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Decadal-Scale Climate Oscillations (North America) -- Summary
Earth's climate is forever changing, on one time scale or another.  Over the past million years, the 100,000-year glacial-interglacial cycle has been the most pronounced of these climatic periodicities; but there is also evidence for a pattern of persistent millennial-scale, centennial-scale and decadal-scale climate oscillations.  Our understanding of these cycles, including their causes and periodicities, is crucial to the global warming debate; for the reality and consistency of these phenomena may suggest that the warming of the past century or so may have been nothing more than the natural "next phase" of climatic progression following on the heels of the Little Ice Age.  In this summary, we thus examine some of these natural climate cycles at decadal time-scales for the region of North America.

A parameter that exhibits decadal-scale oscillations and shows up frequently in the literature of North American climate studies is precipitationZhang et al. (2001), for example, analyzed the spatial and temporal characteristics of extreme precipitation events over Canada for the period 1900-1998, finding that decadal-scale variability was a dominant feature of both the frequency and intensity of the annual number of extreme precipitation events.  In another study, Campbell (2002) analyzed the grain sizes of sediment cores obtained from Pine Lake, Alberta, Canada (52N, 113.5W), discovering periods of both increasing and decreasing moisture availability for this part of North America over the past 4000 years at decadal, centennial and millennial time scales.

Other studies have highlighted decadal-scale oscillations of drought.  Laird et al. (2003), for example, examined diatom assemblages in sediment cores taken from three Canadian and three United States lakes situated within the northern prairies of North America.  The results of their analysis show that "shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia."

In another study, Woodhouse and Overpeck (1998) used various proxy data to study drought variability in the central United States over the past 2000 years.  These data indicated the presence of numerous "multidecadal- to century-scale droughts," leading the authors to conclude that "twentieth-century droughts are not representative of the full range of drought variability that has occurred over the last 2000 years."  In addition, the authors note that the most recent century has been characterized by droughts of "moderate severity and comparatively short duration, relative to the full range of past drought variability."

Much the same conclusions flow from investigations conducted off the eastern seaboard of the United States, where Cronin et al. (2000) studied the salinity gradient across sediment cores obtained from the bottom of Chesapeake Bay in an effort to assess precipitation variability in the surrounding watershed over the past 1000 years.  These investigators found a high degree of decadal and multi-decadal variability between wet and dry conditions, with regional precipitation fluctuating between 25 to 30%, often in extremely rapid shifts that occurred over a single decade.  They also determined that the Chesapeake Bay region has experienced several "mega-droughts," lasting from 60 to 70 years, some of which were more severe than any droughts of the past century.  Such multi-decadal-to-centennial length droughts in the Chesapeake Bay region have also been reported by Willard et al. (2003), who also analyzed sediment cores to derive a 2300-year moisture record.

Toward the south and west of the continent, Ni et al. (2002) developed a 1000-year history of cool-season (November-April) precipitation for each climate division in Arizona and New Mexico, USA, from a network of 19 tree-ring chronologies.  Several periods of both wet and dry conditions were noted at multi-decadal time-scales.

Another climate parameter exhibiting decadal-scale oscillations that regularly appears in the literature for North American is temperature.  According to Higgins et al. (2002), considerable decadal variability in winter surface air temperatures was noted in their study of Jan-Mar daily temperature extremes over the conterminous United States for the 50-year period 1950-1999.  Similarly, Gedalof and Smith (2001) report decadal-scale temperature oscillations in a 400-year study of tree-ring-width chronologies from stands of mountain hemlock growing from southern Oregon to the Kenai Peninsula, Alaska.  Analyzing the data in such a way as to "directly relate changes in radial growth to annual variations in the North Pacific ocean-atmosphere system," the authors determined that "much of the pre-instrumental record in the Pacific Northwest region of North America is characterized by alternating regimes of relatively warmer and cooler SST [sea surface temperature] in the North Pacific, punctuated by abrupt shifts in the mean background state," which were found to be "relatively common occurrences."  The authors concluded, for example, that "regime shifts in the North Pacific have occurred 11 times since 1650" and that "another regime-scale shift in the North Pacific is almost certainly imminent."

In Alaska, Barclay et al. (1999) used tree-ring data to construct a 1119-year temperature record from A.D. 873 to 1991.  The tree-ring record indicated there were a number of intervals with both cooler and warmer temperatures than present: cooler intervals were centered on A.D. 1400, 1660 and 1870, warm intervals at A.D. 1300, 1440 and 1820.  When comparing their proxy temperature record with three major intervals of glacial expansion, the authors noted that, although glacial advances generally coincide with multidecadal periods of cooler temperatures, in two of these instances they overlap with multidecadal warm intervals.

Lastly, decadal-scale oscillations have been observed in United States hurricane activity and ice-sheet elevation changes in southern Greenland.  Elsner et al. (2001) used data for annual U.S. hurricane numbers obtained from the U.S. National Oceanic and Atmospheric Administration and data for average sea surface temperature (SST) anomalies for the region bounded by 6N to 6S latitude and 90W to 180W longitude (called the "cold tongue index" or CTI) obtained from the Joint Institute for the Study of the Atmosphere and the Oceans to see if there is a connection between the number of hurricanes that hit the eastern coast of the United States each year and the presence or absence of El Nio conditions.  Based on data for the period 1901-2000, the authors concluded that "when CTI values indicate below normal equatorial SSTs, the probability of a U.S. hurricane increases."  Or as they describe the relationship in another place, "the annual count of hurricanes is higher when values of the CTI are lower (La Nia events)."  In addition, they determined that the North Atlantic Oscillation is "an additional important factor in explaining U.S. hurricane activity on the decadal scale after accounting for ENSO."

McConnell et al. (2000) derived changes in ice-sheet elevation in southern Greenland for the years 1978-88, using "a physically based model of firn densification and records of annual snow accumulation reconstructed from 12 ice cores at high elevation."  Their results agreed closely with those obtained from satellite measurements of ice-sheet elevation change; and, in the authors' words, "we therefore attribute the changes observed in 1978-88 to variability in snow accumulation."  They also determined, from similar analyses of longer ice-core records, that "the decadal-scale changes in ice-sheet elevation that occurred during 1978-88 are typical over the last few centuries and well within the natural variability of accumulation-driven elevation change."

In view of these many real-world observations of decadal-scale climate variability in North America, as well as other parts of the planet that will be reviewed in subsequent summaries, it should be clear we must have a solid understanding of their past history and current status before anything can be said about the cause of what climate alarmists claim to be the unprecedented warming of the past two decades, which they are much too quick to attribute to anthropogenic CO2 emissions.

References
Barclay, D.J., Wiles, G.C. and Calkin, P.E.  1999.  A 1119-year tree-ring-width chronology from western Prince William Sound, southern Alaska.  The Holocene 9: 79-84.

Campbell, C.  2002.  Late Holocene lake sedimentology and climate change in southern Alberta, Canada.  Quaternary Research 49: 96-101.

Cronin, T., Willard, D., Karlsen, A., Ishman, S., Verardo, S., McGeehin, J., Kerhin, R., Holmes, C., Colman, S. and Zimmerman, A.  2000.  Climatic variability in the eastern United States over the past millennium from Chesapeake Bay sediments.  Geology 28: 3-6.

Elsner, J.B. Bossak, B.H. and Niu, X.F.  2001.  Secular changes to the ENSO-U.S. Hurricane Relationship.  Geophysical Research Letters 28: 4123-4126.

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

Higgins, R.W., Leetmaa, A. and Kousky, V.E.  2002.  Relationships between climate variability and winter temperature extremes in the United States.  Journal of Climate 15: 1555-1572.

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.

McConnell, J.R., Arthern, R.J., Mosley-Thompson, E., Davis, C.H., Bales, R.C., Thomas, R., Burkhart, J.F. and Kyne, J.D.  2000.  Changes in Greenland ice sheet elevation attributed primarily to snow accumulation variability.  Nature 406: 877-879.

Ni, F., Cavazos, T., Hughes, M.K., Comrie, A.C. and Funkhouser, G.  2002.  Cool-season precipitation in the southwestern USA since AD 1000: Comparison of linear and nonlinear techniques for reconstruction.  International Journal of Climatology 22: 1645-1662.

Willard, D.A., Cronin, T.M. and Verardo, S.  2003.  Late-Holocene climate and ecosystem history from Chesapeake Bay sediment cores, USA.  The Holocene 13: 201-214.

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.

Zhang, X., Hogg, W.D. and Mekis, E.  2001.  Spatial and temporal characteristics of heavy precipitation events over Canada.  Journal of Climate 14: 1923-1936.