The Present carbon cycle

(Flux in GtC/yr) 1 GtC (gigaton carbon) = 109 metric tons =1012 kg =1015 grams (1 petagram [Pg]) [1 Tg (teragram) = 1012 grams] Fast exchange (75 m.) (GtC) Permanent sink CO2 fixation Anthropogenic sources -fossil fuels + cement [6.3 GtC/yr] -permanent deforestation [1.6 GtC/yr] Fast exchange Permanent sink CO2 fixation Anthropogenic sources -fossil fuels + cement [6.3 GtC/yr] -permanent deforestation [1.6 GtC/yr] Fate of anthropogenic CO2: 42% stays in air longer than 1 yr 29% absorbed by ocean surface 29% Northern Hemisphere land Fast exchange Permanent sink Residence time (yr) = steady-state concentration (ppm)/ input rate (ppm/yr) Can also be defined with respect to output rate Can be well-defined if there is a known removal mechanism (example – CH4 is removed by reaction with hydroxyl radical in the troposphere in a reaction with established kinetics) For CO2, the removal mechanisms are complex: (i) photosynthesis cycle; terrestrial biosphere absorption (fast) (ii) dissolution in oceans (hundreds of years) (iii) reactions in ocean to equilibrate with CaCO 3 (thousands of yrs) (iv) weathering reactions (hundreds of thousands of years) •After 1000 years: 15-30% of CO2 remains in the atmosphere •After 100,000 years: 7% of CO2 remains in the atmosphere Long-term fate of atmospheric CO2 following a large release from fossil fuel burning today Climatic Change 90, 283-297 (2008) LOSU = assessed level of scientific understanding [At T = 288°K, Iout = 390 W/m2] Radiative forcing – imbalance in the energy equilibrium of Earth •change in solar insolation rate •change in IR absorption/reradiation (natural or anthropogenic) Requires then a reequilibration (fast in stratosphere; generally slow in troposphere) Human and natural influences affect the drivers for climate change, which then in turn affect RF and have lag effects (such as changes in evaporation). This leads to a large-scale climate change and response, later modified by human mitigation or a biogeochemical feedback in Nature. FEEDBACK – process whereby the result affects its origin (cause-effect loop) -can be positive (amplifies original phenomenon) or negative (dampens original phenomenon) ICE ALBEDO FEEDBACK original cause: anthropogenic fossil fuel burning affected climate change driver: CO2 trapping of IR light radiative forcing is the primary effect climate response: increased temperature feedback: positive: increased melting of ice decreases overall Earth albedo, leading to more overall absorption of Isolar, producing higher temperatures, leading to more ice melting… lag effect: positive, increased evaporation of H2O leading to further IR trapping.  Implicates another positive feedback: water vapor positive feedback. FEEDBACK – process whereby the result affects its origin (cause-effect loop) -can be positive (amplifies original phenomenon) or negative (dampens original phenomenon) EXAMPLES OF NEGATIVE FEEDBACKS hydrological cycle: prevents runaway of water vapor positive feedback radiative damping: increased T leads to increased Iout: Iout = seT4 LOSU = assessed level of scientific understanding [At T = 288°K, Iout = 390 W/m2] Radiative forcing – imbalance in the energy equilibrium of Earth •change in solar insolation rate •change in IR absorption/reradiation (natural or anthropogenic) Requires then a reequilibration (fast in stratosphere; generally slow in troposphere) Methane levels - 7.66 mm absorption band - about 2.5-fold increase since 1800 - leveling off since 1999 is not well understood, but may be decreased inputs to balance sinks (Bousquet et al., Nature 443, 439 (2006); Wuebbles & Hayhoe, Earth-Sci Rev. 57, 177-210 (2002)) Chief atmospheric sink for methane: CH4 + ·OH  ·CH3 + H2O (residence time ~ 12 years) •Effectiveness over 100 years: 23x higher than CO2 (per-molecule basis) •Note also the water generation (stratosphere) Nature 443, 71-75 (2006) also: Science 318, 633 (2007) •Warming-induced permafrost melting and lake expansion •Increased rates of methane bubbling •Methane exists as a caged frozen hydrate with ice (methane hydrate) •Methane hydrates may be the world’s largest fossil fuel resource!! Possible methane hydrate release from shallow ocean sediments This scenario is very unlikely in the next century But thawing of the Arctic permafrost and large-scale CH4 release is possible without coupling to sea level rise Sources of methane – anthropogenic and natural “marsh gas” Anthropogenic sources Natural sources Approximate overall estimates Natural: 140-235 Tg CH4/yr Agriculture: 141 Tg CH4/yr Anthro, non-Ag: 217 Tg CH4/yr What is the ultimate origin of CH4? ANAEROBIC CARBON AND HYDROGEN CYCLES Clostridia anaerobic environment •Reduced CH2O from photosynthesis undergoes fermentation (disproportionation) reactions to generate a variety of small-molecule products •Methanogenesis exploits this process: 4H2 + CO2  CH4 + 2H2O •Methanogens are chemoautotrophs and depend on H2 generated by other microorganisms, such as Clostridia, or on geologic H2. •Redox half-reactions: 4 H2  8 H+ + 8 eCO2 + 8 H+ + 8 e-  CH4 + 2 H2O E° ~ 400 mV E° ~ -250 mV Methanococcus jannaschii •Strict anaerobe •Optimal growth temperature = 85°C •Autotrophic, nitrogen fixer •4H2 + CO2  CH4 + 2H2O •Exclusively hydrogenotrophic •Central role in global carbon cycling Methanogens account for 70% of the total global methane source “black smoker chimney” East Pacific Rise Methanogenesis Microbial biogeochemistry: coupled reactions in distinct microorganisms Methane monooxygenase enzyme (MMO): catalyzes CH3OH production from CH4. Overall pathway: CH4 + O2  CO2 + H2O Methanococcus, Methanosarcina, etc Clostridia Anaerobic reactions in the lake sediment are dependent on the detritus from photosynthesis, which is drifting downward LOSU = assessed level of scientific understanding [At T = 288°K, Iout = 390 W/m2] Radiative forcing – imbalance in the energy equilibrium of Earth •change in solar insolation rate •change in IR absorption/reradiation (natural or anthropogenic) Requires then a reequilibration (fast in stratosphere; generally slow in troposphere) Nitrous oxide, N2O Bond stretch at 7.8 mm Angle bend at 8.6 mm Preindustrial concentration: 270 ppb Atmospheric half-life: 114 years Effectiveness compared to CO2: 296x •Anthropogenic driver: microbial (increased fertilizer use) •Implicates the global nitrogen cycle in climate change Global warming potentials for CFC’s and related compounds CFC’s data from IPCC4 •per-molecule effects can be several thousand fold greater than CO2. •residence times are 1-1700 yrs. •SF6: residence time = 3200 ; efficiency = 37000x CO2. Used as an insulating gas in semiconductor industry. Now recycled, not vented. Effects of ozone (O3). Stratospheric ozone – anthropogenic effect is toward cooling due to depletion by CFC’s and similar compounds. Tropospheric ozone – arises from photochemical smog as a secondary ground-level pollutant, from NO and volatile organics under the influence of sunlight. Significant warming effect. Controllable with smog reduction technology. Albedo – influence on heat balance •Varies greatly over the Earth’s surface (average = 30%) -as low as 6% in the oceans (most energy absorbed) -agricultural and forest land are low albedo (10-20%) -deserts are higher (25-40%) -global average for clouds (35-40%) -snow and ice cover (40-80%) Cropland/pasture in 1750: 6-7% of global land surface Cropland/pasture in 1990: 35-39% of global land surface Decrease in forest cover: by 8% of global land surface Estimated total effect: small net cooling (increased albedo) “Black carbon on snow” – increases melting, small net warming (decreased albedo) Solar irradiance changes (natural forcing) data from IPCC4 Possibly a small influence toward warming; poorly understood LOSU = assessed level of scientific understanding [At T = 288°K, Iout = 390 W/m2] Radiative forcing – imbalance in the energy equilibrium of Earth •change in solar insolation rate •change in IR absorption/reradiation (natural or anthropogenic) Requires then a reequilibration (fast in stratosphere; generally slow in troposphere) Effects of particles and clouds on albedo Cloud types •Clouds dominate albedo: reflect incoming sunlight, but reradiate the IR radiation coming from the Earth’s surface •Clouds form from water condensation, but depend also on particles •Biggest unknown in climate modeling is the effect of clouds, which in turn depend significantly on presence of aerosols Effects of particles and clouds on albedo Cloud types •Cirrus: very thin  little reflection of Isolar. But absorbs Iup (IR) from warmer ground and reradiates colder light  net warming. Reradiation to space is from cloud top, so that T is relevant. Jet airplane contrails Climate effects similar to cirrus clouds: cause surface warming. How? Jet emissions particles nucleate condensation of atmospheric water Dissipation within a few days: 9/11 effect Effects of particles and clouds on albedo Cloud types focused convection broad, diffuse convection •Cirrus: very thin  little reflection of Isolar. But absorbs Iup (IR) from warmer ground and reradiates colder light  net warming. Reradiation to space is from cloud top, so that T is relevant. •Cumulus and stratus: hold much more water. 2/3 of Earth albedo derives from these clouds, so very small changes in extent have large effects. Little IR effect. General effects of aerosols to promote cooling Particles in the atmosphere absorb light depending on their albedo Size of particle is important: larger particles absorb more light, and smaller particles tend to scatter (reflect) light. Most are submicron  scattering (cooling) dominates Black soot absorbs much light  local warming in the atmosphere Sulfate-rich aerosols reflect light because of high albedo  cooling effect on the air near ground level. Both natural and anthropogenic SO2 emissions oxidize to H2SO4 in small aerosol droplets. Overall, anthropogenic pollution increases particles in the atmosphere by an estimated 25-50% [“global dimming”] Natural condensation nuclei: sea salt, dust, pollen, smoke, biogenic sulfur Mount Pinatubo eruption, Philippines, 15 June 1991 ejected 10 Tg SO2 (1 Tg = 1012 g); slowed the rise of global warming Approximate albedo increase of 0.5%  T decrease of 0.5 K SO2 (g)   H2SO4 (aq)  acid rain H2S DMS FeS2 1012 gram = 1 teragram (Tg) Sulfur emissions occur from both anthropogenic and natural sources Anthropogenic generation of SO2 •Anthropogenic sources: from coal (mainly) and oil combustion •H2S is also an anthropogenic source (from oil and CH4 processing; natural gas reservoirs often contain significant H2S) Sulfide ores are also a source of SO2 when processed to yield the elemental metal, for example: 2NiS(s) + 3O2(g)  2NiO(s) + 2 SO2(g) •The SO2 can be oxidized to SO3, then converted to commercial H2SO4 •SO2 in power plant emissions is “scrubbed” by reaction with CaCO3 to yield CaSO4 (+CO2) •Paradox: reduced sulfur emissions alleviate particulate pollution, acid rain, but also exacerbate global warming. incorporation of S into protein, etc. (+6) 1. +6 -2 3. 4. 2. (-2) •Energy-requiring conversion from SO42- to H2S occurs in the sulfate-reducing bacteria (steps 1 - 4). •Very common in anaerobic marine sediments: SO42- + 2CH2O + 2H+  H2S + 2H2O + 2CO2 •The energy is provided by oxidation of reduced carbohydrate •Sulfate-reducing bacteria (SRB) often live in consortium with methaneoxidizing bacteria (methylotrophs) •If oxygen is available, methylotrophs carry out: CH4 + 2 O2  CO2 + 2 H2O •In a consortium of SRB with methylotrophs, anaerobic oxidation of CH 4 is coupled to reduction of SO42-. Example of syntrophy. Anaerobic oxidation of methane Syntrophic interaction between SRB and methylotrophs Nature 407, 623-626 (2000). Scale bar = 5m. Fluorescent probes to species-specific 16S rRNAs •Sulfate-reducing bacteria: SO42- + 2CH2O + 2H+  H2S + 2H2O + 2CO2 •If oxygen is available, methylotrophs carry out: CH4 + 2 O2  CO2 + 2 H2O •But in a consortium of SRB with methylotrophs, anaerobic oxidation of CH4 is coupled to reduction of SO42-. Occurs in marine sediments. Example of syntrophy. Blocks escape of CH4 to the atmosphere. •Proposed overall reaction: CH4 + SO42-  HCO3- + HS- + H2O •Issues: what intermediates are transferred? How to maintain favorable energetics? •Sulfide oxidation mediated by bacteria can occur from iron sulfide ores •H2S oxidation produces an acidic environment: acidothiobacillus also oxidizes Fe2+ to Fe3+. Oxidation can be via O2 or other oxidizers. Volatile Organic Sulfur bacterial bacterial H2S is rapidly oxidized photochemically in the atmosphere  inorganic S does not have a significant global effect But dimethyl sulfide (DMS) does enter the atmosphere  photochem conversion to CH3SO3- (methane sulfonic acid)  cloud condensation nuclei Link between methane production rates and industrial sulfur emissions? Schimel J, PNAS 101, 12400 (2004) industrial emissions microbial ecology Competition between methanogens and sulfate reducing archaebacteria, for nutrients in anaerobic environments, is affected by sulfur emissions. In turn, levels of atmospheric methane are affected.