The next 2 weeks. Reading: IPCC (2007), Chap 7 (sections 7.4 and 7.5)

Transcription

1 PCC 588 Jan 15 The next 2 weeks Th. Jan 15: non-co 2 greenhouse gases CH 4 and N 2 O Tu. Jan 20: non-co 2 greenhouse gases: ozone, halocarbons Th. Jan 22: Aerosols and Climate Tu. Jan 27: Paper discussion Reading: IPCC (2007), Chap 7 (sections 7.4 and 7.5) Centuries Decades Months/Weeks Weeks/Days IPCC (2007) SPM.2 Radiative forcing for 2005 relative to 1750: CO 2 = 1.66 W/m 2 CH 4 +N 2 O+halocarbons+tropospheric ozone stratospheric ozone = = 1.28 W/m 2 1

2 TIME SCALES FOR HORIZONTAL TRANSPORT 1-2 months 2 weeks 1-2 months 1 year Atmospheric Chemistry and Greenhouse Gases Natural emissions Anthropogenic emissions CH 4 N 2 O Chemical + physical loss Anthropogenic emissions Chemical production Chemical production CFCs halons HFCs PFCs SF 6 Stratospheric ozone Anthropogenic emissions (CH 4, NOx, hydrocarbons) Tropospheric ozone Chemical loss Anthropogenic emissions (halocarbons) Chemical + physical loss Chemical + physical loss 2

3 Human influence on Methane Increase from 700 ppb 1770 ppb Historical methane trend Recent methane trend IPCC, 2001 Biological formation of methane Anaerobic decomposition of organic material by methanogenic bacteria (flooded soils, wetlands, landfills, digestive tracts..) CH 2 O ½CH 4 + ½ CO 2 Methane production is temperature dependent, with maximum growth C 3

4 Present-day global sources of CH 4 (Tg CH 4 /yr) Natural Sources Wetlands Termites Ocean Hydrates Geological sources Wild animals Wildfires 200(±80) Anthropogenic Sources 330 (±100) Fossil fuels 100 Landfills & Waste Ruminants Rice agriculture Biomass burning % of present sources are anthropogenic + direct aerobic emissions from plants ( Tg/yr) [Keppler et al., 2006] highly controversial IPCC, 2008, Chap 7 CH 4 concentrations (ppb) Changes in methane since 1900 x 3! Atmospheric methane: Its role in the global environment Khalil (ed.) 2000 x 2! 4

5 Methane sinks Dominant loss (90%) of methane by oxidation in the troposphere: CH 4 + OH CH 3 + H 2 O CO 2 + products ~ 8 year lifetime Minor sinks (~10%) Soil uptake (biological oxidation in soils) Chemical reactions in the stratosphere Production of tropospheric OH O 3 + sunlight O 2 + O* 290 nm <λ<330 nm ~1% O* + M O + M M ( third body ) = O 2 or N 2 O+ O 2 + M O 3 + M O* = Electronically excited oxygen, O( 1 D) O* + H 2 O OH + OH (~99%) Solar radiation (290<λ<330 nm) (~1%) 2 OH 5

6 Sinks of OH Reactions with CO and CH 4 are the dominant sinks of OH in the troposphere: CO + OH CO 2 + H CH 4 + OH CH 3 + H 2 O (~30% of OH sink) In the lower troposphere over continents, reaction with nonmethane hydrocarbons (NMHCs) is also important: NMHC + OH products Lifetime of OH ~ 1 second! Chemical feedback of methane on its lifetime Increase in CH 4 emissions increase in CH 4 concentrations decrease in OH levels increase in the lifetime of CH 4 further increases in CH 4 concentrations (positive feedback) Example: 1% increase in CH 4 results in a 0.32% decrease in OH effective CH 4 lifetime ~ 12 years (compared to 8 years without feedback) 6

7 Seasonal cycle and interhemispheric gradient Northern Hemisphere Global Southern Hemisphere Dlugokencky et al., 1994 Satellite observations of CH 4 ( ) Frankenberg et al., JGR,

8 How is the CH 4 budget determined? Ice-core record assess human influence Bottom-up studies: Extrapolate direct flux measurements and observations Process-based models (1,2,3-box models or global chemical transport models) Inverse modeling (top-down approach): Spatial distribution of observations latitudinal distribution of sources Use temporal evolution of CH 4 sources Use observations of isotopes: Separate fossil fuel and biomass burning from bacterial sources 13 C-CH 4 : enriched = natural gas, biomass burning vs. depleted = bacterial sources 14 C -CH 4 : enriched (modern) vs. depleted (fossil) Atmospheric 13 CH 4 over the last millennium Houweling et al., GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 22, C-CH 4 : enriched = natural gas, biomass burning vs. depleted = bacterial sources 8

9 IPCC A1B projection (1840 ppb) Recent trends in CH 4 decreasing rate of increase? large interannual variability Effects of climate of CH 4 emissions Effect of climate on biogenic CH 4 emissions from natural (wetlands) and anthropogenic (landfills, rice agriculture, biomass burning): temperature and moisture Wetlands: in a warmer+wetter world increase extent of wetlands and their CH 4 emissions (30-80% increase for a C warming) [IPCC, 2007] Climate affects stability of methane hydrates in ocean sediments highly uncertain but potentially large 9

10 Learning from the past The ice core record of CH 4 : glacial-interglacial cycles Spahni et al., 310 (25), Science, Rapid variations between 350 ppbv and 700 ppbv during glacial-interglacial + remarkable correlation with temperature proxy How can we explain the changes in CH 4 between glacial and interglacial periods? Change in wetland area and productivity caused by changes in temperature and precipitation Changes in CO 2 Changes in the sink of CH 4, the hydroxyl radical 10

11 Changes in wetlands CH 4 emissions from wetlands Present day Last Glacial Maximum 15% larger wetland area but 24% lower CH 4 emissions Kaplan, Geophys. Res. Lett., 29, explains only half of the 50% decrease between present day and LGM did OH change too? Nitrous oxide (N 2 O) 0.8 ppbv/yr increase ~0.2%/year ~15% increase since pre-industrial times 11

12 Nitrification and denitrification: microbial source of N 2 O in soils and oceans Denitrification: NO 3 - N 2 anaerobic conditions N 2 O Nitrification: NH 4 + NO 3 - Oceans: nitrification ~ 4 TgN/yr Soils: nitrification and denitrification ~7 TgN/yr Anthropogenic sources Fossil Fuel Combustion & Industrial processes Agriculture Biomass and biofuel burning Rivers, estuaries, coastal zones Atmospheric deposition Global budget of N 2 O (TgN/yr) (0.2-1) 2.8 ( ) 0.7 (0.2-1) 1.7 ( ) 0.6 ( ) Natural Sources Soils (undisturbed) Oceans Atmospheric chemistry (3.3-9) 3.8 ( ) 0.6 ( ) Sink: N 2 O photolysis in stratosphere (120 year lifetime) IPCC, 2007, Chap 7 12