The dynamic biosphere
By INEZ FUNG
Earth's
atmosphere and biosphere
exchange energy, water, carbon dioxide, and other trace substances on all space and time
scales. These linked exchanges depend on, and in turn alter, the states of the atmosphere and
biosphere themselves. Because the atmosphere-biosphere is thus a coupled and dynamic
system, no global scientific or socioeconomic question can be usefully examined in isolation;
our knowledge of the processes that maintain planetary equilibrium--and of the events that
could disturb that balance--is necessarily interdisciplinary. For this reason,
21stC has assembled a wide range of topics, ideas, and perspectives in this
special issue, "Biospheres," to illuminate what contemporary research tells us about these
interlocking global issues.
From leaf level to climate level, and vice versa
IT'S ESSENTIAL to appreciate how small-scale events can influence larger-scale events in a
complex system. The most frequently cited example of this principle is a butterfly's flapping
wings provoking a hurricane thousands of miles away, but a better-understood case might be
the relations between plant physiology and global climate. Temperature at the land surface
results directly from competition between the energies gained and energies lost by the
biosphere. The solar energy absorbed is the difference between incident and reflected energy,
and the reflectivity of the biosphere is governed by leaf phenology and soil conditions.
In most plants, the exit points for water, stomates on the undersurfaces of leaves, are also the
entry points for CO2 for photosynthesis. The flow rates are
determined by the stomatal closure, which plants adjust according to the ambient climate and
CO2 concentrations so as to use water most efficiently, or gain
the maximum carbon for the water loss. In this way, the atmosphere-biosphere exchanges of
energy, water, and carbon are intricately linked, and perturbation of one cycle invariably
perturbs the others. Changes in one aspect of climate, e.g., atmospheric CO2 levels, may affect water use efficiency of plants and the allocation of
plant material to leaves and roots, altering the reflective ability, transpiration, and
photosynthesis of the entire plant. These in turn would foster further feedback effects on the
climate. Science has begun to understand these processes in detail, but there is much
information to accumulate and synthesize on a systemic level. Columbia's Biosphere 2 facility for ecological
research and education, as discussed in several articles in this issue, now makes it possible to
connect leaf-level events with observations at a full ecosystemic scale, offering the hope that
research can explicitly address questions whose answers we have only been able to infer
piecemeal.
Change, the only constant EVIDENCE
FOR THE co-variation of the atmosphere and biosphere abounds in the historical and
paleo-records. In the past 40 years of continuous CO2
measurements, the atmospheric CO2 increase is only about half
On longer time scales, the paleo-data show that vegetation distributions have varied with
climate. About 6,000 years ago, when the Earth's orbit around the sun produced enhanced
seasonality in the Northern hemisphere, and summer temperatures over the Northern
continents were 2 to 4 °C warmer than the present, boreal forests extended 300 to 500
km further north, and the Sahara desert was smaller. About 20,000 years ago, during the Last Glacial Maximum, low-density
vegetation, typical of those in cold, dry climates, covered the ice-free portions of the Earth's
surface.
These planetary-scale changes in vegetation distribution translate immediately to, and result
from, changes in the energy, water, and carbon transfers between the atmosphere and the
biosphere on the leaf scale. Equally important, as the amount of carbon stored in the
biosphere is altered, mass balance requires alteration of the atmospheric and oceanic carbon
reservoirs as well: CO2 was about 25 percent lower during the
Last Glacial Maximum than in the pre-industrial era. Cooling was maintained not only by the
highly reflective ice cover, but also by the reduced levels of CO2 and water vapor in the atmosphere and the more reflective vegetation, which
absorbed less sunlight.
The coupled climate-biosphere changes in the past show clearly that neither system has
remained untouched while the other was disturbed. The terrestrial biosphere has been
important in determining the levels of CO2 and water vapor in
the atmosphere, which in turn have contributed to defining the climate for the biosphere. The
biosphere acted to magnify in some cases, and temper in others, the climate changes.
Humanity's impact OUR SPECIES IS the
agent of the most extensive and most rapid change in the biosphere--the productivity of which
sustains our existence. The annual net primary
productivity (NPP) of the biosphere, i.e., the amount of matter fixed by photosynthesis
after accounting for autotrophic respiration, is about 130 gigatons per year. We directly use 3
percent of the annual NPP as food, materials, or fuel, but we destroy or turn over nearly 40
percent of the terrestrial and aquatic NPP in acquiring that 3 percent. Meeting the needs of
an expanded human population thus presents a risk to the biosphere's self-regulating processes
as well as a strain on consumable resources, as Drs. Joel Cohen, John Bongaarts, and Allan
Rosenfield discuss in their conversation about the processes and implications of population
growth. Agricultural issues are far from peripheral concerns for an urban research
university, as Scott Veggeberg points out in describing Columbia's research into the relations
of agriculture, plant biology, and climatology. While increasing CO2 in the atmosphere and the anticipated climate change may benefit some agricultural
practices in some regions, they are not likely to meet the needs of the expanded population.
Agriculture has taken over most areas suitable for cultivation, and there is little potential for
expansion. Already, about 50 percent of the Earth's land surface--or 75 percent of the surface
excluding rock, ice, and uninhabitable barren lands--has undergone some degree of human
disturbance; what we do with the rest remains an open question. The remaining
undisturbed habitats are found in the rain forests of South America, the arid and
semi-arid regions of Australia, and the taiga and tundra areas of the Arctic. In Brazil in 1991
alone, about 11,100 km²--half the size of Massachusetts--was destroyed. However,
only about 30 percent of the deforestation in
Amazonia is by small farmers in their quest for arable lands; the remainder occurs on
larger properties for cattle ranching and land speculation. Natural erosion and erosion of the
cultivated and abandoned areas also diminish arable land. To date, global economic processes
have fared poorly in protecting such regions, but efforts are under way to marshal market
forces to value, rather than undervalue, public goods such as land and air quality. Whether
human economic institutions are well suited to this task is the subject of Doug Henwood's
examination of the theories of Columbia economist Graciela Chichilnisky. A related
question, the reduction of biodiversity, poses different threats to our well-being: In defending
our species against its microbial enemies, we rely on nature's chemical laboratory more than
most of us are aware. A genetically impoverished biosphere, as Dickson Despommier
illustrates, would rob us of potential armor that we may deeply regret losing; preserving
diversity benefits all species, including humans. The outlook:
stewardship, like it or not LIFE IS CONTINGENT upon the
availability within a tolerable range of resources, from climate to nutrients to DNA. Alteration
of these resources leads to alteration in the structure and functioning of life. The
atmosphere-biosphere system has danced and lurched through its own rhythm of natural
oscillations, through which each adjusts to changes in the other in an attempt to achieve a
temporary equilibrium. Humans, through their increasing demands for energy, food, and other
resources, have set into motion biogeophysical and biogeochemical feedbacks that can disturb
the energy, water, and carbon cycling of the entire system. The human perturbations are
happening at a magnitude and pace faster than any known natural oscillation. In this way, our
pressures on the environment are also taxing resources we need to survive. Predicting how the
Earth system may respond to this deliberate and unprecedented perturbation, and how
humans can sustain themselves, presents an inescapable challenge. Knowledge of how
our biosphere has behaved in the past, as reconstructed by Wallace Broecker in his discussion
of Earth's oxygen reserves, provides an indispensable context for our understanding of that
An essential
task for researchers in a democracy is educating the public about earth science. In an age when
the public consciousness floats on unprecedented tides of information and
pseudo-information, the "Publisher's Corner" editorial by Maxine Singer argues forcefully that
scientists bear a responsibility to be more informative, not just impressive, in addressing
laypeople. 21stC's Metanews department also presents
three cases in which the press has handled stories involving global research--including one that
continues to rattle the foundations of international politics--with varying degrees of
responsibility. Educators and scholars at Biosphere 2, the newest and perhaps most
adventurous of Columbia's research enterprises, take seriously the idea that the scientific
community and the community as a whole should comprehend each other, expressing this
commitment by combining the missions of research, academics, and public education. The
research university cannot stand alone among social institutions in preparing citizens for
biospheric leadership, but universities have the distinct privilege and responsibility of
translating the biosphere's complex languages into messages that can motivate everyone who is
willing to listen.
PHOTOS: O.C. Rejlander, adapted by Howard Roberts
Another source of energy is downwelling longwave (infrared) radiation emitted by the
atmosphere; the magnitude of this radiation depends on temperature and amounts of
CO2, water vapor, and other greenhouse gases. The biosphere
cools itself by emitting longwave radiation, sensible heat associated with turbulence, and
latent heat associated with evaporation and transpiration. The thermal and water status of the
biosphere regulate these heat losses.
that from fossil fuel combustion, because the terrestrial biosphere and the oceans have shared
in ameliorating the CO2 growth rate by absorbing some of the
anthropogenic CO2. The year-to-year fluctuations in
atmospheric CO2 are also partially related to varying
photosynthesis and/or respiration in response to climate changes related to El
Niño/Southern Oscillation events and volcanic eruptions. Furthermore, seasonal
amplitudes of atmospheric CO2, a measure of biospheric
breathing, have increased since the 1980s together with surface air temperature, suggesting
that the biosphere has been churning at a faster rate related to the longer growing season at
middle to higher latitudes in the Northern hemisphere. Since transpiration and CO2 absorption are under the same stomatal controls, the CO2 amplitude changes imply feedbacks on the atmospheric water and
energy cycles as well.
challenge. Turning toward the future, some researchers speculate that massive new
geoengineering endeavors might be a way to better handle our responsibilities as stewards of
the Earth; as evinced by the various projects discussed by Patrick Huyghe, geoengineering
offers both great promise and great risk. Regardless of whether humans ever decide to send
huge mirrors aloft or fertilize the oceans with iron, the traces of Homo sapiens's
activities are an ineradicable component of the modern biosphere, and the philosophy of
planetary management--the argument that we should consciously, rather than unconsciously,
shape our planet--is likely to occupy a prominent place in coming debates.
Related links...INEZ FUNG, ScD.,NASA/Goddard Institute for Space Studies and is at
the School of Earth and Ocean Sciences, University of Victoria, Canada. She is an adjunct senior
scientist at Columbia's Lamont-Doherty Earth
Observatory.
