How are past temperatures determined from an ice core?
—G. SPENCER, LONGWOOD, FLA.

Robert Mulvaney, a glaciologist with the British Antarctic Survey, offers this answer:

Temperature is not measured directly but is inferred from the levels of certain isotopes (chemically identical atoms with the same number of protons but differing numbers of neutrons) of water molecules released by melting the ice cores.

Water is composed of molecules comprising two atoms of hydrogen (H) and one of oxygen (O). The isotopes of particular interest for climate studies are 16O (with eight protons and eight neutrons); 18O (eight protons and 10 neutrons); 2H (with one proton and no neutrons); and 2H (one proton and one neutron, also known as deuterium, D).

Using mass spectrometers, researchers measure the ratio of the oxygen and hydrogen isotopes in ice-core samples and compare the result with the isotopic ratio of an average ocean-water standard known as SMOW (standard mean ocean water). The water molecules in ice cores contain slightly less of the heavy isotopes than SMOW, and the difference compared with the standard is expressed as delta (or change of) 18O or D. Both these values tell essentially the same story—namely, that there is less 18O and D during cold periods than during warm ones. Why? Simply put, it takes more energy to evaporate the water molecules containing heavy isotopes from the surface of the ocean. As the moist air is transported poleward and cools (loses energy), those water molecules containing heavier isotopes are preferentially lost in precipitation.

Plotting either delta 18O or delta D with depth along the length of an ice core can reveal seasonal oscillations in temperature and longer-term shifts in the climate. From the very deepest ice cores, reaching depths of more than three kilometers in the Antarctic ice sheet, we can clearly see the steady pulsing of the ice ages on a period of about 100,000 years. From a site called Dome C in Antarctica, we have recently reconstructed the climate spanning the past three quarters of a million years and have shown seven ice ages, each interspersed with a warm interglacial climate, such as the one we are living in today.

Why do people have different blood types?
—NICHOLE, DETROIT, MICH.

Harvey G. Klein, chief of the department of transfusion medicine for the National Institutes of Health, explains:

The short answer: blood types can aid survival under certain conditions. The specific proteins, glycoproteins and glycolipids found (or expressed) on the surface of red blood cells define blood types, which are inherited. In 1900 Karl Landsteiner described the original classifications—A, B and O—and doctors now recognize 23 blood-group systems with hundreds of different subtypes.

Most such molecules do not seem to be essential for blood cell operation, but some have specific jobs on the red cell membrane. Blood-type factors may be transporters, for instance, allowing materials to enter and exit the red cell, or receptors that permit the binding of certain substances to the cell surface.

Environmental selective pressures clearly play a role in the persistence of some blood types. For example, a “Duffy” blood-type receptor enables certain malarial parasites to enter the red cell. Thus, in some malarial areas of Africa, populations who lack the Duffy blood factor gain a measure of protection against malaria, a distinct survival advantage.

We do not yet know the functions of the A and B blood-group factors. (O blood does not contain A or B factors.) They are probably important in some way, because they appear on many cells and tissues in addition to blood cells and circulate in plasma as well. Also, statistical differences in the frequency of certain malignancies associated with a given A, B or O group suggest that these factors play a role in these diseases.

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