Geoscience Reference
In-Depth Information
sea-level rise, as noted in
Table 7.1
, where the threshold for a major loss ice from
Greenland could well be transgressed within the next couple of decades but the
transition time (before the consequences are fully felt) is many centuries. The
consequences of a 5m sea-level rise, for example, would be enormous if societies
did not proactively plan for it, and loss of ice from the polar ice sheets is essen-
tially irreversible in any timeframe meaningful for human societies.
Even more subtle is the potential for emissions of carbon dioxide and methane
from organic material stored in the frozen soil of the northern high latitudes.
Again, the transition time is potentially long - in the order of a century or two
- but the consequences could be very large. The most recent analysis of perma-
frost carbon (Schuur et al., 2011) found that for a high-emissions/high-warming
scenario - the type of emission trajectory that would lead to a Four Degree
World later this century - emissions of carbon (almost entirely carbon dioxide)
from this frozen source would be 30-63 billion tonnes by 2040 but could reach
232 to 380 billion tonnes by 2100 and 549-864 billion tonnes by 2300, showing
the long lag times involved. By comparison, current total emissions per year of
carbon from fossil fuel combustion are approaching 10 billion tonnes. These
projections reflect a strong feedback loop in which emissions of carbon from
permafrost drive more warming, which in turn drives more emissions of carbon
from permafrost, and so on.
Returning to
Figure 7.4
, at present the feedbacks from climate change to
the other sectors are sometimes significant, but many of the other drivers and
feedbacks are of similar orders of magnitude. That would change dramatically in
a Four Degree World, in which climate change would not only dominate most of
the direct impacts on sectors, but would trigger many more feedback loops than
are currently shown in
Figure 7.4
.
Attempting to predict the types compounding global crises that will emerge in
a Four Degree World (and probably at lower levels of temperature increases) is
virtually impossible, but it is reasonable to say that such crises will be common,
profound and exceptionally difficult to cope with. Given the highly connected
nature of the human enterprise in the twenty-first century, it is difficult to foresee
situations in which a compounding crisis will affect Australia alone; rather, we
will most likely be strongly linked into phenomena that are truly global in scale,
especially those that are strongly manifest in the Asia-Pacific region.
The Earth System as a single, complex system: tipping towards an
uninhabitable state?
The tipping elements shown in
Figure 7.3
and
Table 7.1
are sub-systems of the
Earth System as a whole. What if the Earth System as a whole, including the
climate system, itself has a tipping point? The notion is not as far-fetched as it
may seem. Given the long lifetimes of significant fractions of the additional CO
2
in the atmosphere (Archer and Brovkin, 2008) and the amplifying feedback
mechanisms in the carbon cycle (e.g. Krey et al., 2009; Schurr et al., 2011), it is
likely that strong warming scenarios will result in a long-lasting high temperature
world (e.g. Solomon et al., 2009).
