North America's Medieval Warm Period

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McGann (2008) analyzed a sediment core retrieved from the western portion of South Bay near San Francisco International Airport (37°37.83’N, 122°21.99’W) for the presence of various foraminifers as well as oxygen and carbon stable isotopes and numerous trace elements found in the tests of Elphidium excavatum. She found “the climate of south bay has oscillated numerous times between warm and dry, and cool and wet conditions over the past 3870 years” and “both the Medieval Warm Period and the Little Ice Age are evident.” More specifically, she identifies the MWP as occurring from AD 743 to 1343 and the LIA as occurring in two stages: AD 1450 to 1530 and AD 1720 to 1850. In addition, she states the timing of the MWP “correlates well with records obtained for Chesapeake Bay (Cronin et al., 2003), Long Island Sound (Thomas et al., 2001; Varekamp et al., 2002), California’s Sierra Nevada (Stine, 1994), coastal northernmost California (Barron et al., 2004), and the San Francisco Bay estuary in north bay at Rush Ranch (Byrne et al., 2001), and south bay at Oyster Point (Ingram et al., 1996).” As for the more recent past, McGann notes “near the top of the core” foraminiferal abundances suggest, “once again, regional warming has taken place.” However, that warming does not appear to have returned the region to the level of sustained warmth it enjoyed during the peak warmth of the MWP.

Moving north to Alaska, Clegg et al. (2010) conducted a high-resolution analysis of midge assemblages found in the sediments of Moose Lake (61°22.45’N, 143°35.93’W) in the Wrangell-St. Elias National Park and Preserve in the south-central portion of the state, producing a record of reconstructed mean July temperatures (TJuly) for the past six thousand years. In examining the latter half of that record, as portrayed in Figure 3.1.1, from 2,500 cal BP to the present, there is a clear multi-centennial oscillation, with its peaks and valleys defining the temporal locations of the Roman Warm Period, the Dark Ages Cold Period, the Medieval Warm Period, the Little Ice Age—during which the coldest temperatures of the entire interglacial or Holocene were reached—and, finally, the start of the Current Warm Period, which is still not expressed to any significant degree compared to the Medieval and Roman Warm Periods.

In discussing their results, the seven scientists write, “comparisons of the TJuly record from Moose Lake with other Alaskan temperature records suggest that the regional coherency observed in instrumental temperature records (e.g., Wiles et al., 2008; Gedalof and Smith, 2001; Wilson et al., 2007) extends broadly to at least 2000 cal BP,” while noting that climatic events such as the LIA and the MWP occurred “largely synchronously” between their TJuly record from Moose Lake and a δ18O-based temperature record from Farewell Lake on the northwestern foothills of the Alaska Range.

In considering these findings, it is instructive to note that even with the help of the supposedly unprecedented anthropogenic-induced increase in the atmosphere’s CO2 concentration that occurred over the course of the twentieth century, the Current Warm Period has not achieved the warmth of the MWP or RWP, which suggests the climatic impact of the twentieth-century increase in the air’s CO2 content has been negligible. The warming that defined the Earth’s recovery from the global chill of the LIA—which should have been helped by the concurrent increase in the air’s CO2 content—appears no different from the non-CO2-induced warming that brought the planet out of the Dark Ages Cold Period and into the Medieval Warm Period.

Working nearby in Canada, Edwards et al. (2008) wrote, “Northern Hemisphere climate is believed to have fluctuated from being generally mild on average in the early millennium (the classic Medieval Warm Period) to being cool and variable during the subsequent Little Ice Age, followed by recent warming.” To see to what extent western Canada had followed this basic pattern over the past thousand years, they employed a coupled isotope response-surface model “to resolve multi-dimensional patterns of climate variability using carbon- and water-isotope time series developed from tree-ring cellulose,” based on “16 subfossil snags and living-tree sequences of Picea engelmannii (Engelmann spruce) from upper alpine treeline sites near Athabasca Glacier and subfossil material from the forefield of Robson Glacier plus living and snag material of Pinus albicaulis (whitebark pine) adjacent to Bennington Glacier, spanning AD 951–1990.”

The results of this climate reconstruction revealed that “high inferred winter temperatures ~AD 1100–1250 stand out in particular, corresponding with the Medieval Climate Anomaly,” with the four researchers adding the “climate shifted broadly in western Canada from warm in winter and atmospherically moist during the growth season during medieval times to being cool in winter and atmospherically dry during the growth season in the subsequent Little Ice Age.” Nevertheless, they note “independent proxy hydrologic evidence suggests that snowmelt sustained relatively abundant streamflow at this time in rivers draining the eastern Rockies,” while during the Medieval Warm Period there was “evidence for reduced discharge in rivers draining the eastern Rockies and extensive hydrological drought in neighboring western USA.” Finally, they write, “declining streamflow in rivers draining the eastern Rockies over the past century (Rood et al., 2005) may indicate that conditions are in the process of returning to a similar state,” which suggests the Current Warm Period has not yet achieved the more extreme climatic status of the Medieval Warm Period.

Edwards et al.’s results thus delineate the classic cycling of climate that brought the Earth the Medieval Warm Period and subsequent Little Ice Age as well as the twentieth-century transition to the Current Warm Period, all independent of the air’s CO2 content. Edwards et al.’s data clearly indicate that both the minimum temperature of winter and the yearly average of the winter minimum and summer maximum temperature were greater during the Medieval Warm Period than they were during the late twentieth century, between which times the air’s CO2 concentration rose by approximately 100 ppm and still could not force a temperature increase equal to that of a thousand years ago.

Whitlock et al. (2008) analyzed “geochemical, stable-isotope, pollen, charcoal, and diatom records” further south in North America, from high-resolution cores obtained from Crevice Lake (45.000°N, 110.578°W), with the goal of reconstructing “the ecohydrologic, vegetation, and fire history of the watershed for the last 2650 years to better understand past climate variations at the forest-steppe transition” in “the canyon of the Yellowstone River in northern Yellowstone National Park [YNP].” Their results indicated the Crevice Lake region experienced “a warm interval with dry winters between AD 600 and 850, followed by less dry but still warm conditions between AD 850 and 1100.” In addition, they write, “other studies in YNP indicate that trees grew above present-day treeline and fires were more frequent in the Lamar and Soda Butte drainages between AD 750 and 1150,” citing Meyer et al. (1995).

As for the modern period, the seven researchers say their data indicate “the last 150 years of environmental history since the formation of YNP have not been anomalous within the range of variability of the last 2650 years, and many of the proxy indicators suggest that 19th and twentieth century variability at Crevice Lake was moderate compared with earlier extremes.” In fact, they note that with the possible exception of the charcoal record, “all of the data show greater variability in the range of ecosystem conditions prior to the establishment of the YNP in 1872.”

In another study, based on isotopic soil carbon measurements made on 24 modern soils and 30 buried soils scattered between latitudes 48 and 32°N and longitudes 106 and 98°W, Nordt et al. (2008) developed a time series of C4 vs. C3 plant dynamics for the past 12,000 years in the mixed and shortgrass prairie of the U.S. Great Plains. They did this because, as they describe it, the percent of soil carbon derived from C4 plants corresponds strongly with summer temperatures as reflected in the soil carbon pool, citing the work of Nordt et al. (2007) and von Fischer et al. (2008). As a result, they were able to devise a history of the relative warmth of the climate of the region over this protracted period. This history suggested the region of study was slightly warmer during parts of both the Medieval and Roman Warm Periods than it has yet been in modern times, and that it was significantly warmer during a sizeable portion the mid-Holocene Thermal Maximum or Climatic Optimum, as it is sometimes called.

Other studies have documented a Medieval Warm Period in Greenland. Norgaard-Pedersen and Mikkelsen (2009), for example, measured and analyzed several properties of a sediment core retrieved from the deepest basin of Narsaq Sound (60°56.200’N, 46°09.300’W) in southern Greenland from which they were able to infer various “glacio-marine environmental and climatic changes” that had occurred over the prior 8,000 years. Their results revealed the existence of two periods (2.3–1.5 ka and 1.2–0.8 ka) that “appear to coincide roughly with the ‘Medieval Warm Period’ and ‘Roman Warm Period’” and they identified the colder period that followed the Medieval Warm Period as the Little Ice Age and the colder period that preceded it as the Dark Ages Cold Period.

Citing the works of Dahl-Jensen et al. (1998), Andresen et al. (2004), Jensen et al. (2004), and Lassen et al. (2004), the two Danish scientists said “the cold and warm periods identified in [those researchers’ studies] appear to be more or less synchronous to the inferred cold and warm periods observed in the Narsaq Sound record,” providing even more evidence for the reality of the naturally occurring phenomenon that governs this millennial-scale oscillation of climate that has been identified throughout the world.

A little closer to the present, Vinther et al. (2010) introduced the report of their study by writing, “during the past 10 years studies of seasonal ice core δ18O records from the Greenland ice sheet have indicated, that in order to gain a firm understanding of the relationships between Greenland δ18O and climatic conditions in the North Atlantic region, it is important to have not only annually resolved, but seasonally resolved ice core δ18O data.” Therefore, working with 20 ice core records from 14 different sites, all of which stretched at least 200 years back in time, as well as near-surface air temperature data from 13 locations along the southern and western coasts of Greenland that covered approximately the same time interval (1784–2005), plus a similar temperature dataset from northwest Iceland (said by them to be employed “in order to have some data indicative of climate east of the Greenland ice sheet”), Vinther et al. proceeded to demonstrate that winter δ18O was “the best proxy for Greenland temperatures.”

Based on that determination, plus three longer ice core δ18O records (DYE-3, Crete, and GRIP), the seven scientists developed a temperature history extending more than 1,400 years back in time. From that history they determined “temperatures during the warmest intervals of the Medieval Warm Period,” which they defined as occurring “some 900 to 1300 years ago, “were as warm as or slightly warmer than present day Greenland temperatures.”

Last, Kobashi et al. (2010) write, “in Greenland, oxygen isotopes of ice (Stuiver et al., 1995) have been extensively used as a temperature proxy, but the data are noisy and do not clearly show multi-centennial trends for the last 1,000 years, in contrast to borehole temperature records that show a clear ‘Little Ice Age’ and ‘Medieval Warm Period’ (Dahl-Jensen et al., 1998).” However, they note nitrogen (N) and argon (Ar) isotopic ratios—15N/14N and 40Ar/36Ar, respectively—can be used to construct a temperature record that “is not seasonally biased, and does not require any calibration to instrumental records, and resolves decadal to centennial temperature fluctuations.” Kobashi et al. further describe the development of such an approach, after which they use it to construct a history of the past thousand years of central Greenland surface air temperature, based on values of isotopic ratios of nitrogen and argon previously derived by Kobashi et al. (2008) from air bubbles trapped in the GISP2 ice core that had been extracted from central Greenland (72°36’N, 38°30’W).

Figure 3.1.2 depicts the researchers’ reconstruction of central Greenland’s surface temperature history. As best as can be determined from this representation, the peak temperature of the latter part of the Medieval Warm Period—which actually began some time before the start of their record, as demonstrated by the work of Dansgaard et al. (1975), Jennings and Weiner (1996), Johnsen et al. (2001), and Vinther et al. (2010)—was about 0.33°C greater than the peak temperature of the Current Warm Period and about 1.67°C greater than the temperature of the last decades of the twentieth century. In addition, it is worth noting that between about 1400 and 1460 there was also a period of notable warmth in Kobashi et al.’s temperature reconstruction, which aligns well with the Little Medieval Warm Period, the peak temperature of which was about 0.9°C greater than the temperature of the last decades of the twentieth century and the first decade of the twenty-first century.

These findings, in the words of Kobashi et al., “show clear evidence of the Medieval Warm Period and Little Ice Age in agreement with documentary evidence,” and those data clearly show that the Medieval Warm Period in North America was at times considerably warmer than the Current Warm Period has been to date, and that even the Little Medieval Warm Period was considerably warmer than the last decades of the twentieth century and first decade of the twenty-first century.

References

Andresen, C.S., Bjorck, S., Bennike, O., and Bond, G. 2004. Holocene climate changes in southern Greenland: evidence from lake sediments. Journal of Quaternary Science 19: 783–793.

Barron, J.A., Heusser, L.E., and Alexander, C. 2004. High resolution climate of the past 3,500 years of coastal northernmost California. In Proceedings of the Twentieth Annual Pacific Climate Workshop, edited by S.W. Starratt and N.L. Blumquist, 13–22. U.S. Geological Survey.

Byrne, R., Ingram, B.L., Starratt, S., Malamud-Roam, F., Collins, J.N., and Conrad, M.E. 2001. Carbon-isotope, diatom, and pollen evidence for late Holocene salinity change in a brackish marsh in the San Francisco estuary. Quaternary Research 55: 66–76.

Clegg, B.F., Clarke, G.H., Chipman, M.L., Chou, M., Walker, I.R., Tinner, W., and Hu, F.S. 2010. Six millennia of summer temperature variation based on midge analysis of lake sediments from Alaska. Quaternary Science Reviews 29: 3308–3316.

Cronin, T.M., Dwyer, G.S., Kamiya, T., Schwede, S., and Willard, D.A. 2003. Medieval Warm Period, Little Ice Age and 20th century temperature variability from Chesapeake Bay. Global and Planetary Change 36: 17–29.

Dahl-Jensen, D., Mosegaard, K, Gundestrup, N., Clew, G.D., Johnsen, S.J., Hansen, A.W., and Balling, N. 1998. Past temperatures directly from the Greenland ice sheet. Science 282: 268–271.

Dansgaard, W., Johnsen, S.J., Reech, N., Gundestrup, N., Clausen, H.B., and Hammer, C.U. 1975. Climatic changes, Norsemen and modern man. Nature 255: 24–28.

Edwards, T.W.D., Birks, S.J., Luckman, B.H., and MacDonald, G.M. 2008. Climatic and hydrologic variability during the past millennium in the eastern Rocky Mountains and northern Great Plains of western Canada. Quaternary Research 70: 188–197.

Gedalof, Z. and Smith, D.J. 2001. Interdecadal climate variability and regime scale shifts in Pacific North America. Geophysical Research Letters 28: 1515–1518.

Ingram, B.L., Ingle, J.C., and Conrad, M.E. 1996. Stable isotope record of late Holocene salinity and river discharge in San Francisco Bay, California. Earth and Planetary Science Letters 141: 237–247.

Jennings, A.E. and Weiner, N.J. 1996. Environmental change in eastern Greenland during the last 1300 years: evidence from foraminifera and lithofacies in Nansen Fjord, 68°N. The Holocene 6: 179–191.

Jensen, K.G., Kuijpers, A., Koc, N., and Heinemeier, J. 2004. Diatom evidence of hydrographic changes and ice conditions in Igaliku Fjord, South Greenland, during the past 1500 years. The Holocene 14: 152–164.

Johnsen, S.J., Dahl-Jensen, D., Gundestrup, N., Steffensen, J.P., Clausen, H.B., Miller, H., Masson-Delmotte, V., Sveinbjörnsdottir, A.E., and White, J. 2001. Oxygen isotope and palaeotemperature records from six Greenland ice-core stations: Camp Century, Dye-3, GRIP, GISP2, Renland and NorthGRIP. Journal of Quaternary Science 16: 299–307.

Kobashi, T., Severinghaus, J.P., Barnola, J.-M., Kawamura, K., Carter, T., and Nakaegawa, T. 2010. Persistent multi-decadal Greenland temperature fluctuation through the last millennium. Climatic Change 100: 733–756.

Kobashi, T., Severinghaus, J.P., and Kawamura, K. 2008. Argon and nitrogen isotopes of trapped air in the GISP2 ice core during the Holocene epoch (0–11,600 B.P.): methodology and implications for gas loss processes. Geochimica et Cosmochimica Acta 72: 4675–4686.

Lassen, S.J., Kuijpers, A., Kunzendorf, H., Hoffmann-Wieck, G., Mikkelsen, N., and Konradi, P. 2004. Late Holocene Atlantic bottom water variability in Igaliku Fjord, South Greenland, reconstructed from foraminifera faunas. The Holocene 14: 165–171.

McGann, M. 2008. High-resolution foraminiferal, isotopic, and trace element records from Holocene estuarine deposits of San Francisco Bay, California. Journal of Coastal Research 24: 1092–1109.

Meyer, G.A., Wells, S.G., and Jull, A.J.T. 1995. Fire and alluvial chronology in Yellowstone National Park: climatic and intrinsic controls on Holocene geomorphic processes. Geological Society of America Bulletin 107: 1211–1230.

Nordt, L., von Fischer, J., and Tieszen, L. 2007. Late Quaternary temperature record from buried soils of the North American Great Plains. Geology 35: 159–162.

Nordt, L., von Fischer, J., Tieszen, L., and Tubbs, J. 2008. Coherent changes in relative C4 plant productivity and climate during the late Quaternary in the North American Great Plains. Quaternary Science Reviews 27: 1600–1611.

Norgaard-Pedersen, N. and Mikkelsen, N. 2009. 8000 year marine record of climate variability and fjord dynamics from Southern Greenland. Marine Geology 264: 177–189.

Rood, S.B., Samuelson, G.M., Weber, J.K., and Wyrot, K.A. 2005. Twentieth-century decline in streamflows from the hydrographic apex of North America. Journal of Hydrology 306: 215–233.

Stine, S. 1994. Extreme and persistent drought in California and Patagonia during Medieval time. Nature 369: 546–548.

Stuiver, M., Grootes, P.M., and Brazunias, T.F. 1995. The GISP2 δ18O climate record of the past 16,500 years and the role of the sun, ocean, and volcanoes. Quaternary Research 44: 341–354.

Thomas, E., Shackeroff, J., Varekamp, J.C., Buchholtz Ten Brink, M.R., and Mecray, E.L. 2001. Foraminiferal records of environmental change in Long Island Sound. Geological Society of America, Abstracts with Program 33(1), A–83.

Varekamp, J.C., Thomas, E., Lugolobi, F., and Buchholtz Ten Brink, M.R. 2002. The paleo-environmental history of Long Island Sound as traced by organic carbon, biogenic silica and stable isotope/trace element studies in sediment cores. Proceedings of the 6th Biennial Long Island Sound Research Conference. Groton, CT.

Vinther, B.M., Jones, P.D., Briffa, K.R., Clausen, H.B., Andersen, K.K., Dahl-Jensen, D., and Johnsen, S.J. 2010. Climatic signals in multiple highly resolved stable isotope records from Greenland. Quaternary Science Reviews 29: 522–538.

von Fischer, J.C., Tieszen, L.L., and Schimel, D.S. 2008. Climate controls on C3 vs. C4 productivity in North American grasslands from carbon isotope composition of soil organic matter. Global Change Biology 14: 1–15.

Whitlock, C., Dean, W., Rosenbaum, J., Stevens, L., Fritz, S., Bracht, B., and Power, M. 2008. A 2650-year-long record of environmental change from northern Yellowstone National Park based on a comparison of multiple proxy data. Quaternary International 188: 126–138.

Wiles, G.C., Barclay, D.J., Calkin, P.E., and Lowell, T.V. 2008. Century to millennial-scale temperature variations for the last two thousand years inferred from glacial geologic records of southern Alaska. Global and Planetary Change 60: 115–125.

Wilson, R., Wiles, G., D’Arrigo, R., and Zweck, C. 2007. Cycles and shifts: 1300 years of multi-decadal temperature variability in the Gulf of Alaska. Climate Dynamics 28: 425–440.


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