METR 112 Global Climate Changes – Lecture 8 Aerosols and Radiative Forcing With More on Ozone Depletion Outline ▪ ▪ ▪ ▪ Atmosphere Ozone Layer Aerosols Radiative Forcing Reading: Chapter 4, pages 19-29; Chapter 11, pages 21-24 1 Class Meeting ▪ Watch the videos for this lecture at: 2 Radiative Forcing: What is it? ▪ A value for quantitatively determining a forcing’s effect on climate ▪ The greater the magnitude of the value, the larger the warming/cooling effect ▪ Positive values indicate warming ▪ Negative values indicate cooling Forcing Factors: Warming or Cooling? ▪ Some forcing factors have a warming effect on climate – Examples: Increases in greenhouse gas concentration, increases in sunspot activity ▪ Others have a cooling effect on climate – Examples: Aerosols, stratospheric ozone decrease Atmospheric Layers Ozone layer absorbs ultraviolet radiation, with greater efficiency in the upper region of the stratosphere (more warming) Stratospheric Cooling ▪ While the troposphere is warming over time, the stratosphere may be cooling over time Ozone Layer Depletion ▪ Over time, the amount of ozone in the stratosphere has decreased, due to increases in CFC’s (chlorofluorocarbons) – Chemicals that contain carbon, chlorine, hydrogen, and fluorine ▪ Chlorine breaks the O3 molecule into O and O2 ▪ CFC’s can originate from refrigerants, aerosol sprays, cleaning solvents, and bubbles in foams ▪ Depletion of Ozone became important topic in 1970’s and 1980’s 8 Ozone Layer Depletion Ozone Layer Depletion: Why Antarctica? ▪ Chlorine breaks up from CFC’s more easily at colder temperatures ▪ The presence of polar stratospheric clouds helps speed up the chemical reactions that remove chlorine from CFC’s Stratospheric Cooling ▪ One reason that the stratosphere may cool is due to a decrease in ozone concentration ▪ Another is due to an increase in anthropogenic greenhouse gases ▪ As CO2, CH4, N2O, and other greenhouse gas concentrations go up, less outgoing infrared radiation reaches stratosphere to be absorbed Aerosols ▪ Microscopic solid or liquid particles suspended in the atmosphere, ranging in size from about 0.01 to 10 micrometers ▪ Can act as CCN (meaning?) ▪ Also alter the earth’s radiation budget, through their direct and indirect effects Aerosols ▪ Short atmospheric lifetime, on the order of days to a couple weeks (in the troposphere) – Can stay in the stratosphere for a few years ▪ Can then go back into ground or ocean Aerosol Examples ▪ Dust ▪ Smoke (Sulfate, Black Carbon, Organic Carbon) ▪ Salt 15 MET 112 Global Climate Change 16 MET 112 Global Climate Change Pollution ▪ In our atmosphere, we usually “see” pollution as large clusters of aerosols ▪ Humans can’t see greenhouse gases Aerosol Direct Effect ▪ Can reflect shortwave radiation ▪ Can absorb shortwave radiation ▪ Can absorb longwave radiation ▪ In general, the net effect of aerosols on the energy budget is cooling Aerosol Direct Effect Aerosols ▪ As a result of human activities, they have increased in concentration since the mid 1800’s (Industrial Revolution) Aerosols: Two Indirect Effects ▪ First Indirect Effect: – Increasing anthropogenic aerosols cause a decrease in droplet size (Twomey 1974), resulting in a brighter cloud (higher albedo). – Cooling Effect – Radiative Forcing: Negative ▪ Second Indirect Effect: – Reduced cloud droplet sizes cause a decrease in precipitation efficiency, a longer lasting cloud (Albrecht 1989), as well as a thicker cloud (Pincus and Baker 1994). Sources of Aerosols ▪ Natural sources include – Land (dust) – Ocean (salt) – Volcanic eruptions (smoke) ▪ Anthropogenic sources include – Factories – Cars – Power plants – Deforestation Aerosol Observations from NASA Satellite 23 Cedar Fire 24 Cedar Fire 25 26 Sulfate Aerosols ▪ Natural source: volcanoes ▪ Anthropogenic source: burning of fossil fuels (hydrocarbons), such as coal and oil ▪ The overwhelming amount (90%) of sulfate aerosols are anthropogenic – Example: SO2 released during coal combustion ▪ Reflect incoming solar radiation. ▪ Total effect on Earth’s energy budget – Cooling – Radiative Forcing: Negative 27 Black Carbon Aerosols ▪ Natural source: biomass burning (such as forest fires) ▪ Anthropogenic sources: incomplete combustion from coal and diesel engines, human-induced biomass burning (deforestation) ▪ Also known as “Elemental Carbon” or “soot,” it is potentially harmful if inhaled ▪ Absorbs solar radiation, due to a low albedo. ▪ Total effect on Earth’s energy budget – Warming – Radiative Forcing: Positive 28 Organic Carbon Aerosols ▪ Natural source: Biomass burning ▪ Anthropogenic sources: Fossil fuel burning, deforestation ▪ These aerosols are reflective. ▪ Total effect on Earth’s energy budget – Cooling – Radiative Forcing: Negative 29 Other Aerosols ▪ Sea Salt – Comes from ocean (natural source) – Highly reflective (albedo up to 0.97) – Total effect on Earth’s energy budget ▪ Cooling ▪ Radiative Forcing: Negative ▪ Dust – Comes from natural (soil, volcanic eruptions) and anthropogenic sources (homes, offices) – Uncertain effects on Earth’s energy budget 30 Radiative Forcing ▪ A change imposed upon the climate system which modifies the Earth’s energy (radiative) balance. ▪ Usually given in units of Watts per Square Meter (W/m2). 31 Figure 3.38 Earth’s annual energy budget Radiative Forcing ▪ Changes in individual forcings produce changes to Earth’s energy budget. ▪ The magnitude of the radiative forcing determines how strong the effect is. ▪ Radiative forcing is computed by comparing preindustrial energy balance (1750) with today’s energy balance (2000) 33 Enhanced Greenhouse Gases ▪ Greenhouse gas concentrations have greatly increased over the last 150 years ▪ Radiative Forcing: Positive 34 Ozone ▪ Exists in the upper atmosphere – Stratospheric ozone levels had been declining from 1970 to 2010 – Radiative Forcing: Negative ▪ Exists in the lower atmosphere – Tropospheric ozone can mix with fog and/or haze to produce smog – Tropospheric ozone levels have increased over the last 50 years – Radiative Forcing: Positive 35 Land Use Change ▪ Changes in land use have contributed to albedo changes ▪ Deforestation has been the largest contributor ▪ High latitudes have been most affected – Pre Industrial: Snow covered forests (lower albedo) – Current: Open snow covered areas (higher albedo) ▪ Large uncertainties exist in details of the changes ▪ Radiative Forcing: Negative 36 Radiative Forcing from the IPCC Radiative Forcing from the IPCC 1 2 3 4 5 6 7 8 9 10 Radiative Forcing from the IPCC What does this part of the diagram mean? Increases in atmospheric CO2 concentration between 1750 through 2000 have caused about a 1.66 W/m2 increase in the earth’s radiation budget. This term by itself would warm the earth’s surface. Radiative forcing value is between 1.49 – 1.83 (average is 1.66) Example ▪ Imagine that you and your friend get offers to work for a new environmental company. They use a pay scale with ‘incentives’. You will get paid $35,000  5,000 depending on your performance, and your friend will get paid $75,000  60,000. Calculate you and your friends’ total salary. ▪ The total (combined) salary with no ‘incentives’: – $110,000 ▪ Your salary ranges from – $30,000 to $40,000 ▪ Your friend’s salary ranges from – $15,000 to $135,000 ▪ Maximum possible combined salary: – $175,000 ▪ Minimum possible combined salary: – $45,000 ▪ So, the total possible salary for you two is between – $45,000 and $175,000 Big Uncertainty! Radiative Forcing from the IPCC 1 2 3 4 5 6 7 8 9 10 Metr 112: Activity 6 1. Calculate the best estimate of the total radiative forcing from the provided figure of individual radiative forcings from the IPCC (see slide 37 of Lecture 8). Hint: You don’t need to include the “net anthropogenic” term when conducting this calculation. Please show your work. (3 points) 2. Calculate the range of possible values for the total radiative forcing. Please show your work. (3 points) 3. What conclusions does the total radiative forcing tell you about how the climate has changed? Use 2-4 sentences for your response. (2 points) 4. How does the range of values (or uncertainties) affect the above conclusions? Use 2-4 sentences for your response. (2 points) Discussion Questions 1. Define “radiative forcing” in your own words. 2. Why is the ozone layer beneficial for humans? At what altitude is it found? 3. Why did ozone depletion become an important topic in the 1970’s and 1980’s? 4. Explain the direct and indirect effects of aerosols. 5. Give 5 examples of aerosols. For each, provide a source and the sign of the radiative forcing. 6. Which radiative forcing term has the largest magnitude? Comment on the significance.

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