Box 18-3 Instability of the Earths Climate

Natural oxygen is 99.8% l60, 0.2% lsO, and 0.04%, 70. Small variations in isotope ratios in H20 in the ocean, the atmosphere, and rain and snow depend on local temperature. The warmer the air, the more enriched it is in H2I80. The lsO content of snow preserved in glaciers provides a record of air temperatures for 250 000 years in 2-km-long cores drilled from ice sheets in Greenland and Antarctica. Depth in the ice pack has been correlated with age in years by a variety of measurements. 

The figure shows variations in the, 80 content of ice deposited in Greenland over the past 160 000 years. The measured quantities are depth in the ice and 5lsO, defined as l 000 X [i jce — RKf]/Rref, where Rice is the ratio l80/l60 in ice and Rrct is the l80/160 ratio in a reference material, which is modem seawater. SI80 is the change in 180/160 ratio between ice and the reference material, measured in parts per thousand (%c). The derived quantities are age of the ice and the air temperature at which the snow fell. 

The past 10 000 years have been an exceptionally stable, high-temperature period. Over most of the ice-core record, temperature switched between two quasi-stationary climate stages. 14 From 20 000 to 80 000 years ago. the warm-climate air temperature was about — 35°C and the cold temperature was near —42°C. Transitions between the two climates required as little as 10-20 years, and each climate persisted between 70 and 5 000 years. 

Similar changes are seen in other terrestrial and marine geological records from the North Atlantic region, but not from the Antarctic. Circulation changes in the North Atlantic Ocean are conjectured to be the driving force for the climate changes. 

Equation 18-7 comes from Equations 18-5 and 18-6. If species other than analyte absorbed at the wavelengths of interest, we would have to include them. 

---