Transcript IASOS - chapter 5

Climate Change 2007: The Physical Science Basis
Chapter 5:Observations: Oceanic Climate
Change and Sea Level
The Working Group I Report of the Intergovernmental Panel on Climate
Change Fourth Assessment Report
Nathan Bindoff and lead authors
ACECRC, IASOS, CSIRO MAR
University of Tasmania
TPAC
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IPCC: team effort
Chapter 5 Team
•Lead authors 11
•Review editors 2
•Contributing authors 52
•Four reviews
•~2000 comments
•Every comment
has a response
• ~6000 emails
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Observations: Oceanic climate change and sea level
• Global scale temperature and salinity change
• Regional scale ocean changes
• Ocean bio-geochemical change (ocean carbon cycle)
• Changes in sea level
• Synthesis
“Warming of the climate system is unequivocal, as is
now evident from observations of increases in
global average air and ocean temperatures,
widespread melting of snow and ice, and rising global
average sea level (see Figure SPM-3).”
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Vertical overturning ocean circulation
Blue: Antarctic
Green: North Atlantic
Red : Southern Ocean
Schmitz 1996
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Heat content change: vertical distribution
Linear trend 1955-2003
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Global heat content change: spatial pattern
Linear trend 1955-2003
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•Warming not uniform
•Equatorial Pacific cooling
warming
Heat content change: time evolution
Key points for 19612003:
•consistency of products
• oceans absorbed 0.21 ± 0.04
W m–2 (0-3000m) over the
earth’s surface.
•70% of this energy is
absorbed in top 700 m
•0.1°C warming (0-700m)
•1993-2003 has higher rates
of warming (0.50 ± 0.18 W m–2)
•decadal variability, cooling
since 2003
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Earth’s overall energy balance
Key points:
•> 80% of energy change is
stored in the oceans
•ice sheets, glaciers and ice
caps about 1% energy
•ice sheets, glaciers and ice
caps about 40% sea level
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Salinity change
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Ocean climate change: salinity
Atlantic Ocean
Pacific Ocean
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Ocean climate change: salinity
Summary of salinity changes
• Large-scale, coherent trends of salinity are observed for 1955–
1998
– global freshening in subpolar latitudes
– increasing salinity in shallower parts of the tropics and subtropics.
• Freshening is pronounced in the Pacific while increasing salinities
prevail over most of Atlantic and Indian Oceans.
Relevant to the atmosphere and climate
• These trends are consistent with changes in precipitation and
inferred larger water transport in the atmosphere from low
latitudes to high latitudes and from the Atlantic to the Pacific.
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Observed change in overturning circulation?
“…we assess that over that over the modern instrumental
record no coherent evidence for a trend in the mean
strength of the [Atlantic] MOC has been found.”
Based on:
•1970’s to 1990’s MOC increased by 10% (SST and models)
•1970’s to 1995 convection strong in Labrador sea
(increased MOC) but convection now weak ( decrease in MOC)
•Denmark overflow mean strength unchanged (record to short)
•Atlantic subpolar gyre (from direct measurements)
unchanged in strength
•Hydrographic data at 25°N show a 30% decrease (1957-2004)
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Ocean bio-geochemical changes
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Ocean carbon cycle: surface pCO2, pH
Increased pC02
implies decreased
pH
pH decreasing at a
rate of 0.02 pH
units per decade.
20 years
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Ocean carbon cycle: vertical distribution
Pacific and Indian Ocean
Anthropogenic carbon
Atlantic Ocean
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•Dissolved inorganic
carbon
•Chloroflourocarbons
•Pre-industrial
(~1750)
•Estimate of added DIC
•Water chemistry
Ocean carbon cycle: spatial pattern
Depth integrated Anthropogenic Carbon
Upwelling
Largest zone of carbon storage
is in the Southern ocean.
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Deep overturning
Subduction zone
Ocean carbon cycle: global uptake
It is more likely than not that the fraction of all the
emitted CO2 that was taken up by the oceans has
decreased…..
Implying reduced rates of renewal of key ocean
water masses
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Oxygen changes: North Pacific Ocean
•There is evidence for decreased
oxygen concentrations, likely to
be driven by reduced rates of
water renewal in most ocean
basins from the early 1970’s to
the late 1990’s.
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Global oxygen decreases
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Sea-level rise observations
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The sea level budget
The main contributions to sea level:
Slr = thermal exp. +
(glaciers + ice-caps) + Greenland + Antarctica
+ …….
Focus on two periods in the report:
•1961-2003
•1993-2003
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20th century sea level
Rates of sea level rise:
•1.8 ± 0.5 mm yr-1, 1961-2003
•1.7 ± 0.5 mm yr-1, 20th Century
•3.1 ± 0.7 mm yr-1, 1993-2003
•Consistency of
sea level data
•Variability of
sea level data
•Are rates increasing?
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Thermal expansion’s contribution to sea-level
The. Exp.
SLR
Sea-level rise 1993-2003
Thermal expansion 1993-2003
• Sea level rise is spatially non-uniform
• Thermal expansion controls spatial pattern
• Observed thermal expansion 1.6 ± 0.5 mm yr-1, 1993-2003
0.4 ± 0.1 mm yr-1, 1961-2003
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Glacier contribution to sea-level since 1961
Increased glacier retreat
since the early nineties
Mass loss from glaciers
and ice caps:
• 0.5 ± 0.18 mm yr-1, 19612003
• 0.77 ± 0.22 mm yr-1, 19912003
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Ice sheet contributions to sea level rise
Mass loss of Greenland:
• 0.05 ± 0.12 mm yr-1 SLE,
1961-2003
• 0.21 ± 0.07 mm yr-1 SLE,
1991-2003
Antarctic ice sheet loses mass
mostly through increased glacier
flow
Greenland mass loss is
increasing
Loss: glacier discharge, melting
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Mass loss of Antarctica:
• 0.14 ± 0.41 mm yr-1 SLE,
1961-2003
• 0.21 ± 0.35 mm yr-1 SLE,
1991-2003
Accounting for observed sea level rise
1961-2003: Sea level
budget not quite
closed.
1993-2003: Sea level
budget is closed.
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Has the sea level rise rate increased?
•The 1993-2003 has high
rate of rise compared
with the 1961-2003
period.
Tide-gauges
3.1 mm yr-1
1.8 mm yr-1
Steric Sea-level
•Other periods have had
sea-level rise as high as
1993-2003
•On longer term (since
19th century) sea-level
rise rate has increased
“It is unknown whether the higher rate in 1993–2003 is due to
decadal variability or an increase in the longer term trend.”
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Ocean climate change and sea level
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Synthesis
• The patterns of observed changes in
global ocean heat content and salinity,
sea-level, thermal expansion, water mass
evolution and bio-geochemical parameters
described in this chapter are broadly
consistent with the observed ocean
surface changes and the known
characteristics of the large-scale ocean
circulation.
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Synthesis
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IPCC: process
The IPCC is a “remarkable example” of
mobilizing expert analysis to inform policymakers
Jeffrey Sachs (Nature, 12 August 2004)
The IPCC assessments are “dull as
dishwater”
Tim Flannery, The Weather Makers
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Salinity change: vertical distribution
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Ocean climate change: temperature
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Thermal expansion’s contribution to sea-level
Thermal Expansion 1961-2003
•1961-2003 0.4 ± 0.1 mm yr-1
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Greenland and Antarctic ice sheets of are shrinking
Greenland mass loss is increasing
Loss: glacier discharge, melting
Mass loss of Greenland:
• -0.07 to 0.17 mm yr-1 SLE,
1961-2003
• 0.14 to 0.28 mm yr-1 SLE,
1991-2004
Greenland gains mass in
the interior, but loses
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more
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the margins
Greenland and Antarctic ice sheets of are shrinking
Mass loss of Antarctica:
• -0.28 to 0.55 mm yr-1 SLE,
1961-2003
• -0.14 to 0.55 mm yr-1 SLE,
1991-2004
Antarctic ice sheet loses
mass mostly through
increased glacier flow
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Sea Level: progress since the TAR
In observations:
In interpretation:
•Mass balance of Antarctica
•Assessed errors in observations
are now smaller relative to TAR
•Mass balance of Greenland
•Better records of glaciers
•Extended records of global sealevel to 1870’s
•New records of altimeter data
•Different error analysiscombined in quadrature
• Errors are quoted as 90%
confidence intervals compared
with 95% intervals in TAR
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•Largest uncertainties in thermal
expansion (1993-2003), Antarctica,
and sea level observations
•Causes of difference between sea
level and its contributions in long
term records (1961-2003) is
unresolved - either the
observations or un-quantified
processes.
•Climate contributions to sea level
can explain the observations in the
short term (1993-2003)
Sea-level rise
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Sea-level rise:
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Sea-level rise: at islands
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Steric sea-level rise:
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Sea-level rise: ENSO
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Glacier contribution to sea-level since 1961
Increased glacier retreat
since the early nineties
Mass loss from glaciers
and ice caps:
• 0.5 ± 0.18 mm yr-1, 19612003
• 0.77 ± 0.22 mm yr-1, 19912003
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Ice sheet contributions to sea level rise
Mass loss of Greenland:
• 0.05 ± 0.12 mm yr-1 SLE,
1961-2003
• 0.21 ± 0.07 mm yr-1 SLE,
1991-2003
Antarctic ice sheet loses mass
mostly through increased glacier
flow
Greenland mass loss is
increasing
Loss: glacier discharge, melting
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Mass loss of Antarctica:
• 0.14 ± 0.41 mm yr-1 SLE,
1961-2003
• 0.21 ± 0.35 mm yr-1 SLE,
1991-2003
Accounting for observed sea level rise
1961-2003: Sea level
budget not quite
closed.
1993-2003: Sea level
budget is closed.
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