FROZEN HEAT | Volume 1

2.1 INTRODUCTION

Carbon cycle

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. 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.

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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 33