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Global Climate Change: What do we
know & What don’t we know?*
Klaus Wolter
University of Colorado at Boulder, [email protected]
Kudos to Susan Solomon (most unmarked slides)
(Co-Chair of WG 1, IPCC 4; a.k.a. “The Horse’s Mouth”)
Background on IPCC & Global Change modeling
Observed climate changes & how well they are modeled
Projections through the 21st century
Closing thoughts
* With two nods to our former Secretary of Defense (also: “you
have to analyze climate change with the data you have got…”)
Structure of the IPCC
WG1 - Climate Change:
The Physical
Science Basis
WG2 - Climate
Change: Impacts
and Adaptation
dates, corals,
policy options,
WG3: Mitigation
coastal zone
discount rates,
Three different working groups with quite distinct scientific
purviews and required expertise. IPCC assesses research but it
doesn’t do research. WG1 report is at
The IPCC Sequence
IPCC (1990) Broad overview of climate change science,
discussion of uncertainties and evidence for warming.
IPCC (1995) “The balance of evidence suggests a
discernible human influence on global climate.”
IPCC (2001) “Most of the warming of the past 50 years is
likely (>66%) to be attributable to human activities.”
IPCC (2007) “Warming is unequivocal, and most of the
warming of the past 50 years is very likely (90%) due to
increases in greenhouse gases.”
Intergovernmental Panel on Climate Change, UN-approved
since 1988, 75% of 2007 WG-1 authors did not work on last
assessment (2001,Third Assessment Report).
Latest info
on GHG
There is no debate
whether greenhouse
gases are rising - the
evidence is clear-cut
that their (carbon
dioxide, methane+)
levels have been rising
to (& at) unprecedented
levels (rates) longer
than homo sapiens has
existed (at least 600K,
actually), and as best as
I can tell there is little
debate as to whether
this increase is humancaused.
IPCC, 2007: SPM 2
Carbon Dioxide Amount (ppmv)
Last interglacial
Last Ice Age
[Adapted from Figure 6.3, ©IPCC 2007: WG1-AR4]
Thousands of Years Before Present
Humans are ‘forcing’ the earth system in a new way. CO2 increases
due to fossil fuel burning are the dominant cause of global warming.
CO2 has not been this high in more than half a million years.
Ice ages
are not
They are
in the
Better and longer satellite data about the
Improved assessment:
a) no significant observed trend in solar irradiance since 1978
using high quality inter-calibrated data; b) spectral information c)
solar magnetic flux model rather than proxy data; d) reevaluation of variations in Sun-like stars.
Solar irradiance forcing much smaller than GHG, and then TAR
New information about aerosols, and
major modelling improvements
January to March, 2001
 Observations reveal the presence and provide quantitative aspects.
 Aerosol transport-forcing models better tested and constrained.
 Much improved estimate of the Aerosol Direct Radiative Forcing.
Human and Natural Drivers of Climate
1.6 W m-2 warms
like 1.6 xmas tree
lights over every
m2 on Earth.
Carbon dioxide is
causing the bulk
of the forcing,
and it lives a long
time in our
atmosphere so
every year of
emission means
commitments to
climate change
for future
IPCC, 2007: SPM 2
Figure 10.1
IPCC, 2007: 10.1
Observed climate changes & how well they are modeled
Figure 3.1
IPCC, 2007: 3.1 - Observed temperature changes since 1850 (different QC/averages)
Correlativity of U.S. climate stations (DJF temps 1979-03)
Correlativity tends to be highest in the winter. Iowa winter temperatures could be
monitored by just a few (one?) stations, while Colorado has many more ‘local flavors’.
(Wolter and Allured;
Figure 3.2
IPCC, 2007: 3.2 - land data
IPCC, 2007: 3.9 - °C per century on left/per decade on right (and below); grey = insufficient data
IPCC, 2007: FAQ 3.1 - MSU-based trends for last 25 years (better match than in TAR)
Figure 3.10
IPCC, 2007: 3.10 - 18 years of data (≥2 months per season) requred; sensitive to end points!
Figure 3.12
IPCC, 2007: 3.12 - a lot more “enthusiasm” about wetting trend in/after TAR!
Figure 3.13
IPCC, 2007: 3.13 - note
contrast between two
periods in Western U.S.
and Sahel!
Figure 4.1
IPCC, 2007: 4.1 - Components of cryosphere & their time scales
Figure 4.2
IPCC, 2007: 4.2 (March-August dropping; September-February NOT - WHY?)
Figure 4.8
IPCC, 2007: 4.8 - NH
dropping (spring&
summer!); SH not WHY?
Figure 4.15
IPCC, 2007: 4.15 - specific vs. total glacier mass balance (regional strength of
climate change vs. contribution to sea level rise)
Figure 4.23
IPCC, 2007: 4.23 - Summary slide
• Asks whether observed changes
are consistent with
 expected responses to forcings
 inconsistent with alternative
• Most of the observed increase
in globally averaged
temperatures since the 1970s is
very likely (>90%) due to the
observed increase in
anthropogenic greenhouse gas
Anthro+ Nat forcing
Understanding and Attributing Climate Change
Anthropogenic effect
on warming
averaged over each
continent except
Antarctica is likely
[IPCC, 2007]
Expected for all
Natural forcing
Understanding and Attributing Climate Change
Anthropogenic effect
on warming
averaged over each
continent except
Antarctica is likely
[IPCC, 2007]
Expected for all
Natural forcing
Attribution studies
• Separate time-space patterns
of response.
• Solar response has very
different behavior to GHG,
especially with altitude.
The upper atmosphere would
be expected to be much
warmer than it is if solar
irradiance were the cause of
current surface climate
“All” forcings
Figure 3.27
IPCC, 2007: 3.27 - SLP/Tsfc/Precip vs. SOI - step change around ‘76?!
Figure 3.33
IPCC, 2007: 3.33 - AMO: main driver for Atlantic hurricane trends?!
IPCC, 2007: 3.38 (10th percentile for 1901-50 (black), 51-78 (blue), and 79-03 (red)
Figure 3.39
IPCC, 2007: 3.39 - contribution from very wet days (95th percentile) to annual precip
Figure 3.40
IPCC, 2007: 3.40 - Accumulated Cyclone Energy (ACE); still much debate on how
much of the recent increase in North Atlantic is related to natural climate
variability (AMO) vs. anthropogenic; there are also quite a few (poorly resolved)
inhomogeneities in record (Landsea, 2007; pers. comm.)
Calculus of extremes
The distribution of weather events
around the climatic average often
follows a ‘bell-shaped’ curve.
Climate change can involve change
in the average, or the spread
around the average (standard
deviation), or both.
A shift of 1 standard
deviation makes a
1 in 40 yr event into
a 1 in 6 yr event
Standard deviation
1 in 40 yr high range
A shift in temperature
distribution has much larger
relative effect in the tails
than near the mean. For
instance, huge JJA’03
anomaly in Europe (>5
sigma) is more consistent
with higher sigma than with
higher mean.
Projections through the 21st century
Figure 10.2
IPCC, 2007: 10.2
Committed Warming is Coming
• For the next two decades a warming of about
0.2°C per decade is projected for a range of
emission scenarios.
• Even if the concentrations of all greenhouse
gases and aerosols were to be kept constant
at year 2000 levels, a further warming of
about 0.1°C per decade would be expected.
The Longer Term
Warming will increase if GHGs increase. If GHGs were kept fixed at
current levels, a committed 0.6°C of further warming would be
expected by 2100. Higher emissions translate into more warming.
3.4oC = 6.1oF
2.8oC = 5.0oF
1.8oC = 3.2oF
0.6oC = 1.0oF
Projections of Future Changes in Climate:
Probabilistic Information for the First Time
Changes over next few decades already ‘locked in’ (to the extent that the
models can provide the answer), big differences by end of century
Seasonal moisture changes as projected for DJF and JJA
Wetter in winter & drier in the summer, if you believe the model average…
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Bottomline on regional climates (&trends)
VAR = GHG + Decadal + ENSO + Regional + Local + Noise
(not everything is greenhouse-related, nor will it ever be &
don’t underestimate the “noise”=our (in-) ability to
measure correctly)
We do NOT fully understand all of the above components of
the climate system, leaving room for surprises!
This is my opinion, not the IPCC’s!
Figure 10.14
IPCC, 2007: 10.14 - Sea ice projections
Figure 10.16
IPCC, 2007: 10.16 - are we going to get more/stronger El Niños???
Figure 10.18
Figure 10.32
IPCC, 2007: 10.32 - sea leve change for end of 21st century relative to expected
global average change
Figure 10.38
Figure 11.12
IPCC, 2007: 11.12 - problem with regional ‘down-scaled’ scenarios: models that are
good in present are not guaranteed to stay good; high fidelity in temp reconstructions/
projections does NOT translate into good precip reconstructions/projections
Box 11.1, Figure 2
Some Pressing Needs for a WG1 AR5…. (Susan Solomon)
• Forcing: How well do we understand past history of emissions and
forcing? Future? Aerosol effects on precip? Role of soot? Land use?
Stabilization of GHG (if governments so decide) is linked to
understanding of forcing/feedback relationships (esp. carbon).
• Beyond global warming: The AR5 will likely advance the
understanding of hurricanes, drought, heat waves, other extremes,
precipitation patterns, ocean circulation, and other ‘earth system’
variables, for observations, attribution, and projections. Much more
regional information will likely be sought, and as in the AR4, this
should follow on the basis of physics (not catalogues). Central to
understanding the adaptation/mitigation challenges.
• Near-term: Already committed to more warming (next few decades),
with choices about emissions affecting the longer term more and
more…commitments to SLR? Drought? Heat waves? Hurricanes?
• Long term: SLR changes the face of the planet. The next
assessment will likely advance understanding of ice sheets and SLR.