Book 2.indb - US Climate Change Science Program

More documents

Recommendations

Info

The U.S. Climate Change Science Program Chapter 500.1200 0350 0.05–1increasing evidence for accelerated warming,enhanced precipitation, and widespread permafrostthaw which could lead to an expansion ofwetland areas into organic-rich soils that, giventhe right environmental conditions, would befertile areas for methane production.A prominentconcern aboutmarine methanehydrates is thatwarming at theEarth’s surface willultimately propagateto hydrate depositsand melt them,releasing methaneto the oceanatmospheresystem.10–25 550 8–60050–110150800810 50–2500840905 150–5000Figure 5.8. The firn column of a typical site ona polar ice sheet from Schwander, 2007; reprintedwith permission from Encyclopedia of QuaternaryScience. Abbreviations: m, meter; kg m –3 , kilogramsper cubic meter.3.2 Destabilization of PermafrostHydratesHydrate deposits at depth in permafrost areknown to exist, and although their extent isuncertain, the total amount of methane inpermafrost hydrates is very likely much smallerthan in marine sediments. Surface warmingeventually would increase melting rates ofpermafrost hydrates. Inundation of some depositsby warmer seawater and lateral invasionof the coastline are also concerns and may bemechanisms for more rapid change.3.3 Changes in Wetland Extent andMethane ProductivityAlthough a destabilization of either the marineor terrestrial methane hydrate reservoirs isthe most probable pathway for a truly abruptchange in atmospheric methane concentration,the potential exists for a more chronic, butsubstantial, increase in natural methane emissionsin association with projected changes inclimate. The most likely region to experience adramatic change in natural methane emissionis the northern high latitudes, where there isIn addition, although northern high-latitudewetlands seem particularly sensitive to climatechange, the largest natural source of methane tothe atmosphere is from tropical wetlands, andmethane emissions there may also be sensitiveto future changes in temperature and precipitation.Modeling studies addressing this issue aretherefore also included in our discussion.4. Potential for AbruptMethane Change FromMarine Hydrate Sources4.1 Impact of Temperature Changeon Marine Methane HydratesA prominent concern about marine methanehydrates is that warming at the Earth’s surfacewill ultimately propagate to hydrate depositsand melt them, releasing methane to the oceanatmospheresystem. The likelihood of this typeof methane release depends on the propagationof heat through the sea floor, the migration ofmethane released from hydrate deposits throughsediments, and the fate of this methane in thewater column.4.1.1 Propagation of TemperatureChange to the Hydrate Stability ZoneThe time dependence of changes in the inventoryof methane in the hydrate reservoir dependson the time scale of warming and chemicaldiffusion. There is evidence from paleotracers(Martin et al., 2005) and from modeling(Archer et al., 2004) that the temperature ofthe deep sea is sensitive to the climate of theEarth’s surface. In general, the time scale forchanging the temperature of the ocean increaseswith water depth, reaching a maximum ofabout 1,000 years for the abyssal ocean. Thismeans that abrupt changes in temperature atthe surface ocean would not be transmitted immediatelyto the deep sea. There are significantregional variations in the ventilation time ofthe ocean, and in the amount of warming thatmight be expected in the future. The Arctic isexpected to warm particularly strongly because178
Abrupt Climate ChangeAtmospheric CH 4 (ppb)Max. Observed IceCore MethaneIce Core CH 4 (ppb)Model Time (yr)Figure 5.9. Model simulations of smoothing instantaneous release of methane from clathrates tothe atmosphere, and the ice core response to those events. The ice core response was calculatedby convolving the atmospheric histories in the top panel with a smoothing function appropriate forthe GISP2 ice core. The solid lines are the atmospheric history and smoothed result for the modelof a 4,000 teragram release of methane from Thorpe et al. (1996). The blue solid line representshow an Arctic ice core would record a release in the Northern Hemisphere, and the red solid linerepresents how an Antarctic ice core would record that event (from Brook et al., 2000). The dashedlines represent instantaneous arbitrary increases of atmospheric methane to values of 1,000, 2,000,3,000, 4,000, or 5,000 ppb (colored dashed lines in top panel) and the ice core response (bottompanel, same color scheme).of the albedo feedback from the melting Arcticice cap. Temperatures in the North Atlanticappear to be sensitive to changes in oceancirculation such as during rapid climate changeduring the last ice age (Dansgaard et al., 1989).The top of the hydrate stability zone is at 200to 600 m water depth, depending mainly onthe temperature of the water column. Withinthe sediment column, temperature increaseswith depth along the geothermal temperaturegradient, 30–50 °C km –1 (Harvey and Huang,1995). The shallowest sediments that couldcontain hydrate only have a thin hydratestability zone, and the stability zone thicknessincreases with water depth. A change in thetemperature of the deep ocean will act as achange in the upper boundary condition of thesediment temperature profile. Warming of theoverlying ocean may not put surface sedimentsinto undersaturation, but the warmer overlyingtemperature propagates downward until a newprofile with the same geothermal temperaturegradient can be established. How long this takesis a strong (second order) function of the thicknessof the stability zone, but the time scales arein general long. In 1,000 years, the temperaturesignal should have propagated about 180 m inthe sediment. In steady state, an increase inocean temperature will decrease the thicknessof the stability zone. Dickens (2001b) calculatedthat the volume of the stability zone ought todecrease by about half with a temperatureincrease of 5 °C.4.1.2 Impact on Stratigraphic-TypeDepositsHydrate deposits formed within sedimentarylayers are referred to as stratigraphic-type deposits.After an increase in temperature of theoverlying water causes hydrate to melt at thebase of the stability zone, the fate of the releasedmethane is difficult to predict. The increase inpore volume and pressure could provoke gas179
Page 5 and 6:
TABLE OF CONTENTSAuthor Team for th
Page 8 and 9:
The U.S. Climate Change Science Pro
Page 10 and 11:
PREFACEReport Motivation and Guidan
Page 12 and 13:
Page 14 and 15:
Page 16 and 17:
Page 18 and 19:
Page 21 and 22:
1CHAPTERAbrupt Climate ChangeIntrod
Page 23 and 24:
Abrupt Climate Change-35Arctic temp
Page 25 and 26:
Abrupt Climate ChangeFigure 1.2. Po
Page 27 and 28:
Abrupt Climate Changewell below sea
Page 29 and 30:
Abrupt Climate Changeheterogeneous
Page 31 and 32:
Abrupt Climate Changeapart from dro
Page 33 and 34:
Abrupt Climate Change6. Abrupt Chan
Page 35 and 36:
Abrupt Climate Changeintermediate c
Page 37 and 38:
Abrupt Climate Changeper square met
Page 39:
Abrupt Climate Changeis the norther
Page 42 and 43:
Page 44:
Page 47 and 48:
Abrupt Climate ChangeRegardless how
Page 49 and 50:
Abrupt Climate Changecentrations, a
Page 51 and 52:
Abrupt Climate ChangeBox 2.1. Glaci
Page 53 and 54:
Abrupt Climate Changetime. Degree-d
Page 55 and 56:
Abrupt Climate Changetion, and accu
Page 57 and 58:
Abrupt Climate ChangeBox 2.2. Mass
Page 59 and 60:
Abrupt Climate Changeof the glacier
Page 61 and 62:
Abrupt Climate ChangeFigure 2.6. Ra
Page 63 and 64:
Abrupt Climate Changewater) or, mor
Page 65 and 66:
Abrupt Climate ChangeFigure 2.7. Re
Page 67 and 68:
Abrupt Climate Changeresults in a l
Page 69 and 70:
Abrupt Climate Changeis not providi
Page 71 and 72:
Abrupt Climate Changemeasurements c
Page 73 and 74:
Abrupt Climate Changemore than 10,0
Page 75 and 76:
Abrupt Climate Changeocean models h
Page 77 and 78:
Abrupt Climate Changeto atmosphere-
Page 79 and 80:
3CHAPTERHydrological Variability an
Page 81 and 82:
Abrupt Climate Change• The integr
Page 83 and 84:
Abrupt Climate Changethe soil (King
Page 85 and 86:
Abrupt Climate Changeon (Kaplan et
Page 87 and 88:
Abrupt Climate Changeorbital-time-s
Page 89 and 90:
Abrupt Climate ChangeFigure 3.4. (t
Page 91 and 92:
Abrupt Climate Changemodels forced
Page 93 and 94:
Abrupt Climate ChangeBox 3.2. Waves
Page 95 and 96:
Abrupt Climate Changebecause (1) th
Page 97 and 98:
Abrupt Climate ChangeHarrison et al
Page 99 and 100:
Abrupt Climate Changethat even the
Page 101 and 102:
Abrupt Climate Changethat seen afte
Page 103 and 104:
Abrupt Climate Changeentire period
Page 105 and 106:
Abrupt Climate Changelong-lived gre
Page 107 and 108:
Abrupt Climate Changeplanetary-scal
Page 109 and 110:
Abrupt Climate ChangeBox 3.3. Paleo
Page 111 and 112:
Abrupt Climate Changerelated to the
Page 113 and 114:
Abrupt Climate Changeinto the North
Page 115 and 116:
Abrupt Climate ChangeThe summertime
Page 117 and 118:
Abrupt Climate Change(Anderson et a
Page 119 and 120:
Abrupt Climate ChangeHere the model
Page 121 and 122:
Abrupt Climate Changetropical tropo
Page 123 and 124:
Abrupt Climate Changefunctions; Koc
Page 125 and 126:
Abrupt Climate Changechanges than t
Page 127:
Abrupt Climate Changeboundary condi
Page 130 and 131:
Page 132 and 133:
Page 134 and 135:
Page 136 and 137:
Page 138 and 139:
Page 140 and 141: The U.S. Climate Change Science Pro
Page 214 and 215: 202REFERENCESChapter 1 ReferencesAl
Page 240 and 241:
Page 242 and 243:
Page 244 and 245:
Page 246 and 247:
Page 248 and 249:
Page 250 and 251:
Page 252 and 253:
Page 254 and 255:
Page 256 and 257:
Page 258:
U.S. Climate Change Science Program
show all

Book 2.indb - US Climate Change Science Program

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?