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From ocean to ozone, the limits of our planet

Transformative changes must be considered to keep Earth safe for the future

The population of vertebrate
species on Earth in the wild
saw a dramatic fall of about
30% between 1970 and 2006, with
the worst eects being in the trop-
ics and in freshwater ecosystems.
Destruction of species’ habitats by
pollutants and land-use change are
obliterating ora and fauna at un-
precedented rates. In fact, the eco-
logical footprint of humanity — the
natural habitats, such as water and
land, transformed or destroyed as
a result of human activity — far ex-
ceeds the biological capacity of the

In an attempt to understand the
natural world, its relationships
with human societies and limits, in
2009, Johan Rockström and others
from the Stockholm Environment
Institute described elements of the
biophysical world that link us to-
gether. Often regarded as a “safe
operating space for humanity”,
these planetary boundaries in-
clude loss of biodiversity, land-use
change, changes to nitrogen and
phosphorus cycles, ocean acidic-
ation, atmospheric aerosols load-
ing, ozone depletion, chemical

production, freshwater use and, of
course, climate change.
In the course of 12,000 or so
years after the last ice age, the
Holocene epoch has oered a
stable climate, a period of grace for
humanity to grow and to ourish,
with settlements, agriculture and,
more recently, economic and pop-
ulation expansion. This epoch has
since given way to the Anthropo-
cene, the exact beginnings of
which are debated, but which has
led to over-reliance on fossil fuels,
industrial agriculture, pollution in
water, soils and air, loss of species
and so on, which are devastating
for many life forms and connected
ecosystems throughout the planet.
Biophysical considerations
Many of these conditions respond
in a non-linear manner to changes.
This means, for instance, that eco-
systems that are stressed by their
exposure to pollutants may not re-
cover once the pollutants are re-
moved. Or, some systems may col-
lapse precipitously under
conditions referred to as
thresholds. We understand many
of these thresholds and how they
interact with each other, but not

When ecological thresholds or
tipping points are crossed, signic-
ant large-scale changes may occur,
such as breakdown of glaciers in
Greenland and the Antarctica, the
dieback of rainforests in the

Amazon, or failure of the Indian
monsoons. Since these boundaries
interact with one another and
cause changes across scales, cross-
ing a threshold in one domain can
speed up or undermine processes
in another subsystem. For in-
stance, greenhouse gas (GHG)
emissions increase ocean acidica-
tion, land-use change often in-
creases GHG emissions, and in-
creasing nitrogen and phosphorus
deplete species biodiversity and
freshwater resources and increase
warming from climate change.


Boundaries and limits
According to Mr. Rockström and
others, we are already at critical
levels of concern for climate
change, fresh water, species biod-
iversity and changes to nitrogen
and phosphorus cycles, which are
reaching tipping points. For ex-ample,

GHG emissions have led to
average atmospheric carbon diox-
ide concentrations being about 410
ppm. This is well above the 350
ppm level considered a ‘safe’ limit,
and the earth is already about a de-
gree Celsius warmer than average
pre-industrial temperatures.
Since publication of these stud-
ies by Mr. Rockstrom and others,
there has been plenty of discus-
sion, even strong disagreement, re-
garding the boundaries. Some sci-
entists, such as Kate Raworth, have
expanded them to reect and in-
clude several social dimensions
such as equity and gender justice
that were subsequently placed in
the centre of a schematic repres-
entation of the boundaries as a
circle with a hole or as a doughnut.
One may regard planetary
boundaries as support systems for
life on Earth or view them as ex-
pressing “carrying capacity” and
dening “limits to growth”. The
latter is a thesis that was originally
published nearly half a century ago
by the Club of Rome as a book in
1972. It described the situation we
would nd ourselves in with expo-
nential population and economic
growth. While the “limits to
growth” argument was challenged
for good analytical reasons, it still
provided a lens through which to
view the changing world of the 21st
century. It also oered the idea of
thinking about a system as a whole
— systems thinking — not just as
separate parts and feedback mech-
anisms as valuable processes in
considering long-term change.


On sustainability
The idea of sustainability has been
embedded in the human imagina-
tion for a very long time and is ex-
pressed through our ideas of
nature, society, economy, environ-
ment and future generations. But it
became formally a part of interna-
tional agreements and discourse
when it was recognised at the
Earth Summit of 1992 in Rio de

This systems view and the recog-
nition of interlinkages among the
social, environmental, and eco-
nomic pillars of sustainability, and
between biophysical planetary
boundaries and social conditions,
are essential to have a chance of
keeping the world safe for future
generations. It is telling that schol-
ars who work on planetary bound-
aries regard climate change as one
of the easiest to manage and

In thinking about these planet-
ary limits then, researchers and
policymakers should reect on
multiple systems and the linkages
among them, and whether step-by-
step or transformative changes
must be considered to keep the
planet safe for the future.

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