01 March 2007Royal Society Meeting Climate Change 2007: The Physical Science Basis Chapter...

01 March 2007 Royal Society Meeting

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


IPCC: team effort

•Lead authors 11•Review editors 2•Contributing authors 52


“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


Vertical overturning ocean circulation

Blue: AntarcticGreen: North AtlanticRed : Southern Ocean

Schmitz 1996


Heat content change: vertical distribution

Linear trend 1955-2003


Global heat content change: spatial pattern

Linear trend 1955-2003 •Warming not uniform•Equatorial Pacific cooling warming


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


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


Salinity change


Ocean climate change: salinity

Atlantic Ocean

Pacific Ocean


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.


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)


Ocean bio-geochemical changes


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


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


Ocean carbon cycle: spatial pattern

Depth integrated Anthropogenic Carbon

Upwelling Subduction zone

Deep overturning

Largest zone of carbon storage is in the Southern ocean.


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


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.


Sea-level rise observations


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


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?


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.


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


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


Accounting for observed sea level rise

1961-2003: Sea level budget not quite closed.1993-2003: Sea level budget is closed.


Ocean climate change and sea level


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.


•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.”


Synthesis


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


Salinity change: vertical distribution


Ocean climate change: temperature


Thermal expansion’s contribution to sea-level

Thermal Expansion 1961-2003

•1961-2003 0.4 ± 0.1 mm yr-1


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


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


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)


Sea-level rise


Sea-level rise:


Sea-level rise: at islands


Steric sea-level rise:


Sea-level rise: ENSO


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


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


Accounting for observed sea level rise

1961-2003: Sea level budget not quite closed.1993-2003: Sea level budget is closed.

01 March 2007Royal Society Meeting Climate Change 2007: The Physical Science Basis Chapter...

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