Transcript Document

NATS 101
Lecture 32
Climate Change (cont’d)
Outline
• IPCC summary from Kevin Trenberth
• Current energy flow and balance
• How changing the GHG concentrations
causes climate to change
– Stephen Boltzmann law (reminder)
• Feedbacks and subsequent changes
Role of the IPCC:
The role of the IPCC is to assess on a
comprehensive, objective, open and transparent
basis the scientific, technical and socioeconomic information relevant to understanding
the scientific basis of risk of human-induced
climate change, its potential impacts and options
for adaptation and mitigation.
Review by experts and governments is an
essential part of the IPCC process.
1988 - The establishment of the IPCC
WMO, UNEP
1990 - First IPCC Assessment Report
1992 - IPCC Supplementary Reports
1992- Adoption of the UNFCCC
1994- Entry into force of the UNFCCC
Ratified by 189 countries
1994 - IPCC Special Report
1995 - Second IPCC Assessment Report
1996 - COP-2, 1997 - COP-3
1997- Adoption of Kyoto Protocol at COP-3
2005 Feb 16- Kyoto Protocol ratified by 164 countries
(But not by USA or Australia)
2001 - Third IPCC Assessment Report
2002 - COP-8, 2003 - COP-9
2007 - Fourth IPCC Assessment Report
Scenarios of future emissions
of greenhouse gases, aerosols
Scenarios of future concentrations
of greenhouse gases and aerosols
Mitigation
Policy options
Adaptation
Assessment of observations,
processes and models
Impacts
Projections of future climate:
The response, global, regional
A major strength of the IPCC process
has been the intergovernmental process, through
reviews and then approval of the Summary for Policy
Makers on a word-by-word basis.
But it has also been subject to criticism as it is much
more political. In principle, this process is designed to
provide a report in which the content is determined by
the science while how it is stated is determined jointly
with the governments. Hence it aids communication
between scientists and politicians.
IPCC 2007 Report on Climate
 The recent IPCC report has clearly
stated that “Warming of the climate
system is unequivocal” and it is “very
likely” caused by human activities.
 Moreover, most of the observed
changes are now simulated by climate
models over the past 50 years adding
confidence to future projections.
IPCC report online
Nobel Peace Prize
2007:
The Nobel Peace Prize goes to the Intergovernmental
Panel on Climate Change (IPCC) and Albert Arnold (Al)
Gore Jr. "for their efforts to build up and disseminate
greater knowledge about man-made climate change, and
to lay the foundations for the measures that are needed
to counteract such change".
GLOBAL Energy Flow Thru Atmosphere
Global Atmo Energy Balance
In a stable climate, Solar Energy IN = IR Energy OUT
IR Out
Ahrens, Fig. 2.14
Solar in
The Natural Greenhouse Effect: clear sky
O3
8%
Carbon
Dioxide
26%
CH4
N20
6%
Water Vapor
Water
Vapor
60%
Carbon Dioxide
Ozone
Methane,
Nitrous Oxide
Clouds also have a greenhouse effect
Kiehl and Trenberth 1997
Changing CO2 concentrations
• CO2 concentrations have varied naturally by a factor of
2 over the past few hundred thousand years
• Fossil fuel burning since the industrial revolution has
created a sharp increase in CO2 concentrations
• CO2 concentrations are now higher than at any time in
past few hundred thousand years
• And concentrations are increasing faster with time
Last 4 Ice Age cycles:
400,000 years
Man made
You are here
See http://epa.gov/climatechange/science/recentac.html
Global Atmo Energy Imbalance
Increasing GHG concentrations decrease Energy out
So Energy IN > Energy OUT and the Earth warms
IR Out
is reduced
Ahrens, Fig. 2.14
Solar in
Atmosphere
Radiative Forcing (RF) Components
{Global-average estimates and ranges; typical geographical
extent and assessed level of scientific understanding}
Stefan-Boltzmann’s Law
(review from Lecture 5)
• The hotter the object,
the more radiation
emitted.
• When the temperature
is doubled, the emitted
energy increases by a
factor of 16!
• Stefan-Boltzmann’s
Law
E= (5.67x10-8 Wm-2K-4 )xT4
E=2x2x2x2=16
4 times
(T is temperature in Kelvin)
Sun Temp: 6000K
Earth Temp: 300K
Aguado, Fig. 2-7
Change in IR Emission to Space
• Notice that because of Earth’s greenhouse gases, 91%
(=64/70) [195/235 = 83%] of the IR emitted to space comes
from the atmosphere and only 9% (=6/70) [40/235 = 17%]
comes from the surface
• When GHG’s are added to the atmosphere, the altitude of
IR emission to space rises
• In the troposphere, air temperature decreases with altitude
• So the temperature of the emission to space decreases
• So the energy emission to space decreases because the
emission energy decreases with decreasing temperature
Change in IR Emission to Space
BEFORE GHG increase IN=OUT
AFTER GHG increase IN>OUT
IR emission
to space
3. IR emission to
space decreases
because of colder
emission
temperature
Altitude
tropopause
NHAltitude of IR
SH
Ahrens, Fig. 2.21
emission to
space
Temperature
Temperature
of IR emission
to space
Temperature
1. Altitude of
IR emission to
space rises
2. Temperature of
IR emission to
space decreases
Change in IR Emission to Space (cont’d)
AFTER GHG increase IN>OUT
Eventual solution IN=OUT
6. IR emission to
space increases
until it matches
the original IR
emission before
GHG increases
3. IR emission to
space decreases
because of colder
emission
temperature
SH
Ahrens, Fig. 2.21
Temperature
1. Altitude of
IR emission to
space rises
2. Temperature of
IR emission to
space decreases
SH
Ahrens, Fig. 2.21
4. Atmosphere
warms until…
5. Temperature of IR
Temperature emission to space
increase to original
temperature
Anthropogenically-Caused Warming
• Initially after increasing GHG concentrations, the IR
radiation to space decreases, such that
Solar in > IR out
– Causing the Earth to start warming
• IF GHG concentrations level off at some point, then
eventually the Earth warms enough that
Solar in = IR out
– “Eventually” depends on how fast the oceans warm
• The warmer Earth represents a new climate regime
– With bad and good consequences that we partially understand
Complexity of Climate System
The climate system involves numerous, interrelated components.
Closer Look at Climate System
Climate Feedback Mechanisms
Positive and Negative Feedbacks
Assume that the Earth is warming.
• Warming leads to more melting of ice
• Less ice reduces Earth’s albedo
• Earth absorbs more sunlight
• Earth becomes warmer melting more ice
Works in the other direction as well:
Cooling makes more ice which reflects sunlight which
makes Earth colder which makes more ice
=>’Positive’ Feedback Mechanism
Positive and Negative Feedbacks
Again assume that the Earth is warming.
• Suppose as the atmosphere warms and moistens,
more low clouds form.
• More low clouds reflect more solar radiation,
which decreases solar heating at the surface.
• This slows the warming, which would counteract
a runaway greenhouse effect on Earth.
‘Negative’ Feedback Mechanism
Positive and Negative Feedbacks
• Atmosphere has a numerous checks and
balances, some that counteract climate
changes and some that enhance
changes
• All feedback mechanisms operate
simultaneously.
• The dominant effect is difficult to predict
with complete certainty.
• Cause and effect is very challenging to
prove.
Complexities of GHG caused Warming
So, as the Earth starts warming, other things start to change…
• The atmosphere can hold more water vapor
– Water vapor is a GHG => more warming (‘positive’ feedback)
– More intense precipitation events and severe weather
• Land heats up faster than the oceans (remember sea breeze)
– Less snowpack => earlier and smaller run-off => drier summers
– Relative humidity over land will likely drop in general
– Continental interiors will generally get drier?
• Clouds will change?????
– Having more clouds increases albedo, cooling the Earth
– More clouds increase Greenhouse effect warming the Earth
• Wind patterns start changing
– Storm tracks and precipitation patterns change.
– Winter storm tracks may move more poleward
• Ice starts melting
– Reducing the albedo => more warming
– Continental ice melt causes sea level to rise
SVP and Temperature (from Lecture 9)
Claussius Clapeyron
equation defines
water vapor
saturation vapor
pressure (SVP)
versus temperature
SVP defines water
holding capacity of air
SVP nearly doubles
with a 10oC warming
Ahrens Fig. 4.5
Water vapor increase at higher temperatures
Water holding capacity of atmosphere increases about 7%
per oC (4% per F) increase in temperature.
Observations show that water vapor in LOWER troposphere
is indeed increasing. Surface temperature increase: 0.6C
since 1970 over global OCEANS and 4% more water vapor.
Total water vapor
Since late 1970’s, ocean surface has been warming at
~0.14 C/decade => ~1% WV increase/decade. Observed WV
trend since 1988 is ~1.2% per decade
From Trenberth/IPCC
The Natural Greenhouse Effect: clear sky
O3
8%
Carbon
Dioxide
26%
CH4
N20
6%
Water Vapor
Water
Vapor
60%
Carbon Dioxide
Ozone
Methane,
Nitrous Oxide
Clouds also have a greenhouse effect
Kiehl and Trenberth 1997
Upper Tropospheric Water Vapor Trends
• Far less water vapor as in lower troposphere because
temperatures are much colder (SVP much less)
• Fractional changes in UT WV are almost as important as
in LT WV because temperatures are much colder
• so IR emitted from upper troposphere is small
reducing outgoing IR to space
• Climate models generally predict UT WV should increase
as climate warms
• Very challenging to measure UT WV
• At least indirect inferences that UT WV is increasing
• Some doubt about the robustness of this result and its
generality
Observational Evidence of Water Vapor Feedback
•
•
•
•
Increasing water vapor concentrations shift the altitude of water IR emission upward which
DECREASES its emission (because of colder temperatures)
Atmospheric temperatures have also been increasing over time (at least in theory) which
should INCREASE the IR emission from water vapor
To isolate the water vapor concentration change, Subtract the IR measured from water
(=‘T12’ from HIRS) from emission measured from O2 (whose concentrations have not
changed) (=‘T2’ from MSU)
If IR from water is becoming less than emission from O2, the atmospheric water
concentrations have increased
=> THIS IS WHAT IS OBSERVED
Soden et al. 2005
Land surface temperatures are rising faster than SSTs
SST
Land
Annual anomalies of global average SST and land surface air temperature
Land increased 0.4oC vs ocean suggesting 3% decrease in RH over land
Trenberth/IPCC
Evidence for reality of climate change
Glaciers melting
Muir Glacier, Alaska
1909
Toboggan
Glacier
Alaska
2000
1900
2003
Alpine glacier, Austria
Snow cover and Arctic sea ice are decreasing
Arctic sea ice
area decreased by
2.7% per decade
(Summer:
-7.4%/decade)
2007: 22% (106 km2)
lower than 2005
Spring snow cover
shows 5% stepwise
drop during 1980s
Trenberth/IPCC
Arctic sea ice disappears in summer by 2050
Already 2007 lowest on record by 22%
Abrupt Transitions in Summer Sea Ice
2007
x
• Gradual forcing results in abrupt Sept ice
decrease
• Extent decreases from 80 to 20%
coverage in 10 years.
• Relevant factors:
• Ice thinning
• Arctic heat transport
• Albedo feedback
Trenberth from Holland et al., GRL, 2006
Sea level is rising:
from ocean expansion and melting glaciers
Since 1993
Global sea level
has risen 43 mm
(1.7 inches)
• 60% from
expansion as
ocean
temperatures
rise,
• 40% from
melting glaciers
from Steve Nerem via Trenberth
Research indicates that less than 8°F of Arctic
warming caused Greenland to lose enough water
to raise sea level by up to 12 feet during the
Last Interglacial Period
Today
125,000 years ago
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Image from Bette Otto-Bliesner, National Center for Atmospheric Research
1 meter
2 meters
www.gfdl.noaa.gov
4 meters
8 meters
Surface melt on Greenland
Increasing melt zones.
Melt descending into a
moulin: a vertical shaft
carrying water to the base
of the ice sheet.
NSIDC (above)
Braithwaite: Univ. Manchester
Greenland ice mass budget
A great deal of effort is going into
estimating how the Greenland ice
sheet is changing
Researchers
Mass Change Method
Time Span
(GT/year)
Krabill et al. 2000
-47
Aircraft Surveys 1994-1999
Velicogna et al. 2006 -200 to -260
GRACE
2002-2006
Luthcke et al. 2007* -145 to -175
GRACE 2003-2006
Zwally et al. 2007*
-80 to -100
ICESat
2003-2005
GRACE is a gravity recovery mission
ICESat is a lidar topographymission
NASA website on Greenland ice sheet
100 GT/yr ~ 0.3 mm/yr sea level rise
Key Points: Climate Change
• Increasing GHG concentrations warms the Earth
• Proxy data are used to infer the past climate.
• Data show that the Earth’s Climate
Has changed in the past
Is changing now
And will continue to change
• Key question is determining whether recent
changes are due to natural causes or man.
Key Points: Climate Change
• The climate system is very complex.
Contains hundreds of feedback mechanisms
All feedbacks are not totally understood.
• Three general climate change mechanisms:
Astronomical
Atmospheric composition
Earth’s surface
Assignment for Next Lectures
• Topic - Anthropogenic Climate Change
• Reading - Ahrens, Ch 14: 373-400 (383412)
• Problems - 14.5, 14.8, 14.10, 14.12, 14.13,
14.15, 14.16, 14.19