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Observed physical and bio-geochemical changes

in the oceanNathan Bindoff

ACECRC, IASOS, CSIRO MARUniversity of Tasmania

TPAC

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“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 changes• Ocean bio-geochemical change (ocean carbon cycle) • Changes in sea level• Two talks in next session (Ridgway, Hobday)

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Global mean temperatures are rising faster with time Warmest 12 years:

1998,2005,2003,2002,2004,2006, 2001,1997,1995,1999,1990,2000

SPM-3a

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•Rate 0.67 C per century, 1901 to 2005•Rate 1.33 C per century, 1979 to 2005•Large scale, warming everywhere

Surface Temperature Changes

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Heat content change: vertical distribution

Linear trend 1955-2003 Depth averaged change 0.1C

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• Spatial patterns: greater warming over land, greater warming at high latitudes

• Albedo changes in high latitudes, less snow and sea-ice.Figure SPM-5,TS-28, 10.8, 10.28

Projections of Future Changes in Climate

HighEmissions

LowEmissions

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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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Projections of Ocean Acidity

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Is Ocean ventilation changing?

•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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Consequence of ocean acidity and renewal change

• Impacts on organsims that have aragonite shells, and structures

• Coral reefs• Changes in upwelling

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Sea-level rise observations

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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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Post 2100 changes, Greenland:

• “…..and that the surface mass balance becomes negative at a global average warming (relative to 1961-1990) in excess of 1.2 to 3.9°C. If a negative surface mass balance were sustained for millennia, that would lead to virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7 m.”

Almost all marker scenarios exceed 1.2 to 3.9 °C tipping points.

• “.. If radiative forcing were to be stabilized in 2100 at A1B levels11, thermal expansion alone would lead to 0.3 to 0.8 m of sea level rise by 2300 (relative to 1980–1999). “

Implication, while not stated, is that there will be large sea level changes beyond 2100 (eg by 2300 something like 1.5 to 3.5m)

Projections of Future Climate: Ice Sheets

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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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• “The balance of evidence suggests a discernible human influence on global climate.” (SAR, 1995)

• “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.” (TAR, 2001)

• “Most of the observed increase in globally averaged temperatures since the mid-20th century is very likelydue to the observed increase in anthropogenic greenhouse gas concentrations.” (AR4, 2007)

• “Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.” (AR4, 2007)

The IPCC WGI “Headlines”

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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 … has implications for transfer of nutrients into the mixed layer…

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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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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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Ocean carbon cycle: spatial patternDepth integrated Anthropogenic Carbon

Upwelling Subduction zone

Deep overturning

Largest zone of carbon storage is in the Southern ocean.

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Attribution

• are observed changes consistent with expected responses to forcings inconsistent with alternative explanations

Observations

All forcing

Solar+volcanic

TS-23

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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)