Journal of Interdisciplinary History, xliv:3 (Winter, 2014), 327–352. THE REAL LITTLE ICE AGE Sam White The Real Little Ice Age Between c.1300 and c.1850 a.d. the world became, on average, slightly but signiªcantly colder. The change varied over time and space, and its causes remain un- certain. Nevertheless, this cooling constitutes a meaningful climate event, with signiªcant historical consequences. Both the cooling trend and its effects on humans appear to have been particularly Downloaded from http://direct.mit.edu/jinh/article-pdf/44/3/327/1706251/jinh_a_00574.pdf by guest on 28 September 2021 acute from the late sixteenth to the late seventeenth century in much of the Northern Hemisphere. This article explains why climatologists and historians are conªdent that these changes occurred. On close examination, the objections raised in this issue of the journal by Kelly and Ó Gráda turn out to be entirely unfounded. The proxy data for early mod- ern global cooling (such as tree rings and ice cores) are robust, and written weather descriptions and observations of physical phenom- ena (such as glacial movements and river freezings) by and large of- fer independent conªrmation. Kelly and Ó Gráda’s proposed alter- native measures of climate and climate change suffer from serious ºaws. As we review the evidence and refute their criticisms, it will become clear just how solid the case for the Little Ice Age (lia) has become. the case for the little ice age The evidence for early modern global cooling comes, ªrst and foremost, from extensive research into physical proxies, including ice cores, tree rings, corals, and speleothems (stalagmites and stalactites). The precise techniques of extracting climate data from these samples are not important in this context. What matters is that each type of sample provides a physical record that correlates reasonably well to a climate vari- able, and that researchers can take reliable high-resolution mea- surements of that record and calibrate them against modern instru- mental measurements. For instance, if instrumental measurements Sam White is Assistant Professor of History, Ohio State University, and co-founder of the Climate History Network (climatehistorynetwork.com). He is the author of The Climate of Rebellion in the Early Modern Ottoman Empire (New York, 2011). The author thanks Franz Mauelshagen, Dagomar Degroot, A. T. White, and the journal editors for their comments on this article. © 2013 by the Massachusetts Institute of Technology and The Journal of Interdisciplinary History, Inc., doi:10.1162/JINH_a_00574 328 | SAM WHITE demonstrate that summer temperature explains much of the var- iation in the annual growth rings of a large sample of Scandinavian trees, researchers can use older growth rings from those trees to es- timate pre-instrumental summer temperatures for the calibrated region.1 No proxy record gives a perfect reconstruction. Yet when used carefully, proxies can produce valuable results. The results of most longer-term proxies and large-scale multiproxy studies (that Downloaded from http://direct.mit.edu/jinh/article-pdf/44/3/327/1706251/jinh_a_00574.pdf by guest on 28 September 2021 is, studies modeling temperatures from many different proxy sources) point overwhelmingly to signiªcant cooling from c.1300 to c.1850, including a distinct cooling in much of the Northern Hemisphere from the late sixteenth to the seventeenth century. The evidence for this trend is by no means limited to a couple of contested studies, as Kelly and Ó Gráda imply, nor only to tree rings in Europe. A wide variety of regional proxies, ranging from Greenland ice cores to South African speleothems, point in the same direction. Even in the Antarctic, borehole temperature data point to the lia as the “largest climate anomaly of the last 1000 years,” with temperatures on average 0.52 Ϯ 0.28°C colder than those of the last century. A German speleothem study now con- ªrms a cooling phase from c.1300 to c.1800 for Europe in particu- lar. The most recent long-term global temperature reconstruction, employing seventy-three world-wide proxy records of different kinds, found unambiguous signs of the lia, particularly from c.1500 to c.1800. Even within error bars, this cooling stands out as undoubtedly the most pronounced global climate anomaly of the past 8,000 years (until contemporary global warming).2 1 For a sense of the range of proxy data, methods, and issues, see, for example, Philip D. Jones et al., “High-Resolution Palaeoclimatology of the Last Millennium: A Review of Cur- rent Status and Future Prospects,” The Holocene, XIX (2009), 3–49. In the four years since that article, the coverage and resolution of proxies has continued to improve apace. 2 Readers concerned that “modeling” is somehow less solid than “measurement” should understand that even contemporary meteorological data routinely combine the two. See Paul N. Edwards, A Vast Machine: Computer Models, Climate Data, and the Politics of Global Warming (Cambridge, Mass., 2010). For examples of the wide range of regional proxies that indicate cooling, see Paul A. Mayewski et al., “Holocene Climate Variability,” Quaternary Research, LXII (2004), 243–255. Anais J. Orsi, Bruce D. Cornuelle, and Jeffrey P. Severinghaus, “Little Ice Age Cold Interval in West Antarctica: Evidence from Borehole Temperature at the West Antarctic Ice Sheet (WAIS) Divide,” Geophysical Research Letters, XXXIX (2012), L09710 (quotation taken from the abstract); Nerilie J. Abram et al., “Acceleration of Snow Melt in an Antarctic Peninsula Ice Core during the Twentieth Century,” Nature Geoscience, VI (2013), 404–411, suggest that the greatest cooling was in the ªfteenth century. Strong conªrmation of Southern Hemisphere cooling also appears in Lonnie G. Thompson et al., “Annually Re- THE REAL LITTLE ICE AGE | 329 Kelly and Ó Gráda respond to such evidence in a mere two footnotes. The ªrst footnote makes the unsupported claim, “While tree rings might seem like a more systematic source [of past cli- mate], annual growth rings only reºect weather for trees at the edge of their geographical ranges that are under stress from cold or arid conditions, which is not the case for most of Europe.” This claim is highly misleading, if not simply false. Climatologists make use of tree rings as temperature proxies across a wide geographical Downloaded from http://direct.mit.edu/jinh/article-pdf/44/3/327/1706251/jinh_a_00574.pdf by guest on 28 September 2021 range for several obvious reasons. First, since different types of trees have different ranges and different sensitivities, much of the tem- perate world is at the edge of at least one species’ range, and re- searchers can create reconstructions using multiple species. Second, trees on mountains and hills (where the oldest trees are often found) are more sensitive to temperature variations, but they can nonetheless offer a reasonable record of regional temperatures. Third, even away from the edge of their range, trees do not grow at the same rate every year. Seasonal temperature and/or precipita- tion in temperate climates determine some part of the variance in annual growth, though probably not as much as in more extreme climates. In any case, climatologists produce reliable tree ring- based temperature records not only for extreme areas, such as Scan- dinavia, but also for more temperate regions, such as Ireland.3 The second problematical footnote tries to dismiss proxy data in general by appealing to a statistical study by McShane and Wyner. Yet the claim that this work refutes evidence of the lia is equally misleading. McShane and Wyner are not primarily con- cerned with whether proxies reconstruct such longer-term trends as the lia but with the supposed inability of proxy-based recon- structions to beat sophisticated null models in reproducing inter- solved Ice Core Records of Tropical Climate Variability over the Past ϳ1800 Years,” Science CCCXL (2013), 945–950. The German study is Jurgen Fohlmeister et al., “Bunker Cave Sta- lagmites: An Archive for Central European Holocene Climate Variability,” Climate of the Past, VIII (2012), 1751–1764. Shaun A. Marcott et al., “A Reconstruction of Regional and Global Temperature for the Past 11,300 Years,” Science, CCCIX (2013), 1198–1201. 3 For a discussion of tree-ring limitations and techniques to overcome them, see A. M. García-Suárez, C. J. Butler, and M. G. L. Baillie, “Climate Signal in Tree-Ring Chronologies in a Temperate Climate: A Multi-species Approach,” Dendrochronologia, XXVII (2009), 183– 198. For a recent example, see Ulf Büntgen et al., “Filling the Eastern European Gap in Mil- lennium-Long Temperature Reconstructions,” Proceedings of the National Academy of Sciences, CX (2013), 1773–1778. Climatologists can now also measure isotopic ratios and maximum latewood density to draw further data from tree rings. 330 | SAM WHITE annual variability over shorter periods in the instrumental record. They even offer four different possibilities for this inability, three of them dealing with how data are handled or whether their own test is too demanding.4 As a worst-case scenario, McShane and Wyner contend, “It is possible that the proxies are in fact too weakly connected to global annual temperature to offer a substantially predictive (as well as re- constructive) model over the majority of the instrumental period.” They Downloaded from http://direct.mit.edu/jinh/article-pdf/44/3/327/1706251/jinh_a_00574.pdf by guest on 28 September 2021 even concede, “This is not to suggest that proxies are unable to de- tect large variations in global temperature (such as those that distin- guish our current climate from an ice age). Rather, we suggest it is possible that natural proxies cannot reliably detect the small and largely unpredictable changes in annual temperature that have been ob- served over the majority of the instrumental period.” In fact, they note, “It is well known and generally agreed that the several hun- dred years before the industrial revolution were a comparatively cool ‘lia.’” Their own Bayesian reconstruction technique for the past millennium of climate produces a result similar to the studies that Kelly and Ó Gráda try to dismiss, albeit with larger uncertainty bands.