Kim M. Cobb

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The science of global warming
Kim M. Cobb kcobb_at_eas.gatech.edu Sustainable
Atlanta Roundtable September 8, 2006
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versus The politics of global warming or The
economics of global warming
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greenhouse gases in the atmosphere trap heat at
the Earths surface and prevent it from escaping.

• These gases include
• Carbon dioxide CO2
• Methane CH4
• Nitrous oxide N2O
• Chlorofluorocarbons
• Water vapor H2O
• (this is the most important one, by far!)

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without greenhouse gases average temp of Earth
would be -18ºC instead of 15ºC
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Charles Keeling
atmospheric CO2 measurements show that CO2 has
been increasing since at least the mid 1950s
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• ice core CO2 records confirm that the CO2 trend
  began in the 1800s
• clear land for agriculture
• Industrial Revolution

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The instrumental record of climate shows a 1ºC
warming over the last century
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Why do 99.999 of climate scientists believe
that anthropogenic CO2 is warming the planet?

• Theory predicts that increasing atmospheric CO2
  should warm the planet.
• Geologic evidence links CO2 and temperature in
  the past.
• The warming is unprecedented in the most recent
  centuries (dwarfs natural variability).
• Climate models show that rising CO2 is necessary
  to simulate
• 20th century temperature trends (solar and
  volcanic minor players).

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Atmospheric CO2 and temperature over the past 650
thousand years
CO2 and temperature are closely linked on
geologic timescales 80ppm CO2 5ºC
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To understand how climate has changed in the
past, we need to use records of climate preserved
in ice cores, ancient tree rings, coral bands,
and other paleoclimatic sources key is to
CALIBRATE to temperature records
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The Hockey Stick
Mann et al., 1999
Key Points error bars increase as you go back in
time natural variability accounts for lt0.5ºC over
the last millennium late 20th century temperature
trend is unprecedented
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solar volcanic only
anthropogenic only
Intergovernmental Panel on Climate Change (IPCC)
2001
solar volcanic anthro
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The uncertain climate future
Modelled global temperature rise
Modelled sea level rise
Scientific Focus
no longer if but how much?, when?,
where? ? regional climate responses? ?
precipitation responses?
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SST
Are hurricanes gaining strength as the tropical
oceans warm? - total not increasing -
intensity of Category 4, 5 storms increasing

• Comments
• this work does not prove GW-hurricane
• causal link
• weak 2006 season does not
• disprove GW-hurricane link

Webster et al., 9/16/05
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Summary Strong, irrefutible evidence supports
the idea that anthropogenic CO2 is warming
the planet. Sea level rise, temperature
increases inevitable (we are not in
equilibrium with current CO2 levels). Future
climate changes in a warming environment are
uncertain - prospect for abrupt climate change
(theyve happened before) -prospect for
increasing storm activity/extreme events -
changes in precipitation patterns certain (but
where, how much, etc?)
My homepage http//shadow.eas.gatech.edu/kcob
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