Reconstructing Climatic and Environmental Changes of the Past 1000 Years: A Reappraisal
Soon, Willie, Sallie Baliunas, Craig Idso, Sherwood Idso, and David R. Legates. 2003. Reconstructing Climatic and Environmental Changes of the Past 1000 Years: A Reappraisal. Energy & Environment, Vol. 14, Nos. 2 & 3, 2003.
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ABSTRACT
The 1000-year climatic and environmental history of the Earth contained in various proxy records is examined. As indicators, the proxies duly represent or record aspects of local climate. Questions on the relevance and validity of the locality paradigm for climatological research become sharper as studies of climatic changes on timescales of 50–100 years or longer are pursued. This is because thermal and dynamical constraints imposed by local geography become increasingly important as the air-sea-land interaction and coupling timescales increase. Because the nature of the various proxy climate indicators are so different, the results cannot be combined into a simple hemispheric or global quantitative composite. However, considered as an ensemble of individual observations, an assemblage of the local representations of climate establishes the reality of both the Little Ice Age and the Medieval Warm Period as climatic anomalies with world-wide imprints, extending earlier results by Bryson et al. (1963), Lamb (1965), and numerous other research efforts. Furthermore, these individual proxies are used to determine whether the 20th century is the warmest century of the 2nd Millennium at a variety of globally dispersed locations. Many records reveal that the 20th century is likely not the warmest nor a uniquely extreme climatic period of the last millennium, although it is clear that human activity has significantly impacted some local environments.
1. INTRODUCTION
Are the Little Ice Age and Medieval Warm Period widespread climatic anomalies? Nearly four decades ago, H. H. Lamb (1965, pp. 14–15) wrote,
“[M]ultifarious evidence of a meteorological nature from historical records, as well as archaeological, botanical and glaciological evidence in various parts of the world from the Arctic to New Zealand… has been found to suggest a warmer epoch lasting several centuries between about A.D. 900 or 1000 and about 1200 or 1300… Both the “Little Optimum” in the early Middle Ages and the cold epochs [i.e., “Little Ice Age”], now known to have reached its culminating stages between 1550 and 1700, can today be substantiated by enough data to repay meteorological investigation… It is high time therefore to marshal the climatic evidence and attempt a quantitative evidence.” …
Thirty-three years later, however, Jones et al. (1998) tentatively concluded that,
“[w]hile the ‘Little Ice Age’ cooling (with the seventeenth century being more severe over Eurasia and the nineteenth century more severe over North America) is clearly evident … we can only concur… that there is little evidence for the ‘Medieval Warm Period’… although the fact that we have only four series before 1400 and the timescale limitations described earlier [i.e., not resolving timescales of multidecades to century with tree ring proxies used in their study] caution against dismissing the feature.” …
These results are but a few of the many that have become available since Lamb’s pioneering analysis. Given advancements in retrieval of information from climate proxies, as well as their extensive surface coverage, we review the accumulated evidence on climatic anomalies over the last 1000 years. ..
What are the regional and global patterns of climatic change over the last 1000 years? Accurate answers to these questions are important, both as benchmarks for the 20th century global average warming exhibited by surface thermometer records and as physical constraints for theories or mechanisms of climate change on timescales of decades to centuries.
To make progress towards this understanding, we address three questions of many individual climate proxies that differ too widely to be quantitatively averaged or compared:
(1) Is there an objectively discernible climatic anomaly occurring during the Little Ice Age, defined as 1300–1900 A.D.? This broad period in our definition derives from historical sea-ice, glaciological and geomorphological studies synthesized in Grove (2001a, 2001b) and Ogilvie and Jónsson (2001).
(2) Is there an objectively discernible climatic anomaly occurring during the Medieval Warm Period, defined as 800–1300 A.D.? This definition is motivated by Pfister et al. (1998) and Broecker (2001) and is slightly modified from Lamb’s original study (1965).
(3) Is there an objectively discernible climatic anomaly occurring within the 20th century that may validly be considered the most extreme (i.e., the warmest) period in the record? An important consideration in answering this question is to distinguish the case in which the 20th century warming began early in the century versus after the 1970s, as recorded by surface thermometers. This criterion is necessary in order to judge the influence of 20th century warming by anthropogenic forcing inputs such as increased atmospheric carbon dioxide content.
Anomaly, in our context, is simply defined as a period of 50 or more years of sustained warmth, wetness, or dryness within the Medieval Warm Period, or a 50-year or longer period of cold, dryness, or wetness during the Little Ice Age. …
4. RESULTS
… For questions 1 and 2, we find the answer to be ‘Yes’ when the proxy record shows a period of 50 years or longer of cooling, dryness or wetness during the Little Ice Age and a period of 50 years or longer of warming, wetness or dryness during the Medieval Warm Period. …
… most of the proxy records do not suggest the 20th century to be the warmest or the most extreme in its local representations, which seems surprising until one realizes the more limited and contrary view was drawn primarily from familiar instrumental thermometer records that yield no information on centennial-scale climate variability. … Another interesting feature of the result is that the warmest or most extreme climatic anomalies in the proxy indicators often occurred in the early-to-mid 20th century, rather than throughout the century.
4.1. Glaciers – Worldwide
Broadly, glaciers retreated all over the world during the Medieval Warm Period, with a notable but minor re-advance between 1050 and 1150 A.D. (Grove and Switsur 1994). Large portions of the world’s glaciers, both in the Northern and Southern Hemispheres, advanced during the 1300–1900 A.D. period (Grove 2001b; see also Winkler 2000). The world’s small glaciers and tropical glaciers have simultaneously retreated since the 19th century, but some glaciers have advanced (Kaser 1999; Dyurgerov and Meier 2000; D. Evans 2000). …
Additional proxy records used here reveal that the climatic anomaly patterns known as the Medieval Warm Period (circa 800–1300 A.D.) and the Little Ice Age (1300–1900 A.D.) occurred across the world. …
4.2.2. North Atlantic and other oceans
The colonization of Greenland’s coastal area by the Vikings starting in 986 A.D. is well documented; and the generally mild and benign climatic conditions from about 800–1200 A.D. that helped to sustain the settlement are also well supported by ice core and borehole proxy information (Dansgaard et al. 1975; Dahl-Jensen et al. 1998). The Norsemen’s ‘Western Settlement’ (around the Godthab district) was mysteriously abandoned sometime between 1341 and 1362 A.D., while the ‘Eastern Settlement’ (actually near the southernmost tip of west Greenland, around the Narssaq and Julianehab districts) died out between 1450 and 1500 A.D. (Grove 1996; Ogilvie et al. 2000). The timing of the abandonment of the settlements coincided with a general cooling over Greenland, as established by both ice-core isotopic and borehole thermometry (Dansgaard et al. 1975; Stuiver et al. 1995; Dahl-Jensen et al. 1998). …
Over the equatorial Central Pacific, around the NINO3.4 (5N–5ºS; 160ºE–150ºW) region, Evans et al. (2000), in their skillful reconstruction of the ENSO-like decadal variability of the NINO3.4 sea surface temperature (SST), found an apparent sustained cool phase of the proxy NINO3.4 SST variability from about 1550 A.D. to approximately 1895 A.D., thereby extending the geographical area covered by the Little Ice Age Climate Anomaly. …
4.2.3. Asia and Eastern Europe
From 49 radiocarbon-dated subfossil wood samples, Hiller et al. (2001) determined that the alpine tree-limit on the Khibiny low mountains of the Kola Peninsula was located at least 100–140 meters above the current tree-limit elevation during the relatively warmer time between 1000 A.D. and 1300 A.D. The summer temperatures corresponding to the tree-line shift during this warm time are estimated to have been at least 0.8ºC warmer than today. …
Middle Russia (around 50–60ºN and 30–50ºE) seems to have experienced its coolest winters around 1620–1680 A.D., its coolest summers and springs around 1860–1900 A.D., and distinctively warm conditions during the first half of the 16th century, similar to conditions for western Europe described above. …
Based on less precise climate proxies like cherry-blossom-viewing dates, lake freezing dates and historical documentation of climate hazards and unusual weather, Tagami (1993, 1996) found that a warm period prevailed between the 10th and 14th centuries, and a cold period between the late 15th and 19th centuries, over large parts of southern Japan. …
4.2.4. North America
From an extensive collection of multiproxy evidence, Stine (1998) concluded that during the Medieval Warm Period prolonged intervals of extreme drought affected California, the northwestern Great Basin, and the northern Rocky Mountains/Great Plains, while markedly wetter regimes persisted over the Upper Midwest/sub-arctic Canada and Southern Alaska/British Columbia regions. …
Graumlich’s (1993) reconstruction of summer temperature and winter precipitation from trees in the Sierra Nevada confirmed the overall warm and dry conditions for California during Medieval times, when two of the warmest and driest 50-year intervals occurred – at 1118–1167, 1245–1294 A.D. and 1250–1299, 1315–1364 A.D., respectively. …
Hu et al. (2001), based on their high-resolution (multidecadal) geochemical analysis of sediments from Farewell Lake by the northwestern foothills of the Alaska Range, also found pronounced signatures of the Medieval Warm Period around 850–1200 A.D. During the Little Ice Age, the surface water temperature of Farewell Lake fell to a low in 1700 A.D. that was estimated to be about 1.75ºC cooler than at present. …
4.3. Southern Hemisphere
4.3.1. New Zealand
In New Zealand, the O-18 concentration in a stalagmite record from a cave in northwest Nelson shows the coldest times during the Little Ice Age to be around 1600–1700 A.D., while exceptionally warm temperatures occurred around 1200–1400 A.D., in association with the general phenomenology of the Medieval Warm Period (Wilson et al. 1979). …
4.3.4. Antarctica
The last important source of geographical information for conditions during the Medieval Warm Period and the Little Ice Age in the Southern Hemisphere is obtained from glaciers, ice cores and sea sediments on and around Antarctica. …
For the Little Ice Age, advances of glaciers on South Georgia Island (which is half-covered by glaciers) began after the late 13th century, with a peak advancement around the 18th–20th centuries (Clapperton et al. 1989). Glacier retreats occurred after about 1000 A.D., which corresponds to the timing for the Medieval Warm Period. Baroni and Orombelli (1994) noted a similar scenario for glacier advances and retreats during the Little Ice Age and Medieval Warm Period for the Edmonson Point glacier at the Terra Nova Bay area of Victoria Land on the Antarctic continent (East Antarctica). The Edmonson Point glacier retreated in two distinct phases, around 920–1020 A.D. and 1270–1400 A.D., and then advanced at least 150 meters after the 15th century. …
5. DISCUSSION
The widespread, but not truly global, geographical evidence assembled here argues for the reality of both the Little Ice Age and the Medieval Warm Period, and should serve as a useful validation target for any reconstruction of global climate history over the last 1000 years. …
Another significant problem is the indication that an anthropogenic influence may have already left its fingerprint on the recent growth of trees across the Northern Hemisphere. If this anthropogenic effect were present in tree ring data, then the calibration and verification procedure designed for extended paleoclimatic reconstructions would be significantly corrupted by further uncertainties (Idso 1989). …
Karlén (2001), for example, notes that according to the Vostok ice core record of atmospheric carbon dioxide, the present concentration of atmospheric CO2 is about 100 ppmv higher than it was during any previous interglacial during the last 400,000 years. Thus, if climate were to respond sensitively to carbon dioxide, global temperatures, or at least Vostok temperature, today ought to be considerably higher than previous interglacials. Yet evidence exists to suggest that the “present interglacial [at least for conditions around Vostok] has been about 2ºC cooler than the previous one and the climate is now, in spite of the recent warming, cooler than it was at the beginning of this interglacial” (Karlén 2001). …
6. CONCLUSIONS
This paper presents a survey of site-specific paleoclimatic reconstructions, then considers whether they indicate that the Medieval Warm Period and the Little Ice Age were observed on broad area of the globe. We conclude that the Medieval Warm Period and Little Ice Age are widespread climatic anomalies, although we emphasize the complex nature of translating the proxy changes into convenient measures like temperature and precipitation as well as confirming their spatio-temporal representation and resolution. …
The picture emerges from many localities that both the Little Ice Age and Medieval Warm Period are widespread and perhaps not precisely timed or synchronous phenomena, easily within the margin of viewpoints conceived by Bryson et al. (1963), Lamb (1965) and numerous other researchers like J. Grove (1996, 2001a, 2001b). Our many local answers confirm that both the Medieval Climatic Anomaly and the Little Ice Age Climatic Anomaly are worthy of their respective labels. Furthermore, thermometer warming of the 20th century across the world seems neither unusual nor unprecedented within the more extended view of the last 1000 years. Overall, the 20th century does not contain the warmest or most extreme anomaly of the past millennium in most of the proxy records. …
However, it is also clear that human activity has shaped almost every aspect of past environmental and climatic changes on local and regional spatial scales …
It might seem surprising or frustrating that paleoclimatic reconstruction research has not yet provided confident and applicable answers to the role of anthropogenic forcing on climate change. This point is particularly sharp when considering the fact that even though some proxy records (e.g., those from Overpeck et al. 1997) show unprecedented 20th century warmth with most of the increase occurring in the early to mid-decades of the 20th century, when the amount of anthropogenic CO2 in the air was less than 20–30% of the total amount there now. Unless there are serious flaws in the timing of the early-to-middle 20th century surface thermometer warming, or unknown anthropogenic mechanisms that caused a large amplification of surface temperature of the then-small increase in anthropogenic atmospheric CO2, then the early part of the 20th century warming must be largely dissociated from anthropogenic CO2 emissions. Other anthropogenic factors still need to be studied on a case by case basis. …
