Transcript Notes - The University of Arizona Department of Atmospheric

NATS 101-06
Lecture 20
Anthropogenic
Climate Change
What is Climate Change?
• Climate change - A significant shift in the mean state
and event frequency of the atmosphere.
• Climate change has been a normal component of the
Earth’s natural variability.
• Climate change occurs over a wide range of time and
space scales.
• A plethora of evidence exists that indicates the climate
of the Earth has changed over time.
• Humans are radically altering the concentrations of
greenhouse gases in the atmosphere which is likely
causing a new type of climate change
Our changing climate:
Key Questions
• Climate modelers have predicted the Earth’s
surface will warm because of manmade
greenhouse gas (GHG) emissions
• So how much of the warming is manmade?
• How serious are the problems this is
creating?
• What, if anything, can and should we do?
Climate Change:
Changing Earth’s Energy Balance
• Changing the
– Incoming solar energy
– Outgoing energy (IR emitted to space)
– Internal transport of energy within the Earth;s
atmosphere and oceans
will cause a shift in climate
Global Energy Balance
In a stable climate, Solar Energy IN = IR Energy OUT
IR Out
Ahrens, Fig. 2.14
Solar in
Global Energy Imbalance
Increasing GHG concentrations decrease the IR Energy out
So Energy IN > Energy OUT and the Earth warms
IR Out
is reduced
Ahrens, Fig. 2.14
Solar in
Atmosphere
Change in IR Emission to Space
• Notice that because of the greenhouse gases in Earth’s atmosphere,
91% (=64/70) of the IR emitted to space comes from the atmosphere and
only 9% (=6/70) comes from Earth’s surface
• When additional GHG’s are added to the atmosphere, the altitude of IR
emission to space rises and less IR from the surface makes it to space
• Since air temperature in the troposphere decreases with altitude, the
temperature of the emission to space decreases
• Therefore Earth’s energy emission to space decreases because the IR
energy flux emitted by an object decreases with decreasing temperature
• Therefore total Energy flux IN > Energy flux OUT
• Earth warms until Energy flux IN = Energy flux OUT
Change in IR Emission to Space
BEFORE GHG increase: IN=OUT
AFTER GHG increase
Altitude
IR emission
to space
NHAltitude of IR
3. IR emission to
space decreases
because of colder
emission
temperature
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
increases to original
temperature
Change: Atmo IR Emission to Surface
Altitude
BEFORE GHG increase:
Energy DOWN=UP at surface
Atmospheric
IR emissionNH
to surface Altitude of IR
emission to
surface
Temperature
Temperature
of IR emission
to space
AFTER GHG increase
more IR emission into surface
SH
Ahrens, Fig. 2.21
Temperature
3. Atmo IR emission
to surface increases
because of warmer
emission temperature
1. Altitude of
IR emission to
surface
decreases
2. Temperature of
IR emission to
surface increases
Out-of-Equilibrium Issues
• Land temperature increases faster than Ocean
temperature
• Atmospheric lifetime of CO2 is ~50 years
• System response is NOT instantaneous
• System takes a while to heat up in response to
increase in CO2
• Initial temperature increase is smaller than final
Anthropogenic Climate Change
The data indicate that global-mean land and sea-surface
temperatures have warmed about ~0.6oC during the last 35
years.
Is this the early stages of a man-made global warming?
Two main anthropogenic forcing mechanisms:
Greenhouse gas concentrations => rising.
Aerosol concentrations => also increasing.
We will focus attention on CO2 increases.
Greenhouse Gas Concentrations
• The two dominant greenhouse gases are H2O
and CO2.
• There are several other GHGs with smaller
concentrations: O3, CH4, N2O
• H2O vapor is categorized separately because it
is controlled indirectly by evaporation and
condensation and changes in response to other
changes in climate (temperature, circulation)
Absorption
Visible
IR
Ahrens, Fig. 2.9
20% of incident Visible (0.40.7 m) is absorbed
O2 an O3 absorb UV (shorter
than 0.3 m)
Infrared (5-20 m) is
selectively absorbed
H2O & CO2 are strong
absorbers and emitters of
IR
Little absorption of IR
around 10 m –
atmospheric window
Global Warming Potential (GWP)
Different gases has different warming potentials which
are defined relative to the warming effect of CO2
Gas
Carbon dioxide (CO2)
Methane (CH4)
Nitrous oxide (N2O)
Hydrofluorocarbons
Ahrens, Fig 2.10
Perfluorocarbons
Sulfur hexafluoride
GWP
1
21
310
560-12,100
6,000-9,200
23,900
CO2 makes the biggest
contribution to the
climate forcing
Ice Core from Vostok, Antarctica
During last ice age (>18,000
years ago)
Temps 6oC colder
CO2 levels 30% lower
CH4 levels 50% lower
H2O levels were lower
than current interglacial.
135,000 years ago it was a bit
warmer than today
50% increase in CO2 was
associated with 6-8oC
increase in temperature
6-8oC decrease in temperature
produced incredibly
different climate: Ice Age
Changing CO2 concentrations
• CO2 concentrations have varied naturally by ~30-50%
over the past few hundred thousand years (ice ages)
• Fossil fuel burning since the industrial revolution has
created a recent 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
Changing CO2 concentrations
• CO2 concentrations
measured very
precisely since 1958
• Over past 45 years
they’ve increased by
~21%
• Presently increasing
at 0.6%/yr
You are here
See http://www.cmdl.noaa.gov/ccgg/trends/co2_data_mlo.php
US (5% of world population) now causes 24% of total pollution.
See http://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions
• Currently, 7 gigatons per year of CO2 are injected into the
air by burning fossil fuels (80%) and forests (20%).
• Half accumulates in atmosphere, where it resides for 50+
years.
• If the burning fossil fuels and forests totally ceased, it would
still take 50 years for CO2 levels to return to 50% above preindustrial levels.
Missing
Carbon
Sink
• CO2 is accumulating in the atmosphere more slowly than
expected (believe it or not)
• Based on our understanding of CO2 emissions and ocean
and atmosphere uptake, there is a missing sink/uptake of
about 25%
NASA OCO mission
Natural Variations in Climate
Milankovitch Theory of Ice Ages
• Attempts to explain ice ages by
variations in orbital
parameters
• Three cycles:
Eccentricity (100,000 yrs)
Tilt (41,000 yrs)
Precession (23,000 yrs)
• Changes the latitudinal and
seasonal distributions of solar
radiation.
Milankovitch Theory of Ice Ages
• Most recent Ice Ages have
occurred for past 2 million
years
• Ice Ages occur when there is
less radiation in summer to
melt snow.
• Partially agrees with
observations, but many
questions unanswered.
What caused the onset of the
first Ice Age?
Milankovitch
Theory
Change in daily
solar radiation at
top of atmosphere
at June solstice
Changes as large
as ~15% occur
Global Temperatures and CO2
There is a very strong
relationship between
CO2 levels and past
global temperatures.
CO2 levels are now higher
than during any period
of the past 450,000 years.
Will global temperatures
responding accordingly?
Long-Term Climate Change
NA
E-A
SA Af
180 M BP
India
Aus
Ant
NA
E-A
Af India
SA
Today
Aus
Ant
Ahrens, Fig 13.6
250 million years ago, the world’s landmasses were joined
together and formed a super continent termed Pangea.
As today’s continents drifted apart, they moved into
different latitude bands.
This altered prevailing winds and ocean currents.
Long-Term Climate Change
• Circumpolar ocean current
formed around Antarctica 4055 MY ago once Antarctica
and Australia separated.
• This prevented warm air
from warmer latitudes to
penetrate into Antarctica.
• Absence of warm air
accelerated growth of the
Antarctic ice sheet.
http://www.ace.mmu.ac.uk/eae/Climate_Change/Older/Continental_Drift.html
Long-Term Climate Change
• Circumpolar seaway leads to
large latitudinal temperature
gradient.
• Circum-equatorial seaway
leads to small latitudinal
temperature gradient
http://www.ace.mmu.ac.uk/eae/Climate_Change/Older/Continental_Drift.html
Climate System Feedbacks
CO2 and the Greenhouse Effect
If the atmosphere were
dry, we could predict
with high confidence
that a doubling of CO2
(likely before 2100)
would increase the
global mean surface
temperature by ~2C.
Ahrens, Fig 2.10
The presence of oceans,
ice, water vapor and
clouds complicates the
analysis significantly.
Complexity of Climate System
The climate system involves numerous, interrelated components.
Closer Look at Climate System
Climate Feedback Mechanisms
Climate System Feedbacks
•
Feedbacks are what makes the climate problem so
difficult
• Positive: Increase the impact of the original change
• Negative: Reduce the impact of the original change
• EXAMPLE: Ice albedo (Positive Feedback):
– Temperatures increase because of increased CO2
1. Ice melts
2. Reduces Earth’s reflectivity (albedo)
3. Earth absorbs more sunlight
4. Earth warms
5. More ice melts….
Positive and Negative Feedbacks
• Assume that the Earth is warming.
- Warming leads to more evaporation from oceans, which
increases water vapor in atmosphere.
-More water vapor increases absorption of IR, which
strengthens the greenhouse effect.
-This raises temperatures further, which leads to more
evaporation, more water vapor, warming…
“Runaway Greenhouse Effect”
Positive Feedback Mechanism
NET EFFECT: Water vapor feedback increases the warming of
the Earth, roughly doubling the effect of CO2 increase alone
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
Cloud Feedback
• Clouds affect both solar & IR radiation
– Clouds increase solar reflection (albedo) cooling the Earth
– Clouds absorb and emit IR radiation like a GHG
• Currently their net effect is to cool the Earth
– Solar effect > IR effect
• Not clear what they will do in the future
– If the amount of low clouds increases, the albedo effect wins and
they cool the Earth (Negative feedback)
– If the amount of high clouds increases, the IR effect wins and
the Earth warms (Positive feedback)
• There are other subtler cloud feedbacks as well involving
aerosols in particular
Positive and Negative Feedbacks
• Atmosphere has a numerous checks and balances that
counteract climate changes.
• All feedback mechanisms operate simultaneously.
• All feedback mechanisms work in both directions.
• The dominant effect is difficult to predict.
• Cause and effect is very difficult to prove at the
“beyond a shadow of a doubt” level.
Climate Model Predictions
Projections of Global Warming
• Atmosphere and coupled Atmo-Ocean models are
run for hundreds of years to simulate future climates
• They assume continued increases in the levels of
greenhouse gasses in the atmosphere.
• The models have sophisticated physics…
But they have coarse spatial grid separations!
Atmosphere: 250 km horizontal, 30 levels vertical
Ocean: 125-250 km horizontal, 200-400 m vertical
How Well Do Models Capture
Current Observed Climate?
• Performance Varies by
Weather Element
In general…
Excellent for Surface
Temperature
Skillful for Sea-Level
Pressure (SFC Winds)
Marginal Skill for
Precipitation
Fig. 8.4 IPCC Report
Global Temperature Outlook
Fig. 9.3 IPCC Report
Assume CO2 levels rise at
rate of 1% per year until
2070.
• Good agreement for past
climate and CO2 levels
leads to high confidence.
• Rather close agreement
among models.
Consensus of several
model runs indicates an
average warming of 2oC
Global Precipitation Outlook
Fig. 9.3 IPCC Report
• Marginal performance
for past climate and
CO2 levels means low
confidence in outlook.
• Large differences exist
among models.
Consensus of several
model runs indicates an
average increase of 2%
in global precipitation
Regional Consistency: Warming
Fig. 10.1.1 IPCC Report
A + or - symbol denotes 7 out of 9 models agree.
A2: no sulphate aerosols; B2: has sulphate aerosols
Regional Consistency: Rainfall
Fig. 10.1.2 IPCC Report
A + or - symbol denotes 7 out of 9 models agree.
A2: no sulphate aerosols; B2: has sulphate aerosols
GDFL Model
Lets look at some details from a 500-year
simulation by a specific climate model.
Examine Two Scenarios:
1% CO2 increase per year for 70 years
2X Total Increase in CO2
1% CO2 increase per year for 140 years
4X Total Increase in CO2
Good agreement for CO2
levels of the past 150 yrs
Mandatory before use as
global warming model
www.gfdl.noaa.gov
CO2 Increases
1% per Year
Average Global Surface
Temperatures…
Warm 2oC for 2X CO2
Warm 4o C for 4X CO2
Sea Level Rises…
Equilibrium Not Reached
until after 500 years
North Atlantic Ocean…
Circulation Weakens
www.gfdl.noaa.gov
CO2 Increases 1% per Year
• Surface Temperatures for GFDL Model
2X CO2 Temperature Animation Remote
2X CO2 Temperature Animation Local Disk
BIG File (8.2 MB)!!!
Climate response is out of equilibrium
for a while
•
Continental temperatures rise faster than ocean temperatures
– Water has much larger heat capacity than land
– Evaporation cools the ocean surface
• Ocean temperatures take long time to reach a new equilibrium
1. Warming we have seen thus far is NOT the total warming even
if GHG concentrations stop increasing
• Water saturation vapor pressure over land increases faster than
that over the oceans
• Most of the atmospheric water vapor ultimately comes from the
ocean
• Therefore expect the relative humidity over land to decrease in
general, at least until ocean temperatures catch up with the land
temperatures.
2. Continental interiors will get drier in general
Annual
Energy
Balance
Radiative
Cooling
NH
Radiative
Warming
Ahrens, Fig. 2.21
Radiative
Cooling
SH
Latitudinal imbalance between visible in and IR out is compensated
by meridional energy fluxes
Heat transfer from low latitudes to high latitudes is accomplished
by winds and ocean currents
Differential heating and spinning Earth drive winds and currents
This horizontal energy transfer will likely change as climate
changes
Mid-Latitude Cyclones
•Winter storms move tropical air poleward and polar air toward the
tropics.
•Net result is to transport energy poleward
mP
cP
mT
Ahrens, Meteorology Today, 5th Ed.
Ocean Currents of World
Ahrens Fig. 7.24
www.gfdl.noaa.gov
4X CO2 Sea Ice Animation
4X CO2 Sea Ice Local Link
CO2 Increases
1% per Year
July Temperature over
Southeastern U.S.
• Warms 5-9oC
July Heat Index over
Southeastern U.S.
• Rises 7-14oC due to
increase H2O vapor
Consistent Signal
Warmer Southern U.S.
www.gfdl.noaa.gov
CO2 Increases
1% per Year
Soil Moisture
• Much Drier over U.S.
60% Soil Moisture Decrease
Higher Evapo-Transpiration
Altered Balance between
Evaporation-Precipitation
• Agricultural Implications
How do we hydrate and feed
ourselves?
www.gfdl.noaa.gov
Increasing CO2 concentrations
• How high will they go? How warm will it get???
• If CO2 concentrations stay within factor of 2 of pre-industrial,
then warming of 3+1oC is expected
• If concentrations go still higher => larger uncertainty because
the climate is moving into unprecedented territory
See
http://epa.gov/climatechange/science/futureac.html
You are going to be
somewhere in here
Last 4 Ice Age cycles:
400,000 years
Man made
You are here
Ice age CO2 range