Kim M. Cobb

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The Science of Climate Change
Kim M. Cobb kcobb_at_eas.gatech.edu
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• Which of the following are scientific statements?
• Reducing CO2 emissions would hurt the economy.
• 2) Improved technology is the best way to slow
  global
• warming.
• 3) A warming of 1ºC over the next 50yrs is
  dangerous.
• 4) Global temperatures were 5ºC colder during the
  Last
• Glacial Maximum.
• 5) Hurricane Katrina was caused by global warming.

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• Which of the following are scientific statements?
• Reducing CO2 emissions would hurt the economy.
• 2) Improved technology is the best way to slow
  global
• warming.
• 3) A warming of 1ºC over the next 50yrs is
  dangerous.
• 4) Global temperatures were 5ºC colder during the
  Last
• Glacial Maximum.
• 5) Hurricane Katrina was caused by 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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1
without greenhouse gases average temp of Earth
would be -18ºC instead of 15ºC
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2
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greenhouse gases trap heat because they absorb
radiation in the infrared range, according to
specific bond geometries and vibrational modes
(ex CO2 below)
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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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Who is responsible for CO2 emissions?
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Total CO2 emissions (in thousands metric tons/yr)
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Per capita CO2 emissions (in tons/per capita per
yr)
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The instrumental record of climate shows a 1ºC
warming over the last century
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A paleo perspective glacial-interglacial cycles
80 ppm
CO2 range 200 to 280ppm
Temperature range 5ºC
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But why doesnt an 80 ppm change in CO2
correspond to a 5 C change?
The climate system does not reach equilibrium
instantaneously Other processes can change the
equilibrium temperature.
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Why do 99.999 of climate scientists believe
that 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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Ice core climate and CO2 records
tiny gas bubbles in the ice trap ancient air
samples
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Atmospheric CO2 and temperature over the past 650
thousand years
CO2 and temperature are closely linked on
geologic timescales
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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
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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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Intergovernmental Panel on Climate Change (IPCC)
2001
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• Consequences of Global Warming (IPCC SPM-AR4)
• Reduced uptake of CO2 by land and ocean in warmer
  climate
• Rising sea levels (0.3 to 0.6m by 2100). at a
  minimum
• Ocean pH will decrease by 0.14 to 0.35 (already
  down 0.1)
• Snow cover will decrease, permafrost melt, sea
  ice melt
• Extreme events (temperature and precipitation)
  will become more frequent
• Tropical cyclones will become more intense
• Storm tracks will move poleward
• Rainfall will increase in the high latitudes,
  decrease in the subtropics
• Meridional overturning of Atlantic ocean will
  decrease

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The uncertain climate future
Range of CO2 emissions scenarios Strict
international agreements ? CO2 at 600ppm by
2100 390ppm today Mid-ground ? 850ppm by
2100 280ppm 1800 Business as usual ? 1550ppm
by 2100
Lower limit 1C by 2100 Upper limit 6.5C by
2100
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Projected temperature change global view
Take-homes -poles warm more -land warms
more -ocean warming patchy and complex
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Regional models use global model output, run at
high-resolution (5km) grid
Length of heat waves increase (
days/event) Peak temperatures increase
Diffenbaugh et al, 2005
US
http//www.usgcrp.gov/usgcrp/nacc/se-mega-region.h
tm
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Projected precipitation change global view
white models disagree color models mostly
agree stippled models agree
Projecting precipitation is VERY uncertain
business, yet extremely critical to human impacts.
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Projected precipitation change regional view
change in yearly average precipitation
dry days
heavy rain days
mm/day
days/yr
days/yr
Diffenbaugh et al, 2005
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IPCC says increase in hurricane
intensity likely (66)
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CERTAIN
Warming of 1-6C by 2100. Sea levels will rise
by 6 to 30 inches by 2100. Precipitation
patterns will change. More erratic
precipitation. Extreme events will increase,
hurricanes more intense. Prospect of abrupt
climate change.
UNCERTAIN
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Can we do anything about it?
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What is a country to do?

• There are only three (prudent) options
• use less energy
• - drive less, drive smaller (plug-in?) cars
• - conserve electricity
• - recycle, reuse
• 2) make clean energy
• - solar power, wind power, nuclear energy
• 3) take CO2 out of the sky (much more difficult)

. but how much at what cost?
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• Some clear lessons
• 1) efficiency makes
• gains from cellulosic
• biofuels likely (not
• corn ethanol!)
• 3) taking CO2 out of the sky
• is more costly than
• not putting it in the
• atmosphere to begin with