By Dr. Glen Barry
Independent Political Ecology Scientist
glen.barry@gmail.com
Excerpt of m
anuscript accepted for publication mid-2014 by Management of Environmental Quality
Abstract
Planetary boundary science defines key
thresholds in the Earth System's biogeochemical conditions that precede
ecosystem collapse and threaten human well-being. Terrestrial ecosystems enter
into the nine originally defined planetary boundaries only indirectly, through
boundaries such as biodiversity and land use. This study proposes a measurable terrestrial
ecosystem boundary to answer the question: what extent of landscapes,
bioregions, continents, and the global Earth System must remain as connected
and intact core ecological areas and agro-ecological buffers t sustain local and
regional ecosystem services as well as the biosphere commons? Two preeminent
considerations are connectivity of large ecosystem patches, enabling them to
persist as the matrix for the landscape, and critical collapse of the dominant
large habitat patch – or "percolating cluster" – into smaller, more
isolated habitats, amid a matrix of human development. This transition, found
to occur at about 40% habitat loss in landscapes and bioregions, is likely to
be similar at continental and global scales.
A new planetary boundary threshold is proposed
based on ecology’s percolation theory: that across scales 60% of terrestrial
ecosystems must remain, setting the boundary at 66% as a precaution, to
maintain key biogeochemical processes that sustain the biosphere and for ecosystems
to remain the context for human endeavors. Strict protection is proposed for
44% of global land, 22% as agro-ecological buffers, and 33% as zones of
sustainable human use.
Because humanity is now the major force shaping
the biosphere, up to 50% of Earth's land surface has already been cleared of
natural ecosystems; thus the biosphere may already have lost its global
percolating cluster. If so, with diminished connectivity, the global ecological
system is now composed of islands of nature within a sea of humanity, meaning
critical water, climate, soil, and other ecosystems processes are at risk. This
observation suggests that to sustain the biosphere it is urgent to protect
remaining large, relatively intact terrestrial ecosystems, especially old-growth
and primary forests. This will require accelerating current approaches such as
biosphere reserves, and taking up new polices such as a carbon tax to fund
protection and restoration of natural and agro-ecological terrestrial ecosystems.
To ensure global ecological sustainability, it will be necessary to reconnect
matrices of intact ecosystems across scales, so that globally the biosphere and
its constituent ecological processes and patterns can percolate back to
connected nature as the context to all life. Otherwise, it is hypothesized the
global biosphere may collapse and the Earth System perish.
Introduction to Planetary Boundaries
From Malthus (1798), through Aldo Leopold's land
ethic (1949), to The Limits to Growth
(Meadows et al. 1972), the Millennium
Ecosystem Assessment (2005), and finally current planetary boundary and global
change science (Rockström et al. 2009a,
2009b) runs a strand of concern about human growth's impacts upon Earth's
biophysical systems – terrestrial ecosystems in particular – and about
requirements for global ecological sustainability, while avoiding biosphere
collapse. Our biosphere is composed of Earth's thin mantle of life present at,
and just above and below, the Earth's surface. Some have indicated that human
impacts upon the biosphere are analogous to a large, uncontrolled experiment,
which threatens its collapse (Trevors et
al. 2010). Little is known regarding what collapse of the biosphere would
look like, how long it would take, what are its ecosystem and spatial patterns,
and whether it is reversible or survivable. But it is becoming more widely
recognized that Earth's ecosystem services depend fundamentally upon holistic,
well-functioning natural systems (Cornell 2009).
Accelerating human pressures on the Earth System
are exceeding numerous local, regional, and global thresholds, with abrupt and
possibly irreversible impacts upon the planet's life-support functions (UNEP
2012). Planetary boundaries provide a framework to study these phenomena, by
defining a "safe operating space for humanity with respect to the Earth
System" (Rockström et al. 2009a).
Planetary boundary studies seek to set control variable values that are a safe
distance from thresholds of key biophysical processes governing the planet's
self-regulation to maintain conditions conducive to life (Rockström et al. 2009b). This builds upon landmark
efforts by Meadows et al. (1972) to
first define global limits to growth. Their prediction that key resource
scarcities would emerge has proven remarkably accurate (Turner 2008), albeit
delayed – but not avoided – through the advent of computer technology.
Ecological and economic warnings since at least Malthus have called attention
to economies' dependence upon natural resources. The observation that
near-exponential growth of human population and economic activity cannot be
sustained, far from being disproven, is more valid than ever (Brown et al. 2011). Those who deny limits to
growth are unaware of biological realities (Vitousek 1986).
The initial planetary boundary exercise
identified nine global-scale processes, including climate change, rate of
biodiversity loss (terrestrial and marine), nitrogen and phosphorus cycles,
ozone depletion, ocean acidification, freshwater, land use change, chemical
pollution, and atmospheric aerosol loading (Figure 1). Preliminary safe
planetary thresholds were established for seven of these, and three – rate of
biodiversity loss, climate change, and the nitrogen cycle – were found to have already
surpassed such a threshold (Rockström et
al. 2009a). Many such changes occur in a nonlinear, abrupt manner; others
are more incremental and subtle. Yet both types of change threaten the
viability of contemporary human societies by diminishing or destroying
ecological life-support systems. If one or more of these boundaries are
crossed, it could be "deleterious or even catastrophic" as nonlinear,
abrupt environmental change occurs at the continental to planetary scale
(Rockström et al. 2009b).
Here an ecologically rich revision to the planetary boundary framework is
proposed – in the tradition of political ecology, not ignoring politics – to set
the threshold of how many intact terrestrial ecosystems are required to sustain
the biosphere. It is not possible to carry out controlled experiments upon our
one biosphere to know at what point collapse occurs. We are thus left with
observational studies and synthesis papers regarding what is known about
ecosystem collapse at other scales. This paper first reviews what is known
about biodiversity and old-growth forest loss, abrupt climate change, and ecosystem
collapse as ecological systems are diminished at lesser scales. Next, the
critical phase shift seen as landscapes percolate from nature surrounding humanity,
to small reserves surrounded by human works, is presented as analogous to outcomes
for the biosphere, whose terrestrial ecosystems are after all simply a
large-scale landscape.
The remainder of the paper synthesizes these findings regarding ecosystem
loss and thresholds in loss of ecosystem connectivity into a rationale for
recognition of a 10
th planetary boundary in regard to terrestrial
ecosystem loss. It is suggested that some two-thirds of Earth’s land surface should
be protected totally (44%) or partially (another 22%) to avoid biosphere
collapse. Given current best estimates are that approximately one-half of Earth’s
terrestrial ecosystems have already been lost, the discussion centers around biocentric
policy measures required to protect and restore terrestrial ecosystem
connectivity in order to maintain global ecological sustainability.
Figure 1: Proposing a Terrestrial Ecosystem Loss Planetary Boundary. Currently nine planetary systems are recognized as
providing a safe operating space for humanity, as long as boundaries are not
exceeded. It is thought three systems (denoted with +) have already surpassed
their boundaries. This paper proposes a Terrestrial Ecosystem boundary of 66%
ecosystem land cover (44% as intact natural ecosystems and 22% as agro-ecological
buffers) to avoid biosphere collapse. Best estimates are that about 50% of
terrestrial ecosystems have been lost; thus this boundary has been surpassed
too, albeit full impacts may not yet be realized due to time lags (adapted from
Rockström
et al. 2009a).
Coming Soon: the rest of this journal article as published mid-2014, including these headings:
Biodiversity and Old-Growth Forest Loss, Abrupt Climate Change, and Ecosystem Collapse
Percolation Theory and Landscape Connectivity
Terrestrial Ecosystem Loss as a Planetary Boundary
Biocentric Discussion on Achieving Global Ecological Sustainability
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