Transcript PowerPoint - Susan Schwinning

Earth’s Energy Balance
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A system is in energy balance (equilibrium) when incoming
energy equals outgoing energy. A system in balance does
not get colder or hotter on average.
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Earth receives energy as radiation from the sun (shortwave).
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Earth emits energy also as radiation: reflected shortwave and
longwave radiation.
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Changes in either energy input or output can lead to changes
in earth temperature, thus climate.
Stefan Boltzmann Law
All heated bodies emit energy proportional to the 4th power of
absolute temperature.
Hot bodies (sun)  high energy  short wavelengths
Colder bodies (earth)  lower energy  longer wavelengths
31% of incoming shortwave
radiation is back scattered.
49% is absorbed by the earth
surface.
20% is absorbed in the
atmosphere.
69% escapes as longwave
radiation.
31% +69% = 100%
Earth is approximately in
balance.
If more energy is absorbed, to
remain in balance, earth must
increase longwave emissions,
by increasing temperature.
From Chapin et al 2001
Humans have interfered with earth’s energy
balance in principally two ways:
1. Increasing the absorption of longwave radiation by
accumulation of greenhouse gases in the atmosphere.
2. Increasing the reflective properties of land cover by turning
dark forest into cropland.
Albedo:
The shortwave reflectance of a surface.
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Oceans and lakes
Sea ice
Fresh snow
Tundra
Conifer forest
Broadleaf forest
Desert
0.03-0.10
0.30-0.45
0.75-0.95
0.15-0.20
0.09-0.15
0.15-0.20
0.20-0.45
Greenhouse Gases
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Gases in the atmosphere that absorb longwave radiation,
including CO2, H2O, methane, ozone and others.
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Greenhouse gases are essential for life: without atmospheric
interception of backradiation, the planet would be much
colder (-18 °C).
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However, small changes in the concentration of greenhouse
gases an have large effects on earth temperature.
The Greenhouse effect
Radiative forcing:
A measure (in units of energy) of change in earth’s
radiation budget.
A positive number implies a change towards greater
energy absorption and higher temperature.
A negative number implies the opposite.
Greenhouse gases compared by radiative
forcing
Gas
Preindustrial
level
Current level
Radiative forcing
(W/m2)
Carbon dioxide
280 ppm
387ppm
1.46
Methane
700 ppb
1745 ppb
0.48
Nitrous oxide
270 ppb
314 ppb
0.15
0
533 ppt
0.17
CFC-12*
* Dichlorodifluoromethane (“freon”)
2005 radiative forcings using 1990 baseline
Greenhouse gas emissions by country
CO2 emission per person per year
(in carbon equivalents)
USA: 5.7 t
France: 2.0 t
Average non-Annex 1 countries:
0.4 t (7% of US average)
William Ruddiman, a climate scientist at the University of Virginia in
Charlottesville, argues that the next ice age should have started 8000 ago, but
didn’t because of the farming activities of early farmer in Europe, India and
China, which released CO2 and methane into the atmosphere.
New Scientist 2005
Climate prediction is complex because of :
FEEDBACKS
A feedback is a process in which changing one quantity changes
a second quantity, and the change in the second quantity in turn
changes the first.
Positive feedbacks are amplifying:
Quantity 1
+
+
Quantity 2
Negative feedbacks are dampening.
-
Quantity 1
Quantity 2
Negative feedbacks in the earth-atmosphere system:
 Ocean solubility pump: more CO2 dissolves in water at higher CO2
concentrations in air
 Carbon fertilization effects: higher CO2 levels accelerate
photosynthesis , drawing more CO2 out of the air, portions of it are
sequestered in oceans and on land
Positive feedbacks in the earth-atmosphere system:
 Thawing permaforst in the artic releases methane, a greenhouse gas
 Warming peatlands release CO2
 Shrinking sea ice and glacial cover reflects less solar radiation
(albedo = reflectance of solar radiation)
 Ocean warming reduces solubility of CO2
 Warmer temperatures on water and land increases water
evaporation, increasing air humidity and the greenhouse effect
 The effect of increasing cloud cover is unclear
So is it getting warmer?
2005 AND 2010 TIED FOR WARMEST YEAR ON RECORD.
Before that the warmest year was 1998.
2002 and 2003 were the second and third warmest years, respectively
2004 was the forth warmest year on record.
Climate Prediction
 Done by global climate models, there are several (8) using different
approaches and parameters, and arriving at different conclusions.
 Tested by “hindcasting” – how good they are predicting past climate
that we know.
 The IPCC* weighs the prediction of these models according to their
ability to hindcast, putting together the most highly supported
scenario
*IPCC: Intergovernmental Panel of Climate Change
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An international body for the assessment of climate change
Established by the United Nations Environment program and the World Meteorological Organization
in 1988
Produce assessment reports every 5 years.
Reports have been puublished in 1990, 1995, 2001, 2007.
The organization was honored with the Nobel Peace Price in 2007.
Climate predictions depend strongly on “scenarios” =
assumptions about political response to climate change
The 2001 IPCC Report had 3 scenarios:
“Business as usual” prediction:
Global mean temperature could rise by 4° C by 2100.
The 2007 IPCC Report had more complex scenarios:
A1 story line: rapid economic growth, population peaks in mid-century, rapid
development of new and more efficient technologies, increased global equity.
A2 story line: heterogeneous world, nations progress at their own pace, global
population keeps growing.
B1 story line: convergent world as in A1, but with transformation into a service
and information economy and less material intensity.
B2 story line: focus on local solutions to social economic and environmental
sustainability.
Worst scenario: A1FI : rapid economic growth, globalization, fossil
fuel intensive development
Best scenario: B1:
sustainable and equitable growth, globalization
Model predictions vary, but they all predict very significant global
temperature rise (worst case, A1FI scenario).
Since 1979, the size of the summer polar ice cap has shrunk
more than 20 percent. (Illustration from NASA)
What could happen in the next 100 years
(rising sea levels):
Precipitation change through global warming:
Projection for Australia, 2030, A1B scenario,% change
Anticipated effects of unchecked climate
change by 2100:
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Melting of glaciers, polar caps and and sea ice.
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Global sea level rise by as much as 6 m.
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Redistribution of global climate zones, especially at high
latitudes)
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More extreme weather: longer droughts, more violent storms
(flooding)
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Massive population migrations and increases in civil war
activities.