Transcript Met10_lecture_16

Global Climate Change
Dr. Craig Clements
San José State University
MET 10 Global Climate Change-Chapter 14
Review: Why is CO2 So Important?
 Carbon Dioxide is a greenhouse gas.
 Greenhouse gases are those gases that cause
the greenhouse effect.
 The greenhouse effect makes a planet’s
surface temperature warmer than it would
otherwise be.
 The stronger the greenhouse effect, the warmer
the surface (other factors being equal).
 Consider the blanket analogy
Earth’s Energy Balance
 Energy entering top of atmosphere
= Energy leaving top of atmosphere
 Energy entering the Earth’s surface
= Energy leaving Earth’s surface
Conservation of Energy
Absorption of
Radiation in
the Earth’s
Atmosphere
Incoming solar radiation
 Each ‘beam’ of incoming sunlight can be either:
– Reflected back to space: Albedo
 Clouds
 Atmosphere
 Surface
– Or absorbed; either by atmosphere (e.g.
clouds or ozone) or Earth’s surface.
Some surface
radiation
escapes to
space
Most
outgoing
longwave is
absorbed in
atmosphere
(by
greenhouse
gases)
Longwave radiation is
emitted from surface.
Some
atmospheric
radiation
escapes to
space
Greenhouse
gases emit
longwave
upward and
downward
Some
atmospheric
radiation is
absorbed at the
surface
Greenhouse Effect
Sequence of steps:
1. Solar radiation absorbed by earth’s surface.
2. Earth gives off infrared radiation.
3. Greenhouse gases absorb some of the Earth’s infrared
radiation.
4. Greenhouse gases (water and CO2) give off infrared
radiation in all directions.
5. Earth absorbs downward directed infrared radiation
Result: warmer surface temperature
Energy Balance
 Assume that the
Earth’s surface is in
thermodynamic
equilibrium:
 Thermodynamic
Equilibrium:
– The flow of energy
away the surface
equals the flow of
energy toward the
surface
Surface
Average surface temperature = 15°C
Sudden Removal of all Greenhouse
Gases
Removal of greenhouse
gases would decrease
downward flow of energy;
now energy away from
surface is greater than
energy toward surface.
Sudden Removal of all Greenhouse
Gases
Removal of greenhouse
gases would decrease
downward flow of energy;
now energy away from
surface is greater than
energy toward surface.
Thus, average surface temperature starts to decrease.
Sudden Removal of all Greenhouse
Gases
As surface cools, emission
of radiation decreases until
balance is restored. At this
point, cooling stops
Result: A Very Cold Planet!
As surface cools, emission
of radiation decreases until
balance is restored. At this
point, cooling stops and
equilibrium is restored.
Average surface temperature = -18°C
Earth’s Greenhouse Effect
 Without the greenhouse effect, the surface
temperature of Earth would be
– Way Cold (-18°C)
 Greenhouse gases play an important role in
shaping climate.
– More GHGs – warmer climate
– Less GHGs – cooler climate
Recent Climate Change
Modeled
temperature
changes
IPCC (2007)
(b) Additionally, the year by year (blue curve) and 50 year average (black curve) variations of the
average surface temperature of the Northern Hemisphere for the past 1000 years have been
reconstructed from “proxy” data calibrated against thermometer data (see list of the main proxy
data in the diagram). The 95% confidence range in the annual data is represented by the grey
region. These uncertainties increase in more distant times and are always much larger than in the
instrumental record due to the use of relatively sparse proxy data. Nevertheless the rate and
duration of warming of the 20th century has been much greater than in any of the previous nine
centuries. Similarly, it is likely7 that the 1990s have been the warmest decade and 1998 the
warmest year of the millennium.
Latest global temperatures
…“Over the last 140 years, the best estimate is that the
global average surface temperature has increased by
0.6 ± 0.2°C” (IPCC 2001)
 So the temperature trend is:
0.6°C ± 0.2°C
 What does this mean?
 Temperature trend is between 0.8°C and 0.4°C
 The Uncertainty (± 0.2°C ) is critical component to the
observed trend
CO2 Concentration in Atmosphere
Short Term Carbon Cycle
 One example of the short term carbon cycle involves plants
 Photosynthesis: is the conversion of carbon dioxide and
water into a sugar called glucose (carbohydrate) using
sunlight energy. Oxygen is produced as a waste product.
 Plants require
 Sunlight, water and carbon, (from CO2 in atmosphere or
ocean) to produce carbohydrates (food) to grow.
 When plants decay, carbon is mostly returned to the
atmosphere (respiration)
 During spring: (more photosynthesis)
 atmospheric CO2 levels go down (slightly)
 During fall: (more respiration)
 atmospheric CO2 levels go up (slightly)
Current CO2: ~387 ppm
What Changed Around 1800?
 Industrial Revolution
– Increased burning of fossil fuels
 Also, extensive changes in land use began
– the clearing and removal of forests
Burning of Fossil Fuels
 Fossil Fuels: Fuels obtained from the earth
are part of the buried organic carbon
“reservoir”
– Examples: Coal, petroleum products,
natural gas
 The burning of fossil fuels is essentially
– A large acceleration of the oxidation of
buried organic carbon
Land-Use Changes
 Deforestation:
– The intentional clearing of forests for
farmland and habitation
 This process is essentially an acceleration of
one part of the short-term carbon cycle:
– the decay of dead vegetation
 Also causes change in surface albedo
(generally cooling)
Climate Feedbacks
Earth’s Climate
 The Earth’s climate is fairly stable in terms of temperature
 This can be visualized using in the following system diagram.
 The idea is that even though the system may change away
from it’s initial point, it will have the tendency to go back to
‘normal’ eventually.
2
3
1
Stable
Stable
Stability versus instability
 Stable:
– Given a perturbation, the system tends to return to original
state
 Instability:
– Given a perturbation, the system moves to another state.
Stable equilibrium
Unstable equilibrium
States of equilibrium
– The system may have multiple states of equilibrium
2
3
1
Stable to small perturbations, until a big force perturbs the
system into a new equilibrium
Climate Stability
 The Earth’s climate changes as a result of internal/external
forcing:
– Changes in solar radiation
– Changes in the earth’s orbit
– Plate tectonics
– Volcanoes
– Human pollution etc.
 These forcings can be thought of as a perturbation (or push)
to climate stability.
 These changes can be enhanced or diminished by positive
or negative feedbacks
Climate Stability
 Internal Forcing mechanisms
- processes that are internal to the climate system that
involve the various elements: ice, water vapor, CO2
 External Forcing mechanisms
- some forcing that can alter the system without itself being
altered.
- solar variability, axis wobble, etc.
Climate Feedbacks
 Positive feedback:
– initial change reinforced by another process.
– Trends towards instability
 Negative feedback:
– initial change counteracted by another process.
– Trends towards stability
Positive Feedbacks
 Processes that accelerate a change
– Note: Feedbacks cannot initiate change; they
can only alter the pace of change
 Important climate examples:
– Ice-albedo feedback
– Water-vapor feedback
– Cloud feedback
Ice-Albedo Feedback (Cooling)
Initiating Mechanism
Earth Cools
Ice Coverage Increases
Albedo Increases
Absorption of Sunlight Decreases
Positive Feedback
Somehow this
happens
Fill in the blanks
1.
increases,
decreases,
decreases
2.
Decreases,
decreases,
increases
3.
Increases,
increases,
increases
4.
Decreases,
decreases,
decreases
Initiating Mechanism
Earth Warms
Ice Coverage ___________
Albedo _____________
Absorption of Sunlight _______
Ice-Albedo Feedback (Warming)
Initiating Mechanism
Ice Coverage Decreases
Albedo Decreases
Absorption of Sunlight Increases
Positive Feedback
Earth Warms
Fill in the blanks
Initiating Mechanism
1.
2.
3.
4.
Increases, increases, increases
Increases, decreases, decreases
Decreases, increases, increases
Decreases, decreases, decreases
Earth Warms
Evaporation
Atmospheric Water Vapor Content
Greenhouse Effect
Water Vapor Feedback (Warming)
Initiating Mechanism
Evaporation Increases
Atmospheric Water Vapor Content Increases
Greenhouse Effect Strengthens
Positive Feedback
Earth Warms
Water Vapor Feedback (Cooling)
Initiating Mechanism
Evaporation Decreases
Atmospheric Water Vapor Content Decreases
Greenhouse Effect Weakens
Positive Feedback
Earth Cools
Negative Feedbacks
 Processes that reduces an imposed change
- Trends towards stability
 Important examples:
– Cloud feedback
– Chemical weathering
 Note: Positive/negative feedbacks have no relation to
‘good versus bad’, but are about how a system
responds to a change.
or cooling
Possible Role of Cloud in Warming or
Cooling the Atmosphere
Figure 12.7 Role of cloud in both warming and cooling the atmosphere.
Figure 12.7 Role of cloud in both warming and cooling the atmosphere.
Which feedback is positive?
1. Left
2. Right