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Growing Season - Summary
Climate model predictions of global warming suggest that a number of climate, weather and biological phenomena will be affected by the atmosphere's increasing atmospheric CO2 concentration.  One such phenomenon is the length of the growing season, which is projected by the models to increase in direct response to a rise in global temperature.  In this summary, we review several studies that have examined growing season trends and whether they can properly be attributed to CO2-induced global warming.

For the period 1930 to 1998, Kozlov and Berlina (2002) examined several phenological variables to look for possible changes in the length of the growing season in the taiga forests of northern Russia.  No trend in the date of first snow was detected, but the date of permanent snow cover in the forests began 13 days earlier at the end of the study period than at its beginning.  In addition, snow around tree-trunks was found to melt 16 days later in the spring at the end of the record.  The duration of the snow-free period in the forests also decreased by 20 days over the 68-year period, while the ice-free period of lakes decreased by 15 days.  Comparison of the above trends with seasonal precipitation data failed to provide an explanation for the observations.

Kozlov and Berlina note that the results of their study "clearly contradict the expected regional warming" that is championed by believers in CO2-induced global warming.  In fact, the data represent such a dramatic contradiction of the climate-alarmist thesis that the authors openly questioned whether something was wrong with their data.  However, as they report, "close scrutiny of the original records, protocols, and other relevant information did not reveal any possible source of error." Thus, they confidently concluded that the length of the growing season on the Kola Peninsula "really declined during the past 60 years due to both delayed spring and advanced autumn/winter."

Elsewhere, Menzel and Fabian (1999) report different results for the growing season in Europe, although their findings apply to a much shorter period of time.  In a study of 30 years of phenological data derived from observations of identical clones of trees and shrubs maintained by the European network of the International Phenological Gardens - which network is located within the area bounded by latitudes 42 and 69 N and by longitudes 10 W and 27 E - they determined that the mean date of spring bud-break had advanced by fully six days since the early 1960s, while leaf senescence in the fall had been delayed by an average of 4.8 days over the same period.  Thus, for this much shorter interval of time, Menzel and Fabian reported an approximate eleven-day increase in the growing season.

What is the source of the apparent discrepancy between the results of the two papers noted above?  The answer may lie in the degree to which the North Atlantic Oscillation influences climate at the two locations.  According to D'Odorico et al. (2002) - who investigated the possibility that earlier onsets of the growing season in Europe are due to warmer winters that are associated with a change in the phase of the North Atlantic Oscillation (NAO) - "spring phenology in Europe is found to be significantly affected by the North Atlantic Oscillation," with high-NAO (warm) winters hastening the occurrence of spring phenophases (budburst and bloom), as well as the production, release, dispersal and transport of pollen.  In fact, they describe the relationship between the dependence of the onset of the pollen season on the phases of the NAO as nothing short of remarkable.  They also identified "a significant degree of dependence between NAO and spring cryophenology in northern-central Europe," with high-NAO phases being characterized by warmer winters leading to earlier dates of ice breakup.  In accomplishing this task, D'Odorico et al. determined the NAO index dependency of the dates of first leafing and blooming in a number of different plants, the time of pollen season initiation, and the beginning dates of ice breakup on several lakes.  Hence, if changes in the NAO largely explain "both the high- and the low-frequency variability of plant phenology," as these authors have shown, there's not much need to invoke anything else as their cause, including global warming.

Moving to the United States, Robeson (2002) used daily minimum air temperature data for the period 1906-1997 obtained from 36 U.S. Historical Climatology Network stations in the state of Illinois to calculate the date of last spring freeze, the date of first fall freeze, and the resulting length of the freeze-free growing season.  They report that, "(1) the date of the last spring freeze is nearly one week earlier now than it was 100 years ago, (2) fall freeze dates have not changed in a systematic fashion, and (3) the growing season is nearly one week longer now," which directly follows from observations 1 and 2.

With respect to the first of these observations, Robeson notes that it is driven by the century-long amelioration of the very coldest spring minimum temperatures and not by a uniform upward shift (warming) of the entire distribution of all minimum temperatures for the month in which they normally occur, i.e., April.  Likewise, he notes that the second phenomenon is a result of the fact that the very coldest autumn minimum temperatures have not changed all that much over the century of record, in spite of the fact that the entire distribution of all minimum temperatures for the month in which they normally occur, i.e., October, actually cooled at a very significant rate.  The complexity of the results makes it difficult to point to a particular forcing mechanism that could be responsible for the observed trends.

Lastly, White et al. (1999) investigated growing season length over an 88-year period (1900-1987) for twelve sites in the eastern deciduous broadleaf forest of the United States, noting that ten-day growing season length decreases were characteristically observed over periods of one to two decades throughout the 88-year study period, while increases of the same magnitude occurred in as little as four to six years.  Thus, recent observations of seven- to eight-day increases in growing season length in high northern latitudes over the past decade or so (Myneni et al., 1997; Zhou et al., 2001), which have been suggested by some to be evidence of CO2-induced global warming, are, according to White et al., "neither unusual nor necessarily a sign of permanent climate change."

In conclusion, it does not appear that changes in the length of the growing season can be construed as evidence of CO2-induced global warming.

References
D'Odorico, P., Yoo, J-C. and Jaeger, S.  2002.  Changing seasons: An effect of the North Atlantic Oscillation?  Journal of Climate 15: 435-445.

Kozlov, M.V. and Berlina, N.G.  2002.  Decline in length of the summer season on the Kola Peninsula, Russia.  Climatic Change 54: 387-398.

Menzel, A. and Fabian, P.  1999.  Growing season extended in Europe.  Nature 397: 659.

Myneni, R.C., Keeling, C.D., Tucker, C.J., Asrar, G. and Nemani, R.R.  1997.  Increased plant growth in the northern high latitudes from 1981 to 1991.  Nature 386: 698-702.

Robeson, S.M.  2002.  Increasing growing-season length in Illinois during the 20th century.  Climatic Change 52: 219-238.

White, M.A., Running, S.W. and Thornton, P.E.  1999.  The impact of growing-season length variability on carbon assimilation and evapotranspiration over 88 years in the eastern US deciduous forest.  International Journal of Biometeorology 42: 139-145.

Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B.  2001.  Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981 to 1999.  Journal of Geophysical Research 106: 20,069-20,083.