Cold Climate During the Closest Stage 11 Analog to Recent Millennia

ARTICLE IN PRESS

Quaternary Science Reviews ] (]]]]) ]]]–]]]

Cold climate during the closest Stage 11 analog to recent Millennia

William F. RuddimanÃ

Department of Environmental Sciences, McCormick Road, University of Virginia, Clark Hall, Charlottesville, VA 22904, USA

Received 23January 2004; accepted 8 October 2004

Abstract

The early part of marine isotopic Stage 11 near 400,000 years ago provides the closest analog to Holocene insolation levels of any interglaciation during the era of strong 100,000-year climatic cycles. The CH4 concentration measured in Vostok ice fell to 450 ppb, and CO2 values to 250 ppm. These natural decreases contrast with the increases in recent millennia and support the early anthropogenic hypothesis of major gas emissions from late-Holocene farming. During the same interval, dD values fell from typical interglacial to nearly glacial values, indicating a major cooling in Antarctica early in Stage 11. Other evidence suggests that 18 new ice was accumulating during the closest insolation analog to the present day: a major increase in d Oatm at Vostok, a similar increase in marine d18O values, and re-initiation of ice rafting in the Nordic Sea. The evidence permits extended (420,000 year) intervals of Stage 11 interglacial warmth in the Antarctic and North Atlantic, yet it also requires that this warmth ended and a new glacial era began when insolation was most similar to recent millennia. The Holocene CO2 anomaly was produced only in part by direct anthropogenic emissions; over half of the anomaly resulted from the failure of CO2 values to fall as they had during previous interglaciations because of natural responses, including a sea-ice advance in the Antarctic and ice-sheet growth in the northern hemisphere. r 2004 Elsevier Ltd. All rights reserved.

1. Introduction difference between the observed rises and the predicted drops—were estimated at 40 ppm for CO2 and 250 ppb Ruddiman (2003a) proposed that early farming for methane. The CH4 anomaly was attributed mainly produced enough CO2 and CH4 during the millennia to irrigation for rice, and the CO2 anomaly to clearance prior to the industrial era to boost gas concentrations in of forests. the atmosphere and counter the natural decreases that Another part of the early anthropogenic hypo- would otherwise have occurred. Natural CO2 and CH4 thesis was the inference that these early emissions trends for the late Holocene were estimated from those averted a natural global cooling that would otherwise in early interglacial marine isotopic Stages 5, 7, and 9, have initiated a new glacial cycle in the northern when concentrations in Vostok ice reached late-deglacial hemisphere (Fig. 2). This conclusion was based in maxima and then fell for more than 10,000 years early in part on an energy-balance model study showing each interglaciation (Petit et al., 1999). During the most that northeast Canada has been close to a state of recent deglaciation, CH4 and CO2 concentrations also glacial inception in recent centuries (Williams, 1978)and reached a late-deglacial peak and began similar that a relatively small cooling would be needed to decreases, but gas levels then rose rather than falling start a new glacial cycle in North America or Eurasia. during recent millennia (Fig. 1). The total gas anomalies Additional support came from ice-volume trends reached just before the start of the industrial era—the simulated by a numerical model that estimated time-dependent d18O responses to summer insolation ÃTel.:540 348 1963; fax:+434 982 2137. changes in the northern hemisphere (Imbrie and Imbrie, E-mail address: [email protected] (W.F. Ruddiman). 1980).

Solar Radiation Years Ago Years Future Observed CH 10,0000 10,000

) 4

2 700 500 Tmax Pmax ) 2 (ppb) 520 490 600 Stage 1

250 ppb (W/m N Tmax P atural Methane max 500 Tmin 480 CH Solar Radiation (Watts/m 4 trend 500 Stage 11 Pmin 480 10,000 5,000 0

(a) Years Ago Insolation N July °

65 Tmin 290 460 Pmin

Observed CO2 280 410,000 400,000 390,000 Natural Years Ago 270 CO2 peak Fig. 3. Mid-July 651N insolation trends during the Holocene and in (ppm)

2 40 ppm isotopic Stage 11. The closest Stage 11 analog to modern values 260

CO occurred 397,000 years ago (Berger and Loutre, 2003). Natural CO

250 2 trend tude than those during the three previous climatic cycles, 240 10,000 5000 0 although moving in the same direction. As a result, the (b) Years Ago early anthropogenic hypothesis can be challenged: the reduced amplitude of forcing during the Holocene might Fig. 1. Observed late-Holocene increases in concentrations of (a) CH4 have resulted in natural climatic responses smaller than (Blunier et al. 1995) and (b) CO2 (Indermuhle et al., 1999) compared to predicted natural decreases from the early anthropogenic hypothesis those during the last three interglaciations.

(Ruddiman, 2003a). The estimated sizes of the anthropogenic CH4 and Isotopic Stage 11 is a better analog to the Holocene. CO2 anomalies are the differences between the observed and the Because of comparably low eccentricity at that time, predicted trends. insolation variations were smaller in amplitude and closer to modern values (Fig. 3). The analog is not perfect: the tilt and July precession maxima were out of phase on the Stage 12/11 deglaciation (termination V)

Warmer but were very nearly in phase on the Stage 2/1

Observed climate deglaciation, while the subsequent insolation minima at tilt and precession were nearly in phase during Stage 11 but are out of phase now. Still, Stage 11 is the closest Anthropogenic available analog to Holocene insolation levels within the Glaciation threshold interval of dominant 100,000-year climatic cycles. Greenhouse One method used to compare Stages 1 and 11 is to Natural Solar radiation Gases determine the length of the Stage 11 interglaciation and and Greenhouse-gas Trend use it to forecast the duration of the current interglacia-

Colder tion. This approach has led to a widespread view that 8000 6000 4000 2000 0 Stage 11 was an unusually long interglaciation, and that Years Ago by implication the current interglaciation could last Fig. 2. According to the early anthropogenic hypothesis, a large many thousand of years into the future (Burckle, 1993; natural Holocene cooling would have initiated a glaciation within the EPICA Community Members, 2004). Some simulations last several thousand years, but anthropogenic greenhouse-gas of ice volume with zonal climatic models even show an emissions countered enough of the cooling to prevent a glacial inception. ice-free interval lasting for tens of thousands of years (Loutre and Berger, 2000). Although these results seem to refute the overdue-glaciation hypothesis, Sections 2–5 The choice of the first part of marine isotopic Stages of this paper will show that such a conclusion is 5, 7, and 9 as analogs to the Holocene can be unjustified. questioned. Eccentricity modulates precession at the The first issue considered here is the response of the 413,000-year period, and insolation variations during climatic system during the part of Stage 11 that provides the current interglaciation have been smaller in ampli- the closest insolation analog to the recent millennia of ARTICLE IN PRESS W.F. Ruddiman / Quaternary Science Reviews ] (]]]]) ]]]–]]] 3 the Holocene (Berger and Loutre, 2003). During this Years ago Years ago closest-analog interval, spanning the millennia just prior 10,000 0 10,000 0 Stage I Stage 1 to 397,000 years ago, July 651N summer insolation Pre-industrial 700 Pre-industrial values fell to a minimum very near the present-day level 280 (Fig. 3). Incoming July radiation was 14 W/m2 below 2 the modern level, compared to values 20–35 W/m lower 600 during the most similar parts of interglacial isotopic (ppb) (ppm)

4 Stages 5, 7, and 9. 2

CH 260 CO Stage 11 The behavior of the climate system during this closest- 500 Stage 11 analog portion of Stage 11 provides a stringent test of the early anthropogenic hypothesis. If the trends during Predicted Predicted this time match those during Stage 1, it will be difﬁcult 400 240 405,000 400,000 405,000 400,000 to argue that the Holocene trends are anomalous (a) Years ago (b) Years ago compared to the natural behavior of the climate system, and the early anthropogenic hypothesis will be refuted. 10,000 0 10,000 0 But if the Stage 11 trends show major reductions in 2 Stage 1 greenhouse gases and a substantial climatic cooling as Stage 1 0 predicted in Figs. 1 and 2, the hypothesis that the 0 Holocene trends are anthropogenic rather than natural (% ) in origin will be supported. 0.25 -2 atm 0 18 δ Temperature (ûC) Temperature

-4 2. Comparison of Stages 11 and 1: Vostok ice Stage 11 0.5 Stage 11 11.24

-6 The occurrence of typical full-interglacial values of 405,000 400,000 405,000 400,000 395,000 18 CH4,CO2, dD, and d Oatm in the lower tens of meters (c) Years ago (d) Years ago of Vostok ice indicates that it reaches deep into Stage 11. Petit et al. (1999) pinned the lower portion of their GT4 Fig. 4. Climatic proxy trends in Vostok ice (Petit et al., 1999) during the Holocene and the analogous interval early in isotopic Stage 11. time scale at Vostok by correlating a feature near 3265 m ice depth to marine isotopic substage 11.24 of Bassinot et al. (1994), who had refined the earlier SPECMAP time scale of Imbrie et al. (1984). This choice linked the scale at Vostok back to 340,000 years ago by Ruddiman first major shift of climatic proxy signals in Vostok ice and Raymo (2003). Using this same rationale, the from full-interglacial (substage 11.3) values to partly prominent early methane minimum during Stage 11 in glacial (substage 11.24) values with the equivalent Vostok ice falls at 397,000 years ago, the age of a transition in marine d18O records. significant precession insolation minimum. Other The Petit et al. (1999) choice was later verified by two proxies like CO2 and dD that have long been considered independent methods. Shackleton (2000) tuned the early responders (Lorius, et al., 1985; Jouzel et al., 1987; 18 Vostok d Oatm record to northern hemisphere autumn Waelbroeck et al., 1995) also reach minimum values at (early September) insolation, and Bender (2002) corre- or near the level of this CH4 minimum. lated O2/N2 variations in Vostok ice to changes in The classic ‘late responder’ in the climate system is summer (December 21) insolation at 781S. These time northern hemisphere ice volume, which lags 5000 years scales confirmed that the interval of Vostok ice showing behind northern hemisphere summer insolation (Milan- the first major shift of proxy signals away from full kovitch, 1941; Hays et al., 1976; Imbrie et al., 1984). The interglacial values dates from 400,000 to 390,000 years proxy in Antarctic ice cores tied most closely to northern 18 ago, the interval that includes the closest insolation hemisphere ice volume is d Oatm, which records analog to the late Holocene and the near future (Fig. 3). changes in average seawater d18O (a measure of ice The time scale used to plot the Vostok records in volume) and in monsoon-driven biomass (Bender et al., Fig. 4 uses two minor refinements of these time scales 1994). During climatic transitions, the slow ice-volume 18 based on separating the proxy climatic signals in Vostok component embedded in the d Oatm signal should cause ice into ‘early’ and ‘late’ responders. A key early it to lag behind the faster-responding parts of the responder is atmospheric methane, which varies in climate system. The transition from the substage 11.3 phase with July insolation at the precession cycle, as interglacial peak into substage 11.24 at Vostok (Petit et 18 shown by the 10,500-year age of the last CH4 al., 1999) shows the expected lag: maximum d Oatm maximum in annually layered GRIP ice (Blunier et al., values are reached a few 1000 years later than minima in 1995). This July phasing was used to tune the CH4 time CH4,CO2 and dD. ARTICLE IN PRESS 4 W.F. Ruddiman / Quaternary Science Reviews ] (]]]]) ]]]–]]]

Bassinot et al. (1994) dated the substage 11.24 d18O The d18O trend in air from Vostok ice can be used to maximum to 390,000 years ago, consistent with the test the prediction that a late-Holocene glacial cycle is SPECMAP time scale (Imbrie et al., 1984). Petit et al. overdue in the northern hemisphere (Fig. 2). As noted 18 (1999) then used this age estimate to anchor the GT4 earlier, the d Oatm signal in ice is controlled both by time scale for Vostok ice. Later studies have suggested sea-water d18O(ice volume) and biomass changes that the SPECMAP time scale is too old by an average linked to tropical monsoons (Bender et al., 1994). The 18 of 1500–2000 years (Pisias et al., 1990; Shackleton, 2000; average glacial–interglacial amplitude of the d Oatm Ruddiman and Raymo, 2003). Allowing for this signal for the last several climatic cycles at Vostok is adjustment, the likely age of the substage 11.24 feature 1.5%. An estimated 1.05% of the 1.5% change at the is probably nearer 392,000 years. Stage 2/1 deglaciation can be attributed to changes in Based on the above considerations, the time scale global ice volume (Duplessy et al., 2002; Schrag et al., used to create Fig. 4 places the Stage 11 CH4 minimum 2002), leaving the remaining 0.45% as a monsoon- 18 at 397,000 years ago and the d Oatm maximum at related biomass contribution. Between substages 11.3 18 392,000 years ago. Ages are linearly interpolated and 11.24, d Oatm values increased by 0.65% (Fig. 4d). between these levels, and ages older than 397,000 years Subtracting the (estimated maximum) biomass compo- 18 ago are derived by aligning the lowermost region of high nent of 0.45% from 0.65% leaves a residual d Oatm CH4 values at Vostok with the precession insolation increase of 0.2% that can plausibly be attributed to ice maximum 408,000 years ago, using the method of growth. This result suggests that new ice was growing in Ruddiman and Raymo (2003). For the key interval the northern hemisphere during and after the closest between 400,000 and 392,000 years ago, these various Stage 11 analog to the late Holocene. If the Dole effect time scales disagree by no more than a few thousand were assumed to have been at less than maximum years. strength during the substage 11.3/11.24 transition, a The greenhouse-gas concentrations at 397,000 years larger ice-volume residual would be required. ago, the time of the closest insolation analog to modern levels, conﬁrm the predictions of the early anthropogenic hypothesis from Fig. 1:CH4 values fell to 3. Comparison of Stages 11 and 1: EDC ice 445 ppb (Fig. 4a), just below the predicted value of 450 ppb, and CO2 values fell to 250 ppm (Fig. 4b), just EPICA Community Members (2004) published pre- above the predicted range of 240–245 ppm. Compared liminary results from the EDC ice core extending back to the measured concentrations actually reached prior to 740,000 years. The dD and dust-concentration ana- the industrial era (282 ppm for CO2 and 680 ppb for lyses span all of Stage 11, while the published CO2 and CH4), the Stage 11 values indicate that late-Holocene CH4 analyses cover only the Stage 12/11 transition and concentrations were anomalously high by more than the early part of Stage 11. The EPICA group concluded 30 ppm for CO2 and 235 ppb for methane, close to the that warmer interglacial temperatures in Antarctica size of the anomalies predicted by the hypothesis. during Stage 11 lasted for 28,000 years. Because only Other proxy climatic signals in Vostok ice record 12,000 years of similar interglacial conditions have major shifts toward glacial conditions at high southern elapsed in Stage 1, they concluded that the current latitudes. The dD signal decreased from typically interglacial warmth will persist for 16,000 years more, in interglacial values to 75% of full-glacial values, and disagreement with the overdue-glaciation hypothesis. estimates of Antarctic temperature derived from the dD On the other hand, the major drop in dD values in the signal dropped to near-glacial levels (Fig. 4c). In EDC record that marks the end of this interval of addition, the Na+ (sea-salt ion) concentration increased Antarctic warmth began before 400,000 years ago and halfway from interglacial to glacial values (Petit et al., was nearly complete by 390,000 years ago, a timing that 1999). These observations indicate that shortly after is close to that observed at Vostok (Fig. 4c), given the 400,000 years ago the Antarctic region had shifted considerable dating uncertainties at EDC. As a result, toward a cold glacial state in an insolation regime the EDC dD record supports the evidence in Section 2 similar to that today and at greenhouse-gas levels very that peak interglacial warmth had ended by 397,000 close to those that should prevail today in the absence of years ago, the time of the closest insolation analog to the anthropogenic overprints. present day. In contrast, the Holocene dD (and Na+) signals at Both of these predictions obviously cannot be correct. Vostok drifted only slightly toward glacial values Does the current interglaciation still have 16,000 years (Fig. 4c). The failure of these proxies to shift farther left to run, or should it have ended during the last few toward glacial values during the Holocene suggests that millennia? today’s Antarctic climate is anomalously warm com- The resolution of this dilemma lies in the alignment pared to the natural trends observed during the most used by EPICA to compare Stages 11 and 1. Their analogous part of Stage 11. choice (Fig. 5 of their paper) places the present-day dD ARTICLE IN PRESS W.F. Ruddiman / Quaternary Science Reviews ] (]]]]) ]]]–]]] 5

Years Ago Years Future Stage II 5000 0 10,000 Interglaciation 26,000 years ) 2 520 Stage 11 EPICA Pmax (2004) Stage I will last 500 Interglaciation 16,000 more years (A) 480 Stage II N July Insolation (W/m N July

û Stage 1 P Interglaciation

65 min 460 410,000 400,000 Ruddiman Years Ago (2005) Stage I has Fig. 5. The alignment of dD trends in EDC ice used by EPICA Interglaciation ended Community Members (2004) positions the present-day 651N insolation (B) minimum with a Stage 11 insolation maximum 409,000 years ago. Fig. 6. Comparison of the Stages 1 and 11 alignments chosen by (A) EPICA Community Members (2004) and (B) this paper (as well as Berger and Loutre, 2003). The alignment in B is consistent with the value opposite the Stage-11 dD value at 409,000 years 651N summer insolation trends (see Fig. 3); the alignment chosen by ago, but that selection results in the alignment of the EPICA is not (see Fig. 5). modern-day 651N insolation minimum with a 651N insolation maximum in Stage 11 (as shown in Fig. 5 of time allows that warmth to come to an end during the this paper). This alignment is clearly incorrect by a very time that was the closest insolation analog to the present large amount. As Berger and Loutre (2003) noted, the day. This alignment supports the hypothesis that the closest Stage 11 analog to the modern-day insolation Stage 1 peak of interglacial warmth in Antarctica should minimum occurred 397,000 years ago (Fig. 3), well after now be over, with a major cooling well underway. For the alignment chosen by EPICA. the rest of this paper, the prominent cooling at the This error on EPICA’s part resulted from their substage 11.3/11.24 transition is used as the boundary decision to align terminations V and I and to assume between peak-interglacial conditions in ‘early’ Stage 11 that the two interglaciations then developed in complete and the ‘late’ portion of Stage 11 that lasted until the parallel (Fig. 6, top). This approach carries considerable Stage 11/10 transition near 352,000 years ago (Imbrie risk. The timing of terminations is thought to be et al., 1984). critically dependent on the close alignment of insolation maxima from both precession (which dominates the 651N insolation signal) and from obliquity (Imbrie et al., 4. Comparison of Stages 11 and 1: marine sediments 1992; Raymo, 1997). Termination I, centered at 13,000 18 years ago, resulted from the nearly coincident insolation The conclusion from the d Oatm evidence at Vostok maxima for precession (11,000 years ago) and obliquity that new ice sheets began growing at the substage 11.3/ (10,000 years ago). 11.24 transition is at odds with the interpretation that In contrast, for the interval near termination V, the Stage 11 was a uniquely long interglaciation in which no insolation maximum from the obliquity signal fell at new ice grew in North America and Eurasia (Burckle, 418,000 years ago, midway between insolation maxima 1993). In apparent support of this interpretation, a high- from the June 21 precession signal at 429,000 and resolution study of Stage 11 in the subpolar North 408,000 years ago. SPECMAP (Imbrie et al., 1984) Atlantic showed that the ocean surface remained warm placed marine d18O termination V at 415,000 years ago for at least 30,000 years after termination V (McManus near the obliquity insolation maximum, a choice that et al., 2003). However, closer scrutiny shows that ice was still remains uncertain. Because the relative forcing from not absent from North America and Eurasia for this obliquity and precession were so different on these two entire interval. McManus et al. (2003) actually stated terminations, aligning the two interglacial stages at the that that the ‘‘relatively ice-free portion’’ of Stage 11 terminations is risky. EPICA’s attempt to do so ‘‘lasted longer than other peak interglacials’’. This produced the implausible misalignment of insolation careful wording permits a relatively early onset of new trends shown in Fig. 5. ice in Stage 11. The correct alignment of Stage 1 against the latter Another problem with the interpretation of a long ice- (rather than the earlier) part of Stage 11 (Fig. 6, bottom) free interglaciation is that North Atlantic sea-surface allows for an unusually long (420,000 year) interval of temperatures cannot be used as a sensitive indicator of warmth in Antarctica during Stage 11, but at the same northern hemisphere ice volume. Lagging warmth (and ARTICLE IN PRESS 6 W.F. Ruddiman / Quaternary Science Reviews ] (]]]]) ]]]–]]] the absence of ice-rafted debris) often characterized the ODP sites 846 and 849 (Mix et al., 1995a, b) register a North Atlantic south of Iceland during intervals when d18O change of 1.61% from Stage 2 to Stage 1, implying ice sheets were growing on nearby landmasses (Ruddi- a maximum T–S overprint of 0.5670.1%. During the man and McIntyre, 1979). During isotopic substage 5a, transition into substage 11.24, d18O values increased by the subpolar North Atlantic was at or close to full- 0.72–0.74%. Subtracting the maximum T/S overprint of interglacial warmth (Sancetta et al., 1972), even though 0.56% leaves a residual of 0.16–0.18% as the estimated sea level was 15–20 m below its modern position ice-volume effect. This independent estimate is close to 18 (Chapell and Shackleton, 1986; Bard et al., 1990). The the 0.2% ice-volume residual estimated from the d Oatm volume of ice required to drop sea level by 15–20 m was trend at Vostok. If each 0.1% change in d18O represents equivalent to the glacial-maximum Scandinavian ice 11 m of sea-level change, the growth of ice during sheet, yet the Atlantic was warm. substage 11.24 would have amounted to 20 m of sea- Several marine proxies further support the Vostok level fall. 18 d Oatm evidence that new ice appeared on land during This amount of ice growth would have accumulated the substage 11.3/11.24 transition. One line of evidence by the substage 11.24 peak at 392,000 years ago, a time comes from marine d18O signals, which record changes equivalent to roughly 5000 years from now according to in ice volume and variations in local ocean temperature the insolation trends in Fig. 3. More pertinent to the and salinity. The maximum size of the temperature–- early anthropogenic hypothesis is the question of salinity (T–S) overprint at any location can be estimated whether or not ice was already growing 397,000 years from changes that occurred during the most recent ago, the time when the insolation configuration was deglaciation. As noted earlier, the ice-volume contribu- most analogous to the present day. Several lines of tion to the d18O shift across termination I is estimated at evidence indicate that it was. 18 1.05% (Duplessy et al., 2002; Schrag et al., 2002). Any First, the d Oatm trend at Vostok had already additional isotopic change on termination I beyond this increased by more than 0.55% by the time of the amount can be attributed to the combined effects of minima in CH4,CO2, and dD 397,000 years ago 18 18 local temperature and salinity, and this ‘excess’ d O (Fig. 4). At least 0.10% of this d Oatm increase (the change on termination I can be used to estimate the amount in excess of the assumed 0.45% biomass maximum T–S overprint present elsewhere in the overprint) should represent increased ice volume. record, including isotopic Stage 11. The tacit assump- Second, Bauch et al. (2000) dated the renewal of ice- tion in this method is that the temperature–salinity rafted deposition in Nordic Sea sediments to 398,000 overprint added to the d18O signal in Stage 11 is unlikely years ago. In order to calve icebergs to the ocean by that to have been larger than the amount removed during a time, ice growth must have been underway in the north full deglacial transition. for several 1000 years. Third, a sensitivity-test experi- Marine cores in the equatorial Atlantic (Tiedemann et ment with the GENESIS 2 climate model showed that al., 1994, Bickert et al., 1997) and tropical Indian Ocean lowering greenhouse-gas concentrations to the levels (Bassinot et al., 1994) record d18O increases as large as reached during the Stage 11 insolation analog to the 1.0% during the substage 11.3/11.24 transition. Because present day resulted in year-round snow cover in parts d18O shifts during the last deglaciation in these regions of Baffin Island, Canada (Ruddiman et al., 2005). were also very large (2%), indicating T–S overprints of Finally, it seems unlikely that 20 m of sea-level- about 1%, it is just barely possible to avoid a equivalent ice volume could have accumulated by requirement for ice growth by invoking the maximum 392,000 years ago without any of the ice having grown T–S overprint. Observed d18O changes in the eastern by 397,000 years ago. tropical Pacific do, however, require ice growth In summary, marine evidence requires the growth of (Table 1). Benthic foraminiferal d18O signals from ice sheets equivalent to 20 m of sea level drop by

Table 1 Estimate of the ice-volume component of the measured d18O increases (%) from substage 11.3to substage 11.24, based on subtracting the maximum temperature–salinity (T–S) overprint on the ice-volume component of the Stage 2/1 deglacial transition.

Stage 2/1 transition Substage 11.3/11.24 transition

ODP Measured Max. T–S Measured Minimum site d18O increase contributiona d18O increase ice contribution ODP 846b 1.61 0.56 0.72 0.1670.1 ODP 849c 1.61 0.56 0.74 0.1870.1

aAssumed ice-volume contribution of 1.05% (Duplessy et al., 2002; Schrag et al., 2002). bMix et al. (1995a). cMix et al. (1995b). ARTICLE IN PRESS W.F. Ruddiman / Quaternary Science Reviews ] (]]]]) ]]]–]]] 7 substage 11.24 (392,000 years ago). The evidence Years Ago Years in the Future further indicates that some of this ice was growing by 10,0000 20,000 40,000 60,000 397,000 years ago, the closest insolation analog to the ) present day. 2 520

Stage II 5. Comparison of Stages 11 and 1: ice-volume simulations 500

1 During the current July 65 N insolation minimum, 480 solar radiation values lie 2/3of the way toward the Stage I N July Insolation (W/m N July glacial extreme of the orbital range over the last several ° 65 hundred thousand years, yet no new ice has appeared in 460 response to this forcing. McManus et al. (2003) termed 400,000 380,000 360,000 340,000 this absence of new ice a ‘skipped beat’ in orbital forcing (a) Years Ago of ice volume. This ‘skipped beat’ creates a dilemma for modeling Years Ago Years in the Future 10,0000 20,000 40,000 60,000 efforts that attempt to simulate ice-sheet growth and 0 decay in response to orbital forcing. The dilemma is that 651N summer insolation will not fall as low as it is now ) Stage I for the next 55,000 years (Fig. 7a). With no ice today, 3 10 Stage II Km

and with higher summer insolation levels far into the 6 δ180 evidence future, there seems to be no reason to expect ice to grow (10 20 for a very long time. Some simulations with time- Stage II Ice Growth dependent ice-sheet models project no ice growth for Ice volume Simulated that entire interval (Loutre and Berger, 2000; Berger and 400,000 380,000 360,000 340,000 Loutre, 2003). Added to the 6000 years of the Holocene (b) Years Ago without significant ice on North America and Eurasia, Fig. 7. Model simulations of insolation forcing of ice volume. (a) the current interglaciation would then last more than Insolation trends at 651N during Stage 11 are similar to those during 60,000 years. the Holocene and future. (b) Model simulations that do not initiate ice But a 60,000-year interglaciation would be without growth in the late Holocene also fail to simulate ice growth early in Stage 11 (Berger and Loutre, 2003). Yet evidence cited in the text precedent in all 2.75 million years of the northern indicates that ice sheets were growing earlier in Stage 11. hemisphere ice age. Even during the smaller ice-volume fluctuations between 2.75 and 0.9 million years ago, ice sheets grew and melted mainly at a 41,000-year cycle, On the other hand, ice-volume simulations that with only about 20,000 years between glaciations. include CO2 forcing based on the Vostok ice record During the 100,000-year cycles of the last 0.9 million have been able to match the evidence for ice growth years, ice sheets have been present in the northern earlier in Stage 11, thus avoiding an unrealistically long hemisphere for 90% of the time, with interglaciations ice-free interval (Berger and Loutre, 2003). These lasting an average of just 10,000 years (Shackleton, results, combined with evidence in Sections 2 and 4, 1987). No interglaciation has ever lasted 60,000 years, suggest a solution to the dilemma of the long ice-free and no obvious reason exists to think that the natural Holocene and future. If the large anthropogenic over- climate system has just entered such an unprecedented prints of CO2 and CH4 during the late Holocene were ice-free era. removed, thereby allowing natural gas concentrations to Stage 11 points the way to an answer to this dilemma. fall to levels like those early in Stage 11, the resulting Insolation trends late in Stage 11 were very similar to model simulations should produce results similar to those during the Holocene and the future (Fig. 7a). Not Stage 11, with early growth of ice and no implausibly surprisingly, the same model simulations of Berger and long ice-free interval. Loutre (2003) that produced no ice growth late in Stage This nucleus of late-Holocene ice would be important 1 and for 55,000 years into the future also simulated no to the progression of the next (current) glacial cycle. ice-sheet growth until the Stage 11/10 boundary near Growing ice sheets create positive feedbacks that favor 350,000 years ago (Fig. 7b). Yet the evidence from additional glaciation, including albedo-temperature 18 changes in Vostok d Oatm (Section 2) and in marine feedback, elevation-temperature feedback, orographic d18O and Nordic Sea ice rafting (Section 4) indicates precipitation along south-facing slopes, and slow radial that new ice was already growing before 392,000 years ice flow across unglaciated terrain (Andrews and ago, in disagreement with the simulation shown in Mahaffy, 1976). After ice first appeared during the Fig. 7b. transition into substage 11.24, ice growth appears to ARTICLE IN PRESS 8 W.F. Ruddiman / Quaternary Science Reviews ] (]]]]) ]]]–]]] have been nearly continuous for the rest of Stage 11 and to this impasse may lie in processes that contribute to all of Stage 10, in part because of ice feedbacks on the the size of the CO2 anomaly without requiring direct climate system. The same process would presumably be emissions of CO2. underway now if natural forcing had prevailed. Ruddiman (2003a) defined the Holocene anthropogenic CO2 anomaly as the difference between the observed CO2 rise and the predicted CO2 fall (Fig. 6. Discussion 1b). The predicted drop in CO2 was based on natural trends measured during the early parts of previous Comparison of late-Holocene climatic trends with interglaciations, when concentrations fell by 30–50 ppm those during the closest Stage 11 insolation analog (Petit et al., 1999). The Stage 11 trend shown in Fig. 4b supports the major predictions of the early anthropo- shows a natural drop from 282 to 250 ppm. genic hypothesis: CH4 and CO2 levels dropped to levels Implicit in this definition of the Holocene CO2 close to those predicted (Fig. 4a and b); the early anomaly are two distinct components: (1) direct responding Southern Hemisphere cooled dramatically anthropogenic carbon emissions that caused CO2 values (Fig. 4c); and ice sheets began to grow in the northern to rise, and (2) natural factors that prevented CO2 values hemisphere (Fig. 4d). In contrast, during the middle and from falling as they had in previous interglaciations. The late Holocene, gas concentrations rose, south-polar resolution of the impasse noted above could lie in the climate cooled only slightly, and northern ice sheets second group of mechanisms. If the natural processes did not grow. that drove CO2 lower during previous interglaciations The cause of these very different Holocene trends is were overridden during the Holocene, the prevention of not likely to lie in natural factors. Not only were a natural drop in CO2 would have added to the size of insolation trends similar during the two intervals the CO2 anomaly, but would not have required direct (Fig. 3), but key terrestrial boundary conditions early emissions of carbon. Such an explanation would be in Stage 11 were nearly identical to those in the consistent with the claim that the entire CO2 anomaly is Holocene: northern ice sheets had melted, sea level anthropogenic in origin, yet it would not violate the was high, and vegetation had reached a full-interglacial carbon-budget and d13C constraints. state. Any differences in insolation and in terrestrial In the concept proposed here (Fig. 8), direct emissions boundary conditions seem too small to account for the from human activities account for most of the 250-ppb fundamentally different direction of the Holocene CH4 anomaly (Ruddiman and Thomson, 2001), trends. If natural forcing can be ruled out, the although the magnitudes are difficult to quantify. In anomalous Holocene gas increases have to be anthro- the case of CO2, direct emissions only account for a pogenic in origin. fraction of the estimated anomaly of 32–40 ppm. The This conclusion is seemingly at odds with the analysis part of the Holocene CO2 anomaly that obviously of Joos et al. (2004), who concluded that direct requires direct anthropogenic emissions is the 14-ppm anthropogenic emissions of CO2 from forest clearance increase between the natural CO2 peak of 268 ppm are insufficient to explain the proposed 40-ppm that occurred 10,500 years ago and the 282 ppm value anthropogenic CO2 anomaly. They estimated that reached just before the industrial era (Fig. 1b). For a full 550–700 Gt C of direct CO2 emissions from forest anthropogenic anomaly of 32–40 ppm, the 14-ppm burning would be needed to explain a 40-ppm anomaly, increase from direct emissions would then represent well above the 250–320 GtC calculated by Ruddiman 35–45% of the total anomaly. (2003a) based on Indermuhle et al. (1999), and even farther above their own estimate of 60–80 Gt C based on DeFries et al. (1999). Joos et al. (2004) also noted that the small amplitude of the negative d13C trend in late- Holocene CO2 found by Indermuhle et al. (1999) CH4 Warming No Northern Natural appears to limit terrestrial carbon emissions to amounts and CO2 Counters Most of Ice Sheets; Drop in Emissions Natural Cooling Smaller Southern CO smaller than those needed to account for a 40-ppm CO2 2 Trend Sea-ice Advance Prevented anomaly. These estimates of Gt C emissions are all based on carbon-cycle models that include both fast exchanges among surface carbon reservoirs as well as DIRECT INDIRECT ANTHROPOGENIC ANTHROPOGENIC the slow uptake of CO2 by the deep ocean and EFFECT EFFECT subsequent dissolution of seafloor CaCO3. These findings seem irreconcilable. How can the Fig. 8. Direct anthropogenic emissions explain most of the Holocene CH4 anomaly but only part of the CO2 anomaly. The rest of the CO2 Holocene CO2 anomaly be entirely anthropogenic in anomaly may result from the prevention of a natural CO2 drop that origin if direct anthropogenic carbon emissions from occurred in previous interglaciations but was overridden in the humans are too small to account for it? The resolution Holocene by anthropogenic warming. ARTICLE IN PRESS W.F. Ruddiman / Quaternary Science Reviews ] (]]]]) ]]]–]]] 9

These direct emissions of CH4 and CO2 would have millennia would have eliminated any ice-driven CO2 produced an anomalous (anthropogenic) warming effect decrease. that prevented Holocene climate from following the If these two mechanisms prevented Holocene CO2 major cooling trend that had occurred during previous levels from falling as they did in the analogous part of interglaciations, including early Stage 11. Suppression of Stage 11, the result would have been an indirect this natural cooling would have stifled the natural anthropogenic contribution to the size of the Holocene internal responses that would have driven CO2 values CO2 anomaly (Fig. 8). Assuming that direct CO2 still lower and added to the size of the anthropogenic emissions explain 14 ppm out of a total anomaly of CO2 anomaly (Fig. 8). 32–40 ppm, this indirect contribution would then be Two climate-system responses that could have pro- 18–26 ppm. Given that CO2 values fell by 30–50 ppm for duced a natural CO2 drop are evident in the Vostok entirely natural reasons during similar intervals in the trends from early in Stage 11 (Fig. 4). Stephens and four previous interglaciations, an indirect effect of Keeling (2000) proposed that advances of Antarctic sea 18–26 ppm seems plausible. Based on model results ice are a plausible mechanism for driving down atmo- from Joos et al. (2004), a 14-ppm anomaly would spheric CO2 values by reducing carbon exchanges require slightly less than 200 Gt C of direct emissions, a between Southern Ocean surface water and the atmo- value midway between the estimates of Ruddiman sphere. The deuterium-based temperature trends in the (2003a) and Joos et al. (2004). Antarctic show a major cooling to near-glacial levels In summary, the use of Stage 11 as an analog for the early in Stage 11, but little cooling during the Holocene natural behavior of the climate system during the (Fig. 4c). A cooling to near-glacial levels during Stage 11 Holocene supports the major predictions of the early must have caused a major advance of sea ice in the anthropogenic hypothesis. During the last several Southern Ocean, but any Holocene advance has been far millennia, CO2 and CH4 levels should have fallen, smaller. If anthropogenic emissions averted most of the climate should have cooled, and ice sheets should have natural Holocene sea-ice advance, the natural reduction begun to grow. The failure of these changes to occur is in atmospheric CO2 from this process must have been best explained by early anthropogenic intervention in largely overridden. This interpretation fits a long history the operation of the global climate system. This of Southern Ocean studies. The southern hemisphere intervention included both direct greenhouse-gas emis- ocean has been regarded as a locus of early responses to sions from human activities and indirect climate-system orbital forcing since the work of Hays et al. (1976),as feedbacks that resulted from the direct emissions. well as later studies (CLIMAP, 1984; Waelbroeck et al., 1995; Brathauer and Abelmann, 1999). Antarctic temperatures reconstructed from deuterium signals are Acknowledgements also regarded as a part of this early response (Jouzel et al., 1987; Waelbroeck et al., 1995). I thank Robert Smith for the graphics. Discussions A different mechanism may have been at work in the with Maureen Raymo and Richard Alley broadened the Northern Hemisphere on the slower time scale of scope of this paper, as did reviews by Jerry McManus response of northern ice sheets. Imbrie et al. (1992) and a’semi-anonymous’ person. 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