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Climate in Medieval Time Tative Paleoclimatology in Which Proxy Raymond S P ERSPECTIVES these moons—which might also be found laboratory materials is hampered by uncer- radar remote sensing—the first time all in other Saturnian moons—must be differ- tainties in the absorption by atmospheric three techniques have been used simultane- ent from the clean, cold ice that makes methane and the absorption and scattering ously to explore a planetary or lunar sur- Jupiter’s icy satellites up to 15 times as by the haze. Furthermore, these effects face. In January 2005, the Huygens probe radar-bright as Titan. Perhaps ammonia, a themselves are not uniform across Titan, will parachute down through the haze to microwave-absorbing nitrogen compound which has a strong seasonal cycle. The ex- one of Titan’s darker spots. The radar data that may have been the source of Titan’s at- istence of discrete, time-variable methane of Campbell et al. (1) suggest that on Titan mosphere, is locked in ices on Titan and clouds beneath the haze poses another itself, as well as in the terrestrial media, Iapetus, making them radar-dark but opti- challenge to infrared observations. this event will make quite a splash. cally bright. As for Titan’s dark regions, In contrast, radar can penetrate the at- quantitative analysis (8) of infrared data mosphere completely, returning an echo References suggests that they are <5% reflective, con- from the surface and perhaps the first few 1. D. B. Campbell, G. J. Black, L. M. Carter, S. J. Ostro, sistent with organic matter like tar or seas meters below it. As when fishermen use po- Science 302, 431 (2003); published online 2 October 2003 (10.1126/science.1088969). of liquid hydrocarbons. larized sunglasses, surface reflections can 2. D. O. Muhleman, A. W. Grossman, B. J. Butler, M. A. This interpretation is consistent with the be discriminated from subsurface scattering Slade, Science 248, 975 (1990). most striking feature in the new radar data: using the polarization of the radar echo. 3. D. O. Muhleman, A. W. Grossman, B. J. Butler, Annu. the transient sharp spikes in the reflected Campbell et al. found a low polarization ra- Rev. Earth Planet. Sci. 23, 337 (1995). 4. R. D. Lorenz, J. Mitton, Lifting Titan’s Veil (Cambridge spectrum, which suggest specular reflec- tio for Titan, suggesting that most of the Univ. Press, Cambridge, 2002). tions (see the figure) from smooth, dark ar- echo is from surface reflection. In contrast, 5. J. I. Lunine, D. J. Stevenson, Y. L. Yung, Science 222, eas 50 to 150 km across. These features highly polarized radar echoes have been re- 1229 (1983). may be impact craters—of which, extrapo- ceived from the icy galilean satellites, 6. C. Sagan, S. F. Dermott, Nature 300, 731 (1982). 7. P. H. Smith et al., Icarus 119, 336 (1996). lating from other saturnian moons (11), where subsurface scattering is important. 8. S. G. Gibbard et al., Icarus 139, 189 (1999). one might expect around 80 with a diame- Better signal-to-noise ratios and spatial 9. R. D. Lorenz, J. I. Lunine, Planet. Space Sci. 45, 981 ter of 150 km and thousands of smaller resolution are needed to make more confi- (1997). ones—that have filled to form lakes and dent interpretations. The limits of what can 10. G. J. Black, D. B. Campbell, L. M. Carter, S. J. Ostro, Bull. seas (12). The radar data suggest that as be achieved from Earth have essentially Am. Astron. Soc. 34, 882 (2002). 11. R. D. Lorenz, Planet. Space Sci. 45, 1009 (1997). much as 75% of Titan’s surface could be been reached. Further advances can be ex- 12. R. D. Lorenz, E. Kraal, E. Asphaug, R. Thomson, Eos 84, covered in this way. pected when the Cassini spacecraft makes 125 (2003). Further subtleties and surprises will un- its first close reconnaissance of Titan in 13. C. A. Griffith, T. Owen, T. Geballe, J. Rayner, R. Rannou, doubtedly emerge from further studies, and October 2004—the first of more than 40 Science 300, 628 (2003). no single data set is unambiguous. The flybys in its 4-year nominal mission. Published online 2 October 2003; conversion of infrared observations (13) in- The Cassini-Huygens mission will in- 10.1126/science.1090464 to reflectivities that can be compared with vestigate Titan with optical, infrared, and Include this information when citing this paper. CLIMATE CHANGE clonic circulation in summer and persistent westerly airflow in winter. Lamb’s studies predated modern quanti- Climate in Medieval Time tative paleoclimatology in which proxy Raymond S. Bradley, Malcolm K. Hughes, Henry F. Diaz records of climate change are calibrated against instrumental observations. The limate in Medieval time is often said must focus on three issues: the timing of temperature change that he attributed to to have been as warm as, or warmer the purported temperature anomaly, its ge- the MWE (1° to 2°C above average) was Cthan, it is “today.” Such a statement ographical extent, and its magnitude rela- based largely on his own estimates and per- might seem innocuous. But for those op- tive to temperatures in the 20th century. sonal perspective. Lamb alluded to a few posed to action on global warming, it has The latter issue is especially important, be- studies in other parts of the world where become a cause célèbre: If it was warmer cause advocates of a warm Medieval conditions appeared to have been warm at in Medieval time than it is today, it could episode commonly argue that solar irradi- this time, but never attempted to estimate not have been due to fossil fuel consump- ance was as high in Medieval time as in the the magnitude of a global or even hemi- tion. This (so the argument goes) would 20th century. They maintain that 20th-cen- spheric Medieval temperature anomaly. demonstrate that warming in the 20th cen- tury global warming was largely driven by His estimates pertain only to western tury may have been just another natural this solar forcing, not by increasing green- Europe. fluctuation that does not warrant political house gas concentrations. Lamb compared past temperatures with action to curb fossil fuel use. The concept of a Medieval Warm mean temperatures from 1900 to 1939, Careful examination of this argument Epoch (MWE) was first articulated by which he referred to as the “modern nor- Lamb in 1965 (1). Lamb based his argu- mal” period (3). Because of the pro- ment almost exclusively on historical anec- nounced rise in temperature in the late 20th R. S. Bradley is in the Climate System Research dotes and paleoclimatic data from western century, the period that Lamb considered Center, Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA. E-mail: Europe. Using these data to construct in- “normal” was ~0.3°C cooler over Europe [email protected] M. K. Hughes is in the dices of “summer wetness” and “winter than the past 30 years. Laboratory of Tree-Ring Research, University of severity,” he found evidence for warm, dry Since Lamb’s analysis, many new paleo- Arizona, Tucson, AZ 85721, USA. E-mail: mhughes@ summers and mild winters centered around temperature series have been produced. How- ltrr.arizona.edu H. F. Diaz is in the Climate Diag- nostics Center, Office of Atmospheric Research, 1100 to 1200 A.D. (the “High Medieval”) ever,well-calibrated data sets with decadal National Oceanic and Atmospheric Administration, (2). In Europe, such conditions would have or higher resolution are still only available Boulder, CO 80303, USA. E-mail: [email protected] been associated with a prevailing anticy- for a few dozen locations (see the figure). 404 17 OCTOBER 2003 VOL 302 SCIENCE www.sciencemag.org P ERSPECTIVES When was it warm? The warmest 30-year periods prior to 1970 A.D. 23 22 from a variety of ice core, tree ring, speleothem, sedimentary, and doc- 21 umentary records. Gray diamonds denote first year of record. 1: δ18O 20 18 18 19 from Quelccaya Ice cap, Peru. 2: δ O from Sajama, Bolivia. 3: δ O 18 from Huascaran, Peru. 4: Inverted mean of eight tree-ring indices from 17 δ18 16 northern Patagonia (Argentina and Chile). 5: Speleothem O from 15 South Africa. 6: Austral summer temperatures from a New Zealand 14 δ 13 tree-ring series. 7: Tree-ring indices, Tasmania. 8: D Talos Dome, 12 Antarctica. 9: δ18O from Guliya, W. China. 10: δ18O from Dunde, W. 11 δ18 10 China. 11: O from Dasuopu, W. China. 12: Summer temperature 9 from three tree-ring series in the Sierra Nevada, California. 13: 8 7 Speleothem annual layer thickness, Beijing, China. 14: Winter temper- 6 atures from historical documents, E. China. 15: Lamination thickness in 5 4 lake sediments, Baffin Island, N. Canada. 16: Tree-ring indices from a 3 site in Mongolia. 17: Mean annual temperature of Northern 2 Hemisphere from multiproxy composite. 18: Regional curve-standard- 1 ized (RCS) temperature-sensitive tree-ring chronology from the Polar 600 800 1000 1200 1400 1600 1800 Urals. 19: RCS temperature-sensitive tree-ring chronology from the Years A.D. Taimyr Peninsula. 20: RCS temperature-sensitive tree-ring chronology from Tornetrask, Northern Sweden. 21: Lake sediments, Ellesmere Only a few of these records are from the Recent modeling Island, N. Canada. 22: δ18O from Summit (GISP2), C. Greenland. 23: tropics, and only a handful from the studies show that in- Solar activity from 10Be. For sources of data, see (16). Southern Hemisphere. Furthermore, some creased solar irradiance records provide estimates for a particular does not cause surface warming in all loca- more than 10 times as many people on season, making comparisons with other tions.
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