1Announcements• Homework 1 will be posted online today; duein class in 1 week.• Presentation topics will be posted Friday;please email us (all 3) with your top threepicks and the name of your partner (if youknow this). We will assign topics Tuesday ifwe do not hear from you.• Did you receive my email yesterday? If not,send me your contact information please -thanks.Radiative Forcing and Global Change• Earth’s energy budget• Forcing vs feedback• Electromagnetic radiation - basics• Calculating temperature fromradiative forcing• Aerosols• Greenhouse gasesImage Courtesy of SOHO/EIT. NASANASA2Earth’s energy budgethttp://www.agci.org/classroom/atmosphere/index.phpFeedbacks vs forcings as causes ofenvironmental change• Forcing: a primary cause of change– Changing solar radiation or other energy imbalance• Feedback: process triggered by an initial change, whicheither amplifies or dampens the initial change• A positive feedback enhances the initial change:– example: warming climate melts ice, which reduces albedo, furtherwarming the planet.• A negative feedback dampens the initial change:– example: as atmospheric CO2 increases, more plants may grow, takingin CO2 and reducing atmos. levels.Most predicted climate change is the result of feedbacks,which tend to be positive (for climate)What is radiative forcing?• Net (down-up) change inradiation, measured attropopause• Used to compare differentcauses of changes inradiative balance– E.g. solar intensity,greenhouse gases, aerosols.• Measured in W/m2 (wattsper meter squared)• Positive = expected warming, negative = expectedcoolingElectromagnetic radiation•! All objects with temperature > absolute zero (0 Kelvin)emit electromagnetic radiation• Electromagnetic radiation travels at the speed of light(c = 3 x 108 m/sec) and is characterized by awavelength (λ) and a frequency (υ): λυ = c• The important wavelengths of radiation, for ourpurposes, are in the near-infrared to ultraviolet range(0.2-100µm).Shortwave - UV and visible; the Sun’s radiationLongwave - near IR to IR; the Earth’s radiationLow-frequency, Long wavelength High-frequency, Short wavelength3http://universe.gsfc.nasa.gov/lifecycles.htmlLow energyHigh energySimple behavior of radiation• Reflection– No change in wavelength– No impact on surface• Absorption– Reradiation at newwavelength - depends ontemperature– Changes temperature ofsurfaceOutgoingwavelengthIncomingwavelength• Refraction– Change in angle asradiation passes thrumaterial (not wavelength)Blackbody radiation a helpful idealization: absorbs all radiation that falls on itand re-emits at wavelength that depends on temperature• Wavelength of max emission depends on temperature• Intensity depends on temperature (thus on wavelength)• Sun: T about 6000K; wavelength about 0.5 µm• Earth: T about 300 K, wavelength about 10µmArea under curve =amount radiationemittedRelevant laws and equations• Objects radiate at a wavelength that is a function oftheir temperature, according to Wien’s law:λ(max) = 2898 µm K/T (where T is in Kelvin, K)• The Stefan-Boltzmann law says that the flux ofradiation is proportional to temperature: F = σT4 (where σ is the S-B constant = 5.67x10-8 W/m2K4)Reminder: temperature scale in Kelvin has absolute zero as 0, whichis -273°C. One degree C is the same as one Kelvin.4Radiation-temperature calculationAssuming steady state, radiation (in) = radiation (out), wecan calculate temperature of Earth from simple radiationparameters.Incoming from Sun: 1370 W/m2 (treating Earth as a flatdisk facing Sun, which is how it receives energy).Area of disk = πR2Outgoing from Stefan-Boltzman law: Flux = σT4This occurs over full surface area of Earth(surface of a sphere = 4πR2) Rad (in) = Rad (out) πR2 x 1370 W/m2 = 4πR2 x σT4Solve for T… answer is around 5°C. Is this right?Albedo…Earth reflects 30% ofincoming radiation backto space.How to account for this?Use 70% of the solarconstantDoes this help or make iteven worse?http://veimages.gsfc.nasa.gov/2429/globe_west_540.jpgRadiation scenarios so far…IncomingAll absorbedReradiatedIncomingReradiatedReflected70% absorbed,30% reflectedEarth T ~5°C Earth T ~-18°COther complications?• Greenhouse gases: these absorb much of theoutgoing longwave radiation and send it backto Earth surface• Water vapor most important; also CO2, CH4,O3, N2O, CFC’s…•Naturally occurring greenhouse gases causethe Earth’s temperature to be about 33°Cwarmer than it would otherwise be due toradiation alone.–(this number includes the longwave effect ofclouds)5Radiative forcing and climate change• Temperature record reflects result of changes inforcings AND feedbacks• How has radiative forcing changed ?• How will it change in future?NASA/GISShttp://carto.eu.org/article2504.htmlIPCC 2007Anomaly for 2005relative to 1750LOSU = level ofscientific understandingSurface albedo• Fresh snow: 75-95%• Old snow: 40-60%• Sand: 18-28%• Grassland, open forests, crops: 15-25%• Cities: 14-18%• Dense forest: 5-10%• Water 2-6%• Average Earth surface: 15%• Average top of atmosphere: 30%Why are last two different?Cloud radiative forcingsALBEDO:• Thin stratus: 30%• Thick stratus: 60-70%• Cirrus: 20%• Nimbostratus: 70%Clouds also absorb infrared radiation and reradiateto space at colder temperatures (greenhouse)High clouds: greenhouse >> albedo; net warmingLow clouds: albedo >> greenhouse; net coolingEffects are large in both directions; total effect ofclouds today is a small cooling (but can change)6Radiative forcing: a changing Sun• Solar radiation varies on all time scales, but particularly on decade-century.• High sunspot activity: higher radiative output– dark spots are surrounded by brighter faculae• Total amount of radiation variance is small: 0.2-0.4% over centuries– Long-term variability is really hard to pin down• Can a small change in Sun’s output create observable changes inEarth’s climate?http://www.windows.ucar.edu/tour/link=/sun/activity/solar_variation.htmlSolar irradiancechanges since 1600Lean 2010• Sunspot cycles wellobserved• Background changes lesswell known• Big uncertainty overcenturies• Paleodata suggest a link toclimate that is small butvisible• How can small changes(0.2%) influence climate?– Stratosphere; circulation;cloudsHas the Sun caused the recentwarming of climate?• NO - if it had,atmosphere would bewarming