Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 2The most recenttime when nopermanent iceexisted on the planet(sea level = +73 m)occurred >35 millionyears ago whenatmospheric CO 2was 1,250±250 ppmV.2.3 Sea Level Changes during the PastThe record of past changes in ice volume providesimportant insight to the response of largeice sheets to climate change. Our best constraintscome from the last glacial cycle (120,000 yearsago to the present), when the combination ofpaleoshorelines and the global δ 18 O record providesreasonably well-constrained evidence ofchanges in eustatic sea level (Fig. 2.1). Changesin ice volume over this interval were paced bychanges in the Earth’s orbit around the sun(orbital time scales, 10 4 –10 5 a), but amplificationfrom changes in atmospheric CO 2 is requiredto explain the synchronous and extensiveglaciation in both polar hemispheres. Althoughthe phasing relationship between sea level andatmospheric CO 2 remains unclear (Shackleton,2000; Kawamura et al., 2007), their records arecoherent and there is a strong positive relationbetween the two (Fig. 2.2).A similar correlation holds for earlier times inEarth history when atmospheric CO 2 concentrationswere in the range of projections for theend of the 21st century (Fig. 2.2). The mostrecent time when no permanent ice existedon the planet (sea level = +73 m) occurred>35 million years ago when atmospheric CO 2was 1,250 ± 250 ppmV (Pagani et al., 2005). Inthe early Oligocene (~32 million years ago),atmospheric CO 2 decreased to 500 ± 50 ppmV(Pagani et al., 2005), which was accompaniedby the first growth of permanent ice on theAntarctic continent, with an attendant eustaticsea level lowering of 45 ± 5 m (DeConto andPollard, 2003). The fact that sea level projectionsfor the end of the 21st century (Meehl etal., 2007; Rahmstorf, 2007; Horton et al., 2008)are far below those suggested by this relation(Fig. 2.2) reflects the long response time of icesheets to climate change. With sufficient time atelevated atmospheric CO 2 levels, sea level willcontinue to rise as ice sheets continue to losemass (Ridley et al., 2005).During the Last Interglaciation Period (LIG),from ~130,000 years ago to at least 116,000years ago, CO 2 levels were similar to preindustriallevels (Petit et al., 1999; Kawamuraet al., 2007), but large positive anomalies inearly summer solar radiation driven by orbitalchanges caused arctic summer temperatures tobe warmer than they are today (Otto-Bliesner etal., 2006). Corals on tectonically stable coastsindicate that sea level during the LIG was 4to 6 m above present (Fig. 2.1) (Stirling et al.,1995, 1998; Muhs et al., 2002), and ice-corerecords (Koerner, 1989; Raynaud et al., 1997)and modeling (Cuffey and Marshall, 2000;Otto-Bliesner et al., 2006) indicate that muchof this rise originated from a reduction in thesize of the Greenland Ice Sheet, although somecontribution from the Antarctic Ice Sheet maybe required as well.At the Last Glacial Maximum, about 21,000years ago, ice volume and area were about2.5 times modern, with most of the increaseoccurring in the Northern Hemisphere (Clarkand Mix, 2002). Deglaciation was forced bywarming from changes in the Earth’s orbitalparameters, increasing greenhouse gas con-Figure 2.2. Relation between estimated atmosphericCO 2 and the ice contribution to eustatic sea level indicatedby geological archives and referenced to modern(pre-industrial era) conditions [CO 2 =280 parts permillion by volume (ppmV), eustatic sea level = 0 m].Horizontal gray box represents range of atmosphericCO 2 concentrations projected for the end of the 21stcentury based on IPCC emission scenarios (lower endis B1 scenario, upper end is A1F1 scenario) (Nakicenovicet al., 2000). The vertical red bar represents the IPCCFourth Assessment Report (AR4) estimate of sea levelrise by the end of the 21st century (Meehl et al., 2007).The difference between the IPCC AR4 estimate and thehigh paleo-sea levels under comparable atmosphericCO 2 levels of the past (blue dots with vertical bar givenas uncertainties) largely reflects the long response timeof ice sheets. A central question raised by the dynamicchanges in ice sheets described in this chapter (and thatare not included in the IPCC AR4 estimates) is howmuch they will reduce the ice-sheet response time toclimate change.36