THE RECORDS OF CO2 ABUNDANCE AND GLOBAL TEMPERATURES IN MODERN TIMES

Ice cores contain trapped bubbles of air which, provided they can be properly dated, represent a record of the composition of air over time. Because of the weight of overlying layers of ice, compressing the pores in the ice, it is very difficult to extend the record back as far as that for temperature derived from the isotopic composition of the water itself. In fact, the manner in which the air bubbles were originally trapped in ice results in their movement upward or downward relative to the ice itself, making age determination a challenge.

Figure 22.1 displays CO2 values from an ice core collected in Greenland. The dating of the air was achieved by taking advantage of a byproduct of nuclear weapons testing: The isotope 14C reached a peak in Earth's atmosphere, from the detonation of nuclear bombs, in 1963. Using this peak in heavy carbon, geochemists M. Wahlen of Scripps Institution of Oceanography and colleagues determined that the trapped air was displaced by the equivalent of 200 years relative to the ice surrounding it.

With this important correction, the figure shows that, during the Little Ice Age, CO2 values were fairly constant. Beginning in the mid-1800s, carbon dioxide began to increase. Direct measurements from a station in Hawaii, selected to be high above any local industries and hence sampling worldwide CO2 borne by the trade winds, show that the increase accelerates after World War II.