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Little Ice Age (Conterminous United States) -- Summary
Working within the Tappan Zee area of the Hudson River estuary (New York, USA), Carbotte et al. (2004) located fossil oyster beds via chirp sub-bottom and side-scan sonar surveys, after which they retrieved sediment cores from the sites that provided shells for radiocarbon dating.  This effort indicated, in their words, that "oysters flourished during the mid-Holocene warm period," when they note that "summertime temperatures were 2-4C warmer than today (e.g., Webb et al., 1993; Ganopolski et al., 1998)."  Thereafter, however, they found that the oysters "disappeared with the onset of cooler climate at 4,000-5,000 cal. years BP," but that they "returned during warmer conditions of the late Holocene," which they specifically identify as the Roman and Medieval Warm Periods, as delineated by Keigwin (1996) and McDermott et al. (2001), explicitly stating that "these warmer periods coincide with the return of oysters in the Tappan Zee."  Unfortunately, they report that their shell dates suggest a final "major demise at ~500-900 years BP," which timing they describe as being "consistent with the onset of the Little Ice Age," noting further that within nearby Chesapeake Bay, "Cronin et al. (2003) report a sustained period of cooler springtime water temperatures (by ~2-5C) during the Little Ice Age relative to the earlier Medieval Warm Period."  Last of all, they add that "similar aged fluctuations in oyster presence are observed within shell middens elsewhere along the Atlantic seaboard," citing results obtained from Maine to Florida.

This intriguing study of the periodic establishment and demise of oyster beds in the Hudson River estuary and elsewhere along the east coast of the United States paints a clear picture of alternating multi-century warm and cold intervals over the past two millennia that is vastly different from the 1000-year-long "hockeystick" temperature history of Mann et al. (1999) and the 2000-year-long temperature history produced by Mann and Jones (2003), wherein Northern Hemispheric and global mean temperatures experience essentially no low-frequency variability until the advent of the 20th century, when temperatures are portrayed as rising dramatically, allowing them to claim that 20th-century warming was driven by anthropogenic CO2 emissions.  With respect to this contention, however, they are most certainly wrong, as papers that tell essentially the same climatic story as that of the Carbotte et al. study are published almost weekly in the scientific literature, as one can readily verify by perusing our website archives; and this ever-growing mountain of real-world evidence clearly shows that late 20th-century warmth was by no means unique over the past two millennia, having been equaled, and often surpassed, at various times throughout both the Medieval Warm Period of a thousand years ago and the Roman Warm Period of two thousand years ago, during both of which epochs the air's CO2 concentration was fully 100 ppm less than what it is today, signifying that the Modern Warm Period is nothing more than the most recent high-temperature phase of this natural non-CO2-driven millennial-scale oscillation of earth's climate.

Working a little further down the US Atlantic seaboard, Brush (2001) obtained sediment cores from marshes and tributaries, as well as the main stem, of Chesapeake Bay, analyzing them for various paleoecological indicators of regional climate change over the past thousand years.  In doing so, they found, in the words of Brush, that "the Medieval Climatic Anomaly," which most people refer to as the Medieval Warm Period, "and the Little Ice Age are recorded in Chesapeake sediments by terrestrial indicators of dry conditions for 200 years, beginning about 1000 years ago, followed by increases in wet indicators from about 800 to 400 years ago."

These findings are but one more example of the reality of both the Medieval Warm Period and the Little Ice Age, which are the two preeminent climatic anomalies of the past thousand years.  Their significance resides in the fact that the earth cycles back and forth between warm and cool periods (of which these named intervals are typical) on a millennial timescale, which suggests there is nothing unusual about the global warming of the past century or so, as it represents the planet's natural recovery from the global chill of the Little Ice Age and the start of its return to Medieval Warm Period-like conditions.

Far to the west, out on the northern prairies of the mid-continental United States, Laird et al. (2003) studied diatom assemblages in sediment cores taken from three lakes, finding that "shifts in drought conditions on decadal through multi-centennial scales have prevailed in this region for at least the last two millennia," specifically noting the abrupt change "at or near the termination of the Medieval Warm Period (ca. AD 800-1300) and the onset of the Little Ice Age (ca. AD 1300-1850)."  Likewise, in studying an ice core from Wyoming's Upper Fremont Glacier by means of electrical conductivity measurements, scanning electron microscopy, energy dispersive analysis, and isotopic and chemical analyses, Schuster et al. (2000) also found that the termination of the Little Ice Age "was abrupt with a major climatic shift to warmer temperatures around 1845 AD," and that "a conservative estimate for the time taken to complete the Little Ice Age climatic shift to present-day climate [was] about 10 years."

A little further west, in central Idaho, Pierce et al. (2004) dated fire-related sediment deposits in alluvial fans in a research program designed to reconstruct Holocene fire history in xeric ponderosa pine forests and to look for links to past climate change.  Their work centered on tributary alluvial fans of the South Fork Payette (SFP) River area, where fans receive sediment from small but steep basins, in weathered batholith granitic rocks that are conducive to post-fire erosion.  Altogether, they obtained 133 AMS 14C-derived dates from 33 stratigraphic sites in 32 different alluvial fans.  In addition, they compared their findings with those of Meyer et al. (1995), who had earlier reconstructed a similar fire history for nearby Yellowstone National Park in Wyoming, USA.

Pierce et al.'s work revealed, in their words, that "intervals of stand-replacing fires and large debris-flow events are largely coincident in SFP ponderosa pine forests and Yellowstone, most notably during the "Medieval Climatic Anomaly (MCA), ~1,050-650 cal. yr BP."  What is more, they note that "in the western USA, the MCA included widespread, severe miltidecadal droughts (Stine, 1998; Woodhouse and Overpeck, 1998), with increased fire activity across diverse northwestern conifer forests (Meyer et al., 1995; Rollins et al., 2002)."

Following the Medieval Climatic Anomaly (= Medieval Warm Period) and its frequent large-event fires was the Little Ice Age, when, as Pierce et al. describe it, "colder conditions maintained high canopy moisture, inhibiting stand-replacing fires in both Yellowstone lodgepole pine forests and SFP ponderosa pine forests (Meyer et al., 1995; Rollins et al., 2002; Whitlock et al., 2003)."  Subsequently, they report that "over the twentieth century, fire size and severity have increased in most ponderosa pine forests," although their summed probability distributions for both the SFP and Yellowstone data sets make the 20th century literally pale in comparison to the Medieval Warm Period in this regard.

We conclude our trip across the conterminous United States with a review of the findings of Cook et al. (2004), who concluded from a study of centuries-long annually resolved tree-ring records that century-scale warm periods such as the Medieval Warm Period tend to experience more severe and longer-lasting large-scale droughts than do century-scale cool periods such as the Little Ice Age.  In coming to this conclusion, the parameter used by Cook et al. to represent drought was the summer-season Palmer Drought Severity Index (PDSI).  For the entire western half of the United States plus adjacent strips of Canada and Mexico - hereafter simply called the West - they constructed a 103-point 2.5 by 2.5 grid that covers the time interval AD 1380 to 1978, where 68 of the grid points possess PDSI reconstructions stretching all the way back to AD 800.  Then, using instrumental data, they extended the PDSI histories of each grid point from 1979 to 2003, which allowed them to compare the recent severe multiyear drought in the West with droughts of the Medieval Warm Period that occurred within the same region.

As serious as the recent drought may have been, Cook et al. found that it "pales in comparison to an earlier period of elevated aridity and epic drought in AD 900 to 1300, an interval broadly consistent with the Medieval Warm Period."  Within that period of exceptional warmth, they identified four megadroughts centered on AD 936, 1034, 1150 and 1253.  Their data also revealed "an abrupt change to persistently less arid conditions after AD 1300 that lasted for ~600 years," essentially concurrent with the duration of the Little Ice Age, after which they found that "overall aridity in the West has increased in an irregular manner," broadly coincident with 20th-century global warming.

Commenting on the obvious relationship between drought and temperature that is evident in their data, Cook et al. state that "the overall coincidence between our megadrought epoch and the Medieval Warm Period suggests that anomalously warm climate conditions during that time may have contributed to the development of more frequent and persistent droughts in the West."  Then, after recounting several possible reasons for such a relationship, they say that "large-scale warming, such as what plausibly occurred during the Medieval Warm Period, is again suggested as a contributor to the AD 900 to 1300 epoch of elevated aridity and epic drought in the West."  Hence, they suggest that the severe drought currently afflicting the West may be a consequence of 20th-century global warming.

The upshot of these many observations linking drought with warming is that the Medieval Warm Period must have experienced much warmer temperatures than those yet experienced in the Modern Warm Period.  This implication is also evident in still other of Cook et al.'s comparisons of the droughts of each period: (1) "Compared to the earlier 'megadroughts' ... the current drought does not stand out as an extreme event, because it has not yet lasted nearly as long," (2) the Medieval Warm Period droughts "dwarf the comparatively short-duration current drought in the 'West'," (3) "more intense droughts of longer duration have occurred in the past and could occur in the future," and (4) "the epoch of unprecedented aridity revealed in [the Medieval Warm Period] might truly be a harbinger of things to come in the West."

All of these observations suggest that the droughts of the past century have been nowhere near as significant as those of the Medieval Warm Period; and since all of the evidence discussed by Cook et al. tends to attribute drought in the western half of the United States and adjacent parts of Canada and Mexico to warming, the body of evidence they describe suggests that the warmth of the past century has also been nowhere near as significant as that of the Medieval Warm Period.  Interestingly, this is also the take-home message of the study of Pierce et al.

In light of the findings of the several studies discussed above, it is clear that the Little Ice Age was a major climatic phenomenon across the entire conterminous United States, and that its thermal and hydrological characteristics strongly contrasted with those of the warm periods that preceded and followed it, especially the Medieval Warm Period, which was generally found to be much more extreme than the Modern Warm Period in terms of both temperature and precipitation.  Hence, there would appear to be no compelling reason to attribute any of the warming of the 20th century to the concomitant rise in the air's CO2 content, for it is likely not nearly as warm now as it was a thousand years ago, when the atmosphere's CO2 concentration was approximately 100 ppm less than it is currently.

Brush, G.S.  2001.  Natural and anthropogenic changes in Chesapeake Bay during the last 1000 years.  Human and Ecological Risk Assessment 7: 1283-1296.

Carbotte, S.M., Bell, R.E., Ryan, W.B.F., McHugh, C., Slagle, A., Nitsche, F. and Rubenstone, J.  2004.  Environmental change and oyster colonization within the Hudson River estuary linked to Holocene climate.  Geo-Marine Letters 24: 212-224.

Cook, E.R., Woodhouse, C.A., Eakin, C.M., Meko, D.M. and Stahle, D.W.  2004.  Long-term aridity changes in the Western United States.  Science 306: 1015-1018.

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.

Ganopolski, A., Kubatzki, C., Claussen, M., Brovkin, V. and Petoukhov, V.  1998.  The influence of vegetation-atmosphere-ocean interaction on climate during the mid-Holocene.  Science 280: 1916-1919.

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

Laird, K.R., Cumming, B.F., Wunsam, S., Rusak, J.A., Oglesby, R.J., Fritz, S.C. and Leavitt, P.R.  2003.  Lake sediments record large-scale shifts in moisture regimes across the northern prairies of North America during the past two millennia.  Proceedings of the National Academy of Sciences USA 100: 2483-2488.

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.

Mann, M.E. and Jones, P.D.  2003.  Global surface temperatures over the past two millennia.  Geophysical Research Letters 30: 10.1029/2003GL017814.

McDermott, F., Mattey, D.P. and Hawkesworth, C.  2001.  Centennial-scale Holocene climate variability revealed by a high-resolution speleothem δ18O record from SW Ireland.  Science 294: 1328-1331.

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.

Pierce, J.L., Meyer, G.A. and Jull, A.J.T.  2004.  Fire-induced erosion and millennial-scale climate change in northern ponderosa pine forests.  Nature 432: 87-90.

Rollins, M.G., Morgan, P. and Swetnam, T.  2002.  Landscape-scale controls over 20th century fire occurrence in two large Rocky Mountain (USA) wilderness areas.  Landscape Ecology 17: 539-557.

Schuster, P.F., White, D.E., Naftz, D.L. and Cecil, L.D.  2000.  Chronological refinement of an ice core record at Upper Fremont Glacier in south central North America.  Journal of Geophysical Research 105: 4657-4666.

Stine, S.  1998.  In: Issar, A.S. and Brown, N. (Eds.), Water, Environment and Society in Times of Climatic Change.  Kluwer, Dordrecth, The Netherlands, pp. 43-67.

Webb III, T., Bartlein, P.J., Harrison, S.P. and Anderson, K.H.  1993.  Vegetation, lake levels, and climate in eastern North America for the past 18000 years.  In: Wright, H.E., Kutzbach, J.E., Webb III, T., Ruddiman, W.F., Street-Perrott, F.A. and Bartlein, P.J. (Eds.) Global Climates Since the Last Glacial Maximum, University of Minnesota Press, Minneapolis, Minnesota, USA, pp. 415-467.

Whitlock, C., Shafer, S.L. and Marlon, J.  2003.  The role of climate and vegetation change in shaping past and future fire regimes in the northwestern US and the implications for ecosystem management.  Forest Ecology and Management 178: 163-181.

Woodhouse, C.A. and Overpeck, J.T.  1998.  2000 years of drought variability in the central United States.  Bulletin of the American Meteorological Society 79: 2693-2714.

Last updated 21 December 2005