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OC450: Climatic Extremes (Winter 2008)
• Profs: Paul Johnson and Paul Quay
• School of Oceanography
• Taught class for ~10 years
• Typically ~10 students
• Time/Place: 205 OTB (M, W, Th, F at 11:30)
• Web Page:
http://courses.washington.edu/ocean450/
What do we want to learn?
• What does the record of past climate change on
earth tell us about future climate change?
• What climate records are available to study past
climate change?
• What are the important processes of the earth’s
climate system that control climate?
• What are the major feedbacks in the earth’s
climate system?
Course Components
• Syllabus
-Lecture topics, lecturer and corresponding chapters in
Textbook for each week (download from Web Page)
-Follow the textbook: Earth’s Climate: Past and Future by
W.F. Ruddiman (2000). Very readable. ~ 2 Chapters/week
• Lectures (M, W and F)
- Read textbook chapters ahead of time, if possible
- The figures used in lectures will be posted on the class web
page ahead of time. Bring to class.
Course Components
• Paper Discussions (~8 total, on Thursdays)
-Papers distributed (posted on Web Page) a week
ahead.
-Three or so questions to answer (briefly) in writing.
-Oral discussion of paper
-randomly pick ~5 students to lead discussion
-focus on key figures and questions
• Problem Sets (~6 total, due on Fridays)
- receive problem a week ahead
- quantitative examples of concepts discussed in
lectures
Course Components
• Exams: Midterm and Final
-Midterm (Week 6) and Final (finals week)
-both descriptive and quantitative questions
• Grading
Problem Sets 25%
Paper Discussions/Questions 25%
Midterm 25%
Final 25%
Climate
• Climate represents average environmental conditions
-primary characteristics: temperature, precipitation,
-other characteristics: sea level, ice sheet extent, aridity,
cloudiness, plant cover extent, greenhouse gas
concentrations, winds and ocean currents
• Spatial and temporal scales of climate indicators
-large spatial scale: global, ocean basins, continental,
regional
-long time scales: millions years, millennial, century,
decadal, annual.
• Weather, in contrast, focuses on local spatial scales and
short term (daily or weekly) variations in atmospheric
conditions (temperatures, precipitation, winds)
Climate System on Earth
Anthropogenic CO2 Emissions
Past Climate Change
• What were the conditions on earth during previous
periods of extreme climate?
-e.g., temperature, precipitation, atmospheric CO2 levels,
position of the continents, vegetation distribution, ice sheet
extent, etc.
• What processes affected climate in the past?
-e.g., weathering rates, ocean and atmospheric circulation
rates, solar insolation rates, photosynthesis rates, plate tectonic
rates, etc.
• How important are the time scales of theses processes?
-e.g., the position of the continents change at a much
slower rate than the growth of ice sheets
Climate Proxies
• A key part of reconstructing climate in the past is the
use of climate proxies.
• For climate studies, a proxy is a record that represents
the actual climate characteristic.
-e.g., use the oxygen isotope composition of ice in
Greenland and Antarctica ice sheets to reconstruct air
temperature record over the last 750,000 years
-e.g., use the oxygen isotope composition of
CaCO3 in corals to reconstruct sea surface temperature in
the ocean
-e.g., use the distribution of continentally derived
minerals in deep sea sediments to reconstruct the presence
of icebergs in the N. Atlantic Ocean.
Current and Future Climate Change
• Anthropogenic Impacts on Climate
- greenhouse gas concentrations (CO2, CH4, N2O)
- aerosol concentrations
• Natural Variations in Climate
- El Nino Events (every few years)
- Ice Age Transitions (every 100,000 years)
• Future Climate………The BIG question!
-Can we accurately predict future climate change?
-Can we reduce the impact of climate change?
Importance of Climate and Climate Change
• Climate affects our quality of life
-e.g., economy, food production, energy consumption,
frequency of catastrophes, environmental diversity.
• Climate change will change how we live
-e.g, global warming will change local precipitation
rates and temperature, sea level, extent of sea ice and
permafrost, coral reefs, ecosystems, fish populations, etc.
• Climate will change in the future
-due to both natural variations and anthropogenic
effects
Questions about Future Climate Change
• Scientific questions
- What can we learn about future climate change from past
climate change?
- How accurately can we separate current and future climate
change into natural and anthropogenic causes?
- Can we use geoengineering and advances in energy
production to significantly reduce climate change?
• Social/economic/political questions:
- Do we have the political and personal will to reduce
greenhouse gas emissions?
- Can we successfully adapt to climate change?
Climate Change in the Pacific Northwest
• The Pacific NW is an excellent example of a region that
will be significantly impacted by climate change
-this region is affected by both natural and
anthropogenic causes of climate change
• El Nino and Pacific Decadal Oscillation are natural
oscillations in the atmosphere/ocean circulation scheme
that causes changes in climate of the Pacific NW
-affect temperature and precipitation rates in the
region
• UW’s Climate Impacts Group
http://www.cses.washington.edu/cig/
Climate Change in the Pacific Northwest
• Region’s history and economy has evolved based on the
availability of water throughout the year
-water for hydroelectric power (cheap electricity)
-water for agriculture (summer crop irrigation)
-water for salmon (spawning in fall)
• How will global warming (predicted 3-4ºF over next 50
years) affect the Pacific NW’s water cycle?
-loss of snow pack
-reduced summer stream flow
• What will be the impact on hydroelectricity, irrigation,
salmon, forest fire frequency, recreation, quality of life?
Trends in Temperature and Snow Pack in
the Pacific Northwest during last 50-80 years
Pacific NW is warming and losing snow pack.
Comparing
Impacts of
Natural and
Anthropogenic
Climate
Change in the
Pacific NW
Predicted Columbia River Discharge
Changes over the next 50 years
Reduced snow pack changes the shape of the hydrograph.
Climate Models Predict Climate Change
• What are climate models?
-mathematical representation of the earth’s climate system
(typically this includes atmosphere, land and oceans)
-climate models are essential for predicting future change
• How accurate are model predictions?
-policy makers want to know the likelihood or
probability of a climate change prediction
• How sensitive are local or regional climate
predictions to global conditions?
-e.g., how do predicted changes in the circulation of
the equatorial Pacific Ocean affect climate in the Pac NW
Simple Reservoir or Box Models
General Circulation Models (GCMs)
Model
Hindcasts
for
Validation
Climate System on Earth
Anthropogenic CO2 Emissions
Response Times of the Earth’s Climate System
Response Time
Illustration
The magnitude of
temperature response
of the water depends
on:
-heating rate
-period of heating
-size of reservoir
being heated
Response
versus
Forcing
The size of the reservoir
relative to the magnitude
and frequency of the
forcing determines the
magnitude of change.
Generally, the larger the
reservoir, the smaller the
response.
Cyclical Response vs Forcing
The time history of response does not necessarily overlap the
time history of forcing. Reservoir response can lag the forcing.
Reservoirs and Budgets
Reservoir’s Heat Budget
- The rate of change of heat content of the reservoir depends on the
heat input and heat loss rates.
ΔHeat /Δtime = Heat Input – Heat Loss (or Output)
- Units: Heat is in Joules
- Heating rate in Joules/s or Watts (1 Watt = 1 Joule/sec)
- When the heat input rate equals the heat loss rate then the total
amount of heat in the reservoir does not change (ΔHeat/Δtime=0)
- The condition is referred to as steady-state, where the inputs
equal the losses.
-Steady-state concept applies to any material in a reservoir.
Earth’s Heat Budget
If the heat input from solar radiation exactly equaled the heat loss
from long wave radiation back to space then neither the heat
content nor the mean temperature on earth would change with time.
Earth’s Heat Budget Terms
• At Steady-state: heat input equals heat output
ΔHeat/Δtime = Solar Insolation Input – Long Wave Back
Radiation Output
• Solar Insolation = 342 Watts/m2
- heat input expressed per unit surface area
• Long Wave Radiation depends of the temperature of the
radiator
LW Radiation = σ * T4 (ºK),
where σ = Stefan-Boltzman constant (5.67x10-8 W/m2/ºK4) and
Temperature (T) is in degrees Kelvin (ºK = ºC +273)
Earth’s Heat Budget
ΔHeat/Δtime = Heat Input - Heat Output
ΔHeat/Δtime = I*(1 - α) – f * σ *T4,
-where I = solar insolation from sun, α = reflectivity of
incoming short wave radiation (~0.3) and f = transmissivity
(~0.6) of atmosphere to long wave radiation
-the reflectivity depends primarily on the amount of clouds
in the atmosphere and the proportion of ice, ocean and land
-the transmissivity of the atmosphere depends inversely on
the amount of greenhouse gases in the atmosphere
Earth’s Heat Budget….calculating Temperature
• At steady-state, a heat balance implies ΔH/Δt = 0
-solve for Temperature (T)
-thus T (ºK) = [I*(1 - α) / (f * σ)]0.25
-Units = [(W/m2) / (W/m2 K4)]0.25 = ºK
• For the earth under present conditions:
α = 0.30 (30% of incoming SW insolation reflected
back into space)
f = 0.61 (61% of LW radiation reaches space)
• Under these conditions, the temperature of the earth needed to
maintain a steady-state balanced heat budget is 288 ºK (or 15ºC).
Earth’s Heat Budget
• A steady-state temperature of 288 K or 15ºC
applies at earth’s surface.
• If there were no greenhouse gases in the earth’s
atmosphere (f = 1, rather than 0.61) the surface of
the earth would be 255 K or –18 º C.
• Like the surface of the moon
• Thus the natural level of greenhouse gases in the
Earth’s atmosphere keeps the surface of the earth
33ºC warmer than it would be in their absence.
Earth versus Venus
• Venus Surface Temperature = 460ºC (vs 15ºC on earth)
• Closer to sun, but that’s not the reason.
• Venus’ atmosphere is 90x denser than earth’s and 96%
CO2. (f = 0.008) (Earth’s atmosphere is 0.03% CO2)
• Venus’ Heat Budget
ΔHeat/Δtime = I*(1 - α) – f * σ *T4
T (ºK) = [I*(1 - α) / (f * σ)]0.25 (at steady-state)
T = [645 W/m2*(1-0.8) / (0.008* 5.67x10-8 W/m2/ºK4)]0.25
T = 733 K or 460ºC
• Atmospheric composition is key factor controlling
temperature.
Only Certain
Gases are
Greenhouse
Gases
Water Vapor (H2O)
Carbon Dioxide (CO2)
Methane (CH4)
Nitrous Oxide (N2O)
Greenhouse Gas Heating in the Atmosphere
Impact of Adding Greenhouse Gases
• Greenhouse gases (GHGs) reduce the transmissivity of
LW radiation through the atmosphere (decrease f) by
increasing the ability of air to adsorb long wave radiation.
• Thus by increasing the concentrations of GHGs in the
atmosphere, f decreases and the heat loss is reduced which
causes the earth to gain heat (warm).
ΔHeat/Δtime = Heat Inputs – Heat Outputs > 0
• At a lower value of f, the temperature of the earth’s
surface must increase in order to maintain a steady-state
balanced heat budget.
T (ºK) = [I*(1 - α) / (f * σ)]0.25
Predicting Warming Effect of GHGs
• The uncertainty in predicting the future warming
rate on earth is primarily a result of our
uncertainty in predicting the rate at which the
reflectivity (α) and transmissivity (f) will change.
– α depends on the amount (and type) of clouds, ice, etc.
– f depends on the future GHG composition of the earth’s
atmosphere (mainly depends on future CO2 input and
loss rates, which are uncertain)
– the change in solar insolation (I) can be accurately
predicted from the earth’s orbital characteristics
Feedbacks in the Earth’s Climate System
Complicate Temperature Predictions
Positive Feedback accelerates change.
Negative Feedback decelerates change.
Earth’s Carbon Cycle
• Importance: controls the concentration of CO2
(and CH4) in the atmosphere, which primarily
control the transmissivity (f) of the atmosphere.
• Variations in the earth’s carbon cycle in the past
have changed the concentration of CO2 (and CH4)
in the atmosphere and, thus, temperature of the
earth.
• Major Carbon Reservoirs: rocks, ocean, soils,
plants, atmosphere.
• Major Exchange Pathways: weathering,
volcanism, photosynthesis/respiration, atmosphereocean (air-sea) CO2 gas exchange.
Carbon Reservoirs on Earth
Reservoir Sizes
Units: Gigatons C, 1015 gms
C or Pedagrams (Pg)
Reservoir Exchange Rates
Gigatons C/yr or 1015g C/yr
Atmosphere’s Carbon Reservoir Budget
ΔCarbon/Δtime = Inputs – Outputs (units: Gtons C /yr)
ΔCO2atm/Δt = - Photosynthesis + Respiration + Ocean CO2 Gas
Evasion – Ocean CO2 Gas Invasion Weathering
= -100 + (50+50) + 74.6 – 74 – 0.6 Gtons C/yr
= 0 Gtons C /yr
• In this situation, the atmosphere’s carbon (CO2) reservoir is at
steady-state, that is, the CO2 inputs equal the CO2 outputs and the
amount of CO2 in the atmosphere would not change over time.
• However, human activity is adding 7 Gtons C/yr of CO2 to the
atmosphere as a result of fossil fuel combustion. What does this do
to the atmosphere’s CO2 budget?
• What would happen to atmospheric CO2 levels if there was a
Residence (Turnover) Time for a Reservoir
• Residence Time = Reservoir Amount/ Input (or Output) Rate
For example: Vegetation on Earth
Residence Time = 610 Gtons C / 100 Gtons/yr
= 6.1 years
• This means that the average time a carbon atom spends in
plants on earth is 6.1 years.
-Does this seem reasonable?
• The residence or turnover time of a reservoir is a rough
estimate of that reservoir’s response time to a perturbation.
-the time it takes for the amount of material in the
reservoir to adjust to a change in the input (or output)
Residence Time for CO2 in the Atmosphere
• CO2 Residence Time (τ) =Amount / Input (or Output)
τ = 600 GTons / (100 + 74.6 GTons/yr)
= 3.4 years
• This means that CO2 molecules remain in the
atmosphere for an average of 3.4 years before being either
photosynthesized by plants or adsorbed in the ocean.
• This short residence time implies that the response of
the atmospheric CO2 pool will be quick to changes in
either CO2 inputs or outputs.
Long Term Controls on Atmospheric CO2
• Weathering, volcanism and sedimentation are processes that
exchange carbon between the atmosphere and rock
reservoir.
• These exchange rates are very slow (~0.2 Gtons C/yr)
• Thus the atmospheric CO2 response time to changes in rates
of weathering, sedimentation or volcanism is long
Tau = 600 Gtons C / 0.2 Gtons C/yr = 3000 years
• However, on geologic time scales (millions of years) this
atmospheric CO2 response time is extremely fast.
• Thus over the history of the earth, changes in weathering,
volcanism and sedimentation rates is an effective way to
change atmospheric CO2 levels.