Search

To many of us the virtues of our day-to-day changeable weather are manifest. Seventy years of standardized meteorological records give us a good idea of the limits within which any day's events are likely to fall. In the southern English lowlands a fall of two feet of snow, a shade temperature of 970, or a month with 70 per cent. of the possible sunshine, represent extremes unlikely to be surpassed. Superimposed upon our day-to-day weather how? ever we now recognize that small but definite fluctuations of climate exist and that since about 1925 an appreciable amelioration has set in round the North Atlantic. Professor Ahlmann has recently reviewed the many associated events.1 It remains for us to consider the possible amplitude of the variation of our climate in Britain, as distinct from the weather. I shall first comment on the magnitude of the present fluctuation in Britain, in terms of overall mean temperature. I shall follow with some evidence for past fluctuations provided by the instrumental records since 1700, relating them to the earlier climatic recessions demanded by the pollenanalysts in historic and prehistoric time. In the second part of the paper I shall address myself more to those technical experts who are acquainted with the geological and botanical background, and I shall give estimates for the temperature of the various phases since the last Glacial maximum in order to compare the range of variation over which our climate appears at intervals to have changed. In my view it is high time such estimates of the mean tem? perature of summer and winter were made, and an opportunity of doing this for Late Glacial times is now provided by using Professor Ahlmann's relationships between the height of the snow-line, the amount of accumulated pre? cipitation and the mean temperature of the ablation season. Comparisons between present-day fluctuations and past changes may throw light on the possible causes of climatic variation; some will be led to consider whether the present amelioration in north-west Europe will con? tinue, and for how long. Throughout this paper I shall largely confine atten? tion to the mean temperature of January and July as a sufficient initial indication of winter and summer characteristics. Periods of disturbed weather when the oceanic influence is greater are characterized by a diminished January-July range; in quieter and more "continental" phases the range increases. Meteorological aspects of this "varying vigour of the atmospheric circulation" are discussed by Angstrom * and Willett.3 1 H. W:son Ahlmann, "The present climatic fluctuation," Geogr.J. 112 (1948) 165-95. 3 A. Angstrom, "The change in the temperature climate in present time," Geogr. Ann. 21 (1939) 119. 3 H. C. Willett, "Long-period fluctuations of the general circulation of the atmo? sphere," Journ. Met., 6, 1949, pp. 34-50; "Solar-variability as a factor in the fluctua? tions of climate," Geogr. Ann. 31 (1949) 295. See also Q. J. R. Met. S., Centenary Proceedings, 1950.

Photosynthesis of leafy shoots was determined under a range of CO2 concentrations that spanned past and likely future levels of atmospheric CO2. The temperature regime was also manipulated. Response of photosynthesis to CO2 was pronounced, and differed markedly among species. A substantial rise in atmospheric CO2 is likely to produce shifts in species composition of southern California mountain and foothill vegetation, with several conifer species being disadvantaged. INTRODUCTION How much has the well-advertised increase in atmospheric CO2 affected photosynthesis of land plants? What sorts of response can we expect from further increases in CO2? What might be the outcome in a forest? These questions are explored in this paper. In the late 1800's, concentration of atmospheric CO2 was ca. 285 ppm (Callendar, 1958). According to recent careful measurements (Keeling, 1970), the current mean world CO2 level is close to 324 ppm. Annual rate of CO2 increase is approaching 1 ppm and is rising. Levels of 400 ppm by the year 2000, and 500 ppm by 2020, are predicted (Peterson, 1969). Increased burning of fossil fuel is at the root of this increase. Carbon that was gradually withdrawn from global circulation over several hundred million years is suddenly being returned to the carbon cycle. It can be absorbed by sea water, or even contribute to deposition of new fossil fuel, but such processes take thousands to millions of years (Bolin, 1970). If the earth's fossil fuels are fully exploited over the next several hundred years, we face a precipitous rise in atmospheric C02, followed by an almost equally precipitous drop (Revelle, 1965). It is well known that CO2 is one of the principal limiting factors governing rates of photosynthesis on land. A number of studies have shown that plants respond to increased CO2 up to several thousand ppm (Hesketh, 1963; Madsen, 1968; Gordon and Gatherum, 1969). Enhancement of CO2 in greenhouses by artificial means is now accepted practice in many places. However, little is recorded concerning the response of native vegetation to small increments in atmospheric C02, such as have been experienced in the past 100 years or may be expected in the 50 years to come. A field study in the oak-pine forest on Long Island, New York, indicated that enhancement of CO2 up to about 100 ppm above ambient levels produced a sharp linear rise in photosynthesis of leafy twigs (Wright and Woodwell, 1970). The present study was designed to explore the response of a number of western species to modest CO2 enhancement, and to assess the relation of CO2 response to composition of the vegetation.