Methane-related issues

	CH4 is of particular interest because of its importance in the 
radiation budget. A molecule of CH4 contributes about 20 times as much 
as a CO2 molecule to radiative forcing, over a 100 year timescale. It 
is also a key species in the chemistry of the atmosphere. CH4 has 
been the most rapidly increasing greenhouse gas, rising 145% since the 
beginning of the industrial revolution. After years of nearly steady 
growth rates, ~12 ppbv/yr in the 1980's, growth rates became highly 
variable.  CH4 increased up to 15 ppbv/yr in 1991 and 1998 and did 
not increase at all in 2000. The changes in growth rate for atmospheric 
concentrations are not well understood, and we cannot confidently 
predict future increases or decreases.  

	As an end product of anaerobic metabolism, CH4 is an analogue 
to CO2 in aerobic metabolism.  Aerobic consumption of CH4 is common 
in soils and aquatic enviroments and this consumption is a critical 
control on overall emissions.  The biochemistry of CH4 production 
and consumption at the cellular level is relatively well understood 
however the link between the microbial processes and ecosystem level 
processes such as net primary production are not well quantified 
and modeled.

	In the USA, over 70% of CH4 emissions are anthropogenic, 
dominated by biogenic sources.  The five major sources of CH4 in 
North America are (1) wetlands, (2) landfills, (3) enteric fermentation 
in animals, (4) animal waste disposal, and (5) fossil sources including 
leakage from natural gas distributions systems and coal production.  
Other sources such as (6) sewage disposal, (7) paddy rice cultivation, 
(8) biomass burning may be important regionally and/or seasonally.  
The agricultural sector accounts for ~50% of the human-induced CH4, 
and ~30% of total CH4 emissions in the US. Agricultural sources of 
CH4 include concentrated (e.g., feedlot) and diffuse (nonpoint source) 
emissions, which are affected by production practices such as 
applications of water, fertilizers, and manures.  Some of these sources 
are already well-quantified based on detailed process studies and extensive
statistics on activities of economic importance.  For example, production of 
CH4 directly from enteric fermentation in animals can be broken down by 
animal species on an annual basis to the county level (e.g. Westberg et 
al. 2001).  CH4 emission from waste lagoons and other waste processing 
facilities from animal management are not nearly as well quantified as 
the direct emissions.  Precise determination of agricultural CH4 
emissions is needed to quantify the North American and global carbon budgets.

	Natural wetlands account for more than 20% of the global CH4 source, 
largely from northern peatlands and tropical wetlands.  CH4 exchange 
from these environments is intimately linked to hydrology, system 
productivity and carbon accumulation and balance.  At the regional scale 
CH4 emission for many landscapes in N. Amermica are dominated by natural 
sources (wetlands, lakes and coastal waters). Different mixes of 
anthropogenic and natural sources and sinks determine the net fluxes in 
different regions.  For example, in New England peatland sources dominate 
CH4 emissions in Maine but landfills and energy use dominate in 
Massachusetts and south (Blaha et al.).  

	There are only two important sinks for atmospheric CH4.  The major 
sink is reaction of CH4 with hydroxyl radical.  The lifetime of CH4 
against this sink is approximately 8-10 years and accounts for ~90% of 
CH4 destruction.  CH4 is consumed by aerobic microbial activity in soils.  
The soil CH4 sink is surprisingly fragile and sensitive to management 
perturbations such as nitrogen fertilization (Mosier et al. 1990).  
Biological consumption of CH4 is critical to the regulation of almost 
all sources.  Net fluxes may be only 50% to 10% of gross CH4 production.

	The NACP requires detailed information on the sources of CH4 and 
estimates of these sources need to be resolved at a 500-1000 km scale 
spatially and sub-daily temporally to integrate with the atmospheric 
measurements at tall towers and aircraft profile sites.  The spatial 
requirements will be even finer for intensive campaigns over limited areas.  
The requirements for integration with atmospheric measurements demands a 
geographically resolved accounting of sources in the categories discussed 
above.  Process information may already be adequate to represent some 
of these sources such as direct emissions from animals (Westberg et al. 2001)
and rice paddies (Sass et al.).  In contrast, we do not have process models 
for reliable prediction of CH4 emissions from natural wetlands.  
Ultimately, we need adequate models to estimate CH4 sources and 
non-atmospheric sinks for 500-1000 km scale regions.  

	The NACP atmospheric measurements should be complemented by surface 
observations at representative sites to enable optimal evaluation of 
source/sinks. The challenge is to ensure that the combine atmospheric 
and surface measurements quantitatively resolve the major elements that 
produce the net flux in order to obtain accurate assessments of fluxes 
and their feedbacks.  Importantly, identifying the sources and developing 
long time series of observations (a stated aim of the program) will 
determine how well we will be able to develop models that quantify the 
variability and resolve processes at inter-annual to decadal timescales.

	Development of improved process models for a number of sources such 
as wetlands is required.  Model development depends upon the study of a 
number of well characterized sites with frequent flux meausurements.  For 
example, in natural wetlands appropriate measurement technologies such as 
automated chambers and eddy correlation could be applied to sites where 
ecosystem processes are also being well-characterized.  Models should include 
both gross production and the important consumption processes.  Small 
modifications in ecosystems such as changes in water level can lead to large 
modifications in the consumption processes leading to enormous changes in net 
fluxes. The new models should take some hydrological features, especially water 
table dynamics, into account.  Appropriate data on soil moisture and water 
table dynamics will be required.

	Using a combination of information on land use derived from economic 
statistics, land cover and use surveys, and remote sensing analysis and 
process models of CH4 emission, a continental bottom-up CH4 inventory 
will be constructed.  The bottom-up models can be compared directly to 
the analyses from atmospheric methods.  Additionally, the process models 
for CH4 may be analyzed in a data assimilation framework for integration 
with the atmospheric measurements.