Greenhouse gases in the Quaternary: constraining sources, sinks, feedbacks and surprises Eric Wolff 1 and the QUEST-DESIRE team 1. British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK

Why palaeo greenhouse gases? How do recent changes compare with natural variability? Can we understand natural cycles? – important if we want to underpin estimates of (future) feedbacks Does the past constrain the likelihood of high impact, low probability events (surprises)? How did Earth respond to high CO 2 climates? Can we find analogues for large carbon releases in the past, to test effects and recovery?

Plan for today Set the scene by presenting the evidence Challenge the AIMES community to understand (at process level) the records Briefly consider which aspects of the past are most relevant to the future Muse on the extent to which the Quaternary Earth System was deterministic

Recent past – other GHG Etheridge et al 1996 MacFarling Meure et al 2006

Greenhouse gases over the Holocene (10 kyr) Natural or anthropogenic? Ruddiman 2003 ( Clim. Change, 61, )

European Project for Ice Coring in Antarctica (EPICA) Dome C 75ºS; 3233 m asl Mean T:-54.5ºC Core to 3270 m

Estimated Antarctic temperature (based on water isotopes) EPICA Community Members, Nature, 429, , 2004; Jouzel et al., Science, 2007

What does CO 2 do in a changing climate? - CO 2 responsible for 30-50% of the glacial-interglacial warming - probably controlled mainly through processes in the Southern Ocean

CO 2 in the last glacial Ahn and Brook 2008

Dome C detailed CO 2 Monnin et al (2001) Science 291, Phasing is consistent with CO 2 as an amplifier

13 CO 2 Lourantou et al, GBC, In Press (2010)

Estimates courtesy of Andy Ridgwell

But we are out of the range of the last 800 ka Lüthi et al., Nature 2008 (EPICA gas consortium) In rate as well as concentration: –Fastest multicentennial rate in last termination was ~20 ppmv in 1000 years –20 ppmv increase in last 11 years

CH 4

But the late Pleistocene holds no analogue for the present And natural changes are dwarfed by the anthropogenic influence

Rapid events in CH 4 Greenland temperature

Methane: the rapid jumps account for much of the g-ig change

Causes of change (CH 4 ) Sources Wetlands –Northern –Tropical Methane hydrates Biomass burning Others (vegetation,….) Sinks OH change Temperature Water vapour Competition (VOCs) Isotopic evidence suggests no major role for hydrates

δD of CH 4 Sowers, Science 2006 Concludes marine clathrates are not important for these warming events No evidence of fossil 14 C at transition Large blocks from Pakitsoq, Greenland. Petrenko et al., 2009, Science

3-box model Dallenbach et al, GRL, 2000 Combining Dallenbach et al (GRL 2000) and Brook et al (GBC 2000), you would conclude: Interhemispheric differences TropicsNorth DO cold-warm Bolling/Allerod to YD LGM to preBoreal

13 CH 4 - termination I LGM 13 CH 4 ~3.5 heavier than Holocene rapid 13 CH 4 variations during Bolling/Alleroed - Younger Dryas also slow changes during LGM and early Holocene when CH 4 constant D 20 heavier in LGM (Sowers, 2006) Data not really in agreement with conclusions of Schaefer et al (2006) Fischer et al., Nature 452, 864 (2008)

Glacial/interglacial CH 4 source changes 10 Variables: 5(6) potenial preindustrial sources with different isotopic signature 1 atmospheric lifetime 3 different sinks with different contributions 4 Constraints: CH 4 concentration North & South D(CH 4 ) North 13 CH 4 South

Isotopic constraints Box model to estimate range of likely emissions and lifetimes in different time periods Relatively minor changes in isotopic content suggest changes in hydrates and biomass burning emissions were modest Concludes that main changes are in boreal wetlands and in atmospheric lifetime But conclusion on biomass burning seems at odds with terrestrial evidence and lifetime changes are hard to explain (see Levine talk) Also the wetland response seems stronger than bottom- up models would predict

Last interglacial – how did CH 4 react to Arctic warmth?

Summary Ice cores show us the unprecedented extent and rate of the recent increase in greenhouse gases and therefore radiative forcing Large changes in Quaternary challenge us to understand natural cycles and test our knowledge of feedbacks (especially wrt ocean carbon and wetland methane) Last interglacial might be used to seek reassurance against large methane releases under warming For anything approaching an analogue for a world with higher CO 2 we will have to tackle the harder questions posed by older climates

How deterministic is this system?

Thank you