Geoscience Reference
In-Depth Information
when, as climate warmed, it began to slowly rise.
Based on the IPCC Fourth Assessment Report,
the best estimate is that global sea-level rose at
about 1.7mm/year over the twentieth century, but
faster in recent decades. Satellite altimeter data
point to a value of around 3mm/year since 1993.
The primary contributions to sea-level rise
over the period 1961-2003 and from 1993-2003,
based on available estimates compiled for the
IPCC Fourth Assessment Report, are summarized
in
Figure 13.20
. These include:
1
A more vigorous global hydrological cycle
overall.
2
More severe droughts and/or floods in some
places and less severe ones in others.
3
An increase in precipitation intensities with
possibly more extreme rainfall events.
4
Greater hydrological effects of climate change
in drier areas than in wetter ones.
5
An overall increase in evaporation.
6
An increase in the variability of river discharges
along with that of rainfall.
7
A shift of peak snow-melt runoff to earlier in
spring as temperatures rise.
8
The greatest falls in lake water levels in dry
regions with high evaporation.
9
More intense tropical cyclones (still contro-
versial as of writing).
1
Thermal expansion of ocean waters. The upper
ocean has warmed, and warmer water occupies
a larger volume per unit mass than colder water.
2
The melting of glacier and ice caps, which is
transferring water from terrestrial storage into
the ocean.
3
Mass loss from the Greenland and Antarctic ice
sheets, also transferring water from the land to
the ocean For the Greenland ice sheet, this
includes contributions from both liquid water
runoff and the process of iceberg discharge
(calving). For Antarctica, calving dominates.
Note that projections of changes in the
hydrologic cycle and atmospheric circulation
are especially uncertain on the regional scale
and the scales important to human affairs. The
hydrological impacts of climate change may be
greatest in currently arid or semi-arid regions,
implying that the more severe runoff events there
will particularly exacerbate soil erosion.
All of the individual terms are larger over the
second period, especially thermal expansion
(0.42mm/year for 1961-2003 versus 1.6mm/year
for 1993-2003) which is the largest contributor in
the later period. For both periods, the observed
sea-level rise exceeds the change assessed from
adding estimates for the individual components.
The causes of this discrepancy remain to be
resolved. Effects of human impoundments
offer no explanation, as impoundments have a
negative impact on sea-level. Note that both the
observations and estimates for the different
components contain substantial uncertainty.
Difficulties in estimating the magnitude of
thermal expansion include the lack of knowledge
of deep ocean temperature changes and the effects
of oceanic circulations. Uncertainty in the ice
sheet contributions include uncertainties in
accumulation (by snowfall) and ice thickness at
the grounding line where ice sheets float.
2 Global sea-level
The mechanisms influencing global sea-level are
complex and operate over a broad spectrum of
timescales. Over timescales of millions of years,
one must consider issues such as plate tectonics
which alter the shape and size of ocean basins, and
the effects of erosion that slowly fill ocean basins
with sediment. Moving to timescales of thousands
to tens of thousands of years, we know that
following the Last Glacial Maximum, global sea-
level rose rapidly as the major ice sheets of
North America and Northern Europe melted. By
6000 years ago, around the Holocene Thermal
Maximum, sea-level had risen by about 120m
from the glacial low stand. Sea-level then stabilized
around 2000-3000 years ago, not changing
significantly until the late nineteenth century,
