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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
33
Methane is a significant component of the near-surface glob-
al carbon pool, and it can exist as a free gas, dissolved in wa-
ter, or held in gas hydrate. Prior to the Industrial Revolution,
methane accounted for 0.05 Gigatonnes of carbon (GtC) in
the oceans, and 1.5 GtC in the atmosphere (Sundquist and
Visser, 2003). Modern estimates suggest methane now ac-
counts for 3.7 GtC in the atmosphere (IPCC, 2007). By com-
parison, midrange estimates discussed in Volume 1, Chapter
1 suggest global methane hydrates sequester ~5000 GtC, rep-
resenting about a third of the ~15 000 Gt of organic carbon
near the Earth’s surface. Methane in hydrates also accounts
for approximately 10 per cent of the total near-surface carbon
pool (organic and inorganic), which is on the order of 41 000
GtC, with another 5 000 to 10 000 GtC held in coal, gas and
oil (Houghton, 2007).
This chapter focuses on the marine environments, where ~99
per cent of the Earth’s methane hydrate exists (See Volume 1,
Chapter 1). A summary of how and where methane is formed
and consumed is presented, along with a discussion of how
gas hydrates, and also sea floor chemosynthetic communities,
fit into methane’s contribution to carbon cycling between the
atmosphere, hydrosphere, geosphere, and biosphere.
2.1
INTRODUCTION
Figure 2.1:
Global carbon cycle. Carbon moves through the
atmosphere, biosphere, geosphere, and hydrosphere. Gas hydrates
(orange) are shown inmarine sediments, but are also buried beneath
permafrost sediment in Arctic regions. The 5 000 GtC cited for gas
hydrates is a midrange estimate from recent global assessments,
and the ~.004 GtC/year carbon flux from hydrates is taken from
the Intergovernmental Panel on Climate Change (IPCC, 2007). All
other values are compiled from Houghton (2004). Although gas
hydrates are a significant global carbon pool, the precise amount
of carbon, the amount of carbon released from gas hydrates to the
atmosphere, and the extent to which that release could increase as
the global climate changes are all under active debate. Improving
upon the values for gas hydrates used in this figure will require
extensive mapping and research efforts around the world.
Carbon cycle
725
38 000
Deep ocean
Sediments
Ocean surface
Marine biota
3
11
0.2
37
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