ABRUPT CHANGES IN THE EARTH’S CLIMATE SYSTEM
ABRUPT CHANGE IN ATMOSPHERIC
METHANE CONCENTRATION
After carbon dioxide (CO2), methane (CH4)
is the next most important greenhouse gas that humans directly influence,
Methane is a potent greenhouse gas because it strongly absorbs terrestrial infrared
(IR) radiation. Methane’s atmospheric
abundance has more than doubled since the start of the Industrial Revolution
(Etheridge et al., 1998; MacFarling Meure et al., 2006), amounting to a total
contribution to radiative forcing over this time of ~0.7 watts per square meter
(W m-2), or nearly half of that resulting from parallel increase in
the atmospheric concentration of CO2 (Hansen and Sato, 2001). Additionally, CO2 produced by CH4
oxidation is equivalent to ~6% of CO2 emissions from fossil fuel combustion. Over a 100-year time horizon, the direct and
indirect effects on radiative forcing from emission of 1 kg CH4 are
25 times greater than for emission of 1kg CO2 (IPCC, 2007). On shorter time scales, methane’s impact on
radiative forcing is higher.
The primary geological reservoirs of methane that could be
released abruptly to the atmosphere are found in ocean sediments and
terrestrial soils as methane hydrate.
Methane hydrate is a solid in which methane molecules are trapped in a
lattice of water molecules (fig. 1.8).
On Earth, methane hydrate forms under high pressure-low temperature
conditions in the presence of sufficient methane. These conditions are most often found in
relatively shallow marine sediments on continental margins but also in some
high-latitude soil (Kvenvolden, 1993).
Estimates of the total amount of methane hydrate vary widely, from 500
to 10,000 gigatons of carbon (GtC) total stored as methane in hydrates in
marine sediments, and 7.5-400 GtC in permafrost (both figures are
uncertain). The total amount of carbon
in the modern atmosphere is ~810 GtC, but the total methane content of the
atmosphere is only ~ GtC (Dlugokencky et al., 1998). Therefore, even a release of a small portion
of the methane hydrate reservoir to the atmosphere could have a substantial
impact on radiative forcing.
There is little evidence to support massive releases of
methane from marine or terrestrial hydrates in the past. Evidence from the ice core record indicates
that abrupt shifts in methane concentration have occurred in the past 110,000
years (Brook et al., 1996), but the concentration changes during these events
were relatively small. Father back in
geologic time, an abrupt warming at the Paleocene-Eocene boundary about 55
million years ago has been attributed by some to a large release of methane to
the atmosphere. Concern about future
abrupt release in atmospheric methane stems largely from the possibility, that
the massive amounts of methane present as solid methane hydrate in ocean sediments,
and terrestrial soils may become unstable in the face of global warming. Warming or release of pressure can
destabilize methane hydrate, forming free gas that may ultimately be released
to the atmosphere (Fig. 1.9).
Destabilization of
Marine Methane Hydrates
This issue is probably the most well known die to extensive
research on the occurrence of methane hydrates in marine sediments, and the
large quantities of methane apparently present in this solid phase in primarily
continental margin marine sediments.
Destabilization of this solid phase requires mechanisms for warming the
deposits and/or reducing pressure on the appropriate time scale, transport of
free methane gas to the sediment-water interface, and transport through the
water column to the atmosphere (Archer, 2007).
Warming of bottom waters, slope failure, and their interaction are the
most commonly discussed mechanisms for abrupt release. However, bacteria are efficient at consuming
methane in oxygen-rich sediments and the ocean water column, and there are a
number of physical impediments to abrupt release from marine sediments.
On the time scale of the coming century, it is likely that
most of the marine hydrate reservoir will be insulated from anthropogenic
climate change. The exception is in
shallow ocean sediments where methane gas is focused by subsurface
migration. These deposits will very
likely respond to anthropogenic climate change with an increased background
rate of sustained methane release, rather than an abrupt release.
Destabilization of
Permafrost Hydrates
Hydrate
deposits at depth in permafrost soils are known to exist, and although their
extent is uncertain, the total amount of methane in permafrost hydrates appears
to be much smaller than in marine sediments. Surface warming eventually would
increase melting rates of permafrost hydrates. Inundation of some deposits by
warmer seawater and lateral invasion of the coastline are also concerns and may
be mechanisms for more rapid change.
Destabilization
of hydrates in permafrost by global warming is unlikely over the next few
centuries (Harvey and Huang, 1995). No mechanisms have been proposed for the
abrupt release of significant quantities of methane from terrestrial hydrates
(Archer, 2007). Slow and perhaps sustained release from permafrost regions may
occur over decades to centuries from mining extraction of methane from
terrestrial hydrates in the Arctic (Boswell, 2007), over decades to centuries
from continued erosion of coastal permafrost in Eurasia (Shakova et al., 2005),
and over centuries to millennia from the propagation of any warming 100 to
1,000 meters down into permafrost hydrates (Harvey and Huang, 1995).
Changes in Wetland Extent and
Methane Productivity
Although
a destabilization of either the marine or terrestrial methane hydrate
reservoirs is the most likely pathway for an abrupt increase in atmospheric
methane concentration, the potential exists for a more gradual, but
substantial, increase in natural methane emissions in association with projected
changes in climate. The most likely
region to experience a dramatic change in natural methane emission is the
northern high latitudes, where there is increasing evidence for accelerated
warming enhanced precipitation, and widespread permafrost thaw which could lead
to an expansion of wetland areas into organic-rich soils that, given the right
environmental conditions, would be fertile areas for methane production
(Jorgenson et al., 2001, 2006).
Tropical
wetlands are a stronger methane source than boreal and arctic wetlands and will
likely continue to be over the next century, during which fluxes from both
regions are expected to increase. However, several factors that differentiate
northern wetlands from tropical wetlands make them more likely to experience a
larger increase in fluxes.
The
balance of evidence suggests that anticipated changes to northern wetlands in
response to large-scale permafrost degradation, thermokarst development, a
positive trend in water balance in combination with substantial soil warming,
enhanced vegetation productivity, and an abundant source of organic matter will
very likely drive a sustained increase in CH4 emissions from the
northern latitudes during the 21st century. A doubling of northern CH4 emissions
could be realized fairly easily. Much
larger increases cannot be discounted.
SUMMARY
The
prospect of a catastrophic release of methane to the atmosphere as a result of
anthropogenic climate change appears very unlikely. However, the carbon stored as methane hydrate
and as potential methane in the organic carbon pool of northern (and tropical)
wetland soils is likely to play a role in future climate change. Changes in
climate, including warmer temperatures and more precipitation in some regions,
particularly the Arctic, will very likely
gradually increase emission of methane from both melting hydrates and natural
wetlands. The magnitude of this effect cannot be predicted with great accuracy
yet, but is likely to be at least equivalent to the current magnitude of many
anthropogenic sources.
