OC450: Climatic Extremes (Winter 2010). Profs : Paul Johnson and Paul Quay School of Oceanography Taught class for >10 years Time/Place: 205 OTB (M, W, Th, F at 11:30) Web Page : http://courses.washington.edu/ocean450/. What do we want you to learn?.
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-Schedule of lecture topics, lecturer and corresponding chapters in Textbook for each week
-Follow the textbook: Earth’s Climate: Past and Future by W.F. Ruddiman (2001). 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 figures to class.
• Paper Discussions (every Thursday)
-Papers distributed (on Web Page) a week ahead.
-Three or so questions to answer in writing (turn in).
-Oral discussion of paper
-randomly pick ~3 students to lead discussion
-focus on key figures and questions
• Problem Sets(weekly and due on Fridays)
- receive problem a week ahead
- quantitative examples of concepts discussed in lectures
• Exams: Midterm and Final
-Midterm (Week 6) and Final (finals week)
-both descriptive and quantitative questions
Problem Sets 25%
Paper Discussions 25%
• Climate represents average environmental conditions
-primary characteristics: temperature, precipitation,
-other characteristics: greenhouse gas concentrations, sea level, ice sheet extent, cloudiness, winds, 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, interannual.
• Weather, in contrast, focuses on local spatial scales and short term (day or week) variations in atmospheric conditions (temperature, precipitation, winds)
Forcing Feedbacks Response
It’s the multiple pathways of interaction (possible feedbacks) that complicate reconstruction and prediction of climate change.
• What were the conditions on earth during previous periods of extreme climate?
-e.g., temperature, precipitation, atmospheric CO2 and CH4 levels, position of the continents, vegetation distribution, ice sheet extent, etc.
• What processes affected climate in the past?
-e.g., weathering , ocean and atmospheric circulation, solar insolation, photosynthesis, plate tectonics, ice sheets, etc.
• How important are the time scales of theses processes?
-e.g., the position of the continents (100 Myrs) change at a much slower rate than the growth of ice sheets (1000s yrs)
• A fundamental part of reconstructing climate in the past is the use of climate proxies.
• For climate studies, a proxy is a record of a climate indicator that represents but doesn’t directly measure 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 deep sea sediments to reconstruct ice sheet volume and ocean temperature
-e.g., use the concentration of continentally derived minerals in deep sea sediments to reconstruct the presence of icebergs in the N. Atlantic Ocean.
• Anthropogenic Impacts on Climate
- greenhouse gas concentrations in air (CO2, CH4, N2O)
- aerosol concentrations in air (reflectivity and impact on clouds)
• Natural Variations in Climate
- El Nino Events (every few years)
- Ice Ages (every 100,000 years)
• Future Climate (the BIG issue)
- How accurately can we predict future climate change?
• Climate will change in the future
- as a result of both natural variations and anthropogenic effects
- currently, observed changes are happening faster than the predicted changes (faster feedbacks?, lower thresholds?)
• Climate affects our quality of life
-e.g., food production, energy production, energy consumption, water availability, coastal flooding, storm frequency, human health, etc.
• Can society reduce the impact of climate change?
- political will, technological solutions, etc.
• 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
• 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 (irrigation in E. Washington)
-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 river flow
• What will be the impact on hydroelectricity, irrigation, salmon, forest fire frequency, recreation, and quality of life?
Pacific NW is warming and losing snow pack.
Reduced snow pack changes the shape of the hydrograph.
Forcing Feedbacks Response
Anthropogenically (human) produced GHGs like CO2 and CH4 and aerosols are added to atmosphere.
The earth’s climate depends on interplay between processes occurring in the atmosphere, ocean, land surfaces and earth’s interior.
Plate Tectonics 10s-100’s x 106 yrs Position of continents
The magnitude and rate of temperature response of the water depends on:
-duration of heating
-size of reservoir being heated
The size of the reservoir relative to the magnitude and frequency of the forcing determines the magnitude of change.
Generally, as the size of the reservoir increases, the response is smaller and slower.
Why is the relationship between forcing and response important to understand past climate change?
- The rate of change of heat content of the reservoir depends on the heat input and heat loss rates.
ΔHeat /Δtime = Heat Input Rate – Heat Loss Rate
- Units: Heat is in Joules
- Heating rate in Joules/s or Watts (1 Watt = 1 Joule/sec)
ΔHeat/Δtime > 0 when heat input rate exceeds heat loss rate and temperature of reservoir increases
ΔHeat/Δtime < 0 when heat input rate less than heat loss rate and temperature of reservoir decreases.
ΔHeat/Δtime = 0 when heat input rate equals heat loss rate and temperature of reservoir doesn’t change (i.e., steady-state)
Why does the rate of temperature increase slow down?
What does this imply about the heat loss rate versus time?
What does the heat loss vs time look like in the situation where the max and min in the temperature of the reservoir lags the max and min of the heat input ?
(mean = 342 W/m2)
Insolation at high latitudes has much greater seasonality.
Seasonality in insolation rate is a major factor causing large seasonality in temperature at high latitudes in N. Hemisphere.
Why is this region important in the earth’s heat budget?
When the heat input from solar radiation exactly equals the heat loss from long wave radiation back to space then neither the heat content nor the mean temperature on earth would change over time.
Winds and ocean currents redistribute heat from tropics to poles. Changes in currents or winds will change equator to pole temperature gradients.
• At Steady-state: heat input equals heat output
ΔHeat/Δtime = Solar Insolation Input – Long Wave Back Radiation Output
• Solar Insolation = 342 Watts/m2(1 W = 1 Joule/sec)
- heat input expressed per unit surface area of earth
• Long Wave Radiation depends strongly on 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)
-a 10% increase in T (ºK) yields a 50% increase in heat loss
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
• At steady-state, a heat balance implies ΔHeat/Δtime = 0
-ΔHeat/Δtime = I*(1 - α) – f * σ *T4 = 0
-solve for T, where T = [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 288ºK (or 15ºC).
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 ºK or 460ºC
Atmospheric GHG composition is key factor causing temperature difference on Venus vs Earth.
Water Vapor (H2O)
Carbon Dioxide (CO2)
Nitrous Oxide (N2O)
• 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 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 reach a steady-state balanced heat budget.
T (ºK) = [I*(1 - α) / (f * σ)]0.25
• Magnitude of human induced change in earth’s heat budget is small ~ 1.5 W/m2 (since 1800s), but important.
PositiveFeedback accelerates change (as shown above).
NegativeFeedback decelerates change.
Carbon Reservoir Sizes
CO2 Exchange Rates
Units: Gigatons (109 tons ) or 1015 gms (Petagrams, Pg)
Gigatons C/yr or Pg C/yr
Residence Time = Reservoir Amount/ Input (or Output) Rate
• For example: Residence time of 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 surface of 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)
• Processes with long CO2 response times:
-weathering, volcanism, sedimentation.
• The atmospheric CO2 response time to changes in rates of weathering, sedimentation or volcanism is 1000s years.
τ = 600 Gtons C / 0.2 Gtons C/yr = 3000 years
• The oceanic CO2 response time is much slower.
τ = 38,000 Gtons C / 0.2 Gtons C/yr = 200,000 years
• An important point is that the atmospheric CO2 concentration is controlled by the surface ocean CO2 concentration.
• On geologic time scales, changes in weathering, volcanism and sedimentation rates are effective ways to change atmospheric CO2 levels.
• Natural Processes: Photosynthesis, respiration, air-sea CO2 gas exchange, ocean circulation rates.
• Anthropogenic Processes: Fossil fuel combustion, biomass burning.
• The atmospheric CO2 response time to changes in rates of terrestrial photosynthesis and respiration is very fast (~decade).
τ = 600 Gtons C / 100 Gtons C/yr = 6 years
• The oceanic CO2 response time to changes in ocean circulation is relatively fast (millenium).
τ = 38,000 Gtons C / 37 Gtons C/yr = 1000 years
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 now adding ~8 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?
CO2 produced from combustion of fossil fuels and biomass has perturbed the CO2 budget from pre-industrial steady-state.