Transcript Climate Change 2007: The Physical Science Basis Chapter 5

Scientific gaps and vulnerabilities
Observations: Oceanic Climate Change
and Sea Level
Nathan Bindoff
ACECRC, IASOS, CSIRO MAR
University of Tasmania
TPAC
02 October 2007
WCRP Meeting
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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WCRP Meeting
Do we understand the observed heat content record
in the oceans?
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
Key issue:
•Excessive internal variability
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WCRP Meeting
Do we understand the heat content record?
• Issues
– Spatial sampling (eg
Southern Ocean)
• Sub-sampling
experiments (Achatao
et al, 2005 )
– Instrumental biases
• Gouretski and
Koltermann (2006),
others
– Robustness – more
global analyses needed
with closure
– Capturing the deep
ocean change (below
~2000m)
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Gaps:
• Can we close the earth
energy budget? (some
papers)
• How does heat
penetrate the ocean?
• Is heat uptake
slowing?
• Has the ocean
stratified more?
• How does this affect
ocean renewal
(ventilation)
WCRP Meeting
Salinity change
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WCRP Meeting
Ocean climate change: salinity
• Why important
– P-E estimates
– Air-ocean
exchanges
– Combine with
terrestrial
networks
– Potential for
increased ice sheet
melt
Issues and science gaps
are same for heat content
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WCRP Meeting
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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WCRP Meeting
Observed change in overturning circulation?
• Issues
– Difficult to
observe.
– Need data +
models (eg
reanalyses of
available data with
models).
– Southern
Hemisphere OT
changes
Illustrates the power of assimilation to monitor the MOC.
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WCRP Meeting
Ocean bio-geochemical changes
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WCRP Meeting
Is ocean carbon cycle changing?
Depth integrated Anthropogenic Carbon
Upwelling
Deep overturning
Subduction zone
It is more likely than not that the fraction of all the
emitted CO2 that was taken up by the oceans has
decreased…..
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WCRP Meeting
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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WCRP Meeting
Is ocean carbon cycle changing?
• Issues
– Robustness – more
global analyses
needed with closure
– Is the ~10 year
sampling timely
enough given the
increasing urgency,
and increasing
signal?
– Can we analyse the
data faster enough?
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Gaps:
• Can we estimate
changes in ocean
carbon storage?
• Are the changes
consistent with
ventilation
changes?
WCRP Meeting
Can we close the sea level rise budget?
Has the last decade of sea-level rise accelerated?
Are sea-level extremes changing?
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WCRP Meeting
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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WCRP Meeting
The sea level budget
The main contributions to sea level:
Slr = thermal exp. +
(glaciers + ice-caps) + Greenland
+ Antarctica
+ Terrestrial +…….
The solutions for hydrology (recommendation)
• re-invigorate terrestrial analysis on basis
• use multiple analyses (GCM’s), GRACE
•Must be complete (P-E, runoff, hydrology)
Other terms are “in hand”, ie confident
of analysis
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WCRP Meeting
Has last sea-level accelerated over the last decade?
Issues:
•Signal is increasing
•Risk of more rapid
Tide-gauges
change, and poor models
of ice sheets
3.1 mm yr-1
Last decade means rapid
1.8 mm yr-1 analysis
•PMSL is slow in
reporting tide data
•Statistical methods
aren’t good enough
Steric Sea-level
•Should resort to
more
representative
“It is unknown whether the higher rate in 1993–2003
is due to
decadal variability or an increase in the longer termmethods
trend.”
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WCRP Meeting
Are sea-level extremes changing?
Global analysis of
records:
• changes in
extremes in tide
gauges (1 global
study in AR4, many
individual studies)
• Examine impacts of
observed wind
changes and
intensity
• Global maps of
changes in return
periods
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WCRP Meeting
General gaps and recommendations
Science Gaps or needs:
• Response time has to
decrease… interested
in differences on
shorter time scales.
• Greater skill at
separating natural
variations from
climate induced
variations.
• Insufficient global
analyses, too many
regional analyses, with
mixed messages.
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Recommendations:
• Accelerate tide gauge
reporting (?)
• Use more ocean
reanalyses (ECCO,
GECCO, GODAE,
ECMWF) unfold
changes (eg OT, Sea
Level Rise rates)
• Accelerate CO2
analyses of extant
data sets and related
data sets
• Foster global analyses
WCRP Meeting
Three underlying issues for ocean
observations
1. Sustained observations (big risks)
- Transitioning a research endeavor into a
sustained, operational capability
- Insitu programs (ARGO, SOO, etc)
2. Timely data access
– How to ensure timely access to data so
that all may derive benefit
3. The non-specialist needs….this is multidisciplinary research
02 October 2007
Courtesy Stan
WCRPWilson
Meeting
Observational Risk – potential failures
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66°-inclination climate reference orbit
Jason-3
Jason-1
SWOT
RA/ERS-1
Jason-2
High-inclination complementary orbit
Envisat
SARAL
GFO
Sentinel 3 – 2-satellite series
Cryosat-2
HY-2A
HY-2B
HY-2C
On going mission
Approved mission
GMES mission beginning development
Pending Jason follow-on
NASA mission pending approval
02 October 2007
Stan Wilson WCRP Meeting
HY-2D
IPCC: team effort
•Lead authors 11
•Review editors 2
•Contributing authors 52
02 October 2007
WCRP Meeting