Lecture 32Today: Projections of global warming. Some impacts.Tomorrow: Arguments of global warming “skeptics”Monday: Impacts of global warming in the Pacific Northwest. by Dr. Phil Mote (state climatologist for WashingtonState).Next week: More impacts and policy.Human C inputs & where they goInputs: (see Fig 16-1 & text):6.1 GtC/yr burning fossil fuels:2.0 GtC/yr deforestation, land use changeOutputs:3GtC/yr accumulates in the atmosphere2.5 GtC/yr absorbed by the oceans2.6 GtC/yr taken up by soil/forests(i.e. land biosphere is a net sink of 0.6 GtC/yr = 2.6-2.0 GtC/year)Over different timescales (10-106 years), atmospheric CO2will be removed by different processes, which we now considerNote: exact distribution between ocean & forest regrowth uptakeis not precisely known. The above are rough estimates.Box Fig 16-2:Long-term CO2 projections1) Uptake by surface ocean (temporary)Residence time of C with respect to surfaceocean = 760 GtC/ (90 GtC/yr)~ 8 years (exchange time)2) Tranfer to deep ocean (temporary)~100s -1000 yrs via thermohaline circ.3) Dissolution of seafloor carbonatesCO2 + CaCO3 +H2O = Ca2+ + 2HCO3-1000s of years (still temporary)4) Weathering of continental rockscarbonate weathering (same chemistry as 3) temporary, 1000s of yearssilicate weathering permanent removal, 105-106 yearsREMOVAL MECHANISMSassumes fossil fuel reservoirconsumed in next 400 yrsSimplified CO2 scenariosCO2 emissionsresulting CO2 concentrationsa. stop all emissions immediatelyb. begin emissions reduction immediately and go down to 1 Gton C/yr by 2100c. freeze at 1990 emissions (Kyoto Protocol for entire world)d. business-as-usual (CO2 doubles by about 2050)Question: Which of these are realistic or feasible?Estimating the future: other GHGsCO2 is not the whole story.We need to consider1) other greenhouse gases2) aerosols2001 Increased radiative forcing (W/m2)since 1750 (pre-industrial)CO2 1.46 CH4 0.48 N2O 0.15 tropospheric ozone (O3) 0.35 Fig 16-4Right: radiative forcingdue to CO2, CH4, N2Oover last 1000 yearsnon-CO2 forcings: other GHGsother greenhouse gases: • CH4 (methane) Rice paddies (40% of emssions), cow/ruminant flatulence (15%), (world population of cows = 1.3b, 100m in the USA), landfills, coal/gas/oil production• N2O (nitrous oxide) Denitrification (the process by whichbacteria break down nitrogen in organic matter and release N2 and N2O), nitrate fertilizers. (30% anthropogenic, 70% natural).• O3 (tropospheric ozone) Not directly emitted. Formed in photochemical reactions from human and natural precursors. (e.g., reactions between human-emitted NO and hydrocarbons can lead to ozone).(10 ppb in remote areas, 100 ppb downwind of metropolitan areas)• Halocarbons (CFCs and halons) Refrigerants, blowing agents, etc (all human)non-CO2 forcings: aerosolsaerosols: • direct effect: reflect sunlight back to space• indirect effect: modify clouds (more droplets) causing increase in albedo• The addition of aerosol forcing played a key role in the IPCC 1995 report.• Gave greater confidence that the models showed the "signal" of anthropogenic influence on climate had been detected.IPCC 1990: "generally consistent"IPCC 1995: "a discernable influence"IPCC 2001: "new and stronger evidence that most of the observes warming over the past 50 years is attributable to human activities."•Problem: forcing estimates for aerosols are poorly known +0.1 to -3 W/m2 (see Lecture 30, Tad Anderson).• NOTE: However, aerosols get washed out of the atmosphere in ~1 week, butgreenhouse gases continue to accumulate.• If aerosols have a strong negative forcing, climate sensitivity could be higherthan we think.effect of climate sensitivity, lEven for the sameforcing increase,different modelswill give differentmean global temperatureincrease dependingon l.IPCC 2001, SPM, Fig 3 (or textbook Fig 16-5): ForcingsNeed to add all these up to get the overall radiative forcing change, DFPaths to CO2 stabilizationQuestion 1: But what level of CO2constitutes "dangerous interferencewith the climate system"?Question 2: What emission pathwaykeeps us below that level?Note: Question 1 is far moredifficult for science to answer.UN Framework Convention onClimate Change calls to avoid“dangerous interference with theclimate system”UNFCCC signed by 162 countries,including the US, at the 1992 Rio EarthSummit.http://www.unfccc.de/index.htmlFig 16-14Non-CO2 ForcingsTable 2: Climate forcing scenarios Scenario 2050 Emissions (Gton C/yr) 2050 Concentration (ppm) 2050 CO2 forcing (W/m2) 2050 Total Forcing (W/m2) B1 11 485 3.0 3.3 A1B 16.5 520 3.4 4.1 A1F1 24 560 3.7 4.8 DT = l * DF * lag_factor Global Warming Forecast Equation• The current consensus view is that non-CO2 forcings (positive GHGs andnegative aerosols) add up to a net forcing close to zero.( Actually depends on what the aerosol forcing contribution is).•In any case, the question of forcing is primarily a question of CO2 concentration.small difference, i.e.CO2 dominatesIPCC Emission Scenarios: B1 = population peak in mid-21st century, more clean energyA1B = rapid economic growth, balance of energy sourcesA1F1 = rapid economic growth, fossil fuel intensiveGCMs for climate predictionGCMs = General CirculationModelsAtmosphere-Ocean GCMs are used.(see Chap. 6 of the textbook).These computer models calculate thephysics and chemistry of theatmosphere and ocean. The world isdivided up into a 3-D gridand energy and mass fluxes betweengrid boxes are calculated in time.IPCC PredictionsIPCC emission scenarios:CO2, CH4, N2O,SO2 (aerosols)GCMs used byresearchersaround the worldResulting predictions:- global temperature(also regional climate(precipitation, etc.))Emission Scenariosa = major CO2 controls & modest population and economic growthd = business as usual: large growth, fossil fuel intensive IPCC reportFig. 16-6Comparison against recent past changeObservations of climate quantities from numerous sites around the globe are available fromrecent decades. Some records also go back a over a hundred years.Models are compared against a range of different indicators (such as surface temperature,rainfall and surface pressure).Comparison against observed climate variabilityThe climate varies naturally from day to day, month to month, year to year, and on longertimescales. Climate models incorporate enough realistic physics to make sure they can crediblyreproduce the observed