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Was There a 15th-Century "Little" Medieval Warm Period?
Volume 7, Number 26: 30 June 2004

In one of the more intriguing aspects of his study of global climate change over the past three millennia - of which he amazingly makes no particular mention - Loehle (2004) presents a graph of the Sargasso Sea and South African temperature records of Keigwin (1996) and Holmgren et al. (1999, 2001) that reveals the existence of a major spike in surface air temperature that began sometime in the early 1400s. This abrupt and anomalous warming pushed global air temperatures considerably above the peak warmth of the 20th century, after which they fell back to pre-spike levels in the mid-1500s, in seeming confirmation of the work of McIntyre and McKitrick (2003), who found a similar period of higher-than-current temperatures in their reanalysis of the data employed by Mann et al. (1998, 1999) in creating their controversial "hockeystick" history of Northern Hemispheric temperature, which gives no indication whatsoever of the existence of this decadal-to-century-scale high-temperature regime.

In another paper we recently reviewed, D'Arrigo et al. (2004) developed a maximum latewood density (MXD) chronology for the period AD 1389 to 2001, based on cores obtained from white spruce trees growing near the elevational treeline on the eastern Seward Peninsula of Alaska, a portion of which data (1909-1950) were calibrated against May-August monthly temperatures obtained at Nome, which were then used to convert the entire MXD chronology to warm-season temperatures. In viewing the final result, it can readily be seen there was a two-decade period of close-to-20th-century warmth in the mid-1500s that was preceded by a decade of warmth in the latter part of the 1400s that was actually greater than that of the mid-20th century.

In yet another pertinent study, Fleitmann et al. (2004) developed a stable isotope history from three stalagmites in a cave in Southern Oman that provided an annually-resolved 780-year record of Indian Ocean monsoon rainfall. Over the last eight decades of the 20th century, when global temperatures rose dramatically as the earth emerged from the Little Ice Age and entered the Modern Warm Period, Indian Ocean monsoon rainfall declined dramatically, while the other most dramatic decline in monsoon rainfall coincided with the major temperature spike that is evident in the temperature records of Keigwin, Holmgren et al. and McIntyre and McKitrick.

Finally, in a study reminiscent of the repeat photography project of Idso and Idso (2000), Munroe (2003) replicated and analyzed six photographs taken in 1870 near the subalpine forest-alpine-tundra ecotone in the northern Uinta Mountains of Utah, USA, in an attempt to quantify the redistribution of vegetation that occurred there between the end of the Little Ice Age and the current stage of the Modern Warm Period. After achieving this objective, he used his findings to infer the nature of regional climate change over the last 130 years; but before concluding - and almost as a afterthought - he directed his attention to what he describes as "downed logs, in situ stumps, and upright delimbed boles on the north side of Bald Mountain [that] indicate a treeline up to 60 m higher than the modern level," which he determined, on the basis of the modern atmospheric lapse rate, "corresponds to an increase of mean July temperature of 0.4C."

With respect to these subfossil relics, Munroe writes that many of them "have been severely abraded by windblown ice, giving the impression of considerable antiquity," noting that "similar wood from elsewhere in the Rocky Mountains has been taken as evidence of higher treeline during the early Holocene climatic optimum, or 'altithermal' (Carrara et al., 1991)." However, he reports that a sample cut from one of the stumps was radiocarbon dated to only about AD 1550, and that "the actual germination of the tree may have occurred a century or more before AD 1550," which places the warm period indicated by the subfossil wood at approximately the same place in time as the warm periods identified in all of the prior studies we have discussed. And, in fact, Munroe himself independently concludes that "a higher treeline in the northern Uintas shortly before AD 1550 is consistent with contemporaneous evidence for warmer-than-modern climates in the southwestern United States (Dean, 1994; Petersen, 1994; Meyer et al., 1995; Pederson, 2000)."

In light of these several observations, of which there may well be many others that suggest the same thing, we wonder if the approximate one-hundred-year rise and fall of the mean global air temperature that began in the 15th century and ended in the 16th century was an independent phenomenon or perhaps the "last hurrah" of the Medieval Warm Period before it relinquished control of the climate to the Little Ice Age that subsequently held sway over the planet. Whatever may be the case, it is beginning to look like the Medieval Warm Period proper and the earlier Roman Warm Period were not the only eras to exhibit surface air temperatures that equaled or eclipsed those of the 20th century.

Sherwood, Keith and Craig Idso

References
Carrara, P.E., Trimble, D.A. and Rubin, M. 1991. Holocene treeline fluctuations in the northern San Juan Mountains, Colorado, U.S.A., as indicated by radiocarbon-dated conifer wood. Arctic and Alpine Research 23: 233-246.

D'Arrigo, R., Mashig, E., Frank, D., Jacoby, G. and Wilson, R. 2004. Reconstructed warm season temperatures for Nome, Seward Peninsula, Alaska. Geophysical Research Letters 31: 10.1029/2004GL019756.

Dean, J.S. 1994. The Medieval Warm Period on the southern Colorado Plateau. Climatic Change 25: 225-241.

Fleitmann, D., Burns, S.J., Neff, U., Mudelsee, M., Mangini, A. and Matter, A. 2004. Palaeoclimatic interpretation of high-resolution oxygen isotope profiles derived from annually laminated speleothems from Southern Oman. Quaternary Science Reviews 23: 935-945.

Holmgren, K., Karlen, W., Lauritzen, S.E., Lee-Thorp, J.A., Partridge, T.C., Piketh, S., Repinski, P., Stevenson, C., Svanered, O. and Tyson, P.D. 1999. A 3000-year high-resolution stalagmite-based record of paleoclimate for northeastern South Africa. The Holocene 9: 295-309.

Holmgren, K., Tyson, P.D., Moberg, A. and Svanered, O. 2001. A preliminary 3000-year regional temperature reconstruction for South Africa. South African Journal of Science 99: 49-51.

Idso, C.D. and Idso, K.E. 2000. The Greening of the American West. Center for the Study of Carbon Dioxide and Global Change, Tempe, AZ, USA.

Keigwin, L.D. 1996. The Little Ice Age and Medieval Warm Period in the Sargasso Sea. Science 274: 1504-1508.

Loehle, C. 2004. Climate change: detection and attribution of trends from long-term geologic data. Ecological Modelling 171: 433-450.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

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.

Munroe, J.S. 2003. Estimates of Little Ice Age climate inferred through historical rephotography, Northern Uinta Mountains, U.S.A. Arctic, Antarctic, and Alpine Research 35: 489-498.

Pederson, J.L. 2000. Holocene paleolakes of Lake Canyon, Colorado Plateau: paleoclimate and landscape response from sedimentology and allostratigraphy. Geological Society of America Bulletin 112: 147-158.

Petersen, K.L. 1994. A warm and wet Little Climatic Optimum and a cold and dry Little Ice Age in the southern Rocky Mountains, U.S.A. Climatic Change 26: 243-269.