Effects of climate change in North America

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Past 1,000 Years

For full article see Past 1,000 years in North America

In an effort to determine whether these climate-driven millennial-scale cycles are present in the terrestrial pollen record of North America, Viau et al. (2002) analyzed a set of 3,076 14C dates from the North American Pollen Database used to date sequences in more than 700 pollen diagrams across North America. Results of their statistical analyses indicated there were nine millennial-scale oscillations during the past 14,000 years in which continent-wide synchronous vegetation changes with a periodicity of roughly 1,650 years were recorded in the pollen records. The most recent of the vegetation transitions was centered at approximately 600 years BP (before present). This event, in the words of the authors, “culminat[ed] in the Little Ice Age, with maximum cooling 300 years ago.” Prior to that event, a major transition that began approximately 1,600 years BP represents the climatic amelioration that “culminat[ed] in the maximum warming of the Medieval Warm Period 1000 years ago.”

And so it goes, on back through the Holocene and into the preceding late glacial period, with the times of all major pollen transitions being “consistent,” in the words of the authors of the study, “with ice and marine records.” Viau et al. additionally note that “the large-scale nature of these transitions and the fact that they are found in different proxies confirms the hypothesis that Holocene and late glacial climate variations of millennial-scale were abrupt transitions between climatic regimes as the atmosphere-ocean system reorganized in response to some forcing.” They go on to say that “although several mechanisms for such natural [our italics] forcing have been advanced, recent evidence points to a potential solar forcing (Bond et al., 2001) associated with ocean-atmosphere feedbacks acting as global teleconnections agents.” Furthermore, they note that “these transitions are identifiable across North America and presumably the world.”


For full articles see Temperature in North America and Urban heat islands in North America

In introducing the rational for their study, Wiles et al. say that “increased understanding of solar variability and its climatic impacts is critical for separating anthropogenic from natural forcing and for predicting anticipated temperature change for future centuries.” In this regard, it is most interesting that they make no mention of possible CO2-induced global warming in discussing their results, presumably because there is no need to do so. Alaskan glacial activity, which, in their words, “has been shown to be primarily a record of summer temperature change (Barclay et al., 1999),” appears to be sufficiently well described within the context of solar and PDO variability alone. Four years later, Wiles et al. (2008) reconfirmed this Alaska solar-climate link in a separate study.

It is clear that broad-scale periods of warmth in North America have occurred over and over again throughout the Holocene—and beyond (Oppo et al., 1998; Raymo et al., 1998)—forced by variable solar activity. This suggests that the Current Warm Period was also instigated by this recurring phenomenon, not the CO2 output of the Industrial Revolution.

Additionally, the urban heat island effect (a powerful anthropogenic but non-greenhouse-gas-induced effect of urbanization on the energy balance of watersheds and the temperature of the boundary-layer air above them) begins to express itself with the very first hint of urbanization and, hence, may be readily overlooked in studies seeking to identify a greenhouse-gas-induced global warming signal. In fact, the fledgling urban heat island effect may already be present in many temperature records that have routinely been considered “rural enough” to be devoid of all human influence.

A case in point is provided by the study of Changnon (1999), who used a series of measurements of soil temperatures obtained in a totally rural setting in central Illinois between 1889 and 1952 and a contemporary set of air temperature measurements made in an adjacent growing community (as well as similar data obtained from other nearby small towns), to evaluate the magnitude of unsuspected heat island effects that may be present in small towns and cities that are typically assumed to be free of urban-induced warming. This work revealed that soil temperature in the totally rural setting experienced an increase from the decade of 1901-1910 to the decade of 1941-1950 that amounted to 0.4°C.

This warming is 0.2°C less than the 0.6°C warming determined for the same time period from the entire dataset of the U.S. Historical Climatology Network, which is supposedly corrected for urban heating effects. It is also 0.2°C less than the 0.6°C warming determined for this time period by 11 benchmark stations in Illinois with the highest quality long-term temperature data, all of which are located in communities that had populations of less than 6,000 people as of 1990. And it is 0.17°C less than the 0.57°C warming derived from data obtained at the three benchmark stations closest to the site of the soil temperature measurements and with populations of less than 2,000 people.


For full articles see Precipitation trends in the United States and Precipitation trends in Canada and Mexico

Cowles et al. (2002) analyzed snow water equivalent (SWE) data obtained from four different measuring systems—snow courses, snow telemetry, aerial markers and airborne gamma radiation—at more than 2,000 sites in the eleven westernmost states over the period 1910-1998. This work revealed that the long-term SWE trend of this entire region was negative, but with some significant within-region differences. In the northern Rocky Mountains and Cascades of the Pacific Northwest, for example, the trend was decidedly negative, with SWE decreasing at a rate of 0.1 to 0.2 inches per year. In the intermountain region and southern Rockies, however, there was no change in SWE with time. Cowles et al. additionally note that their results “reinforce more tenuous conclusions made by previous authors,” citing Changnon et al. (1993) and McCabe and Legates (1995), who studied snow course data from 1951-1985 and 1948-1987, respectively, at 275 and 311 sites. They too found a decreasing trend in SWE at most sites in the Pacific Northwest but more ambiguity in the southern Rockies.

These findings are particularly interesting in light of the fact that nearly all climate models suggest the planet’s hydrologic cycle will be enhanced in a warming world and that precipitation will increase. This prediction is especially applicable to the Pacific Northwest of the United States, where Kusnierczyk and Ettl (2002) report that climate models predict “increasingly warm and wet winters,” as do Leung and Wigmosta (1999). Over the period of Cowles et al.’s study, however, when there was well-documented worldwide warming, precipitation that fell and accumulated as snow in the western USA did not respond as predicted. In fact, over the Pacific Northwest, it did just the opposite.

These studies reveal nothing unusual about precipitation in the U.S. during the twentieth century, the latter two decades of which the IPCC claims comprise the warmest such period of the past two millennia. Cronin et al.’s work indicates, for example, that both wetter and drier intervals occurred repeatedly in the past in the Chesapeake Bay watershed. There is reason to believe such intervals will occur in the future … with or without any further global warming.

Extreme Weather

For full articles see Floods in North America, Droughts in the United States, Droughts in Canada, and Droughts in Mexico

Lins and Slack (1999) analyzed secular streamflow trends in 395 different parts of the United States that were derived from more than 1,500 individual streamgauges, some of which had continuous data stretching to 1914. In the mean, they found that “the conterminous U.S. is getting wetter, but less extreme.” That is to say, as the near-surface air temperature of the planet gradually rose throughout the course of the twentieth century, the United States became wetter in the mean but less variable at the extremes, which is where floods and droughts occur, leading to what could well be called the best of both worlds, i.e., more water with fewer floods and droughts.

There still were some significant floods during the last decade of the past century, such as the 1997 flooding of the Red River of the North, which devastated Grand Forks, North Dakota, as well as parts of Canada. However, as Haque (2000) reports, although this particular flood was indeed the largest experienced by the Red River over the past century, it was not the largest to occur in historic times. In 1852 there was a slightly larger Red River flood, and in 1826 there was a flood that was nearly 40 percent greater than the flood of 1997. The temperature of the globe was colder at the times of these earlier catastrophic floods than it was in 1997, indicating that one cannot attribute the strength of the 1997 flood to higher temperatures that year or the warming of the preceding decades. We also note that Red River flooding is also linked to snow melt and ice jams because itflows northward into frozen areas.

Similarly, the most severe droughts to have occurred in Mexico took place during the Little Ice Age and the latter part of the Dark Ages Cold Period. These observations do much to discredit the model-based claim that droughts will get worse as air temperatures rise. All of the Mexican droughts of the twentieth century (when the IPCC claims the planet warmed at a rate and to a level that were unprecedented over the past two millennia) were much milder than many of the droughts that occurred during much colder centuries.

Andreadis and Lettenmaier (2006) examined twentieth century trends in soil moisture, runoff, and drought over the conterminous United States with a hydro-climatological model forced by real-world measurements of precipitation, air temperature, and wind speed over the period 1915-2003. This work revealed, in their words, that “droughts have, for the most part, become shorter, less frequent, less severe, and cover a smaller portion of the country over the last century.”

Taken together, the research described in this section suggests that North American flooding tends to become both less frequent and less severe when the planet warms, although there have been some exceptions to this general rule. We would expect that any further warming of the globe would tend to further reduce both the frequency and severity of flooding in North America.


For full article see Glaciers in North America

The history of North American glacial activity also fails to support the claim that anthropogenic CO2 emissions are causing glaciers to melt. Dowdeswell et al. (1997) analyzed the mass balance histories of the 18 Arctic glaciers with the longest observational records, finding that just over 80 percent of them displayed negative mass balances over the last half of the twentieth century. However, they note that “ice-core records from the Canadian High Arctic islands indicate that the generally negative glacier mass balances observed over the past 50 years have probably been typical of Arctic glaciers since the end of the Little Ice Age.” They say “there is no compelling indication of increasingly negative balance conditions which might, a priori, be expected from anthropogenically induced global warming.”


Andreadis, K.M. and Lettenmaier, D.P. 2006. Trends in 20th century drought over the continental United States. Geophysical Research Letters 33: 10.1029/2006GL025711.

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.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Changnon, S.A. 1999. A rare long record of deep soil temperatures defines temporal temperature changes and an urban heat island. Climatic Change 42: 531-538.

Changnon, D., McKee, T.B. and Doesken, N.J. 1993. Annual snowpack patterns across the Rockies: Long-term trends and associated 500-mb synoptic patterns. Monthly Weather Review 121: 633-647.

Cowles, M.K., Zimmerman, D.L., Christ, A. and McGinnis, D.L. 2002. Combining snow water equivalent data from multiple sources to estimate spatio-temporal trends and compare measurement systems. Journal of Agricultural, Biological, and Environmental Statistics 7: 536-557.

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.

Dowdeswell, J.A., Hagen, J.O., Bjornsson, H., Glazovsky, A.F., Harrison, W.D., Holmlund, P., Jania, J., Koerner, R.M., Lefauconnier, B., Ommanney, C.S.L. and Thomas, R.H. 1997. The mass balance of circum-Arctic glaciers and recent climate change. Quaternary Research 48: 1-14.

Haque, C.E. 2000. Risk assessment, emergency preparedness and response to hazards: The case of the 1997 Red River Valley flood, Canada. Natural Hazards 21: 225-245.

Kusnierczyk, E.R. and Ettl, G.J. 2002. Growth response of ponderosa pine (Pinus ponderosa) to climate in the eastern Cascade Mountain, Washington, U.S.A.: Implications for climatic change. Ecoscience 9: 544-551.

Leung, L.R. and Wigmosta, M.S. 1999. Potential climate change impacts on mountain watersheds in the Pacific Northwest. Journal of the American Water Resources Association 35: 1463-1471.

Lins, H.F. and Slack, J.R. 1999. Streamflow trends in the United States. Geophysical Research Letters 26: 227-230

McCabe, A.J. and Legates, S.R. 1995. Relationships between 700hPa height anomalies and 1 April snowpack accumulations in the western USA. International Journal of Climatology 14: 517-530.

Oppo, D.W., McManus, J.F. and Cullen, J.L. 1998. Abrupt climate events 500,000 to 340,000 years ago: Evidence from subpolar North Atlantic sediments. Science 279: 1335-1338.

Raymo, M.E., Ganley, K., Carter, S., Oppo, D.W. and McManus, J. 1998. Millennial-scale climate instability during the early Pleistocene epoch. Nature 392: 699-702.

Viau, A.E., Gajewski, K., Fines, P., Atkinson, D.E. and Sawada, M.C. 2002. Widespread evidence of 1500 yr climate variability in North America during the past 14,000 yr. Geology 30: 455-458.

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 indicated from glacial geologic records of Southern Alaska. Global and Planetary Change 60: 115-125.

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