard, indivisible sphere. By 1911 Rutherford showed indisputably that most of the mass of the atom was found in the improbably tiny nucleus over ten thousand times smaller than the atom, which is only fifty-millionths of an inch across.
The nucleus contains the positively charged protons and electrically neutral neutrons. One element in the familiar periodic chart of the elements differs from another mainly by the number of protons packed in a nucleus, and that number is the atomic number in the periodic chart. For example, a hydrogen atom (atomic number 1) has one proton and no neutrons, while an oxygen atom (atomic number 8) has eight protons and eight neutrons.
Oxygen in the water in each layer of an ice core sample can reveal the contemporary air temperature, layer by layer going back in time. The key is that a naturally occurring but rare form of oxygen also exists, with eight protons and ten neutrons, designated oxygen-18, by the atomic weight - i.e., the sum of the protons and neutrons. This isotope is slightly heavier than the common oxygen-16.
When the air is warmer, the heavier water molecules containing an oxygen-18 atom more readily condense out to form the snow that falls in that season's layer than when it is colder. Thus, ice core layers with an enriched ratio of oxygen-18 to oxygen-16 indicate times of warmer air temperature. By measuring changes in the ratio of the oxygen isotopes, scientists infer the fluctuations in air temperature, going back hundreds of thousands of years. Other isotopes -- hydrogen and nitrogen, for instance -- can help confirm the temperature changes inferred from the oxygen measurements.
And the latest ice core records confirm that air temperature and carbon dioxide content tend to vary together. But the advance of the new records is the detail of the timing of the changes between the air temperature and carbon dioxide content: often the temperature changes first, then the air's carbon dioxide concentration follows suit, with an average lag of several hundred years. That means the concentration of carbon dioxide in the air responds to -- but does not cause -- those temperature fluctuations.
And there are examples in the ice core records when carbon dioxide concentration remained high in the air, but temperature dropped steadily, as it did between 130,000 and 110,000 years ago as the previous warm period, the Eemian, dwindled into the most recent glacial period. Researchers at Scripps Institute of Oceanography expect that both the ocean and plant life absorb and release the carbon dioxide in response to the changing air temperature.
The ice core records highlight the complexity of how carbon dioxide is absorbed and interacts with the climate system - including the interlocking reservoirs of the ocean, soils and vegetation. The topic remains a frontier of research. But it is clear that the hard-won information preserved in the polar ice cannot readily support the idea of catastrophic global warming from the increased content of carbon dioxide in the air.
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