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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.