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Global Warming - 1
An Assessment
The balance of the evidence ...
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 Greenhouse
gases warm the earth,
increasing evaporation.
 Do you think changes in cloudiness will
cool or warm the earth? What is your
reasoning?
Key Points
Scientific assessment describes present state of
knowledge
 Global temperature has increased during 20
century, continuing through the 1990s

–

beyond reasonable doubt ( virtually certain)
Some of this increase is due to human activities
– the balance of the evidence (likely)
Impacts are complex, vary with region, very
difficult to quantify reliably
 Any action to limit future increases will take
decades to become effective

– certain (virtually certain)
Scientific Assessments
There are accepted standards of what
constitutes scientific evidence
 Review and discussion by knowledgeable
peers establishes consensus

–
–
–

What is thoroughly established
What is plausible, but definitive evidence lacking
What is poorly understood or legitimately
controversial
Scientists are paid to be skeptical. Some will
always disagree
The Intergovernmental Panel
on Climate Change

IPCC95 Cover
Prediction IPPC 1990 Report


Latest predictions show less warming
Uncertainties were underestimated

IPCC90 Fig 8
Notes on
Science Assessment
 Current
–
consensus on attribution
Balance of the evidence
 Regional
predictions much less secure
than global average
 Precipitation predictions much less
secure than temperature
 Impacts on ecosystems, agriculture, etc
only in general terms
Judgements of Confidence
IPCC Third Assessment Report-2001
Summary for policymakers - Footnote 7
 Statement
–
–
–
–
–
–
–
Virtually Certain
Very Likely
Likely
Medium Likelihood
Unlikely
Very unlikely
Exceptionally
unlikely
 Chance
–
–
–
–
–
–
–
> 99%
90-99%
60-90%
33-66%
10-33%
1-10%
< 1%
True
Key Points
Scientific assessment describes present state of
knowledge
 Global temperature has increased during 20
century, continuing through the 1990s

–

beyond reasonable doubt ( virtually certain)
Some of this increase is due to human activities
– the balance of the evidence (likely)
Impacts are complex, vary with region, very
difficult to quantify reliably
 Any action to limit future increases will take
decades to become effective

– certain (virtually certain)
Global Average Temperature
Is this a natural fluctuation or attributable to
human activities?
 Sept 1998. Every month for past 19 has been
a record high!

Past NH Summer Temperature

tree rings, ice cores,documents

IPCC95 Fig 10
IPPC 3rd Assessment Report-1
Global temperature increased over the 20th
Century by about 0.6o C :
virtually certain
 This increase is largest for any century within past
100 years:
likely
 Snow cover and ice extent decreased
– Snow by 10% since 1960s
very likely
– Lakes & rivers by 2 wks since 1900 very likely
– Arctic sea-ice by 40% since 1950s
likely
 Global average sea level rose 10-20 cm (4-8
inches) during 20th century
 Cloud cover increased 2% over mid-high latitude
land during 20th century
likely

Key Points
Scientific assessment describes present state of
knowledge
 Global temperature has increased during 20
century, continuing through the 1990s

–

beyond reasonable doubt ( virtually certain)
Some of this increase is due to human activities
– the balance of the evidence (likely)
Impacts are complex, vary with region, very
difficult to quantify reliably
 Any action to limit future increases will take
decades to become effective

– certain (virtually certain)
What Do the Models Say?
Partially Tuned Simulations

What happened 1940-1970?

IPCC95 Fig 15
Model Tuning
Models are simplified representations of
reality
 They represent what are believed to be the
most significant processes
 Representations sometimes involve
coefficients which must be empirically
determined
 Tuning = adjusting coefficients within
plausible ranges to achieve desired model
output

Is the Aerosol Effect Real?
We think we understand it but could be
surprised
 Radiative forcing per unit sulfate aerosol
uncertain a priori
 Good simulations of Mt. Pinatubo
–
aerosol in stratosphere, not troposphere
 Warming
–
in sulfate areas only at night
observed and modeled
 Better
agreement for global rise for past
100 years
–
some tuning
After Eruption of Mt. Pinatubo

IPCC95 Fig 14
Sensitivity to Aerosols -1
for 3 values of cloud feedback

greenhouse gases only

IPCC95 Fig 16a
Sensitivity to Aerosols -2
for 3 values of cloud feedback

greenhouse gases
and aerosols

IPCC95 Fig 16b
Estimated Current Forcing
 IPCC
1995
Key Points
Scientific assessment describes present state of
knowledge
 Global temperature has increased during 20
century, continuing through the 1990s

–

beyond reasonable doubt ( virtually certain)
Some of this increase is due to human activities
– the balance of the evidence (likely)
Impacts are complex, vary with region, very
difficult to quantify reliably
 Any action to limit future increases will take
decades to become effective

– certain (virtually certain)
Impacts of Climate Change
Regional analysis is critical
 Current climate models disagree on regional
changes
 Changes in precipitation at least as important
as changes in temperature
 Impacts on ecosystems, water resources,
agriculture, extreme events (floods, droughts,
hurricanes,…). Some beneficial,vary with
region
 Other stresses use to land use, acid rain
 Less wealthy countries more vulnerable
 Need to build resilience to multiple stresses

Key Points
Scientific assessment describes present state of
knowledge
 Global temperature has increased during 20
century, continuing through the 1990s

–

beyond reasonable doubt ( virtually certain)
Some of this increase is due to human activities
– the balance of the evidence (likely)
Impacts are complex, vary with region, very
difficult to quantify reliably
 Any action to limit future increases will take
decades to become effective

– certain (virtually certain)
IPPC 3rd Assessment Report-2

Atmospheric CO2
– increased 31% since 1750
virtually certain
– present concentrations largest in 420,000 yr and likely
in 20,000,000 yr

3/4 anthropogenic emissions CO2 due to fossil
fuel burning, remainder mainly land use change
 Human
influences will continue to change
atmospheric composition throughout the
21st Century
 Global average temperatures are projected
to rise under all IPPC scenarios.
Changes in Radiative Forcing
Business as Usual Scenario
Projected Future Warming
Intermediate cloud feedback


For high or low feedback multiply by 1.8 or 0.6
Must examine credibility

IPCC95 Fig 18
Emissions Scenarios
CO2 Gt C /y
Concentration Scenarios
CO2 ppmv
Key Points
Scientific assessment describes present state of
knowledge
 Global temperature has increased during 20
century, continuing through the 1990s

–

beyond reasonable doubt ( virtually certain)
Some of this increase is due to human activities
– the balance of the evidence (likely)
Impacts are complex, vary with region, very
difficult to quantify reliably
 Any action to limit future increases will take
decades to become effective

– certain (virtually certain)
Sources of Information
 Horel
and Geisler Chapter 8
 Climate Change 1995 - Houghton et al
Cambridge University Press
 IPCC (Working Group 1) “Third
Assessment Report.” Summary for
Policy Makers
 IPPC “Regional Impacts of Climate
Change” http://www.grida.no/climate/ipcc/regional/
– Introduction