Transcript Observed physical and bio-geochemical changes in the ocean

Observed physical and
bio-geochemical changes
in the ocean
Nathan Bindoff
ACECRC, IASOS, CSIRO MAR
University of Tasmania
TPAC
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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)
“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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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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Surface Temperature Changes
•Rate 0.67 C per century, 1901 to 2005
•Rate 1.33 C per century, 1979 to 2005
•Large scale, warming everywhere
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Heat content change: vertical distribution
Linear trend 1955-2003
Depth averaged change 0.1C
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Projections of Future Changes in Climate
Low
Emissions
High
Emissions
• Spatial patterns: greater warming over land, greater warming at
high latitudes
• Albedo changes in high latitudes, less snow and sea-ice.
Figure SPM-5,
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TS-28, 10.8, 10.28
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
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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Projections of Future Climate: Ice
Sheets
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)
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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 IPCC WGI “Headlines”
• “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 likely
due 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)
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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, 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
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
balanceKey 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 pattern
Depth integrated Anthropogenic Carbon
Upwelling
Largest zone of carbon storage
is in the Southern ocean.
Deep overturning
Subduction zone
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Attribution
• are observed
changes
consistent with
expected
responses to
forcings
inconsistent
with alternative
explanations
Observations
All forcing
Solar+volcanic
TS-2325
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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