Human Influence How Do We Know

1
Human Influence? How Do We Know?
Ben Santer Program for Climate Model Diagnosis
and Intercomparison Lawrence Livermore National
Laboratory, Livermore, CA 94550 Email
santer1_at_llnl.gov Climate Change Science
Workshop Field Museum, Chicago, IL April 18th,
2009
2
Truth in advertising Who do I work for?

• PCMDI Program for Climate Model Diagnosis and
  Intercomparison
• Service
• Coordinate international climate modeling
  simulations (standard benchmark experiments)
• Enable broader science community to analyze and
  evaluate models
• Goal Quantify how well models simulate
  present-day climate and evaluate uncertainty in
  projections of future climate change
• PCMDI was established in 1989
• Has been at Lawrence Livermore National Lab since
  then

3
Detection and attribution research has made
important contributions to the conclusions of
IPCC assessments
The balance of evidence suggests a discernible
human influence on global climate
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
There is new and stronger evidence that most of
the warming observed over the last 50 years is
attributable to human activities
4
Structure of talk

• Climate change 101
• What is climate change detection and attribution?
  Why is it important?
• How do we study the causes of climate change?
  What is fingerprinting?
• Fingerprinting examples
• Are there Inconvenient observations?
• Looking towards the future
• Conclusions

5
Climate Change 101 Natural mechanisms influence
climate

• Changes in the Sun
• Changes in the amount of volcanic dust in the
  atmosphere
• Internal variability of the coupled
  atmosphere-ocean system

Natural mechanisms
6
Climate Change 101 Human factors also influence
climate

• Non-natural mechanisms
• Changes in atmospheric concentrations of
  greenhouse gases
• Changes in aerosol particles from burning fossil
  fuels and biomass
• Changes in the reflectivity (albedo) of the
  Earths surface

Smoke from fires in Guatemala and Mexico (May 14,
1998)
7
Climate Change 101 Computer models can perform
the control experiment that we cant do in the
real world
Average surface temperature change (C)
Meehl et al., Journal of Climate (2004)
8
Climate Change 101 We routinely test how well
current climate models simulate

• Todays annual average climate
• The daily cycle
• The seasonal cycle
• The response to massive volcanic eruptions
• Ocean uptake of products of atmospheric tests of
  nuclear weapons
• The climate changes of the past 30 to 150 years
• Climates of the deep past (e.g., the last Ice
  Age)
• Weather
• Modes of natural climate variability (like El
  Niño)

9
Structure of talk

• Climate change 101
• What is climate change detection and attribution?
  Why is it important?
• How do we study the causes of climate change?
  What is fingerprinting?
• Fingerprinting examples
• Are there Inconvenient observations?
• Looking towards the future
• Conclusions

10
Detection and attribution defined

• Detection of climate change
• The process of showing that an observed change is
  highly unusual in a statistical sense
• Attribution of climate change
• The process of establishing cause and effect
  relationships

11
Why is detection and attribution work important?

• It is another form of model evaluation
• Successful simulation of historical changes in
  climate enhances our confidence in projections of
  future climate change
• In an environment where there is still political
  debate regarding the reality of human effects on
  global climate, it is imperative to have sound
  science on the nature and causes of climate
  change

12
Structure of talk

• Climate change 101
• What is climate change detection and attribution?
  Why is it important?
• How do we study the causes of climate change?
  What is fingerprinting?
• Fingerprinting examples
• Are there Inconvenient observations?
• Looking towards the future
• Conclusions

13
Multiple lines of evidence on which discernible
human influence conclusions are based

• Basic physics evidence
• Physical understanding of the climate system and
  the heat-trapping properties of greenhouse gases
• Circumstantial evidence
• Qualitative agreement between observed climate
  changes and model predictions of human-caused
  climate changes (warming of oceans, land surface
  and troposphere, water vapor increases, etc.)
• Paleoclimate evidence
• Temperature reconstructions enable us to place
  the warming of the 20th century in a longer-term
  context
• Fingerprint evidence
• Rigorous statistical comparisons between modeled
  and observed patterns of climate change

14
What is climate fingerprinting?

• Strategy Search for a computer model-predicted
  pattern of climate change (the fingerprint)
  in observed climate records
• Assumption Each factor that influences climate
  has a unique signature in climate records
• Method Standard signal processing techniques
• Advantage Fingerprinting allows researchers to
  make rigorous tests of competing hypotheses
  regarding the causes of recent climate change

15
Structure of talk

• Climate change 101
• What is climate change detection and attribution?
  Why is it important?
• How do we study the causes of climate change?
  What is fingerprinting?
• Fingerprinting examples
• Are there Inconvenient observations?
• Looking towards the future
• Conclusions

16
Understanding different fingerprints The case of
the Sun
10
28
25
24
50
20
Height above Earths surface (kilometers)
Pressure (hPa)
16
100
12
200
300
8
500
4
700
90N
60N
30N
Eq
30S
60S
90S
-1.0
-0.6
-0.2
0.2
0.6
1.0
-1.2
-0.8
-0.4
0
0.4
0.8
1.2
Temperature change (degrees Celsius per century)
17
Different factors that influence climate have
different fingerprints
10
10
28
28
25
25
24
24
50
50
20
20
Pressure (hPa)
Height (km)
1. Solar
2. Volcanoes
100
100
16
16
12
12
200
200
300
300
8
8
500
500
4
4
700
700
Eq
30S
60S
90S
90N
60N
30N
90N
60N
30N
Eq
30S
60S
90S
10
10
28
28
25
25
24
24
3. Well-mixed greenhouse gases
50
50
20
20
4. Ozone
Height (km)
Pressure (hPa)
100
100
16
16
12
12
200
200
300
300
8
8
500
500
4
4
700
700
Eq
30S
60S
90S
90N
60N
30N
90N
60N
30N
Eq
30S
60S
90S
10
28
25
24
50
20
5. Sulfate aerosol particles
Height (km)
16
100
Pressure (hPa)
12
200
300
8
500
4
700
Eq
30S
60S
90S
90N
60N
30N
-1
-0.6
-0.2
0.2
0.6
1
Santer et al., CCSP Report (2006)
C/century
-1.2
-0.8
-0.4
0
0.4
0.8
1.2
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Fingerprinting with temperature changes in
Earths atmosphere
Model Changes CO2 Sulfate Aerosols
Stratospheric Ozone
50


18
100


14
Height (km)
Pressure (hPa)


200
10
300


6
500


2
850






60S
45S
30S
15S
0
15N
30N
45N
60N


Observed Changes


Height (km)
Pressure (hPa)







Temperature changes in oC
-1.5
-0.9
-0.3
0.3
0.9
1.5
-1.8
-1.2
-0.6
0
0.6
1.2
1.8
Santer et al., Nature (1996)
19
Mythology 101 Changes in the Suns energy output
explain ALL observed warming
CCSP Unified Synthesis Product (2009)
20
Fingerprinting in the ocean Warming of the
worlds oceans over 1955-99
Red Observed Green Model run with human
factors Blue Model control run
Barnett et al., Science (2005)
21
Fingerprinting in the ocean Warming of the
worlds oceans over 1955-99
Green Model run with human factors
Blue Model control run
Red Observed
Depth (m)
Depth (m)
Temperature change (C)
Temperature change (C)
Depth (m)
Depth (m)
Depth (m)
Temperature change (C)
Temperature change (C)
Temperature change (C)
Barnett et al., Science (2005)
22
Fingerprinting with changes in the amount of
water vapor over oceans
Santer et al., PNAS (2007)
23
The climate system is telling us a
physically-consistent story. We have identified
human fingerprints in

• TEMPERATURE FIELDS
• Global-scale surface temperatures (Santer et al.,
  1995 Hegerl et al., 1996, 1997 North and
  Stevens, 1998 Stott and Tett, 1998 Tett et al.,
  1999, 2002 Stott et al., 2000, 2006)
• Regional-scale surface temperatures (Stott, 2003
  Zwiers and Zhang, 2003 Min et al., 2005 Karoly
  and Wu, 2005 Knutson et al., 2006 Bonfils et
  al., 2007, 2008)
• Vertical profiles of atmospheric temperature
  (Santer et al., 1996 Tett et al., 1996 Allen
  and Tett, 1999 Forest et al., 2001, 2002 Thorne
  et al., 2002 Tett et al., 2002 Jones et al.,
  2003)
• Global ocean heat content (Barnett et al., 2001)
• MSU stratospheric and tropospheric temperatures
  (Santer et al., 2003)
• The height of the tropopause (Santer et al.,
  2003, 2004)
• Vertical structure of upper-ocean temperatures
  (Barnett et al., 2005 Pierce et al., 2006)
• SSTs in hurricane formation regions (Santer et
  al., 2006 Gillett et al., 2008)
• Arctic and Antarctic temperatures (Gillett et
  al., 2008)

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Weve moved beyond temperature only fingerprint
detection studies

• ATMOSPHERIC CIRCULATION, SEA-ICE, AND THE
  HYDROLOGICAL CYCLE
• Sea-level pressure (Gillett et al., 2003)
• Continental-scale runoff (Gedney et al., 2006)
• Atmospheric water vapor over oceans (Santer et
  al., 2007, 2009)
• Surface specific humidity (Willett et al., 2007)
• Zonal-mean precipitation (Zhang et al., 2007)
• Hydrologically-relevant climate variables in the
  western U.S. (Barnett et al., 2008 Pierce et
  al., 2008 Hidalgo et al., 2009)
• Arctic sea-ice extent (Min et al., 2008)

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Structure of talk

• Climate change 101
• What is climate change detection and attribution?
  Why is it important?
• How do we study the causes of climate change?
  What is fingerprinting?
• Fingerprinting examples
• Are there Inconvenient observations?
• Looking towards the future
• Conclusions

26
No history of detection and attribution work
would be complete without discussion of the
great MSU debate
Inconvenient observations the apparent lack
of tropospheric warming in satellite data
satellite measurements over 35 years show no
significant warming in the lower atmosphere,
which is an essential part of the global-warming
theory.
James Schlesinger (former U.S. Secretary of
Energy, Secretary of Defense, and Director of the
CIA), Cold Facts on Global Warming, L.A. Times,
January 22, 2004
27
Using microwave sounders to measure atmospheric
temperature from space
Figure and text courtesy of Carl Mears, RSS

• Higher temperatures more microwave emissions
  from oxygen molecules
• By choosing different microwave frequencies,
  different layers in the atmosphere can be
  measured
• Much of the scientific focus has been on
  measurements of the temperature of the lowest 78
  km of the atmosphere

28
Which groups have been involved in constructing
Climate Data Records from MSU information?

• University of Alabama at Huntsville (UAH)
• John Christy and Roy Spencer
• Remote Sensing Systems (RSS)
• Frank Wentz and Carl Mears
• University of Maryland (UMd)
• Konstantin Vinnikov, Norm Grody
• NOAA National Environmental Satellite, Data, and
  Information Service
• Cheng-Zhi Zou, Mitch Goldberg, et al.

29
The UAH satellite dataset implied that the
troposphere cooled as the tropical surface warmed
30
The RSS satellite data showed that the
troposphere warmed by more than the surface
31
What factors contribute to these differences?

• Local measurement time for each satellite drifts
  due to orbital drift
• This leads to drifts in the sampling of the
  Earths daily temperature cycle
• These drifts need to be removed, or they can
  affect long-term trends

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22
NOAA-8
NOAA-6
NOAA-10
NOAA-12
20
Ascending LECT (Hrs.)
18
NOAA-6
16
TIROS-N
14
NOAA-7
NOAA-9
NOAA-11
NOAA-14
12
1980
1985
1990
1995
2000
Year
Figure and text courtesy of Carl Mears, RSS
32
Three papers in Science partially resolved the
great MSU debate
An early satellite-based analysis of the
temperature of the tropical troposphere has a
spurious cooling trend
Weather balloon estimates of the temperature of
the tropical troposphere also contain a spurious
cooling trend
When errors in the satellite and weather balloon
data are accounted for, both models and
observations show warming of the tropical
troposphere relative to the surface
33
Resolution?
Previously reported discrepancies between the
amount of warming near the surface and higher in
the atmosphere have been used to challenge the
reliability of climate models and the reality of
human-induced global warming This significant
discrepancy no longer exists (from Preface of
U.S. Climate Change Science Program Report, May
2006)
34
Structure of talk

• Climate change 101
• What is climate change detection and attribution?
  Why is it important?
• How do we study the causes of climate change?
  What is fingerprinting?
• Fingerprinting examples
• Are there Inconvenient observations?
• Looking towards the future
• Conclusions

35
Looking towards the future

• In a post-IPCC AR4 world, is the science done
  and dusted?
• What will the role of detection and attribution
  research be in AR5?

36
Key scientific issues for future detection and
attribution (DA) studies

• Most fingerprint work has focused on global-scale
  changes in individual, primary climate
  variables
• Can we identify human effects on climate at
  continental to regional scales?
• Can we identify human fingerprints in variables
  of direct relevance to climate-change impacts?
  (e.g., timing of stream flow, snowpack depth)
• Can we attribute shifts in the distributions of
  plant and animal species to human influences?
  (the double attribution problem)

37
Key scientific issues for future detection and
attribution (DA) studies

• We now live in a multi-model world, yet most DA
  studies to date have been performed with
  individual models
• Is it a model democracy (One model, one vote?)
  Or should we pay more attention to models that do
  a better job in capturing aspects of present-day
  climate that we care about?

38
Key scientific issues for future detection and
attribution (DA) studies

• We cannot confidently attribute any specific
  extreme event to human-induced climate change
• But can we make informed scientific statements
  about the influence of human activities on the
  likelihood of extreme events? (the operational
  attribution issue)

39
Evaluation of Fractional Attributable Risk
Risk of European heatwave exceeding 1.6C
threshold with and without human influence
(Stott, Stone, and Allen, Nature, 2004)
0.8
0.6
Average simulation omitting human influence
0.4
Estimated likelihood
Average model simulation with combined human and
natural effects
0.2
0
1
4
10
20
Number of occurrences per 1,000 years
40
Evaluation of Fractional Attributable Risk
Risk of European heatwave exceeding 1.6C
threshold with and without human influence
(Stott, Stone, and Allen, Nature, 2004)
0.8
0.6
Average simulation omitting human influence
0.4
Estimated likelihood
Average model simulation with combined human and
natural effects
0.2
Can we do this type of analysis with other
extreme events?
0
1
4
10
20
Number of occurrences per 1,000 years
41
Structure of talk

• Climate change 101
• What is climate change detection and attribution?
  Why is it important?
• How do we study the causes of climate change?
  What is fingerprinting?
• Fingerprinting examples
• Are there Inconvenient observations?
• Looking towards the future
• Conclusions

42
Conclusions

• We have identified human fingerprints in a
  number of different aspects of the climate system
• We have moved beyond temperature only detection
  and attribution
• Criticisms leveled at IPCC Second Assessment
  Report (you are only looking at surface
  temperature changes) are no longer valid
• The climate system is telling us a physically-
  and internally-consistent story
• The storys message Natural causes alone cannot
  explain the observed changes
• Many scientists at dozens of universities and
  research institutions around the world have
  helped to tell this story

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QA SLIDES

• QA SLIDES

53
THE SUN

• THE SUN

54
ICE AGES

• ICE AGES

55
Mythology 101 Atmospheric CO2 levels lag
temperature levels, so CO2 is not an important
greenhouse gas
Interglacial
Interglacial
Interglacial
Interglacial
Interglacial
CO2
CH4
Proxy for temperature
IPCC Fourth Assessment Report (2007)
56
There are important changes in Earths orbital
parameters on Ice Age time scales
P (Precession) Change in direction of Earths
axis of rotation T (Tilt) Axial tilt
(obliquity) of the Earth E (Eccentricity) How
elliptical Earths orbit is
IPCC Fourth Assessment Report (2007)
57
These slow changes in orbital parameters cause
slow changes in Earths climate
There are periodic variations in Earths orbit
These variations affect the amount and the
distribution (both seasonal and latitudinal) of
solar radiation reaching Earths surface

• On Ice Age timescales, CO2 is a feedback,
  amplifying changes in orbital forcings
• Cooler ocean can absorb more CO2 from the
  atmosphere
• Cooling during an Ice Age reduces CO2 emissions
  from vegetative decay

58
Since the Industrial Revolution, CO2 has been a
forcing, not a feedback

• Changes in Earths orbital parameters are not
  important over the past 150 years
• They do not operate on such short timescales
• The rapid increase in atmospheric CO2 since the
  Industrial Revolution is primarily due to the
  burning of fossil fuels
• For the first time, CO2 and other greenhouse
  gases are primarily acting as forcings in the
  climate system, instead of as a feedback to
  orbital forcing
• The lag between temperature and greenhouse gases
  in ice core records does not cast doubt on the
  importance of CO2 as a forcing of recent changes
  in Earths climate

59
GLOBAL-MEAN TEMPERATURES

• GLOBAL-MEAN
• TEMPERATURES

60
Computer models can perform the control
experiment that we cant do in the real world
Average surface temperature change (C)
Meehl et al., Journal of Climate (2004)
61
Computer models can perform the control
experiment that we cant do in the real world
Average surface temperature change (C)
Meehl et al., Journal of Climate (2004)
62
WATER VAPOR

• WATER VAPOR