In order to better understand the nature and possible causes of modern global warming, it is important to know what preceded it and why. In this summary, we briefly review what has been learned in this regard from pertinent studies conducted in Canada.
Futter (2003) analyzed data on ice break-up dates and length of ice-free season for several lakes in Southern Ontario, Canada. However, only one lake had ice break-up dates extending back beyond 1910 (Lake Simcoe, to 1853), and only one had ice-free season data extending back beyond 1971 (Lake Simcoe, again to 1853). Hence, Lake Simcoe was the only lake that had sufficient data to determine, in the words of Futter, “whether the trends in Lake Simcoe ice phenology were due to the end of the Little Ice Age, or to more recent warming.”
Breaking the Lake Simcoe data into three comparable time intervals (1853-1899, 1900-1949, 1950-1995), Futter determined that “only the period from 1853-1899 showed a statistically significant trend indicative of warming temperatures in both the ice break-up and ice-free season series.” In fact, he reports that the 1900-1949 data actually indicate a cooling trend, and that the data from 1950-1995 “show slight but not statistically significant evidence of warming temperatures.”
These data suggest, as we and many others have long contended, that the so-called unprecedented warming of the 20th century was likely nothing more than the natural recovery of the world from the chilly conditions of the Little Ice Age. This view is well supported by the Lake Simcoe ice data, which only show evidence of significant warming over the period 1853 to 1899. Late 20th century warming, on the other hand (when greenhouse gas effects should have been most evident), was not statistically significant, which finding pretty much speaks for itself for this particular part of the world: although the Little Ice Age was much cooler than it is currently, the most recent 50-year boost to the Modern Warm Period (when CO2 emissions far outpaced all that had preceded them) was next to negligible.
Laird et al. (2003) studied diatom assemblages in sediment cores taken from three Canadian lakes situated within the northern prairies of North America, finding that “shifts in drought conditions on decadal through multicentennial scales have prevailed in this region for at least the last two millennia.” Of particular note, in this regard, was their detection of major shifts near the beginning of the Medieval Warm Period together with abrupt changes 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).” Furthermore, they report that “millennial-scale shifts over at least the past 5,500 years, between sustained periods of wetter and drier conditions, occurring approximately every 1,220 years, have been reported from western Canada (Cumming et al., 2002),” and that “the striking correspondence of these shifts to large changes in fire frequencies, inferred from two sites several hundreds of kilometers to the southwest in the mountain hemlock zone of southern British Columbia (Hallett et al., 2003), suggests that these millennial-scale dynamics are linked and operate over wide spatial scales.”
Girardin et al. (2004) developed a 380-year reconstruction of the July monthly average of the Canadian Drought Code (CDC, a daily numerical rating of the average moisture content of deep soil organic layers in boreal conifer stands that is used to monitor forest fire danger) from 16 well replicated tree-ring chronologies from the Abitibi Plains of eastern Canada just below James Bay. Among other things, their “cross-continuous wavelet transformation analyses indicated coherency in the 8-16- and 17-32-year per cycle oscillation bands between the CDC reconstruction and the Pacific Decadal Oscillation prior to 1850,” while “following 1850, the coherency shifted toward the North Atlantic Oscillation.”
Girardin et al. say their results suggest that “the end of [the] ‘Little Ice Age’ over the Abitibi Plains sector corresponded to a decrease in the North Pacific decadal forcing around the 1850s,” and that “this event could have been followed by an inhibition of the Arctic air outflow and an incursion of more humid air masses from the subtropical Atlantic climate sector.” In this regard, they note that several paleo-climatolgical and paleo-ecological studies have suggested that “climate in eastern Canada started to change with the end of the ‘Little Ice Age’ (~1850),” citing the works of Tardif and Bergeron (1997, 1999), Bergeron (1998, 2000) and Bergeron et al. (2001), while further noting that Bergeron and Archambault (1993) and Hofgaard et al. (1999) have “speculated that the poleward retreat of the Arctic air mass starting at the end of the ‘Little Ice Age’ contributed to the incursion of moister air masses in eastern Canada.”
This substantial group of reports clearly places the “beginning of the end” of the Little Ice Age in the Abitibi Plains of Canada fully half a century before what is suggested by the Mann et al. (1999) reconstruction of mean Northern Hemispheric temperature over the past millennium. Hence, it represents yet another set of studies that testifies against the validity of that IPCC-endorsed representation of earth’s temperature history.
Using new tree-ring data from the Columbia Icefield area of the Canadian Rockies, Luckman and Wilson (2005) developed a significant update to a millennial temperature reconstruction published for this region in 1997. The new update employed a different standardization technique (regional curve standardization) in an effort to capture a greater degree of low frequency variability (centennial to millennial scale) than reported in the initial study. In addition, the new data set added over one hundred years to the earlier chronology, so that the new-and-improved one covers the period 950-1994.
Luckman and Wilson’s new tree-ring record was found to explain 53% of May-August maximum temperature variation observed in the 1895-1994 historical data and was thus viewed as a proxy indicator of such temperatures over the past millennium. Based on this relationship, the record showed considerable decadal- and centennial-scale variability, where generally warmer conditions prevailed during the 11th and 12th centuries, between about 1350-1450 and from about 1875 through the end of the record. The warmest reconstructed summer occurred in 1434 and was 0.23°C warmer than the next warmest summer that occurred in 1967. Persistent cold conditions prevailed between 1200-1350, 1450-1550 and 1650-1850, with the 1690s being exceptionally cold (more than 0.4°C colder than other intervals).
The revised Columbia Icefield temperature record provides further evidence for the natural fluctuation of climate on centennial-to-millennial time scales and demonstrates, once again, that temperatures during the Modern Warm Period have been no different from those observed during the Medieval Warm Period (11-12th centuries) or the Little Medieval Warm Period (1350-1450). And since we know that the air’s CO2 content had nothing to do with creating the warm temperatures of those earlier periods, we cannot rule out the possibility that they may have had nothing to do with creating the warm temperatures of the modern era.
But if not anthropogenic CO2 emissions, then what has produced the planet’s current warmth? According to Luckman and Wilson, the Columbia Icefield reconstruction “appears to indicate a reasonable response of local trees to large-scale forcing of climates, with reconstructed cool conditions comparing well with periods of known low solar activity,” which is something that has been found again and again in study after study. Hence, it is highly likely that cyclical solar activity is the primary driver of the low frequency temperature oscillation that brought the world the Little Ice Age, as well as the adjoining Medieval and Modern Warm Periods, plus all of the similar warm and cold periods that preceded them.
References Bergeron, Y. 1998. Les consequences des changements climatiques sur la frequence des feux et la composition forestiere au sud-ouest de la foret boreale quebecoise. Geogr. Phy. Quaternary 52: 167-173.
Bergeron, Y. 2000. Species and stand dynamics in the mixed woods of Quebec’s boreal forest. Ecology 81: 1500-1516.
Bergeron, Y. and Archambault, S. 1993. Decreasing frequency of forest fires in the southern boreal zone of Quebec and its relation to global warming since the end of the ‘Little Ice Age.’ The Holocene 3: 255-259.
Bergeron, Y., Gauthier, S., Kafka, V., Lefort, P. and Lesieur, D. 2001. Natural fire frequency for the eastern Canadian boreal forest: consequences for sustainable forestry. Canadian Journal of Forest Research 31: 384-391.
Cumming, B.F., Laird, K.R., Bennett, J.R., Smol, J.P. and Salomon, A.K. 2002. Persistent millennial-scale shifts in moisture regimes in western Canada during the past six millennia. Proceedings of the National Academy of Sciences USA 99: 16,117-16,121.
Futter, M.N. 2003. Patterns and trends in Southern Ontario lake ice phenology. Environmental Monitoring and Assessment 88: 431-444.
Girardin, M-P., Tardif, J., Flannigan, M.D. and Bergeron, Y. 2004. Multicentury reconstruction of the Canadian Drought Code from eastern Canada and its relationship with paleoclimatic indices of atmospheric circulation. Climate Dynamics 23: 99-115.
Hallett, D.J., Lepofsky, D.S., Mathewes, R.W. and Lertzman, K.P. 2003. 11,000 years of fire history and climate in the mountain hemlock rain forests of southwestern British Columbia based on sedimentary charcoal. Canadian Journal of Forest Research 33: 292-312.
Hofgaard, A., Tardif, J. and Bergeron, Y. 1999. Dendroclimatic response of Picea mariana and Pinus banksiana along a latitudinal gradient in the eastern Canadian boreal forest. Canadian Journal of Forest Research 29: 1333-1346.
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.
Luckman, B.H. and Wilson, R.J.S. 2005. Summer temperatures in the Canadian Rockies during the last millennium: a revised record. Climate Dynamics 24: 131-144.
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.
Tardif, J. and Bergeron, Y. 1997. Ice-flood history reconstructed with tree-rings from the southern boreal forest limit, western Quebec. The Holocene 7: 291-300.
Tardif, J. and Bergeron, Y. 1999. Population dynamics of Fraxinus nigra in response to flood-level variations, in northwestern Quebec. Ecological Monographs 69: 107-125.