Attribution of Changes

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Attribution of Changes
Primary Source IPCC WG-I Chapter 9 -
Understanding and Attributing Climate Change
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What is climate?
Climate is usually defined as the average
weather, or more rigorously, as the statistical
description in terms of the mean and variability
of relevant quantities over a period of time
ranging from months to thousands or millions of
years. WMO period 30 years
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What is climate change and variability?
Climate change refers to a change in the state of
the climate that can be identified (e.g., by
using statistical tests) by changes in the mean
or variability of its properties, and that
persists for an extended period, typically
decades or longer. Climate variability refers to
variations beyond individual weather events in
the mean state and other statistics of the
climate (such as standard deviations, the
occurrence of extremes, etc.) on all spatial and
temporal scales. This includes internal
variability.
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What is detection and attribution?
Detection of climate change is the process of
demonstrating that climate has changed in some
defined statistical sense, without providing a
reason for that change. Attribution of causes
of climate change is the process of establishing
the most likely causes for the detected change
with some defined level of confidence.
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Net RF 1750 to 2005(from Chapter 2)
Note uncertainties
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Atmospheric Temperature Changes - PCM Model
solar only
volcanic only
GHGs only
O3 only
direct sulphate aerosol
all
1890 to 1999
(C/100Y)
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Reflected Solar Radiation
Satellites
Models
Deceasing Cloud Cover?
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NH ?T Reconstructions vs. EBM
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GCM Simulations Natural Forcings
19 Simulations 5 Models
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GCM Simulations All Forcings
58 Simulations 14 Models
Internal variability?
Aerosols?
Solar, volcanic effects?
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Latitudinal Distribution of Trends
Red All forcings, middle 90 of GCM range (58
simulations by 14 GCMs) Blue Natural forcings
only, middle 90 of GCM range (19 simulations by
5 GCMs)
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Land Ocean Changes
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IPCC Statement
IPCC Most global warming very likely due to
increase in anthropogenic GHG concentrations Likel
y anthropogenic warming on continental scale
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Spatial Distribution of Trends
58 Sim. 14 GCMs
19 Sim. 5 GCMs
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Changes in Tropopause Height
ERA40 Observation-based analysis Model results
(each 4 simulations by PCM) Red - All
forcings Blue - Natural forcings only
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Ocean-Change Signal Strength
Observed Signal Red dots Model results
(individual basins from PCM) Green - All
forcings Blue - Natural forcings only
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Methods of Detection - Optimal Fingerprinting
Linear Regression
Internal variability (noise)
Observed climate patterns
Scale factors
solar
volcanic
Response patterns to forcings - from a GCM
GHGs
O3
Earlier slide
All
Aerosol
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Methods of Detection - Inferring Change
Are at least some of the scale factors
significantly different from zero?
DETECTION
Is a forcing distinguishable from others? Can it
(or some combination) explain the observed
pattern?
ATTRIBUTION
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Scale Factors - 4 GCMs
Bars 5-95 uncertainty range Red GHGs Green
Other human factors Blue Natural
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Inferred Climate Change - 4 GCMs
1990s minus 1900s (decades)
Observed Black line
Trend 1950 - 1999
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Can Greenhouse Warming Cause a Climatic Extreme?
Summer temperatures in Switzerland
A single event cannot be attributed to a forcing
factor, but
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Probability of extreme European summer
temperatures(using the Hadley Centre GCM)
No anthropogenic change
With anthropogenic change
the probability (risk) of an occurrence can
change due to external influences.
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END
Attribution of Changes