Related Greenhouse Research Articles

Climate of the earth: An overview

Climate has varied over a large range in the recent history of the Earth, with extremes represented by equable environments of the Cretaceous and Eocene and the comparatively frigid conditions of the ice ages that punctuated the past few million years. It is suggested that major shifts in climate are controlled largely by variations in CO 2 with related fluctuations in modes of ocean circulation. Changes in climate can proceed rapidly, on time scales as short as centuries or even decades, as indicated by data for the Younger Dryas (a period of globally cold conditions interrupting recovery of the Earth from the last ice age) and the Little Ice Age (a cold snap extending from about 1250 to about 1850 ad). Rapid fluctuations in climate appear to be linked to changes in production of deep water in the North Atlantic, possibly also to variations in circulation of intermediate waters in the Pacific. Mechanisms are discussed whereby changes in ocean circulation can result in shifts of climate on a global scale. A 10 000 year record of climate from Norway is used to provide context for a discussion of possible changes in climate today arising as a result of the build-up of industrially related greenhouse gases. Brief, somewhat pessimistic, comments are offered concerning the prospects for meaningful near-term predictions of the response of climate to increased concentrations of greenhouse gases. Studies of past climates, by drawing attention to important processes and feedbacks, can play a valuable role in the development of credible models for the future.

Leaf carbon isotope discrimination and vegetative responses of Dryas octopetala to temperature and water manipulations in a High Arctic polar semi-desert, Svalbard.

Integrative ecophysiological and vegetative responses of Dryas octopetala were measured in response to field perturbations of temperature, precipitation and their interactions in a polar semi-desert in Svalbard, Norway (79°N, 12°E). Leaf carbon isotope discrimination (Δ), total leaf nitrogen concentration and leaf development were determined for photosynthetic leaves collected during the last week of August 1991, after one season of manipulations. Individual leaf weight and the total mass of leaf tissue were significantly lower when water was added, irrespective of temperature regime. Leaf carbon isotope discrimination and estimated long-term c i/c avalues (the ratio of CO2 concentration in leaf intercellular spaces to that in the atmosphere) were significantly higher under all three field manipulation treatments, and Δ was significantly reduced when Dryas was grown under drought conditions in a related greenhouse study. Nitrogen concentrations of plants from the field experiment were significantly lower under warmed conditions regardless of water regime. Our results indicate that changes in environmental conditions in high arctic settings will result in alterations of Dryas leaf gas exchange, as expressed by increases in carbon isotope discrimination, which may be accompanied by shifts in leaf nitrogen content and leaf biomass.

Growth response of two Sphagnum species to photoperiod

Growth of two Sphagnum species, S. magellanicum Brid. and S. papillosum Lindb., was studied in the laboratory, field, and greenhouse in relation to photoperiod. Short days (less than 10 h light) induce both species into dormancy. Both species exhibit a high correlation of growth in length and biomass with photoperiod, having maximum length and biomass increase under continuous illumination. Likewise, the ability to lower pH increases with photoperiod. The greater increase in biomass of S. papillosum under 18-h photoperiods could provide short-term niche partitioning. Key words: Sphagnum, photoperiod, growth, pH-lowering ability.

On the Cause of the Relative Greenhouse Strength of Gases such as the Halocarbons

This note examines some of the factors important in determining the large radiative impact, relative to carbon dioxide, of increased concentrations of gases in the optically thin limit (such as the halocarbons at their present day concentrations). A narrow-band radiative transfer model is used to show that an absorber with the same integrated band strength as CFC-12, but with almost the same spectral variation of tropopause net flux change as occurs for small variations in carbon dioxide concentration, is 400 times more effective than carbon dioxide, on a molecule-per-molecule basis; this can be compared with the relative strength of 20,000 for CFC-12. This illustrates that the dominant reason for the relative strength of such gases is not their position in the 8-13 [mu]m window. It is not possible to unambiguously separate the possible reasons (spectral position, preexisting amounts and spectroscopic strength) for the variations in relative strength, as they are all related. 9 refs., 6 figs.

EFFICIENCY OF RHIZOBIUM MELILOTI STRAINS AND THEIR EFFECTS ON ALFALFA CULTIVARS

Five alfalfa cultivars were studied in the greenhouse in relation to their effective symbiotic relation with different strains of Rhizobium meliloti. A significant variety × R. meliloti strain interaction for yield was evident in the first cutting, but at the second cutting, some strains of R. meliloti were more effective in their symbiosis with alfalfa cultivars than others.