
July 2021 73
One of the enduring challenges of addressing the
multiple ecological crises that confront us has been to
find a way of seeing them as part of an integrated
system, a ‘spaceship Earth’ floating in the void, with
its crew entirely dependent on the on-board life sup-
port systems and the integrity of the hull, which on a
planetary scale, means the thin atmospheric envelope
that surrounds us.
In 2009, a group of Earth system scientists, led by
Professors Johan Rockström andWill Steen set about
the interdisciplinary task of mapping out a “safe oper-
ating space for humanity”, to help advise governments
and international organisations on how to identify and
remain within this safe zone.
This led to the identification of the nine key ‘plane
-
tary boundaries’, within which Earth systems must
operate, and beyond which lie thresholds or tipping
points with dangerous or even catastrophic conse-
quences when breached.
The boundaries are: climate change, novel entities,
stratospheric ozone depletion, atmospheric aerosol
loading, ocean acidification, biochemical flows of
phosphorus and nitrogen, freshwater use, land-system
change and biodiversity integrity, including functional
and genetic diversity.
The famous pie-shaped visualisation (see graphic,
bottom right) developed by the Stockholm Resilience
Centre is colour-coded green, orange and red, to
demarcate the safe, uncertain and high risk zones. All
nine of these should lie well within the green ‘safe’
zone, the region where Earth systems have remained
throughout the unusually stable 11,700 year recent
epoch known as the Holocene.
“There is increasing evidence that human activities
are aecting Earth system functioning to a degree that
threatens the resilience of the Earth system, its ability
to persist in a Holocene-like state in the face of increas-
ing human pressures and shocks”, according to a 2015
paper authored by Will Steen which aimed to update
and extend the original ‘planetary boundaries’ study
from six years earlier.
While all nine boundaries are vital, two (climate
change and biosphere integrity) are identified as
“core”. If planet Earth were indeed a spaceship, right
now its red warning lamps would be flashing and its
alarm bells buzzing, warning that a hull breach was
imminent unless drastic remedial action were taken.
Given the global focus on the climate crisis, you
might expect its indicators to be the most alarming. At
the moment, our climate-system trajectory is clearly
unstable, but it is rated as still being in the zone of
uncertainty, meaning increasing risk.
The areas that have already smashed into the red
zone of extreme danger are the global biodiversity
crisis and nitrogen and phosphorus flows to the bio-
sphere and oceans. Ecosystem damage resulting from
human activities over the last half century in particular
are “the most rapid in human history, and increase the
risks of abrupt and irreversible changes”. So severe
has the loss of biological diversity and accompanying
pulse of species extinctions been that it has been char-
acterised as the Sixth Mass Extinction event in Earth
history.
To truly grasp the devastation that has already
occurred, consider that 96 per cent – by weight - of
mammals on Earth today consist of humans and their
livestock, with the totality of the world’s remaining wild
mammals a mere 4 per cent. At the start of the Holo-
cene, humans and their livestock would have accounted
for less than one per cent of Earth’s then-teeming popu-
lation of mammals.
Humanity’s race to increase food production has
thrown global cycles of both nitrogen and phosphorus
into chaos. Humans now create more artificial nitrogen
(as a chemical fertiliser) than all Earth’s terrestrial pro-
cesses combined. This is leading to serious and
escalating pollution crises, both globally and as wit-
nessed in Ireland in recent years as nitrogen-dependent
industrial dairying expanded rapidly.
Thanks to decisive intergovernmental action in the
1980s, the ozone crisis has been largely averted, but
other planetary boundaries are under growing pres-
sure. These include ocean acidification (now occurring
at its most rapid rate in 300 million years) and fresh-
water over-extraction and pollution plus atmospheric
aerosols and thousands of novel chemical entities that
have been suddenly introduced into the biosphere.
There remain large areas of uncertainty, but our cur
-
rent trajectory of ever-increasing resource extraction,
consumption, pollution, land use change and spiralling
emissions push us ever further beyond our planetary
safe zone. With wry understatement, Steen warned:
“A continuing trajectory away from the Holocene could
lead, with an uncomfortably high probability, to a very
dierent state of the Earth system, one that is likely to
be much less hospitable to the development of human
societies”.
As Carl Sagan concluded: “There is perhaps no
better demonstration of the folly of human conceits
than this distant image of our tiny world. To me, it
underscores our responsibility to deal more kindly with
one another, and to preserve and cherish the pale blue
dot, the only home we’ve ever known”.
John Gibbons is an environmental writer and
commentator.
96 per cent –
by weight - of
mammals on
Earth today
consist of
humans and their
livestock, with
the totality of the
world’s remaining
wild mammals a
mere 4 per cent.