Lecture 10. Orbital-scale changes in greenhouse gases Ruddiman Chapter 11 The key questions: 1) Role of GHGs in ice ages? 2) Does Milankovitch explain the timing of the ice ages? Bill Ruddiman
Main points: 1) methane and carbon dioxide concentrations fluctuate at Milankovitch rhythms 2) GHG concentrations lower during ice ages 3) deep sea served as reservoir for CO 2 during coldest times
4) Methane concentration is sensitive to strength of north-tropical monsoons.
5) CH 4 responds immediately to precession cycles (21 ka) solar radiation - strength of northern-tropical monsoon amount of CH 4 produced 6) CO 2 lags the precession cycle by about 1 ka
7) Both CH 4 and CO 2 lag the obliquity (41 ka) cycles (they track ice-sheet volume) 8) GHGs can be both forcing and feedbacks to glaciation
ice cores and deep-sea sediment cores are 2 of the most valuable archives of paleoclimate data Glomar Challenger, WIKIPEDIA www.news.brown.edu/pressreleases/2009/06/nitrogen
and they usually agree ice sheet ocean Fluckiger et al 2004 Global Biogeochem Cycles The NGRIP isotopic compared with the planktonic isotopes in the Iberian margin sediment core MD95-202429 (b). The Greenland Dansgaard Oeschger events (interstadials) are numbered along with the associated stadials. The two age scales are independent and match within a few kyr.
Where you core an ice sheet
How you core a glacier. British Antarctic Survey hard at work
Dating an ice core 1) Counts of annual layers 2) Tephra layers of known age 3) Ice-flow models 4) Wiggle matching ice properties with other proxy records (e.g., deep sea del 18 O, Milankovitch cycles)
Annual layers Sometimes annual layers demarcated by dust Volcanic ashes dated elsewhere occur in ice cores
Most of our understanding of GHG concentrations over the last 100,000 yr comes from air bubbles trapped in glacial ice http://www.swisseduc.ch/glaciers/earth_icy http://www.niwa.co.nz/our-science/atmosphere/researchprojects/ice-cores
Accumulating snow/ice samples contemporary atmosphere http://www.swisseduc.ch/glaciers/earth_icy http://www.polartrec.com/e
Age of the gases trapped in ice is younger than the age of the surrounding ice Age difference depends on rate of ice deposition. Several hundred to several thousand years
How do we know the ice bubbles preserve true record of paleo-atmosphere?
Both CO 2 and CH 4 records match-up well with instrumental records
Ok, so we have a valid method for reconstructing the concentrations of CO 2 and CH 4 in ancient atmospheres. What do these records look like? Dye3 GISP2 GRIP Vostok, Byrd, Epica NGRIP
rem: today, CH 4 is at 1800 ppb (!) ppb Greenland YEARS BP ppb Greenland
Vostok Station, East Antarctica
CH 4 and CO 2 both tracked global temperature climate records from the Vostok ice core (Lorius et al., 1985; Barnola et al., 1987; Jouzel et al., 1987; Chappellaz et al., 1990). Temperature data are plotted as deviations from the present day mean annual temperature of -56 d C. Also included is the record of the flux of dust to the area (shown on an inverted scale for comparison purposes) (Petit et al., 1990). less more
CO 2 tracked global ice volumes.
ice age: 350 ppb CH 4 and 180 ppm CO 2 AD 1750: 700 ppb CH 4 280 ppm CO 2 today: 1800 ppb CH 4 and 400 ppm CO 2
Ice age to pre-industrial: CH 4 x 2, CO 2 x 1.5 Ice age to today: CH 4 x 5, CO 2 x 2 Darfur Mexico City suburbs
Main points: 1) methane and carbon dioxide concentrations fluctuate at Milankovitch rhythms 2) GHG concentrations lower during ice ages 3) deep sea served as reservoir for CO 2 during coldest times We are here
What was happening with the carbon geochemical cycle during the ice age?
We know CO 2 concentration in atmosphere was lower during ice age. But where did that CO 2 go?
Review of carbon reservoirs: note characteristic differences in isotope values (del 13 C) billions of tons of C
Global vegetation & soil: were they sucking up more C during cold intervals?
Global vegetation & soil: were they sucking up more C during cold intervals? Unlikely. Less vegetation at high latitudes and increasing drought at middle latitudes during ice ages Probably a 15-30% reduction in size of veg-soil C reservoir during glacial periods.
The ocean mixed layer exchanges CO 2 rapidly with atmosphere, so most likely storage depot = deep sea. Interglacial to glacial changes (billion tons of carbon) Estimates based on variety of observations on land and in ocean cores.
How was this figured out? Organic C in terrestrial vegetation has del 13 C values around -25 parts per thousand (lots of the lighter 12 C). C in the deep sea presently has del 13 C values near 0 parts per thousand
When washed into ocean, terrestrial organic C rapidly converts to inorganic C, but it retains its highly negative del 13 C signature.
Deep-sea foraminifera during Last Glacial Maximum became enriched in 12 C indicating uptake of terrestrially derived C. 0 o/oo (today) to -0.3 o/oo (LGM)
Where was 90 ppm of CO 2 hiding during ice ages? (in the deep sea)
How do you move an 1/3 of the CO 2 and >1/2 of the CH 4 in the interglacial atmosphere into the deep sea reservoir during ice ages? Why is this question relevant today?
3 ways: 1) Cooler ocean holds more dissolved gases But it takes a while (ca. 1000 years) to equilibrate
Atmospheric CO 2 levels drop by 10 ppm for each 1 degree C of ocean cooling. Even tropical oceans cooled by 2-4 d C during ice ages; high latitude oceans cooled much more. Ruddiman: atmospheric CO 2 concentrations fell 20-30 ppm because of ocean cooling during ice ages.
A second mechanism for C transfer to deep sea during glacials: phytoplankton Carbon pumping of photosynthetic products into deep water/sediments
Today, much of the ocean is a desert in terms of C fixation
Factors limiting primary productivity in the ocean mixed layer today: 1) Nutrients (nitrogen, phosphorus, iron) 2) Nutrient supply is from either land (rivers) or upwelling of deep water anchovies Peruvian guano mine (Nat l Geo)
Did increased ice age windiness enhance biological pumping of C into the deep sea reservoir?
Did increased ice age windiness enhance biological pumping of C into deep sea reservoir? Australian outback
Iron fertilization hypothesis Oceanographer John Martin discovered that iron shortage was limiting phytoplankton growth and overall productivity in many regions of ocean phytoplankton bloom off east coast of Argentina
Nature News Ocean-fertilization project off Canada sparks furore Bid to boost salmon stocks relied on hotly debated science and dubious carbon credits. Jeff Tollefson 23 October 2012 October 2012 The first reports about the project, which appeared in British newspaper The Guardian on 15 October, presented it as a rogue geoengineering scheme the largest in history in blatant violation of international treaties. Russ George, a US entrepreneur, had persuaded the Haida Nation village of Old Massett on the Queen Charlotte Islands to fund the project by promising that it would be possible to sell carbon credits for the carbon dioxide taken up by phytoplankton.
There was much more dust in atmosphere during glacial maxima. Gobi Desert Reversed Scale!
What would the presence of continental ice sheets have done to global wind velocities? Austin Post: Vegetation: the scum that covers the Earth between glaciations.
katabatic winds, Antarctica I have never heard or felt or seen a wind like this. I wondered why it did not carry away the earth. Apsley Cherry-Garrard http://hotgates.stanford.edu:3455/southpole/577
3 rd mechanism for transferring more C into the deep sea: changes in ocean currents (rem: #1 = cooler ocean temperature #2 = enhanced biological pump (Fe and more wind stirring and hence upwelling in mixed layer)
Today, N Atlantic deep water has relatively positive del 13 C
Reconstructing paleo-del 13 C using forams in deep-sea cores
ANT deep water little deep water formed in N Atlantic
Conclude: Large-scale changes in global ocean circulation were involved in the ice age transfer of the 90 ppm CO 2 from atmosphere to deep sea
What about methane? Methane concentration has increased by about 150% since AD 1750, and it accounts for 20% of the total radiative forcing from all greenhouse gases.
Unlike CO 2, methane does not cycle into the ocean. Aronson et al (2013). Frontiers in Microbiology
CH 4 conc. In atmosphere responsive to intensity of northernhemisphere monsoons, which respond to precession.
Precession ca. 22,000 year period Strongest effects at low latitudes where it amounts to +- 12% (40 W/m 2 ) of long-term, mean insolation (see Ruddiman Chapter 8)
CH 4 concentrations closely track precession cycle
4) Methane concentration is sensitive to strength of north-tropical monsoons.
5) CH 4 responds immediately to precession cycles (21 ka) solar radiation - strength of northern-tropical monsoon amount of CH 4 produced We are here 6) CO 2 lags the precession cycle by about 1 ka
CO 2 lagged global temperature (based on del 18 O) by roughly 1,000 years Vostok ice core records (Petit et al., 2000 and Barnola et al. 2003).
What might be the processes causing this 1000-yr lag in CO 2 response to precession?
If CO 2 lags temperature, it cannot be the main forcing factor..right?
7) Both CH 4 and CO 2 lag the obliquity (41 ka) cycles (they track ice-sheet volume) We are here 8) GHGs can be both forcing and feedbacks to glaciation
In several older, convoluted articles, Ruddiman asserts: Obliquity changes trigger ice sheet growth, which reduces conc. of GHGs in atmosphere, which causes further ice sheet growth.
41,000 yr Obliquity changes trigger ice sheet growth, which reduces conc. of GHGs in atmosphere, which causes further ice sheet growth. 23,000 yr Precession changes GHG conc. directly and immediately, which then force ice growth/retreat.
The trouble is..detecting lag / lead relationships requires detailed age control.that we still lack.
7) Both CH 4 and CO 2 lag the obliquity (41 ka) cycles (they track ice-sheet volume) Much more on this next time! 8) Ruddiman: GHGs can be both forcing and feedbacks to glaciation