IPCC AR4 WGI Uncertainties and Gaps

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IPCC AR4 WGI Uncertainties and Gaps

• V. Ramaswamy
• NOAA/ Geophysical Fluid Dynamics Laboratory
• plus fellow Authors/ Contributors/ Review
  Editors and Reviewers
• Presentation BASC/ CRC Meeting May 17, 2007

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IPCC AR4 WGI ? Key points

• Unprecedented rise in long-lived anthropogenic
  greenhouse gases ? driver of climate change.
• Warming of the climate system is unequivocal.
  Warming unusual in at least the last 1300 years.
• Most of the increase in global-mean temperatures
  since mid-20th century ?very likely due to
  anthropogenic greenhouse gas increases.
• Better understanding of water vapor feedback
  ?better estimate of the range for climate
  sensitivity.
• Climate projected to warm further increased
  greenhouse gases ? very likely larger changes
  than observed in 20th century, and higher
  confidence in projected patterns of warming.

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Agents of Climate Forcing
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Uncertainties/ gapsDrivers

• Causes of recent changes in methane growth rates
• Roles of different factors in tropospheric ozone
  increase
• Aerosol distributions
• Aerosol-cloud interactions
• Water vapor increases in the stratosphere
• Land-surface properties and land-atmosphere
  interactions.
• Solar irradiance changes on decadal-to-centuries
  scales.
• Emissions, concentrations and forcings in future
  ? GHGs and aerosols

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Land precipitation is changing significantly over
broad areas
Smoothed annual anomalies for precipitation ()
over land from 1900 to 2005 other regions are
dominated by variability.
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Uncertainties/ gaps Atmosphere and surface
observations

• Radiosonde records spatial incompleteness
  reliability
• Satellite and surface observations disagreement
  on total and low-cloud changes over oceans
• Multi-decadal changes in DTR not well understood
  owing to limited observations of cloudiness and
  aerosols
• Difficulty in separating change and variability
  in large-scale atmospheric circulation patterns
  in analyses data
• Difficulty in measuring precipitation ? trends in
  regional and global precipitation
• Short records of soil moisture and streamflow
  affects analyses of changes in drought

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Atmosphere and surface observations ..continued

• Availability of observational data restricts the
  types of extremes that can be analyzed
• Information on hurricane frequency and intensity
  is limited prior to the satellite era affects
  interpretations
• Insufficient evidence for determining whether
  trends exist in tornadoes, hail, lightning and
  dust storms at small spatial scales

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Snow cover and Arctic sea ice are decreasing
Spring snow cover shows 5 stepwise drop during
1980s
Arctic sea ice area decreased by 2.7 per
decade (Summer -7.4/decade)
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Uncertainties/ gaps Observations of snow, ice
and frozen ground

• No global compilation of in situ snow data prior
  to 1960. Well-calibrated snow water equivalent
  data are not available for the satellite era
• Insufficient data to draw conclusions about
  trends in Antarctic sea ice thickness
• Uncertainties in estimates of glacier mass loss
  due to limited global inventory data
• Mass balance estimates for ice shelves and ice
  sheets limited by calibration and validation of
  changes detected by satellite altimetry
• Limited knowledge of basal processes and ice
  shelf dynamics ? uncertainties in ice flow
  processes and ice sheet stability

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Is ocean warming accelerating?

• Causes of decadal variability not well understood
• cooling due to volcanism?
• artefact due to temporally changing observing
  system?

No statement on acceleration possible
Annual ocean heat content 0-700m relative to
1961-90 average
Ishii et al 2006 Willis et al 2004
Levitus WOA
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Sea level is rising in 20th century

• 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

SPM-3b
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Uncertainties/ gaps Observations Oceans and
sea-level

• Limitations in ocean sampling ? evaluations of
  decadal variability in global heat content,
  salinity and sea-level changes only with moderate
  confidence
• Low confidence in observations of trends in the
  MOC
• Global-average 1961-2003 sea level rise appears
  to be larger than can be explained by thermal
  expansion and land ice melting

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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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Uncertainties/ gapsSea Level

• Models do not exist that address key processes ?
  contribute to large rapid dynamical changes in
  the Antarctic and Greenland ice sheets ? could
  increase the discharge of ice into the oceans.
• The sensitivity of ice sheet surface mass balance
  (melting and precipitation) to global climate
  change is not well constrained by observations ?
  large spread in models.
• Large uncertainty regarding the magnitude of
  global warming that, if sustained would lead to
  the elimination of the Greenland Ice Sheet.

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Uncertainties/ gapsPaleoclimate

• Mechanisms of onset and evolution of past abrupt
  climate change and thresholds not well understood
  ? limits confidence in simulation of realistic
  abrupt change
• Degree to and rate at which ice sheets retreated
  in the past, and associated processes not well
  known
• Knowledge of climate variations over more than
  the last few hundred years in SH and tropics is
  limited
• Amplitudes and variability in the different NH
  reconstructions, differences because of proxy
  choice and calibration methods, still to be
  reconciled

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Continental warming
SPM-4

• likely shows a significant anthropogenic
  contribution over the past 50 years

Observations All forcing natural forcing
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Uncertainties/ gapsUnderstanding and attribution

• Confidence in attributing some climate change
  phenomena limited by uncertainties in forcing,
  feedbacks and observations
• Attribution at smaller than continental scales
  and time scales less than 50 years limited by
  larger climate variability on smaller scales,
  uncertainties in small-scale forcing details, and
  uncertainties in simulations at such scales
  including modes of variability
• Less confidence in understanding of forced
  changes in precipitation and surface pressure
  than in temperature
• Incomplete global data for analyses of extremes,
  and model uncertainties still restrict regional
  detection studies of extremes
• Uncertainties in model-simulated internal
  variability still limit some aspects of
  attribution studies e.g., ocean heat content
• Limitations in modeling leading to uncertainties
  in quantifying the anthropogenic contributions to
  sea level rise

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Climate Sensitivity Science Presentation
Ch 10, Fig. Box 10.2, Fig. 2
ECS very unlikely below 1.5C ECS likely range is
2C to 4.5C
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Uncertainties/ gapsModel evaluation and climate
sensitivity

• Proven set of metrics comparing simulations with
  observations, for use in narrowing range of
  climate projections, yet to be developed.
• Most models continue to have difficulty
  controlling climate drift, particularly in the
  deep ocean.
• Problems remain in simulation of some modes of
  variability (e.g., MJO, recurrent blocking,
  extreme precipitation).
• Systematic biases in most models simulation of
  SO ? linked to uncertainty in the transient
  climate response
• Models remain limited by the spatial resolution
  afforded by present computer resources need for
  more ensembles and by the need to include
  additional processes.
• Models differ considerably in the strengths of
  the different climate feedbacks.
• Large uncertainties remain about cloud feedbacks.

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Uncertainties/ gapsGlobal Projections

• Likelihood of a large abrupt change in MOC beyond
  end of 21st C cannot yet be assessed reliably. A
  permanent reduction in MOC cannot be excluded if
  the forcing is strong and long enough
• Model projections for extremes of precipitation
  show larger ranges in amplitude and locations
  than for temperature
• Responses of some major modes of climate
  variability (e.g., ENSO) still differs from model
  to model may be associated with differences in
  space-time representation of present-day
  conditions
• Robustness of many model responses of tropical
  cyclones to climate change is still limited by
  the spatial resolution
• Changes in key processes that drive some global
  and regional climate changes are poorly known
  (e.g., ENSO, NAO, MOC, land-surface feedbacks,
  tropical cyclone distribution)
• Magnitude of future carbon cycle feedbacks is
  till poorly determined

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• Key Points
• Warming pattern similar in all panels, magnitude
  different.
• This pattern will be overlaid with natural
  variability.
• A1B warming middle of the road.
• Land areas tend to warm more than adjacent
  oceans.
• High latitudes tend to warm more than low
  latitudes.

oC
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Percent change
2090-2099 minus 1980-1999

• Key Points
• Precipitation changes more uncertain than
  temperature changes.
• Models do not agree on sign of the change in
  many areas.
• High latitudes tend to receive more
  precipitation, especially in winter.
• The Mediterranean region tends to dry.

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Uncertainties/ gapsRegional projections

• In some regions, there has been only very limited
  study of key aspects of regional climate change,
  particularly with regard to extreme events.
• AOGCMs show no consistency in simulated regional
  precipitation change in some key regions (e.g.,
  northern South America, northern Australia and
  the Sahel).
• In many regions where fine spatial scales in
  climate are generated by topography, there is
  insufficient information on how climate change
  will be expressed at these scales.

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The importance of the future scenarios for
theForcings and Climate Changes(especially
extremes)
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Key Points Most CO2 emission scenarios level
off or decrease by 2100 Most sulfate
emissions decrease by 2030
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A1B Warming (CM 2.1)
2020s
2050
2070s
2090s
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A1B Warming (CM2.1)
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Heat waves are increasing an example
Extreme Heat Wave Summer 2003 Europe
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The End
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IPCC AR4 WGII ? Key points

• Observational evidence from all continents and
  most oceans ? many natural systems being affected
  by regional temperature change.
• Since 1970 ? likely that anthropogenic warming
  has had a discernible influence on many physical
  and biological systems.
• Other aspects of regional change impacts
  emerging, though difficult to discern due to
  adaptation and non-climatic drivers.
• More specific information now available across a
  range of systems, sectors (agriculture, water
  resources, health, industry, ecosystems, coastal
  zones) and regions (all continents, polar, small
  islands).
• Magnitudes of impacts can now be estimated better
  for a range of possible increases in global
  average temperatures.
• Impacts due to altered frequencies and
  intensities of extreme weather and climate events
  ? very large impacts especially after 21st C
  regionality, but very likely impose costs that
  increase with time.

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IPCC AR4 WGII ? Key points

• Some adaptation occurring now will be necessary
  to address impacts due to the past emissions
  more needed to reduce vulnerability exacerbation
  by other stresses dependence on development
  pathway
• Sustainable development can reduce vulnerability
  to climate change.
• Many impacts can be avoided, reduced or delayed
  by mitigation. Can diminish the risks.

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IPCC AR4 WGIII ? Key points

• Global greenhouse gas emissions up by 70
  1970-2004. Continued growth over the next few
  decades
• Substantial economic potential for mitigation of
  global GHG emissions over the coming decades.
• Change in lifestyle and behavior patterns.
• In all analyzed world regions, near-term health
  co-benefits from reduced air pollution as a
  result of actions to reduce GHG emissions can be
  substantial.
• Scale of carbon leaks remains uncertain.
• Energy security-related developments can create
  opportunities to achieve GHG emission reductions.
• Multiple mitigation options in the transport
  sector energy efficiency options for new and
  existing buildings industry agriculture sector
  forest-related mitigation activities.
• Geo-engineering options ? largely speculative and
  unproven lack reliable cost estimates risk ofm
  unknown side-effects.

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• For stabilization of GHG concentrations,
  emissions need to peak, then decline
• Range of stabilization levels achieved by
  deployment of a portfolio of technologies.
• Decision-making about the appropriate level of
  global mitigation over time involves an iterative
  risk management process that includes mitigation
  and adaptation, taking into account actual and
  avoided climate changes, co-benefits,
  sustainability, equity and attitudes to risk.
  Balancing the economic costs of more rapid
  emissions reductions now against the
  corresponding medium-term and long-term climate
  risks of delay.
• A wide variety of national policies and
  instruments are available to governments to
  create incentives for mitigation action.
• Policies that provide a real or implicit price of
  carbon could create incentives for producers and
  consumers.
• Many options for achieving GHG reductions through
  cooperation, agreements. Developments path
  changes. Gaps in mitigation of climate change in
  developing countries.

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Aerosol-cloud interactions
Only the change of cloud albedo induced by
aerosols in the context of liquid water clouds,
is considered to be radiative forcing Other
processes are not considered as radiative
forcings. However, they are included in climate
models that explicitly consider the relevant
processes Aerosol effects on ice clouds are
poorly understood, and are not quantified.
Aerosol cloud interactions Figure 7.20
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Ramanathan et al. (2001)
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0.6 K
- 0.6 K
d s
0.6 K
- 1.9 K
d s i
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Observed Variability of Dust for the last 50 Years
Dust concentration at Barbados (Prospero and
Lamb, 2003)
Factor 4 increase
Sahel drought
Since 1970ies dust concentration in Caribbean
(Prospero and Lamb, 2003) and dust deposition in
French Alps (De Angelis and Gaudichet, 1991) have
increased by a factor 4-5
Correlation at Barbados (Prospero and Lamb, 2003)
Barbados Dust
Sahel Precipitation Index (previous year)
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Global mean temperatures are rising faster with
time
Period Rate Years ?/decade
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North Atlantic hurricanes have increased with SSTs
N. Atlantic hurricane record best after 1944 with
aircraft surveillance.
(1944-2005)
SST
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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 0.35 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

Antarctic ice sheet loses mass mostly through
increased glacier flow Greenland mass loss is
increasing Loss glacier discharge, melting
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Observations

• Anthropogenic greenhouse gas increases very
  likely caused most of the observed warming since
  mid-20th century

All forcing
Solarvolcanic
TS-23
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Explaining the satellite-observed stratospheric
temperature evolution in terms of the
Anthropogenic (ozone depletion, long-lived
greenhouse gases and Natural (solar variations,
major volcanos) forcings
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Ramaswamy et al. Science (2006)
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Results from interactive stratospheric ozone,
dynamics, radiation simulation 48-layer model
with ozone chemistry GFDL simulation ? WMO/
UNEP 2007
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Global-mean Temperature Profile Models vs. Obs.
CCSP SAP 1.1 (2006)
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Tropical Temperature Profile Models vs. Obs.
CCSP SAP 1.1 (2006)