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Proxy (climate)

In climate research, a proxy variable is something that is probably not in itself of any great interest, but from which a variable of interest can be obtained.

Examples include:

1. Isotopic variations in Ice cores can be used to infer temperature changes and ice sheet volume.
2. Beryllium 10 variations can be used to infer past Solar irradiance.
3. Tree ring widths can be used to infer precipitation and temperature changes.

In all cases it is necessary to carefully calibrate the proxy against the variable of interest. Tree growth, for example, is sensitive to precipitation and temperature as well as a number of other signals, and is often most snesitive during certain seasons of the year. Ice core proxies are usually the most direct.

Water isotopes and temperature reconstruction

Ocean water is mostly <math>{H_2}^{16}O<math>, with small amounts of <math>HD^{16}O<math> and <math>H_2^{18}O<math>. In inversion layer, we are left with a linear relation:

δ ¹⁸O = aT + b

which is empirically calibrated from measurements of temperature and δ as a = 0.00067 %/^oC for Greenland and 0.00076 %/^oC for East Antarctica. The calibration was initially done on the basis of spatial variations in temperature and it was assumed that this corresponded to temporal variations (Jouzel and Merlivat, 1984). More recently, borehole themometry has shown that for glacial-interglacial variations, a = 0.00033 %/^oC (Cuffey et al., 1995), implying that glacial-interglacial temperature changes were twice as large as previously believed.

References

1. E. W. Wolff, History of the atmosphere from ice cores; ERCA vlo 4 pp147-177