Geoscience Reference
In-Depth Information
There is considerable evidence from the past that the Earth System behaves as
a single, interacting system (Scheffer, 2009). During the late Quaternary period
in earth history, the period during which hominins and fully modern humans
have evolved, the earth has existed in two states - the long glacial state (ice ages)
and the much shorter warm intervals that punctuate the ice age world (e.g. Petit
et al., 1999). Other features of Earth System behaviour are typical of a complex
system: (i) the close correlation between climate (temperature) and greenhouse
gas concentrations (feedback loops); (ii) the tight bounds on the variation of
temperature and carbon dioxide and methane concentrations between ice ages
and warm periods (limit cycles); and (iii) the c.100,000-year cycling between ice
ages and warm periods, triggered by changes in earth's orbit around the sun but
largely driven by the internal dynamics of the Earth System (phase locking).
The global mean temperature difference between an ice age and a warm period
is approximately 5-6°C. A Four Degree World may mean that the Earth System
has already crossed a global threshold on the way to a much warmer state, a
6-7°C world which would eventually become ice free and which could be stable
for a very long period of time, as it has in the recent past in terms of earth history
(Zalasiewicz et al., 2012). Such a state, however, would be much hotter than the
earth that
Homo sapiens
has evolved in and much hotter than our physiology has
ever experienced across much of our current geographic range. It is hard to image
that contemporary human societies would be able to survive in a planet where
much of its surface would be a hostile environment for our species.
Towards global sustainability
It is clear from this and other chapters in this volume that a Four Degree World
will be very difficult to live, let alone prosper, in, and so effective approaches are
needed to limit the magnitude of warming to significantly lower levels. However,
as this chapter has demonstrated, climate change does not act in isolation from a
large number of other global-level environmental and socio-economic changes.
One approach to dealing with the interacting environmental challenges is
based on a small set of planetary boundaries (Rockström et al., 2009a; 2009b)
that define the safe operating space for humanity. These planetary boundaries are
based on hard-wired features of the Earth System and form the non-negotiable
basis for securing a sustainable future for humanity on the planet. Transgressing
one or more planetary boundaries may be deleterious or even catastrophic due to
the risk of crossing thresholds that will trigger non-linear, abrupt environmental
change within continental- to planetary-scale systems.
Climate change is one of the nine planetary boundaries, and Rockström et al.
recommended a boundary value of 350 ppm CO
2
(or no more that
1
1 W m
2
2
in radiative forcing), which would limit global average warming to very much
below 4°C (and even below 2°C). The complete set of planetary boundaries,
with their control variables and suggested boundaries, are:
●
climate change (CO
2
concentration in the atmosphere <350 ppm and/or a
maximum change of
1
1 W m
2
2
in radiative forcing);
