ScienceWeek

CLIMATOLOGY: CLIMATE IN THE MIDDLE AGES

The following points are made by R.S. Bradley et al (Science 2003 302:404):

1) Climate in Medieval time is often said to have been as warm as or warmer than it is today. Such a statement might seem innocuous. But for those opposed to action on global warming, it has become a cause célèbre: If it was warmer in Medieval time than it is today, it could not have been due to fossil fuel consumption. This (so the argument goes) would demonstrate that warming in the 20th century may have been just another natural fluctuation that does not warrant political action to curb fossil fuel use.

2) Careful examination of this argument must focus on three issues: the timing of the purported temperature anomaly, its geographical extent, and its magnitude relative to temperatures in the 20th century. The latter issue is especially important, because advocates of a warm Medieval episode commonly argue that solar irradiance was as high in Medieval time as in the 20th century. They maintain that 20th-century global warming was largely driven by this solar forcing, not by increasing greenhouse gas concentrations.

3) The concept of a Medieval Warm Epoch (MWE) was first articulated by Lamb in 1965 (1). Lamb based his argument almost exclusively on historical anecdotes and paleoclimatic data from western Europe. Using these data to construct indices of "summer wetness" and "winter severity", he found evidence for warm, dry summers and mild winters centered around 1100 to 1200 A.D. (the "High Medieval") (2). In Europe, such conditions would have been associated with a prevailing anticyclonic circulation in summer and persistent westerly airflow in winter.

4) Lamb's studies predated modern quantitative paleoclimatology in which proxy records of climate change are calibrated against instrumental observations. The temperature change that he attributed to the MWE (1 deg to 2 deg C above average) was based largely on his own estimates and personal perspective. Lamb alluded to a few studies in other parts of the world where conditions appeared to have been warm at this time, but never attempted to estimate the magnitude of a global or even hemispheric Medieval temperature anomaly. His estimates pertain only to western Europe.

5) The author concluded: The balance of evidence does not point to a High Medieval period that was as warm as or warmer than the late 20th century. However, more climate records are required to explain the likely causes for climate variations over the last millennium and to fully understand natural climate variability, which will certainly accompany future anthropogenic effects on climate.(2-5)

References (abridged):

1. H. H. Lamb, Palaeogr. Palaeoclimatol. Palaeoecol. 1, 13 (1965)

2. The historical Medieval lasted from the Fall of Rome (476 A.D.) to the Renaissance (~1500 A.D.). Lamb's MWE was more restricted in time, centering around 1100 to 1200 A.D. (the High Medieval)

3. H. H. Lamb, Arid Zone Research Series 20 (UNESCO, Paris, 1963), p. 125

4. G. H. Haug, K. A. Hughen, D. M. Sigman, L .C. Peterson, U. Röhl, Science 293, 1304 (2001)

5. W. Soon, S. Baliunas, Clim. Res. 23, 89 (2003)

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TOWARD INTEGRATED RECONSTRUCTION OF PAST CLIMATES

The following points are made by K.E. Trenberth and B.L. Otto-Bliesner (Science 2003 300:589):

1) Climate involves interactions among the atmosphere, the oceans, the land surface and its vegetation and hydrology, and the cryosphere. It naturally varies on time scales ranging from interannual (El Nino) to millennia or longer. The instrumental record of a hundred years or so is clearly inadequate to help us understand these processes.

2) Paleoclimate reconstructions fill this void. Made up of estimates of climate variables at times long before the instrumental record, they are based on proxy indicators known to be sensitive to climate. Examples include cores from long-lived trees, ice sheets in Greenland and Antarctica, glaciers at high elevations in the tropics, sediments, and corals. With considerable ingenuity, these proxies have been used to derive information about past climates, natural variability, and global climate change.

3) The reconstruction of a time series of temperature or precipitation at a single location is no mean achievement. To synthesize results from previous reconstructions is even more difficult and has only recently been credibly achieved after considerable work, especially in statistical analysis (1). However, it is becoming clear that a synthesis of data with more physical credibility requires collaboration between paleoclimate and climate dynamics experts (including modelers).

4) Two difficulties faced by climate reconstructions concern chronological dating and what a proxy is really measuring. For example, oxygen isotopes (measured by the O-18 isotopic ratio) in corals are affected by both temperature and salinity of the seawater in which the coral grows, resulting in an inherent ambiguity. Other methods are based on trace element ratios normalized to calcium (such as Sr/Ca and Mg/Ca) in the skeletons of corals and shells. It is assumed that the trace metals are incorporated into the skeleton at concentrations that depend on growth temperature. However, as living organisms are complex, their response varies and empirical calibrations are almost always necessary.(2-5)

References (abridged):

1. M. E. Mann et al., Geophys. Res. Lett. 26, 759 (1999)

2. R. G. Fairbanks, M. Sverdlove, R. Free, P. H. Wiebe, A.W. H. B, Nature 298, 841 (1982)

3. M. Werner, U. Mikolajewicz, M. Heimann, G. Hoffmann, Geophys. Res. Lett. 27, 723 (2000)

4. K. E. Trenberth et al., J. Geophys. Res. 103, 14291 (1998)

5. M. Blackmon et al., Bull. Am. Meteorol. Soc. 82, 2357 (2001)

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