Nathan Bindoff and lead authors ACECRC, IASOS, CSIRO MAR University of Tasmania TPAC
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Transcript of Nathan Bindoff and lead authors ACECRC, IASOS, CSIRO MAR University of Tasmania TPAC
18 April 2007
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 authorsACECRC, IASOS, CSIRO MAR
University of TasmaniaTPAC
18 April 2007
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
18 April 2007
“Warming of the climate system is unequivocal, as is now evident from observations of increases inglobal average air and ocean temperatures, widespread melting of snow and ice, and rising globalaverage sea level (see Figure SPM-3).”
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
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Vertical overturning ocean circulation
Blue: AntarcticGreen: North AtlanticRed : 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 •Warming not uniform•Equatorial Pacific cooling warming
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Heat content change: time evolution
Key points for 1961-2003:•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 instrumentalrecord 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
Atlantic Ocean
Anthropogenic carbon•Dissolved inorganic carbon•Chloroflourocarbons
•Pre-industrial (~1750)
•Estimate of added DIC•Water chemistry
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Ocean carbon cycle: spatial pattern
Depth integrated Anthropogenic Carbon
Upwelling Subduction zone
Deep overturning
Largest zone of carbon storage is in the Southern ocean.
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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 main contributions to sea level:
Slr = thermal exp. +
(glaciers + ice-caps) + Greenland + Antarctica
+ …….
Focus on two periods in the report:
•1961-2003
•1993-2003
The sea level budget
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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
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
SLRThe. Exp.
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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, 1961-2003• 0.77 ± 0.22 mm yr-1, 1991-2003
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Ice sheet contributions to sea level rise
Antarctic ice sheet loses mass mostly through increased glacier flowGreenland mass loss is increasingLoss: glacier discharge, melting
Mass loss of Greenland:• 0.05 ± 0.12 mm yr-1 SLE, 1961-2003• 0.21 ± 0.07 mm yr-1 SLE,
1991-2003
Mass loss of Antarctica:• 0.14 ± 0.41 mm yr-1 SLE,
1961-2003• 0.21 ± 0.35 mm yr-1 SLE,
1991-2003
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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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•The 1993-2003 has high rate of rise compared with the 1961-2003 period.
•Other periods have had sea-level rise as high as 1993-2003
•On longer term (since 19th century) sea-level rise rate has increased
Has the sea level rise rate increased?
Steric Sea-level
3.1 mm yr-1
Tide-gauges
1.8 mm yr-1
“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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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
IPCC: process
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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
18 April 2007
Greenland and Antarctic ice sheets of are shrinking
Greenland mass loss is increasingLoss: glacier discharge, melting
Greenland gains mass in the interior, but loses more at the margins
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
18 April 2007
Greenland and Antarctic ice sheets of are shrinking
Antarctic ice sheet loses mass mostly through increased glacier flow
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
18 April 2007
Sea Level: progress since the TAR
In observations:
•Mass balance of Antarctica
•Mass balance of Greenland
•Better records of glaciers
•Extended records of global sea-level to 1870’s
•New records of altimeter data
•Different error analysis- combined in quadrature
• Errors are quoted as 90% confidence intervals compared with 95% intervals in TAR
In interpretation:
•Assessed errors in observations are now smaller relative to TAR
•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)
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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, 1961-2003• 0.77 ± 0.22 mm yr-1, 1991-2003
18 April 2007
Ice sheet contributions to sea level rise
Antarctic ice sheet loses mass mostly through increased glacier flowGreenland mass loss is increasingLoss: glacier discharge, melting
Mass loss of Greenland:• 0.05 ± 0.12 mm yr-1 SLE, 1961-2003• 0.21 ± 0.07 mm yr-1 SLE,
1991-2003
Mass loss of Antarctica:• 0.14 ± 0.41 mm yr-1 SLE,
1961-2003• 0.21 ± 0.35 mm yr-1 SLE,
1991-2003
18 April 2007
Accounting for observed sea level rise
1961-2003: Sea level budget not quite closed.1993-2003: Sea level budget is closed.