Geoscience Reference
In-Depth Information
all accessible freshwater is appropriated for human purposes (Postel, 1998; Oki
and Kanae, 2006), and the loss of biological diversity is now at least two orders
of magnitude greater than the background extinction rate (MA, 2005).
The human enterprise itself is also under considerable pressure in the 21st
century. Resource constraints are beginning to have a demonstrable effect on
global change. The oil price spikes of 2007-8 and 2011 are examples of how
even a small imbalance in the supply-demand relationship of a finite resource
can propagate rapidly through the global economic system. Less is known about
the potential for 'peak phosphorus' to occur this century (e.g. Cordell et al.,
2009), but its implications in terms of our capability to feed a still-rising human
population are significant. Communication and transportation are changing
at astounding rates, leading to a state of hyperconnectivity that can transmit
information - and misinformation - at phenomenal rates around the globe.
Per capita incomes and rates of consumption in the industrialized world have
increased at a rapid rate since the Second World War, and several large devel-
oping countries, such as China, Brazil, India and South Africa, are now integral
parts of this world. Yet equity issues remain stubbornly difficult to solve and are
driving deleterious social outcomes across the global, even in wealthy countries
(Wilkinson and Pickett, 2009).
Tipping elements in the Earth System
The rapidly changing human enterprise, itself now in a novel situation never
before experienced by humanity, is, in addition, facing a global environmental
context that is also unique. The advent of the Anthropocene implies that the
earth is leaving the current epoch, the Holocene. This exit is no mere fact of
scientific curiosity or quirk of labeling.
The Holocene, the recent 10,000-11,000-year period of relative climate
stability, is the only state of the Earth System that we know for certain is capable
of supporting complex human civilizations (Steffen et al., 2011). It may be
possible that complex societies can continue to prosper and thrive through the
period of rapid changes in the Anthropocene, but that assertion is based on a
rather optimistic leap of faith rather than on a careful, evidence-based analysis.
One of the most prominent causes for concern in the Anthropocene is that
the very severe forcing of the climate system implied by a Four Degree World
would lead to highly nonlinear, abrupt and often irreversible changes to which
contemporary society would find it difficult or impossible to adapt. The notion of
tipping elements
in the Earth System (Lenton et al., 2008; Richardson et al., 2011)
is central to the risk of abrupt and irreversible change.
The concept of tipping elements is based on complex system science (e.g.
Scheffer, 2009) and refers to the observation that systems, and in this case
sub-systems of the Earth System, can have multiple states with abrupt transitions
between them. This behaviour is counterintuitive compared to the much better
known logic of cause-effect, in which the response of a system is proportional to
the amount of forcing it experiences. Systems with tipping points (thresholds)
