Surface Temperature Reconstructions for the Last 2,000 Years (2006)

Historical observations, preserved mainly in documentary form, can provide valuable records about past climate states (Lamb 1982). For example, the schematic temperature curve for the last millennium included in the first Intergovernmental Panel on Climate Change report (IPCC 1990; see also Figure O-3) drew heavily on documentary evidence. In addition to systematic weather recordings, such as those used by Manley (1974) to compile his record of temperatures in central England, there is a wide range of direct and indirect

proxy climate information available (Table 3-1). In a classic early study, Ladurie (1972) used farming and phenological¹ records to document times of feast and famine in western Europe during the Little Ice Age (roughly 1500–1850). Logbooks and diaries, such as the diary kept by Benjamin Franklin when he was American ambassador in Paris during the 1780s, provide another, complementary source of data. Franklin reported a “constant dry fog on which the rays of the sun seemed to have little effect” along with severe late frosts, which we now attribute to the Laki volcanic fissure eruption in Iceland (Grattan and Brayshay 1995).

Many historical documents, rather than recording weather per se, provide indirect evidence of past climatic conditions. Historical paintings of alpine landscapes, for example, allow us to pinpoint the former extent of glaciers at precise moments in time, thus contributing to the temperature reconstructions derived from glacier length records discussed in Chapter 7. Similar, but potentially more continuous, time series of sea ice cover have been derived from Antarctic whaling records and from observations of drift ice around the coast of Iceland (e.g., Ogilvie 1992, de la Mare 1997). In the tropics and in dryland regions, periods of drought and flood are most frequently reported; Endfield et al. (2004), for instance, used archival sources to reconstruct rainfall fluctuations in Spanish colonial Mexico. To quantify long series of documentary data such as these in climatic terms, they, like other proxies, need to be calibrated against instrumental measurements. Brázdil et al. (2005) provide a comprehensive review of the methodological framework within which historical archives and documents are currently utilized.

All historical sources need to be evaluated critically, even for relatively recent times. For example, frost fairs were routinely held on the iced-over surface of the River

Thames in London during the cold winters of the Little Ice Age, with the last one occurring in 1814. It would be quite wrong, however, to attribute their absence since that time solely to a rise in Northern Hemisphere winter temperatures: As London has grown and developed, the “urban heat island” effect has reduced the likelihood of frosts in the city center, and the replacement of the old London Bridge in the 1830s allowed greater uptide incursion of saltwater, which freezes less easily. Manley’s central England temperature series indicates that the winter of 1962–1963 was the third coldest since 1659, yet the Thames did not freeze below its tidal limit (Jones and Mann 2004b).

The problem of quality control becomes even more acute further back in time, so that—in contrast to natural archives such as ice cores or tree rings—historical records generally degrade in their utility as they become older. There are, for example, weather records preserved in Irish and Norse annals back to the middle of the first millennium A.D., but their dating is imprecise and descriptions of weather and climate often are exaggerated. Understandably, historical observations also tend to focus on extreme events rather than climatic averages. For example, it was major storm events that most concerned Venetian traders and mariners; their records were used by Grove (2004) to reconstruct the climate of Crete in the 16th and 17th centuries. Documentary evidence is one of the few kinds available that can register severe floods, hurricanes, and other natural disasters. Consequently, their analysis enables an investigation of the relationship between variations in climate and the frequency and severity of extreme events, a subject that is of major societal concern in relation to projected global warming.

Historical observations are typically discontinuous through time and, as such, one of their most valuable roles is in providing a cross-check on reconstructions based on other proxy records, such as tree rings, and on the validity of paleoclimate model simulations. For example, modeling experiments show marked warming in Siberia during the winters immediately following major explosive volcanic eruptions, such as that of Pinatubo (Shindell et al. 2003). The diaries of travelers passing through northern interior Asia in key years (e.g., 1815–1816, 1883–1884) would allow this prediction to be tested independently.

Europe and East Asia are the two regions of the world where long temperature series have been most successfully developed from documentary evidence in a repeatable and consistent way for periods of more than the last two centuries.

The documentary evidence for Europe as a whole has been reviewed by Brázdil et al. (2005) and for the Mediterranean region by Luterbacher et al. (2006). Seasonal temperature data have been compiled for most areas of central and western Europe back to 1500, and these show that the late 20th and early 21st centuries have been warmer, at high probability for three out of four seasons, than any time period in the last five centuries (Xoplaki et al. 2005) (Figure 3-1). By combining documentary evidence with other proxy data, Luterbacher et al. (2004) were able to map winter and summer temperature anomalies across Europe for individual years back to 1500, along

with area-specific error estimates. This mapping permits a rigorous assessment of the spatial coherency of past annual to decadal climatic changes at a subcontinental scale, and also allowed Pauling et al. (2003) to calculate the best predictors of winter and summer temperatures from the available array of different proxy climate data for different parts of Europe and the North Atlantic Ocean. It shows, for example, tree rings to have been a good predictor of past summer temperatures across northern and central Europe, whereas documentary sources are more reliable for reconstructing wintertime temperatures.

Within Europe, only two continuous records currently extend back before 1500, namely, those from the Czech Republic (Brázdil 1996) and the Low Countries (Netherlands and Belgium; van Engelen et al. 2001), both of which have been incorporated into synthetic large-scale temperature reconstructions (e.g., Jones and Mann 2004b). They mark the 20th century as exceptionally warm but also indicate milder conditions prior to about 1400, while the Czech record also shows higher temperatures around the turn of the 19th century. It is not possible, however, to glean systematic, quantitative temperature data across Europe during the medieval period from historical documents alone. Although historical evidence provides important anecdotal evidence for this era, it is very difficult to know—from these limited and rather imprecise sources alone—if there were medieval time periods lasting a decade or more when the climate was as warm as, or warmer than, the late 20th and early 21st centuries.

The second region for which there exist systematic temperature syntheses of several centuries’ duration is East Asia. In Korea and Japan, for example, the date of the spring flowering of cherry trees has been recorded systematically every year for more than a thousand years (Aono and Omoto 1993). Wang et al. (2001) used documentary records to compile decadal average mean annual temperatures for East and North China back to 1380, and 50-year average temperatures for East China back to A.D. 800. In some cases (e.g., Yang et al. 2002), documentary data have been amalgamated with other proxy-climate data to generate regional composite temperature curves. Results from Ge et al. (2001) using phenological records supplemented by winter snow-day records from historical documents, reproduced in Figure 3-1, show temperatures above the long-term mean from A.D. 950 to 1300, and again after 1925, with Little Ice Age thermal minima in the 17th and 19th centuries. These data from the opposite ends of the Eurasian landmass give support to the idea that medieval warming and the Little Ice Age affected much if not all of the extratropical Northern Hemisphere landmasses, notwithstanding significant differences at annual to decadal timescales in the periods of warmth and cold.

Documentary evidence is generally limited to regions with long written traditions. The historical time depth is probably sufficient in a few other regions of the world to attempt systematic compilation of seasonal or annual temperature time series from documentary evidence. This potential exists, but has yet to be realized, in South and Southeast Asia and in the Middle East. An example of this capacity, albeit for African precipitation, is the Roda Nilometer, which has recorded annual data on the height of the Nile flood in Egypt from A.D. 645 to 1890 (Hassan 1981).

Historical documents, along with archeological and paleobiological evidence, can also reveal how societies and ecosystems have responded to climate variability in the past. This section provides a short illustrative summary of past human responses to climate change. However, it is important to note the danger of circular reasoning in this sphere, in the sense that the same evidence for cultural response cannot also be used to infer climatic causality. It is also clear that past societal responses have in general not been predictable or predetermined in advance. Although societies may have been required to adapt to new conditions, the outcome has depended on the success of the choices that were made (Diamond 2005, Rosen in press).

The implications of changing climatic conditions have often been most immediate for agrarian economies, particularly in environmentally marginal lands, and for long-distance communications. In the former case, there was a widespread contraction of rural settlement in upland regions of Europe to lower-lying terrain, associated with the overall climatic deterioration between the late 16th and mid-18th centuries (Parry 1978). In Iceland, an increase in storminess and in winter sea ice cover during the Little Ice Age hampered seaborne communications across the North Atlantic, on which the island’s population was critically dependent. The 1780s brought not only the most severe pack ice of any decade since the 16th century (Ogilvie 1992) but also poisoning of livestock and humans by hydrogen fluoride gases released by the Laki fissure eruption. In combination, this killed more than 75 percent of Iceland’s livestock and 25 percent of its human population and brought society close to collapse. Other examples where climate change may have played a part in societal collapse include the Classic Maya during the 9th century A.D. and the Anasazi of the American Southwest during the 12th and 13th centuries. Both of these cases were linked to periods of extended drought conditions (Hodell et al. 1995, Dean 1998). Climate-induced stress can also act as a stimulus to innovate; for example, declines in rainfall or shifts in temperature have sometimes been followed by technological developments, such as irrigation (Rosen in press).

It is also possible to find examples of climatic changes that were not accompanied by any obvious direct social consequences, and to find cases where the same climatic change had sharply contrasting consequences for different social groups in the same area. A clear example of contrasting adaptations and success/failure in the same environment is provided by the Inuit and the Vikings in western Greenland and the Arctic during the onset of the Little Ice Age. The Norse settlements of Greenland were always marginal, not only because climatic conditions were poorly suited for agriculture but also because of isolation from their parent cultures in northern Europe. In the face of increasingly harsh climatic conditions, populations declined, the western Viking settlement was abandoned around 1350, and the eastern settlement followed suit about a century later. The Norse perceived the adverse changes in climate as a function of cosmological disorder and built ever more impressive churches, rather than adopting new technologies or searching for new sources of food (Barlow et al. 1997, Buckland et al. 1996, McIntosh et al. 2000, Diamond 2005, Rosen in press).

During the same period of medieval warmth that had encouraged Norse expansion, retreating sea ice appears to have allowed an eastward migration of native Inuits along the Arctic shore from Alaska, and thence southward into the same areas of west Greenland being colonized by the Vikings. And like the Norse, these Thule Inuit

cultures were challenged to adapt constantly in order to exploit the available resources; for example, their methods of whale hunting had to adjust depending on whether the sea ice was close to, or far removed from, the shore (Wohlforth 2004). There appears to have been little contact between the Norse and the Thule peoples and no cultural exchange, so that the Norse may not even have been aware of the successful Inuit adaptations for use of marine resources. During the period of the Little Ice Age, the Inuit peoples had to adapt to changing environmental conditions once again. For example, to continue whaling, their populations on Alaska’s North Slope congregated in the few places on the coast where open water could still be reached, such as Nuvuk (Point Barrow). As a result of this and other choices, the Inuit—unlike the Norse—survived in the Arctic up to modern times.