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Archaea are a major evolutionary lineage distinct from bacteria. The cellular comparison here concerns the prokaryotic members of archaea; see two-domain system for broader clade usage. Their biological cells have a nucleoid rather than a membrane-bounded cell nucleus, yet aspects of DNA replication, transcription and biological translation resemble those of eukaryotes. Their characteristic lipids differ chemically from the usual bacterial membrane lipids, and their cell walls, when present, lack peptidoglycan. See OpenStax Biology 2e, 22.2: archaeal distinctions.

Some archaea perform methanogenesis, producing methane as part of their metabolism. Archaea also inhabit ordinary soils and oceans: extreme habitats do not define the lineage. See OpenStax Biology 2e, 22.1: moderate and extreme environments. Woese and Fox's 1977 analysis of ribosomal RNA distinguished the methanogenic group they called archaebacteria from typical bacteria. Their proposed three primary lines of descent were a historical interpretation, rather than the final resolution of deep phylogenetics. See Woese and Fox, 1977: Phylogenetic structure of the prokaryotic domain.

Warming can mobilize frozen organic material or change sediment conditions, increasing methane emissions and strengthening the greenhouse effect. Production, transport and atmospheric release must be distinguished: methane can dissolve or be oxidized before reaching the atmosphere. Observed seabed bubbles alone do not establish the magnitude or timing of a global atmospheric feedback.

In hot hydrogen-rich thermochemical equilibrium, much carbon can be bound in carbon monoxide, consuming oxygen that is unavailable for water. At a C/O ratio near or above one, water can be strongly depleted. Cooler methane-dominated chemistry can release more oxygen for water at the same elemental inventory. Pressure, quenching, dissociation and atmospheric photochemistry can modify this temperature-dependent partition.

Changes in temperature, water vapor and radical chemistry alter methane removal. Warming can increase its hydroxyl radical reaction coefficient and the hydroxyl radical source from oxygen singlet D atom plus water, tending to reduce chemical lifetime at fixed precursor conditions. Changed emissions and competing radical sinks can modify or reverse that conditional response.

Electron pair geometry describes the arrangement of all electron domains in the VSEPR model, including lone pairs. Molecular geometry instead describes the arrangement of chemical atoms. Four domains have a tetrahedral ideal arrangement, but keeping only the bonded neighbours gives the different molecular geometries of methane, ammonia and water.

A greenhouse gas absorbs and emits planetary thermal radiation and therefore contributes to the greenhouse effect. Its climatic impact depends on absorption spectrum, concentration, lifetime and chemical effects. Carbon dioxide, methane and water vapour are important examples.

In a fixed-production hydroxyl radical box, adding hydrogen by $\Delta [{\mathrm{H}}_2]$ changes $K$ to $K+k_{{\mathrm{H}}_2+OH}\Delta [{\mathrm{H}}_2]$. Methane lifetime scales as $K/P$ when its hydroxyl radical reaction coefficient is held fixed. The pre-existing hydrogen contribution is already in $K$ and must not be counted again.

Light initiates methane chlorination by photodissociation of $\mathrm{C}{\mathrm{l}}_2$ through homolysis. The elementary bookkeeping isHere $h\nu$ denotes the photon energy. The first propagation step replaces a chlorine radical with a methyl radical; the second regenerates a chlorine radical. Adding the two propagation equations cancels both free radicals and gives $\mathrm{C}{\mathrm{H}}_4+\mathrm{C}{\mathrm{l}}_2⟶\mathrm{C}{\mathrm{H}}_3\mathrm{Cl}+\mathrm{HCl}$: chloromethane and hydrogen chloride are made while the chain carrier is regenerated.^[ref]

For example, recombination $2{\mathrm{C}{\mathrm{H}}_3}^{\bullet }⟶{\mathrm{C}}_2{\mathrm{H}}_6$ of methyl radicals makes ethane and a covalent bond from two unpaired electrons, removing two carriers. Recombination of two ${\mathrm{Cl}}^{\bullet }$ also terminates carriers; in the gas phase, stabilization of the product requires disposal of released energy, for example to a third collision partner. The figure omits that partner. Termination competes with propagation, and further chlorination can consume chloromethane, so this cycle alone does not predict a pure product or a measured reaction rate.^[ref]

The e-folding relaxation time of an added methane burden after accounting for chemical feedbacks. It need not equal burden divided by the contemporaneous methane loss rate: extra methane can consume hydroxyl radical and reduce its own removal. A fixed-OH exponential estimate omits that coupled adjustment.

Methanogenesis is biological production of methane by anaerobic microorganisms. Waterlogged soil and lake sediments can favour it. Warming or thaw changes substrate availability, but production is distinct from atmospheric emission because oxidation and transport intervene.

Why do methane, ammonia, and water have different shapes when each central chemical atom is surrounded by eight valence electrons? The answer separates the positions of atomic nuclei from the electron domains used in a VSEPR model.

Molecular geometry describes the three-dimensional arrangement of the atomic nuclei in a molecule, including bond lengths and bond angles. For a larger molecule, a shape name often describes just the neighbours of one central chemical atom. A Lewis structure records connections and electron bookkeeping; its flat layout does not fix molecular geometry.

In the VSEPR model, electron pair geometry includes both bonding electron domains and lone pairs, whereas molecular geometry counts only the chemical atoms. In methane, four bond pairs and no lone pairs give tetrahedral molecular geometry. In ammonia, three bond pairs and one lone pair give trigonal pyramidal molecular geometry. In water, two bond pairs and two lone pairs give bent molecular geometry. All three use the same ideal tetrahedral electron pair geometry; this does not make their actual bond angles equal. See Flowers, Theopold, Langley and Robinson, Chemistry 2e, §7.6: electron-pair and molecular geometries.

A static drawing also needs a physical interpretation. The atomic nuclei undergo oscillations; an arrangement at a minimum of potential energy and an experimentally averaged arrangement need not have identical bond lengths or bond angles. This distinction matters when comparing measurements with quantum mechanics calculations; see the IUPAC Gold Book: experimental bond-length conventions. Gas electron diffraction gives a concrete route from a laboratory signal to molecular geometry rather than inferring shape from a drawing alone.

A warmer troposphere does not imply a warmer stratosphere: increased carbon dioxide can cool the middle and upper stratosphere by enhancing infrared emission. One direct kinetic effect is slower temperature-dependent destruction in the Chapman cycle, tending to increase upper-stratospheric ozone. Two indirect chemical effects can have the opposite sign. If the entry atmospheric cold trap warms, it can admit more water; reaction with an electronically excited oxygen atom then supplies more of the HOx family, enhancing catalytic ozone loss. In cold polar air, changes in water and temperature can favour polar stratospheric clouds, whose surfaces activate halogen reservoirs and can enhance ozone destruction when sunlight returns. These are conditional pathways, not a universal sign for total ozone: cold-trap temperature and circulation, halogen abundance and altitude all matter. Methane oxidation can also supply stratospheric water.^[ref]

For a real nonmagnetic $T_1$ electronic triplet, the symmetric square of a group representation decomposes asThe $A_1$ coordinate shifts all components equally and does not remove the degeneracy. The linear Jahn-Teller selection rule therefore permits $E$ and $T_2$ vibrational modes to couple and split the triplet. Both methane $T_2$ vibrational triplets are symmetry-eligible. A nonzero allowed coupling generally produces a symmetry-lowering instability, although symmetry alone does not determine its strength or the preferred minimum.

In the class order $(E,8C_3,3C_2,6S_4,6{\sigma }_d)$, the 15 Cartesian nuclear displacements of methane have characters $(15,0,-1,-1,3)$. Only nuclei left fixed by an operation contribute to that operation's trace. Subtract translations $T_2$ and rotations $T_1$ to obtain the methane normal-mode representationIts dimension is $1+2+3+3=9=3(5)-6$. There are four frequency families: a nondegenerate symmetric stretch, a doubly degenerate bend, and triply degenerate stretch and bend modes.

Under the proposed steady balance,The air number density cancels from the ratio. ThereforeThe reservoir relaxation test for a chemical steady state gives a hydrogen chloride relaxation time of $1/(k_{10}[\mathrm{OH}])=1.53\times 10^6\mathrm{s}\simeq 17.7$ days. Hydrogen chloride is a reservoir rather than a rapidly equilibrating radical. A steady state is plausible only if chlorine abundance, methane, hydroxyl radical and transport vary slowly compared with several weeks. It is doubtful during rapid vortex breakdown and reservoir reformation, precisely when production and mixing are changing. The numerical hydroxyl radical value is conditional on the steady-state hypothesis, not an independent measurement of hydroxyl radical.

Methane is an infrared-active greenhouse gas with absorption in the terrestrial thermal spectrum. Its direct forcing is supplemented by chemically induced changes in tropospheric ozone and stratospheric water vapor. Its finite atmospheric lifetime makes sustained emissions and emission pulses different experiments: stopping emissions permits the methane burden to decline much faster than the long tail of a carbon-dioxide perturbation. Quantitative comparisons require a specified emission mass, time horizon and treatment of indirect effects, as formalized by global warming potential.

Methane reaching the stratosphere is oxidized and is an important source of stratospheric water vapor. That water can feed HOx radical family chemistry and alter radiative cooling and polar-cloud conditions. Methane also converts chlorine atoms into hydrogen chloride, opposing some chlorine-driven ozone loss. These effects compete: increasing methane does not have one universal ozone sign at every altitude and latitude.

Methane consumes hydroxyl radicals during its initial oxidation and therefore influences atmospheric oxidative capacity and the removal of other trace gases. Subsequent oxidation generates peroxy radicals. In the presence of sufficient nitrogen monoxide, these convert nitrogen monoxide to nitrogen dioxide without consuming ozone, allowing net photochemical ozone production. At low nitrogen monoxide, peroxide formation and ozone-destroying radical reactions compete. Methane oxidation ultimately produces carbon monoxide and carbon dioxide; the sign and rate of the associated ozone response depend on the radical/nitrogen oxides regime, not methane alone.

For the tropospheric OH budget, let $P$ be hydroxyl radical production per unit volume and $K$ its total first-order loss frequency. The present hydroxyl radical steady state hasHere one year is $365.25$ days; using it consistently makes the reported lifetimes in years independent of that conversion convention. Methane's chemical lifetime is ${\tau }_0=1/(k_2[\mathrm{OH}{]}_0)$, givingThe current combined loss frequency already includes existing hydrogen, so add only the incremental hydrogen sink:Holding production and the other losses fixed then gives $[\mathrm{OH}{]}_1=5.107\times 10^5\mathrm{molecules}\mathrm{c}{\mathrm{m}}^{-3}$ andThe rapid hydroxyl radical adjustment time, $1/K$ of order a second, supports its steady-state approximation in this box model. Subsequent methane growth would itself change hydroxyl radical consumption, so the computed increase is not a fully coupled long-term atmospheric prediction. Detailed molecular hydrogen modelling treats changes in methane, ozone and stratospheric water alongside reductions in other precursor emissions.^[ref]

The finite-horizon GWP lifetime sensitivity follows because a longer lifetime leaves more methane from a given pulse at later times and raises the steady burden for a given constant emission rate. For the stipulated exponential pulse response and unchanged greenhouse gas radiative efficiency, the integrated methane forcing over horizon $H$ is proportional to $\tau (1-e^{-H/\tau })$. The same carbon dioxide reference pulse appears in both global warming potential denominators, so it cancels from their ratio:Thus the twenty-year GWP increases by $13.75\%$ under these fixed-efficiency assumptions. This is smaller than the $19.33\%$ lifetime increase because part of the extra forcing occurs beyond the finite horizon. For an infinite horizon the fractional increase would approach the lifetime ratio. Indirect forcing and the distinction between a chemical lifetime and a hydroxyl radical-coupled methane perturbation lifetime require a fuller chemical model; they are held fixed in this comparison.