4.Climate in Medieval Time

气候变化与中华文明

404these moons—which might also be found in other Saturnian moons—must be differ-ent from the clean, cold ice that makes J upiter’s icy satellites up to 15 times as radar-bright as Titan. Perhaps ammonia, a microwave-absorbing nitrogen compound that may have been the source of Titan’s at-mosphere, is locked in ices on Titan and Iapetus, making them radar-dark but opti-cally bright. As for Titan’s dark regions,quantitative analysis (8) of infrared data suggests that they are <5% reflective, con-sistent with organic matter like tar or seas of liquid hydrocarbons.

This interpretation is consistent with the most striking feature in the new radar data:the transient sharp spikes in the reflected spectrum, which suggest specular reflec-tions (see the figure) from smooth, dark ar-eas 50 to 150 km across. These features may be impact craters—of which, extrapo-lating from other saturnian moons (11),one might expect around 80 with a diame-ter of 150 km and thousands of smaller ones—that have filled to form lakes and seas (12). The radar data suggest that as much as 75% of Titan’s surface could be covered in this way.

Further subtleties and surprises will un-doubtedly emerge from further studies, and no single data set is unambiguous. The conversion of infrared observations (13) in-to reflectivities that can be compared with

laboratory materials is hampered by uncer-tainties in the absorption by atmospheric methane and the absorption and scattering by the haze. Furthermore, these effects themselves are not uniform across Titan,which has a strong seasonal cycle. The ex-istence of discrete, time-variable methane clouds beneath the haze poses another challenge to infrared observations.

In contrast, radar can penetrate the at-mosphere completely, returning an echo from the surface and perhaps the first few meters below it. As when fishermen use po-larized sunglasses, surface reflections can be discriminated from subsurface scattering using the polarization of the radar echo.Campbell et al.found a low polarization ra-tio for Titan, suggesting that most of the echo is from surface reflection. In contrast,highly polarized radar echoes have been re-ceived from the icy galilean satellites,where subsurface scattering is important.Better signal-to-noise ratios and spatial resolution are needed to make more confi-dent interpretations. The limits of what can be achieved from Earth have essentially been reached. Further advances can be ex-pected when the Cassini spacecraft makes its first close reconnaissance of Titan in October 2004—the first of more than 40flybys in its 4-year nominal mission.

The Cassini-Huygens mission will in-vestigate Titan with optical, infrared, and

radar remote sensing—the first time all three techniques have been used simultane-ously to explore a planetary or lunar sur-face. In January 2005, the Huygens probe will parachute down through the haze to one of Titan’s darker spots. The radar data of Campbell et al.(1) suggest that on Titan itself, as well as in the terrestrial media,this event will make quite a splash.

References

1.D.B.Campbell,G.J.Black,L.M.Carter,S.J.Ostro,Science 302,431 (2003);published online 2 October 2003 (10.1126/science.1088969).

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3.D.O.Muhleman,A.W.Grossman,B.J.Butler,Annu.Rev. Earth Planet. Sci .23,337 (1995).

4.R.D.Lorenz,J.Mitton,Lifting Titan’s Veil (Cambridge Univ.Press,Cambridge,2002).

5.J.I.Lunine,D.J.Stevenson,Y.L.Yung,Science 222,1229 (1983).

6.C.Sagan,S.F.Dermott,Nature 300,731 (1982).

7.P.H.Smith et al.,Icarus 119,336 (1996).

8.S.G.Gibbard et al.,Icaru s 139,189 (1999).

9.R.D.Lorenz,J.I.Lunine,Planet. Space Sci .45,981(1997).

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Am. Astron. Soc. 34,882 (2002).

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125 (2003).

13.C.A.Griffith,T.Owen,T.Geballe,J.Rayner,R.Rannou,

Science 300,628 (2003).Published online 2 October 2003;10.1126/science.1090464

Include this information when citing this paper.

C

limate 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 op-posed 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 consump-tion. This (so the argument goes) would demonstrate that warming in the 20th cen-tury may have been just another natural fluctuation that does not warrant political action to curb fossil fuel use.

Careful examination of this argument

must focus on three issues: the timing of the purported temperature anomaly, its ge-ographical extent, and its magnitude rela-tive to temperatures in the 20th century.The latter issue is especially important, be-cause advocates of a warm Medieval episode commonly argue that solar irradi-ance was as high in Medieval time as in the 20th century. They maintain that 20th-cen-tury global warming was largely driven by this solar forcing, not by increasing green-house gas concentrations.

The concept of a Medieval Warm Epoch (MWE) was first articulated by Lamb in 1965 (1). Lamb based his argu-ment almost exclusively on historical anec-dotes and paleoclimatic data from western Europe. Using these data to construct in-dices 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 anticy-clonic circulation in summer and persistent westerly airflow in winter.

Lamb’s studies predated modern quanti-tative paleoclimatology in which proxy records of climate change are calibrated against instrumental observations. The temperature change that he attributed to the MWE (1°to 2°C above average) was based largely on his own estimates and per-sonal 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 hemi-spheric Medieval temperature anomaly.His estimates pertain only to western Europe.

Lamb compared past temperatures with mean temperatures from 1900 to 1939,which he referred to as the “modern nor-mal” period (3). Because of the pro-nounced rise in temperature in the late 20th century, the period that Lamb considered “normal” was ~0.3°C cooler over Europe than the past 30 years.

Since Lamb’s analysis, many new paleo-temperature series have been produced. How-ever,well-calibrated data sets with decadal or higher resolution are still only available for a few dozen locations (see the figure).

C L I M AT E C H A N G E

Climate in Medieval Time

Raymond S.Bradley,Malcolm K.Hughes,Henry F.

Diaz

17 OCTOBER 2003VOL 302

SCIENCE

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