Environmental Engineering Reference
In-Depth Information
In summary, there is potential for biogeochemical feedbacks to climate
change through emissions of trace gases but it cannot be quantified at pres-
ent. Such releases could add 2,000 GtC or more to the carbon directly put
into the atmosphere by fossil-fuel burning and land-use change. If some of
this stored carbon is released rapidly in the form of methane, it could lead
to a pronounced transient warming spike above and beyond the more persis-
tent warming caused by increases in atmospheric CO
2
. Local measurements
of releases are important in understanding processes but are currently insuf-
ficient to characterize the significance of sources for the global atmosphere.
Thresholds for large effects are difficult to establish from paleoclimatic
information, since key processes may become important over very slow
time scales of many thousands of years in a warmer world, but may take
place too slowly to be important on the time scales of relevance for human
perturbations (i.e., the very long time scales required for the process to act
may exceed the Anthropocene period over which human carbon emissions
are expected to significantly warm the atmosphere).
Ice Sheets Beyond 2100
The extent to which the great ice sheets of Greenland and Antarctica
will survive the Anthropocene is a question of paramount importance. Pa-
leoclimate reconstructions provide some information regarding the condi-
tions for initiation of these ice sheets, but there are no known instances of
complete deglaciation of these ice sheets that could provide ground-truth
for estimates of the temperature thresholds for deglaciation and the time
required for deglaciation to take place. Therefore, estimates of temperature
and duration thresholds must be drawn from ice sheet models, past partial
deglaciations of Greenland and Antarctica, and past total deglaciations of
the Laurentide or Fenno-Scandian ice sheets. It should be kept in mind that
there are many important physical processes that are not well represented
in current ice-sheet models, so that any thresholds based on models should
be taken as only a general indication of what ice sheets can do, rather than
precise, definitive values.
The waxing and waning of large land ice sheets has a profound effect
on sea level. Moreover, the major categories of past climate states are often
distinguished by the presence of ice near both poles (the Pleistocene and
Holocene), near the South Pole only (the Pliocene), or by ice-free conditions
in both the Arctic and Antarctic (the Eocene). The glacial/interglacial transi-
tions of the Pleistocene involve the growth and decay of the Laurentide and
