Geoscience Reference
In-Depth Information
may be generally unresponsive to changes in a forcing factor for a long time,
until the tipping point is approached. An analogy is the continuous addition of
weight to a bridge. For a while the bridge appears unresponsive and handles the
additional weight without problems. Then, with a small amount of additional
weight, a critical threshold (tipping point) is transgressed, the bridge can no
longer bear the weight, and it suddenly and spectacularly collapses. The general
principle is simple: as the tipping point is crossed, small changes in the forcing
factor lead to rapid and surprisingly large changes in the system.
Figure 7.3
is a global map showing the location of various tipping elements
in the Earth System (Richardson et al., 2011). The existence of such tipping
elements is not based only on theoretical considerations or on model simula-
tions, but rather each of them has been observed in records of earth's history.
They are real phenomena with identifiable risks for human wellbeing should
tipping points be transgressed in a Four Degree World. As shown in
Figure 7.3
,
the tipping elements can be classified into: (i) those based on the melting of ice
or frozen soil or sediments; (ii) those based on rapid changes to atmospheric or
oceanic circulation; and (iii) those based on changes to major biomes or marine
ecosystems.
Table 7.1
provides more details on many of the tipping elements shown in
the figure, including the transition time for the shift from one state to another
of the system, the impacts for humanity that crossing the tipping point would
produce, and the temperature range within which the threshold is likely to lie.
For all but one of the tipping elements shown in the table, temperature is the
Sea-Ice
Greenland
Ice Sheet
Cold Wa ter
Coral Reefs?
Ye doma
Permaforest
Boreal
Forest
Boreal
Forest
Ocean
Methane
Hydrates?
Atlantic
ermohaline
Circulation
Himalayan
Glaciers?
Indian
Summer
Monsoon
SW North
America?
Sahara Greening?
Sahel Drying?
Dust Source
Shutdown?
Tropical
Coral Reefs?
We st African
Monsoon
..
El Nino-
Southern Oscillation
Amazon
Rainforest
Marine Biological
Carbon Pump?
We st Antarctic
Ice Sheet
Population density (persons per km
2
)
Melting
Circulation change
Biome Loss
No data
0
5 020
100
200
300 400
1000
Figure 7.3
:
Tipping elements in the Earth System, overlaid on the human population
density on Earth
Source: Richardson et al., 2011
