What Has Been Learned About The Potential Consequences Of Climate Change?


Significant Human Health Effects May Be Linked To Changing Climate

A link has been suggested between the patterns and impacts of disease and changes in temperature, rainfall, and the pattern of extreme weather events. Potential health effects include not only those associated with heat stress, but increases in vector-borne diseases, such as those carried by mosquitoes and rodents. Evidence that extended heavy rainfall precedes outbreaks of vector borne disease includes the 1974 epidemic of West Nile fever in southern Africa, the 1973 encephalitis outbreak in India, and major epidemics of malaria in Ecuador, Peru, and Bolivia in 1983.
Reference: Health and Climate Change, Epstein, P. and D. Sharp, editors, Special Issue of The Lancet, 1994.

Sea Level Rise Found To Endanger Tidal Marsh Ecosystems

Studies in the state of Louisiana show that tidal marsh deterioration is brought on by the stress of water-logging of vegetation associated with sea level rise. Death of vegetation caused by increased flooding leads to the loss of wetlands and pond formation. Along the Gulf Coast, salinity stress would accompany increased flooding. Significant genetic variation in baldcypress' s tolerance of combined flooding and salinity indicates that certain moderately salt-tolerant baldcypress seedlings could be used in coastal restoration projects. However, permanent flooding renders baldcypress more vulnerable to defoliation by the fruit tree leafroller insect pest. Fifty thousand acres of baldcypress-dominated wetlands in southern Louisiana were nearly totally defoliated by this insect in 1993. The area affected by defoliation is expected to increase with an increase in permanently flooded forests. An increase in atmospheric carbon dioxide is also likely to enhance the production of algae and other plants, possible changing natural communities along the Gulf coast.
Reference: Global Climate Change: Implications for Submerged Aquatic Vegetation, Rizzo, W. M., and H. A. Neckles, in Gale, R. U., W. U. Catallo, R. C. Hohanty, and J. B. Johnston (Eds.), Environmental Concerns, Public Policies and Remediation Technologies, Proceedings of the 3rd Annual Convention of the American Society of Environmental Science, Baton Rouge, LA, 1994.

Sagebrush Replaces Shrubs In Alpine Meadow In Climate Warming Experiment

In experimentally heated plots in a Rocky Mountain meadow that were designed to help study the responses of montane meadow vegetation to climate warming, above-ground biomass of sagebrush increased in the drier habitat, while shrubs increased in the wetter habitats. These results, as well as others, suggest that the increased warming expected under an atmosphere with a concentration of CO2 twice that of pre-industrial levels could change the dominant vegetation of a widespread alpine meadow habitat. The findings suggest that during a climate warming the competitive status of shrubs is enhanced, sagebrush growth is enhanced at the expense of meadow forbs which now dominate such alpine meadows, and elongation rate is enhanced. Pollen records spanning the last 100,000 years of Earth history also show intervals of increased abundance of sagebrush during times of climate warming, consistent with the results of this present work.
Reference: Shifting Dominance Within a Montane Vegetation Community: Results of a Climate-Warming Experiment, Hart, J., and R. Shaw, Science, Vol. 267, pp. 876-880, 1995.

Decline Of Plankton In Southern California Due To Ocean Warming

A recent study based upon 43 years of observations (1951-1993) along the coast of southern California indicates that the biomass of large zooplankton has decreased by 80% during this interval of time. This decline has occurred in conjunction with the surface layers of the ocean having warmed in this region (by 1.5°C in some places), and the warming seems a likely cause of the changes. This decline represents a major disruption in the biotic web of this region because large zooplankton are a significant part of the food web and are the main diet of some birds and many schooling, commercially important species of fish. As a result of the surface-intensified warming, the vertical stratification separating the upper ocean mixed layer from the ocean depths was substantially intensified. Consequently, less cold (and nutrient-rich) waters were carried upward to come into contact with the atmosphere in the upwelling zones. With less upward displacement of water, shallower layers of water bearing fewer nutrients are exposed to light and photosynthetic processes, leading to less new biomass production and ultimately to decreases in zooplankton. Thus, the mechanism is not a decrease in the volume of upwelled waters, but rather, it is that the waters that upwell come from shallower depths, and are therefore, warmer and less nutrient rich. It should be underscored that a suppression of nutrient supply by enhanced oceanic stratification is not a mechanism confined to coastal ocean regions. If there is an average global temperature rise of 1 to 2°C in the next 40 years and stratification becomes more widespread, the impacts on fisheries and other marine life could be significant.
Reference: Climatic Warming and the Decline of Zooplankton in the California Current, Roemmich, D., and J. McGowan, Science, Vol. 267, p. 1324-1326, 1995.

New Indicators Of Fish Distribution Developed By The GLOBEC Program To Study The Effects Of Ocean Climate Changes

The GLOBEC Georges Bank Program has included study of: (i) proliferation of cell nuclear antigen as a measure of growth in fish and zooplankton, (ii) development of new genetic markers to identify populations of copepods around the north Atlantic, and (iii) development of high-frequency acoustics and towed video cameras that provide extraordinarily detailed pictures of the distribution of planktonic organisms. Coupled physical-biological models are now run in prognostic mode, which makes possible the testing of various regional climate-change scenarios, including analysis of how changes in wind speed and direction, storms, increased heating and early water column stratification may influence community structure on Georges Bank. The influence of changes in the position and strength of the Gulf Stream and Scotian Shelf currents will be examined next year. These GLOBEC efforts are being carried out in collaboration with other coastal ocean programs, including a study of the role of heavy fishing in changing ecosystem structure on Georges Bank. Results of these programs will be used to produce integrated assessments of the relative importance of over-fishing vs. climate variability as factors controlling fluctuations in fish populations, and an assessment of how the recent closure of the fishery has changed the socio-economic structure of local fishing communities.
References: (1) Topical studies in Oceanography - U.S. GLOBEC: Global Ocean Ecosystem Dynamics, Deep-Sea Research, Vol. 41, pp. 1-227, 1994; (2) Special Issue on U.S. GLOBEC Georges Bank Program, Deep-Sea Research, in preparation, 1995.

Boreal Forest Growth Rate Stalled

A four-year study of growth rings in trees near the timberline in northern and central Alaska indicates that the response of tree growth to climate change is not simple and linear. Based on records of tree-ring growth and weather station and borehole temperatures, the far-northern climate has warmed by roughly 2°C since the 1880s (mostly in the fall, winter, and spring, as predicted by global climate models). Records from trees indicate that by the middle of this century the boreal regions were warmer than at any other time in the last 300 years. This warming is much faster than is occurring in the rest of the world, providing an important opportunity to examine ecological responses. What has been found is that as high latitudes have warmed over the past 100 years, tree growth at first accelerated, as anticipated, but then flattened, even though the climate continued to warm. Unexpectedly, the most recent decades of warming, instead of encouraging significant growth, may be stressing northern forests by speeding up moisture loss and perhaps subjecting them to more frequent insect attacks, especially by insects that were previously restricted to lower, warmer latitudes. Since the 1970s Alaska' s forests have suffered from severe outbreaks of bark beetles, which have devastated millions of acres of forest. Warmer temperatures can shorten the reproductive cycle of the bark beetle, for example, from two years to one, thus increasing the population of bark beetles significantly. Insect pest survival benefits from warmer temperatures.
Reference: Boreal Forest Growth Rate Stalled, Jacoby, G., and R. D' Arrigo, Global Biogeochemical Cycles, in press, 1995.

Surveys Of Tropical Forests Find That They Are Changing Much More Rapidly Now Than They Have In The Past

The measured turnover rate of tropical forest trees since 1934 suggests that there has been a significant increase in forest turnover since about 1960. The increase appears to have been accelerated during the 1980s. The likely cause is environmental changes from increased extreme weather events, adjacent deforestation, and elevated productivity which has resulted from increased atmospheric CO2. This increased turnover rate has implications for tropical biodiversity and possibly for the tropical carbon cycle. Faster forest turnover could lead to a dominance of climbing plants and gap-dependent tree species, which are most likely to benefit from increased CO2. Many of these species have less dense wood than shade-tolerant species, suggesting that tropical forests could eventually become less of a sink for carbon than they currently are.
Reference: Increasing Turnover Through Time in Tropical Forests, Phillips, O. L., and A. H. Gentry, Science, Vol. 263, pp. 954-958, 1994.

First Visit To North Pole Station By North American Surface Vessels Finds That The Arctic Has A Much Higher Biological Productivity Than Expected

Preliminary biological measurements, made by the joint U.S.-Canadian Arctic Ocean Section of Arctic System Science (ARCSS), suggest that the Arctic Ocean is a far more productive area than had previously been thought, as evidenced by the abundance of phytoplankton and their nutrients. Hydrographic measurements also suggest that the influence of the Atlantic Ocean on the central part of the Arctic Ocean is much larger than suspected from historical data. It is not yet known if this represents a major change in the Arctic Ocean, possibly as a consequence of warming, or a deficiency in the earlier data. The remarkably warm temperatures in the Atlantic layer (ca. 200- 400 meters) were also found on the Eurasian side of the Lomonosov Ridge, where the water in this layer is a full half degree warmer than observed there in 1991.
Reference: Active Cycling of Organic Carbon in the Central Arctic Ocean, Wheeler, P.A., et al., Nature, in press, 1995.

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