Estimating contemporary methane emissions from tropical wetlands using multiple modelling approaches

Abstract

Estimating contemporary methane emissions from tropical wetlands using multiple modelling approaches
Methane gas makes a significant contribution to the greenhouse effect happening on the planet. In modern times, natural wetlands are the biggest source of methane gas as compared to other sources of natural origin. Emissions of methane gas from wetlands rely on the climate and have the ability to give a positive feedback to the changing climate. Nevertheless, there is lot of uncertainty associated with the strength of both the natural emissions and the feedback. Upscaling from site-based observations, process based models and inverse models have variable estimations of methane emission regionally as well as globally. Very few studies are available on tropical regions as compared to higher latitudes. This study has estimated methane emissions from tropical wetlands (30 degrees S to 30 degrees N) by extrapolation of fluxes based on wetland types using the Lehner and Doll (2004) wetland dataset. The information on the tropical wetland vegetation was also derived from the upscaling study carried out and was used to run a version of the LPJ-GUESS model developed for northern wetlands (LPJ-GUESS WHyMe) to estimate methane emissions from the tropics at a resolution of 0.5 degree latitude 0.5 degree longitude. Modelled heterotrophic respiration from LPJ-GUESS (version 2.1) was also used to assess methane release from the tropics in a simple parameterization in which a fixed carbon conversion factor was used to divert 4.15% of the respiration to methane release. The estimated annual methane emissions from tropical wetlands using upscaling of site based fluxes ranged from around 51 to 183 Tg CH₄ per year with largest emissions coming from ‘swamp forest, flooded forest’ wetland type. This range is largely in agreement with inverse modelling studies carried out in recent years. Modelled emissions from LPJ-GUESS WHyMe for the tropics was too high, at 566 TgCH₄ per year, whereas estimation from the simpler method using modelled heterotrophic respiration was 110 Tg CH₄ per year, which is also largely in agreement with inverse modelling studies carried out in recent years. Simulations with LPJ-GUESS WHyMe highlight the need for specific tropical wetland vegetation and their associated parameters, better water table calculations, seasonal inundation inputs, and more analysis of the factors governing the CO₂ to CH₄ production ratio in tropical regions. These factors combined could reduce the emissions to more reasonable values.

Abstract (Swedish)

Metangas bidrar med en signifikant andel till den rådande växthuseffekten på jorden. I nutid är den största källan till utsläpp av metangas våtmarker i jämförelse med andra naturliga utsläppskällor. Emissioner av metangas från våtmarker är beroende av klimatet och har den egenskapen att ge en positiv återkoppling till klimatändringen. Icke desto mindre finns det dock stor osäkerhet associerat med både storleken på de naturliga utsläppen och hur mycket dess återkoppling är. Uppskalning från lokalbaserade observationer, processbaserade modeller och inverse modeller har olika uppskattningar på metanutsläpp, såväl regionalt som globalt. Väldigt få studier finns tillgängliga från tropiska områden jämfört med högre breddgrader. Den här studien har beräknat metanutsläpp från tropiska våtmarker (30 grader S till 30 garder N) genom att extrapolera flöden baserade på våtmarkstyper med våtmarksdataset från Lehner och Doll (2004). Information om den tropiska våtmarksvegetationen härstammar även från uppskalningstudier och användes till att köra en version av LPJ-GUESS utvecklad för nordliga våtmarker (LPJ-GUESS WHyMe), för att uppskatta metanemissioner från tropikerna med en upplösning på 0,5 grader latitud * 0,5 grader longitud. Modellerad heterotrof respiration från LPJ-GUESS (version 2.1) användes också för att beräkna metanutsläpp från tropikerna med en enkel parametrisering, i vilken en fix koldioxidfaktor användes till att avleda 4,15 % av respirationen till metanutsläpp. De från uppskalningsmetoden beräknade årliga etanemissionerna i tropiska våtmarker varierade mellan ca 51 till 183 Tg CH₄/år, där de största utsläppen kom från typer av våtmarker som sumpskog och översvämmad skog. Detta intervall stämmer bra överens med de från inverse modellering utförda under de senaste åren. Beräknade emissioner från LPJ-GUESS WHyMe i tropikerna var betydligt större, nämligen 566 Tg CH4/år, medan beräkningar från den enklare modellerade heterotrofa respirationen var 110 Tg CH₄/år, vilket också stämmer bra överens med studier gjorda de senaste åren av inverse modellering. Simuleringar utförda med LPJ-GUESS belyser behovet av specifik tropisk våtmarksvegetation och relaterade parametrar, till exempel bättre beräknade grundvattennivåer, årliga översvämningsdata och mer analys av de faktorer som styr förhållandet mellan CO₂ och produktion till metan i tropiska områden. Dessa faktorer kombinerade skulle kunna minska utsläppen till mer rimliga värden.

Abstract

Popular science
Global warming is a significant environmental problem now-a-days. Methane is a major greenhouse gas causing global warming by trapping long wave radiations from the Earth. Presently, natural wetlands are responsible for the largest amount of emissions of this gas when compared to other natural sources. Changing climate impacts the emissions from wetlands which in turn further intensify the climate change.

Upscaling methods, use of process based models and inverse models have lot of variation in their assessment of these emissions. Tropical regions have very few studies as compared to higher latitudes. This study has estimated these emissions using upscaling method as well as process based model. Upscaling method is based on extrapolation. Process based model LPJ-GUESS capable of simulating methane emissions from northern wetlands was used for tropical region for the first time by incorporating information on the tropical wetland vegetation. Another simple approach for simulating these emissions was also used where a previous version of LPJ-GUESS without methane emissions module was used. Simulated heterotrophic respiration from the model was used to estimate the release of methane from the tropics by diverting a predicted 4.15% of respiration to methane release.

‘Swamp forest, flooded forest’ wetland type was the largest emitter of methane found in extrapolation method used. Annual methane emissions ranged from around 51 to 183 Tg CH4 per year which is largely in agreement with recent inverse modelling studies. LPJ-GUESS with methane module highly overestimated at 566 TgCH₄ per year whereas simpler method used for LPJ-GUESS without methane module gave a reasonable value of 110 Tg CH₄ per year, again largely in agreement with recent inverse modelling studies. This study can be used to develop LPJ-GUESS model with methane module further to work for tropical region as well.

• author: Lamba, Shubangi
• supervisor: Paul Miller
• organization: Dept of Physical Geography and Ecosystem Science
• alternative title: Large methane emissions from tropical wetlands
• course: NGEM01 20111
• year: 2012
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: LPJ-GUESS, uppskalning, modellering, tropisk våtmark, metanutsläpp, methane emissions, tropical wetlands, modelling, upscaling, physical geography, ecosystem analysis
• publication/series: Student thesis series INES
• report number: 256
• funder: European Commission, Erasmus/Socrates Program
• language: English
• additional info: Funder: Erasmus Mundus External Cooperation Window Lot 15 India
• id: 3095362
• date added to LUP: 2012-09-25 12:18:04
• date last changed: 2012-09-25 12:18:04