ARTICLE IN PRESS Quaternary Science Reviews ] (]]]]) ]]]–]]] Cold climate during the closest Stage 11 analog to recent Millennia William F. Ruddimanà 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). 0277-3791/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2004.10.012 ARTICLE IN PRESS 2 W.F. Ruddiman / Quaternary Science Reviews ] (]]]]) ]]]–]]] 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 (˚C) 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.