IB›IB Physics SL›Mind mapsB.2 Greenhouse effectIB Physics SL: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsEnergy balanceEquilibrium: power absorbed = power radiatedSolar constant about 1361 W m⁻²Mean incoming intensity = S4\frac{S}{4}4SEarth intercepts over πR2\pi R^2πR2, radiates from 4πR24\pi R^24πR2Albedo, emissivityAlbedo = scattered power ÷ incident powerEarth's mean albedo about 0.3Clouds and ice raise albedoe=power per areaσT4e = \frac{\text{power per area}}{\sigma T^4}e=σT4power per areaEquilibrium: (1−α)S4=eσT4(1 - \alpha)\frac{S}{4} = e\sigma T^4(1−α)4S=eσT4Greenhouse gasesMethane: wetlands, livestock, landfillCarbon dioxide: fossil fuels, deforestationNitrous oxide: fertilisers, soil bacteriaWater vapour: evaporation, human effect mostly indirectGreenhouseeffectCO₂CH₄H₂ON₂OInfrared absorptionSun's radiation peaks in visible, passes throughEarth (about 288 K) emits infraredVibrational energy levels match infrared photonsBonds driven into resonance at natural frequencyRe-emission in all directions, some back downEnhanced effectHuman activity raises greenhouse gas concentrationsCO₂: about 280 ppm to over 420 ppmSurface temperature without effect: about 255 KMelting ice lowers albedo: feedbackExam tipsFactor is 4, not 2, for mean intensityModel limits: global averaging, feedbacks, ocean time lag