Global Warming and Antarctica: Happening, Imminent, or Lost in the Climatic Noise

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Global Warming and Antarctica Happening,
Imminent, or Lost in the Climatic Noise?
David H. Bromwich Polar Meteorology Group Byrd
Polar Research Center The Ohio State
University Columbus, Ohio, USA
Average annual near-surface temperature for
Antarctica from three different datasets
Monaghan et al. (submitted), Chapman and Walsh
(2007), and Schneider et al. (2006)
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Background

• Antarctic climate variability is of global
  significance because of the tens of meters of
  global sea level stored in Antarctic land ice.
  However, a system-wide approach toward
  understanding Antarctic climate has received only
  modest attention from the Antarctic research
  community. The following three aspects need
  particular attention

• Combining regional climate records to extend
  continental-scale records of climatic variables
  as far into the past as possible to gain a
  multi-decadal perspective on present events
• More comprehensive exploration of the factors
  governing Antarctic climate variability, such as
  stratospheric ozone depletion, greenhouse gas
  increases, the forcing from the tropical Pacific
  Ocean (El Nino, etc.), the contribution of
  air-sea interactions over the Southern Ocean, and
  ice sheet-ocean interactions.
• Enhancement of the global and regional models
  used to project future Antarctic climate,
  especially dealing with the atmospheric
  hydrologic cycle (clouds, precipitation, etc.)
• The following slides focus on specific questions
  and possible solutions

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1 Combining and extending regional climate
records
Part 1 Combining/extending records
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Problems with existing Antarctic records

• Shortest and sparsest instrumental record on
  earth
• 50-year records of temperature, winds, pressure
  at a handful of stations (surface and upper
  level)
• Observational records with reliable, complete
  spatial coverage of Antarctica are even shorter
  (30 years satellites)
• Basically no instrumental record of snowfall
  variability
• Few records of ocean, sea ice variability prior
  to 1970s
• The short record is confounded by high
  variability
• Temporal (SAM, ENSO, Ozone, GHGs) multidecadal,
  and longer
• Spatial (Peninsula, WAIS, EAIS)
• Current U.S., European, and Japanese global
  reanalyses are plagued by problems over
  Antarctica
• Satellite records are relatively short

Part 1 Combining/extending records
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Potential Solutions

• Blend instrumental observations and ice core
  records with satellite and model fields to
  reconstruct Antarctic climate records
  (atmosphere, ocean, sea ice, chemistry, etc.) for
  the past 200 years with spatial resolution. Such
  a multi-media approach requires collaboration
  across disciplines.
• An Antarctic system model reanalysis during the
  modern instrumental record that builds on the
  shortcomings of past reanalyses, employs higher
  spatial resolution and optimal processes, and
  integrates all components of the climate system.
  This system-wide approach requires
  multidisciplinary cooperation between
  oceanographers, atmospheric scientists,
  glaciologists, atmospheric chemists, and data
  assimilation specialists.

Part 1 Combining/extending records
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2 Understanding mechanisms of change
Part 2 Understanding climate forcing mechanisms
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Question 1 What drives SAM variability?
Annual
DJF
MAM
JJA
SON
SAM index, 1957-2005 Running SAM
Trends through 2005
SAM trends have leveled off since the 1990s,
despite increases in GHG concentrations and
continued ozone depletion. Why?
Part 2 Understanding climate forcing mechanisms
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Question 2 How important is tropical forcing?
(Fogt and Bromwich 2006)
El Nino (SOI) and the Southern Annular Mode (SAM)
become strongly coupled during springtime in the
1990s. Why?
Part 2 Understanding climate forcing mechanisms
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Question 3 Is Direct GHG forcing playing any
role in Antarctic climate variability?
Reconstructed near-surface temperature trends,
1992-2005 (Monaghan et al. submitted)
Observed Near-surface temperature Trends from 3
periods (error bars plt0.1) (Monaghan et al.
submitted)
Since the SAM has leveled off in the early 1990s,
Antarctica has been slightly warming overall.
Are GHGs driving the warming?
Part 2 Understanding climate forcing mechanisms
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Other pressing mechanism questions
How important is the ocean as a mechanism in
forcing observed atmospheric changes, and vice
versa? (i.e., western Antarctic Peninsula) What
is causing the regional ocean warming in the
Bellingshausen Sea? What is causing the change
in ocean circulation that has brought warmer
waters into contact with WAIS? Based on our
understanding of forcing mechanisms, will
Antarctica undergo strong, widespread warming in
this century, as climate models project? If so,
how will the ice sheets react?
Part 2 Understanding climate forcing mechanisms
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Potential Solutions to mechanism questions

• Enhanced/extended reconstructions (suggested
  earlier)
• A climate system reanalysis (suggested earlier)
• Multi-disciplinary field campaigns
• By improving climate model projections

Part 2 Understanding climate forcing mechanisms
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3 Projecting future Antarctic climate Improvin
g GCMs in Antarctica
Part 3 Improving climate models
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Issue 1 Stratospheric Ozone Forcing and the SAM

• Most models have much lower SAM values in the
  early 20th century, thereby producing significant
  long-term trends that are not in the
  reconstruction
• Models with and without time variable
  stratospheric ozone forcing give different trends
  over the last 50 years (not shown), suggesting
  that accurate ozone concentrations are needed for
  accurate 20th and 21st century SAM predictions

Time series of reconstructed SAM (blue). Shaded
region corresponds to SAM calculated from IPCC
AR4 models, red is the grand ensemble mean (Fogt
et al. in preparation).
GCM simulations of the SAM are strongly dependent
on stratospheric ozone. Understanding past and
future SAM variability will depend on accurate
records (and projections) of stratospheric ozone
concentration.
Part 3 Improving climate models
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Issue 2 Spurious water vapor trends in GCMs
b) T vs. LW Down, Cloudy-sky
c) T vs. Precipitable Water Vapor
a) T vs. LW Down, All-sky
Bromwich et al. (in preparation)
GCMs project 20th century warming in Antarctica
that is 2-3 times that observed. The
over-amplified warming appears to be caused by a
LW radiation feedback at the surface that is due
to a monotonic increase in water vapor (not
clouds) in the GCMs. What causes this?
Part 3 Improving climate models
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Issue 3 Clouds
Figure 8.7 Zonally averaged December-January-Febr
uary total cloudiness simulated by ten AMIP1
models. The solid black line gives observed data
from the International Satellite Cloud
Climatology Project (ISCCP). From Gates et al.
(1999). Reproduced from IPCC (2001)
Clouds have always been a weakness in GCMs.
Antarctic clouds are not well observed or
understood.
Part 3 Improving climate models
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Other pressing GCM and regional climate model
issues
Meridional overturning in the Southern Ocean is
not well represented. This appears to be
partially due to a lack of sub-ice-shelf
processes in the GCMs. Antarctic mass balance
changes are not well represented due to the
absence of non-linear ice sheet dynamics in GCMs
and ocean-ice sheet interactions. ??
Part 3 Improving climate models
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Potential solutions to climate modeling issues

• More accurate forcing for benchmark (20th
  Century) and future (21st Century runs).
  Stratospheric ozone comes to mind as a major
  issue.
• Improve and adapt ice sheet models to the point
  that they can be included in IPCC GCMs.
  (addresses sea level)
• Improve and adapt ice shelf models to the point
  that they can be included in IPCC GCMs.
  (addresses meridional overturning)
• Improved observation and modeling of clouds and
  precipitation in the Antarctic (build on Arctic
  lessons) SHEBA-SOUTH aka Antarctic Regional
  Interactions Meteorology Experiment (RIME)?

Part 3 Improving climate models
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Summary

• To date, an integrated systems approach toward
  understanding Antarctic climate has not been a
  major focus of NSF OPP.
• Through AISS, for the first time Antarctic
  scientists will have the resources to tackle the
  most pressing and societally-important science
  questions involving the entire Antarctic climate
  system. Although the physical components have
  been emphasized here, the flora and fauna are
  integral aspects to be synthesized.
• The following aspects would particularly benefit
  from AISS
• Combining and extending regional climate records
  to gain a multi-decadal perspective on present
  events
• More exploration into the causality of Antarctic
  climate variability
• Enhancement of the global and regional models
  used to project future Antarctic climate