Page 368 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

332 part II The Water, Weather, and Climate Systems
F cus Study 11.1 Climate Change
Thawing Methane Hydrates—Another Arctic Methane Concern
in this chapter’s Geosystems Now, we dis- cussed the release of greenhouse gases— CO2 and methane—into the atmosphere as permafrost thaws in arctic regions.
This process is biogenic, produced by living organisms, a result of bacterial ac- tion breaking down organic matter in shallow soils and sediments. Meanwhile, another form of methane exists as a gas hydrate, stored deep beneath permafrost deposits on land in the arctic and in off- shore deposits on the ocean floor. Called methane hydrates, these natural gas deposits consist of methane molecules encased in ice and become destabilized with warming temperatures. The process of melting breaks down the crystalline hydrate structures, releasing a burst of methane—an important greenhouse gas—into the oceans and atmosphere. if these isolated bursts multiply over a large area, atmospheric methane concentra- tions could increase enough to accelerate global warming.
Methane Hydrate Essentials
Methane hydrate is a solid, icy compound containing a central methane molecule surrounded by a structure, or “cage,” of interconnected water molecules. Meth- ane hydrates exist only under conditions of cold temperature and high pressure, usually in subsurface deposits of sedi- mentary rock. The methane in the gas hydrate is formed by the deep burial
and heating of organic matter, a ther- mogenic (heat-related) process similar to that which forms oil. Methane hydrate
deposits are a potential source of en- ergy, although the extraction process is difficult and expensive. Scientists think that over 90% of the world’s gas hydrates occur in deep-ocean settings.
as temperatures rise in the ground and oceans, methane hydrate deposits are at risk of dissociation, or melt, a pro- cess that would release high concentra- tions of methane gas to the atmosphere (Figure 11.1.1). For example, melting 1 m3 of methane hydrate releases about 160 m3 of methane gas. if all of these gas hydrates were to thaw, the result would be a pulse of methane so large that it would almost certainly trigger abrupt climate change. Scientists think that a mas-
sive release of carbon ob-
served in climate records
from ocean sediment
cores about 55 million
years ago may be related
to a gas hydrate dissolu-
tion event (see the tem-
perature spike called the
PeTM in Figure 11.10).
to accelerate global warming. For exam- ple, the dissociation of large gas hydrate deposits—breakdown of the solids into liquids and gases—can destabilize sea- floor sediments, causing a loss of struc- tural support that can lead to subsidence and collapse in the form of submarine landslides. This type of major landslide could release large quantities of methane. a second danger is the potential for mas- sive methane release as a by-product of energy extraction.
Methane hydrates are now thought to be the world’s largest reserve of carbon- based fuel—scientists think that 10 000 gt of methane is trapped in gas hydrates worldwide, an amount that exceeds the energy available in coal, oil, and other
Causes and Effects of Methane Hydrate Thaw
Several dangers exist with regard to the dissocia- tion of methane hydrates and its effect on climate change. One is the pos- sibility of a destabilizing event causing a sudden re- lease of enough methane
▲Figure 11.1.1 The “ice that burns” is extracted from seafloor deposits. Solid methane hydrate extracted from about 6 m beneath the seafloor near Vancouver island, Canada. as methane hydrate warms, it releases enough methane to sustain a flame. [U.S. geological Survey.]
CrITICALthinking 11.2
Thinking through an Action Plan to Reduce Human Climate Forcing
Figure 11.24 illustrates some of the many factors that force cli- mate. let us consider how these variables might inform deci- sion making regarding climate change policy and mitigation. assume you are a policy maker with a goal of reducing the rate of climate change—that is, reducing positive radiative forcing of the climate system. What strategies do you sug- gest to alter the extent of radiative forcing or adjust the mix of elements that cause temperature increases? assign priori- ties to each suggested strategy to denote the most to least effective in moderating climate change. Try brainstorming and discussing your strategies with others. in your opinion, how should the jump to 2.3 W·m−2 in human climate forcing over the past 6 years influence policy and action strategies? •
In Figure 11.24, the factors that warm the atmosphere, causing a positive radiative forcing, are in red, orange, or yellow; those that cool the atmosphere, causing a nega- tive forcing, are in blue. The estimates of radiative forc- ing (RF) in watts-per-square-metre units are given on the x-axis (horizontal axis). The horizontal black lines super- imposed on the coloured bars represent the uncertainty range for each factor (for example, the cloud-albedo effect has a large uncertainty range). In the far-right column, LOSU refers to “level of scientific understanding.” The overall results of this analysis show that the RF of green- house gases far surpasses the RF of other factors, whether natural or anthropogenic in origin. In the 2013 IPCC Fifth Assessment Report, Working Group I states that total net anthropogenic forcing increased to 2.3 W · m−2, compared to 1.6 W·m−2 in 2006 (see notation in the graph).