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Potential limits of human-dominated fossil-energy-based global-ecosystems
J R E Harger 08-04-97
Abstract
Presently atmospheric carbon dioxide (CO2) content is rising in a sustained manner faster than at any time in the recent past (150,000 years) for which we have well documented information Carbon dioxide is also rising even faster than at any time in the past 500 million years. The present sharp rise in CO2, driven mainly by the comparatively rapid combustion of fossil carbon representing the energy deposits arising from slow accumulation from the ecosystems of bygone ages, is expected to have profound effects on the present and future global climate as well as on natural ecosystems, their associated biodiversity and current patterns of agricultural production.
However, there exists an even more serious facet to the problem posed by the necessity to control increasing CO2 and associated global warming and that is the loss of biodiversity (ecosystems, habitats, and species), resulting from the fact that climatic change and biological diversity are intimately inter-linked. Plants are the only organisms capable of removing significant amounts of CO2 from the atmosphere. It is now certain that natural responses for sequestering carbon have been outstripped, therefore humankind will be forced to accelerate the biosphere adjustment processes, i.e. massive reforestation programs and ecosystem engineering in order to buffer atmospheric CO2.
A significant part of this atmospheric carbon was, until recently, sequestered into oil and coal by biological and geological action. Currently, mainly due to industrialization, man is pumping sequestered carbon back into the atmosphere at a rate faster than the present distribution of ecosystems, including agricultural crops, can absorb and is therefor largely responsible for the "greenhouse effect". Moreover, most prehistoric global ecological changes occurred slowly, probably over millions of years and vegetation had sufficient time to either move or adapt. Present changes are expected to include an increase in the average temperature (which will not necessarily be evenly distributed), a subsequent rise of sea level and the redistribution of precipitation. These changes will occur in less than the lifetime of an average tree. It is probable that many species will not have the necessary time to adjust and will perish as their habitats disappear and vegetation belts change. Over 50% of the original forest biomass covering the globe before the advent of modern civilization has already been removed and replaced by human habitats and associated agricultural systems. Even simple solutions to the problem of sequestering carbon through traditional mono-culture plantation developments are no longer tenable because of the excessive geographical areas that would be involved.
Introduction
Is climate change happening ?
Positive indications of global climate change and associated green-house warming are available and include ice-melt and break-up in both the Arctic (Gloerson and Campbell, 1991), and the Antarctic (Doake and Vaughn, 1991) as well as other unsettling indications of temperature shift such as the seasonal development of conditions favoring wide-spread fires in Kalimantan (Borneo). The progressively increasing frequency of cyclone activity in the Pacific (Nunn, 1991) and the intermittent occurrence of tropical coral reef fishes in the waters of the North Island of New Zealand (Francis and Evans, 1991). Over the past three decades, deep water, >=2,000 m from the Western Mediterranean Sea has shown a trend of continuously increasing temperatures (Bethoux et al, 1990) over a range of 0.12 deg C, presumably due to greenhouse warming.
In crude terms the present situation involves the transfer of global primary producer biomass towards a steady increase in human (consumer) biomass with the help of fossil carbon energy itself arising from past epochs. An ecological insight is required which takes into account the whole of the living history of the earth as well as the narrow and precarious present. In one sense, the problem can be analyzed in terms of reactive time-frames. Processes that might formerly have taken tens of thousands of years to run to completion are now forced into a time period of tens or hundreds of years through anthropogenic energy expenditures. Significantly, the available solutions are also framed in terms of tens or hundreds of years at the most and are, for the most part, also biological in form.
Instead of moving into a naturally induced period of cooling, such as was initially prefaced by the "little ice age" of the 17th. century, the world now moves into an increasingly warmer global regime. Humankind, since the dawn of civilization and particularly over the last few centuries has systematically reduced the forests and drained the wetlands. In recent times humankind has also been driven to expose and oxidize the vast carbon deposits laid down, in part, by those same forests in days gone by and has so released carbon based gases and other industrial pollutants such as the nitrogen based gases.
In the geological past, occasions resulting in the accumulation of atmospheric carbon dioxide have occurred before and indeed substantial periods of such concentrations have dominated the earth throughout much of the time when it has supported higher life forms at warmer temperatures than we see today. The early to mid Carboniferous was probably a high CO2 regime (Spicer and Chapman 1990) and the mesozoic is estimated to have had similar periods with carbon dioxide concentration up to 11 times that of today (Arthur et al 1991, Harger et al, 1992). Periods with 4 times current CO2 concentrations have been extensive (Spicer and Chapman 1990).
The carbon cycle
Current releases of carbon in to the atmosphere on an annual basis may be classified as arising from three sources: (1) oxidation of fossil fuels accounting for 5.9 Gt C (Post et al, 1990); (2) biomass burning 1.1-3.6 Gt C (Houghton, 1991) or 1.8-4.7 Gt C (Crutzen and Andreae, 1990), (median 3.9 Gt C) and (3) a component due to the oxidation of soil organic carbon stores estimated (Jenkinson et al, 1991) as up to 1.0 Gt C annually. For the purpose of the analysis reported herein a figure of 10.0 Gt C as an annual release to the atmosphere will be assumed for 1990, the base-year.
The net terrestrial primary production from plants is estimated at 50-60 Gt C/yr although it is also supposed that this is balanced by an equivalent release due to decay. This sector could be converted to accumulate a significant amount of carbon by planting new trees. The atmosphere currently contains 751 Gt C (353 ppmv CO2 (Ashmore, 1990), and 1 Gt C = 0.47 ppmv CO2 (Sundquist, 1985), compared to a pre-industrial content of around 550-590 Gt C, a difference of 165 Gt C or so. From 3 - 3.4 Gt C are accumulated each year into the atmosphere currently and that figure grows by around 0.4% annually (Ashmore, 1990).
There appear to be very few carbon sinks. Fixed carbon can accumulate naturally either by carbonate deposition or by photosynthesis to be then sequestered either in the ground or on the shallow sea floor to be covered up for long-term storage. Excluding the liberation of man-made greenhouse gasses such as CFC's etc., the direct effects of respiration as well as certain other specific considerations such as cement production etc., the major portion of the anthropogenic liberation of carbon as CO2 to the atmosphere proceeds by two main mechanisms: either by combustion and oxidation of fossil or recently living organic carbon; or by destruction of soil carbon reservoirs through agricultural expansion, particularly where former actively accumulating wetlands are involved. The extent to which both mechanisms are active depends directly on population.
The oceans
The world's water bodies also constitute a more immediately dynamic sink for CO2. Measurements are limited but recent empirical estimations of net direct CO2 uptake by the ocean (Tans et al, 1990, Quay et al, 1992) have suggested a figure of 1.6 - 1.9 GtC/yr. It has also been suggested that the ocean may absorb 21%-50% of the CO2 released into the atmosphere by human activities based on various models (IPCC, 1990, Sundquist, 1985). In the ocean mixed layer at the surface, CO2 exchange is rapid and the resulting seawater concentrations are consequently very close to equilibrium with the atmosphere.
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Original presentation:
UNESCO/UNEP/Dept. Environment, Malaysia Regional Workshop on the Carbon Cycle and Global Climate Change Kuala Lumpur, Malaysia October 24-26, 1991.
Published in: Chemosphere Vol 27, No 6, pp 9070945, 1993 (Pergamon)
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