MethanePaleoclimate & Milankovich

Gerrit Lohmann

Carbon Course26. January 2006

@PEP, University of Bremen, Germany

13-C Fractionation

• The photosynthetic activity in the oceans (photic zone) results in a very strong depletion of the surface waters in 12C (captured in organic matter), strong enrichment in 13C.

• Planktonic organisms living in the photic zone, and forming calcareous shells from dissolved inorganic carbon (largely HCO3

- ), thus use carbon that is enriched in 13C

Organic Carbon cycle

Organic Carbon cycle

oxidation

Photosynthesis -> low PO4,

organic matter with 12-C,

residual water high 13-C

``13-C conveyor belt´´

Atlantic Ocean

Pacific Ocean

Pacific Ocean

13-C: Aging

waters out of contact with the surface ocean for a long time:

accumulated much carbon derived from oxidation of organic material

thus their total dissolved inorganic carbon is isotopically light.

Circulation and productivity

13-C: climate changes

Interglacials=warm

Lower photosynthesis

13-C surface-deep neg.

Paleoclimate

Paleoclimate

Anomalien und Spektren

Northern Hemisphere Summer

Boreal Summer

Annual Cycle

Northern Hemisphere Summer

Boreal Summer

Annual Cycle

We have summerThey have summer

Annual Cycle

Fixed axis of Earth rotation

Eccentricity

Obliquity

Precession & Eccentricity Present Orbital Configuration:

Winter Solstice close to the position of Perihelion

Precession

One basic concept of ice ages:

Recfification: Non-symetric response

Ice albedo effect, Growth and decay of ice sheets

Half Wave Rectification

The incorporation of a single diode is the simplest way of rectifying AC.  Only the forward half-cycle is passed.  The reverse is blocked.  The rectified current is a series of pulses. 

Task: calculate the Fourier transformation of the rectified signal in insolation, 20. June at 65 deg N

Planets

 FORTRAN code based upon the Laskar solution is at  http://xml.gsfc.nasa.gov/archive/catalogs/6/6063/index_long.html

Threshold theory

1) Explain why periods of glacial advance in the

higher latitudes tend to occur with warmer winters and cooler summers.

2) Are ice ages in the Northern Hemipshere more likely when:

• a. the tilt of the earth is at a maximum or a minimum?

• b. the sun is closest to the earth during summer in the Northern Hemisphere, or during winter?

Questions:

Effect on climate

Southen margin of the Sahara Desert

Congo Air Boundary (CAB)

Rough locations of the Intertropical Convergence Zone (ITCZ)

Methane

• Methane (CH4) is a greenhouse gas that remains in the atmosphere for approximately 9-15 years.

• One methane molecule will absorb 20 times as much infrared radiation as CO2.

• Actually, methane is the most rapidly increasing greenhouse gas. The concentration is increasing at about 1% per year.

Methane: Sources & Sinks

Methane: Sources

Wetlands

• important natural source of methane to the atmosphere. The amounts of methane produced vary greatly from area to area, with temperature, water level and organic carbon content all being important controlling factors.

• The process of methane production involves the microbial mineralization of organic carbon under anaerobic conditions in the waterlogged soil.

Soil Organic Matter

Landfills (dt. Deponien)

• In an environment where the oxygen content is low or nonexistent, organic materials, such as yard waste, household waste, food waste, and paper, are decomposed by bacteria to produce methane.

• Methane emissions from landfills are affected by such specific factors as waste composition, moisture, and landfill size.

Agriculture

• enteric fermentation in domestic livestock

• decomposition of organic animal waste

• methane emissions from rice cultivation

• emissions from field burning

• Several other agricultural activities, such as irrigation and tillage practices

Agriculture etc.

landfill

oil

Effect on climate

Rough locations of the Intertropical Convergence Zone (ITCZ), the Congo Air Boundary (CAB), and the southen margin of the Sahara Desert for the present-day, and for the monsoonal maximum.

Sinks of MethaneIn the troposphere (below about 10-12 km), the key cycles are mediated above all by the presence of what are called OH radicals.

All hydrocarbon chemical species are broken down (or oxidized) by these radicals to CO2 and H2O.

Around 10% of the CH4 makes it into the stratosphere where it also gets oxidized.

A key point is that in the very dry stratosphere, the water produced from methane oxidation is a big part of the water budget and stratospheric water vapor is a greenhouse gas in it's own right! This indirect process enhances the climate impact of methane changes by about 15%.

Positive Feedback: CH4 sink

• Changing emissions of other chemicals (e.g. carbon monoxide in biomass burning, complex hydrocarbons from vegetation) can compete for OH and again cause CH4 to change proportionately.

• In fact, the CH4 level itself has a positive feedback on it's own lifetime: i.e., the more methane there is, the more OH is "used up" and the longer the methane can stick around.

Methane Clathrates and Climate

• Clathrates are a class of compound that consist of a cage of molecules that can trap gases, such as methane, in a solid form.

• For methane, the most important "cage" is one that is made of water molecules, and so is described sometimes as a hydrate.

• Some key facts about clathrates make them particularly interesting to climatologists. First, they may make up a significant portion of total fossil carbon reserves, including coal and oil. Current best guesses suggest that maybe 500 to 2000 gigatonnes of carbon may be stored as methane clathrates (5-20% of total estimated reserves).

• They occur mainly on the continental shelf where the water is relatively cold, there is sufficient pressure and enough organic material to keep the methane-producing bacteria happy.

• Most importantly, clathrates can be explosively unstable if the temperature increases or the pressure decreases — which can happen as a function of climate change, tectonic uplift or undersea landslides.

Methane Hydrate

Methane hydrate consists of a cage of water molecules trapping a methane molecule within. This can form large crystals of hydrate in cold and heavily pressurized situations (mainly on the continental slope in the oceans).

Earth History• period 55 million years ago at the transition between the Paleocene

and Eocene epochs, carbon isotope ratios everywhere (the deep sea, on land, at the poles and in the tropics) suddenly changed to favour the lighter 12C isotope of carbon at the expense of 13C.

• The rapidity and size of this change was unprecedented in the period since the demise of the dinosaurs, and this excursion was simultaneous with a short period of extreme global warming (around 3 to 4 degrees globally, more in the high latitudes).

• perturbation to the global carbon cycle to fit these data was a massive input of light carbon that had been stored as methane clathrates, which are observed to be particularly high in 12C. Nothing else could be as fast-acting or have enough of the lighter isotope to have had the observed effects. Given that both CH4 and it's oxidization product CO2 are greenhouse gases, this might explain the global warming as well.

Oxygen isotope values (left column) and carbon isotopic values of the deep sea for the Cenozoic, after Zachos, et al.2001

•Note that lower (more negative) d18O values mean that water temperature was higher, or the polar ice sheets were smaller, or both at the same time. Note temperature scale (given for ice-free conditions) below

Cenozoic: deep-sea oxygen isotope • the values for benthic foraminifera reflect the deep-water temperatures

globally, as well as the surface water temperatures at high latitudes.

• During the Cenozoic (after the Cretaceous/Tertiary boundary at 65 Ma) the d18O values of benthic  foraminifera increased, but the values in surface-water planktonic foraminifera at low latitudes showed little to no change. The increases did not occur gradually, but in major steps each lasted about 100,000 years or less:

• in the earliest Oligocene (33.5 Ma): establishment of the east Antarctic ice sheet

• in the middle Miocene (13.6 Ma): increase in volume of the Antarctic ice sheet, possibly establishment of the west Antarctic ice sheet and small northern hemispheric ice sheets

• in the Pliocene (3.0-2.7 Ma): initiation of large-scale northern hemipsheric ice sheets.

• Presently, the relative large 'steps' in the oxygen isotope values (at ~33.5

and 13.6 Ma) are mainly interpreted as resulting from rapid increases in ice volume rather than from decreasing temperatures, based on data on a different temperature proxy which is not ice-volume dependent.

Clathrate gun hypothesis

• methane builds up in clathrates during cold periods, and as a warming starts it is explosively released, leading to enhanced further rapid climate warming.

• This idea is not yet widely accepted, mainly because the records of methane in the ice cores seems to lag the temperature changes, and the magnitudes involved do not appear large enough to significantly perturb the radiative balance of the planet.

• The more conventional explanation is that as the climate warms there is increased rain in the tropics and thus increased emissions from tropical wetlands which need to have been large enough to counteract a probable increase in the methane sink.