THE TENDENCY OF CLIMATE CHANGE OVER THE PAST SEVERAL MILLIONS OF YEARS AND THE CURRENT INTERGLACIAL DURATION. V.A. Dergachev

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1 THE TENDENCY OF CLIMATE CHANGE OVER THE PAST SEVERAL MILLIONS OF YEARS AND THE CURRENT INTERGLACIAL DURATION V.A. Dergachev Ioffe Physical-Technical Institute, St. Petersburg, , Russia, Abstract. The Earth's climate, from regional to global, varies on all time scales. Large-scale climate variations in the past can be related to changes in geological processes (plate tectonic) and orbital cycles upon the Earth s climate. Astronomical theories of paleoclimate attribute climate cycles to changes in the Earth s orbital parameters: eccentricity (~100 kyr), obliquity (~41 kyr), precession (~22 kyr). It has been established from the paleoclimate and paleo-oceanographic data that during last more than 50 millions of years planetary temperatures were several degrees warmer than today, but there has been a progressive decrease in the average surface temperature on this time interval. Substantial glacial and interglacial temperature fluctuations are imposed on this decrease since about 2.8 million years ago. The last interglacial peak (at ~130 kyr ago) was a period with significantly higher temperatures in many parts of the Northern Hemisphere compared to the currant interglacial period, which started ~ 11,000 years ago. A detailed analysis of the recent oscillations in temperature shows a clear 100,000 year correlation with the interglacial periods coinciding with the maxima of the elipticity of the Earth s orbit. To understand better our current interglacial period (the Holocene, MIS-1 [Marine Isotope Stage]) and its future, it is necessary to investigate the response of the climate system to the peaks of interglacials in the past. Start of the last interglacial period occured at ~130 kyr ago (MIS-5). Similar to the Holocene, the latitudinal and seasonal distributions of the incoming solar radiation show two interglacials: MIS 11 (start kyr ago) and MIS 19 (start kyr ago). However, it is difficult to find in the available climate data a complete analogue to the current interglacial climate. The available data on climatic changes and the cyclic influence of solar radiation on the climate change are analyzed and the problem of current interglacial duration is discussed. Introduction The climate of the distant past (the first billion years of the Earth existence), where geological records are almost non-existent or sparse, is very poorly understood. During the evolution of our planet its climate was characterized by a large variety of climate stages. There were periods of increased variability or regular oscillations or almost quiet phases. Over the entire time coverage of the Earth s history the main external and internal natural mechanisms of climate variability are related to changes in plate tectonics, orbital changes with different periodicities, the sun s output on different time scales, and volcanism. Natural climate variability in the past was the rule and not the exception and the evolution of life on Earth was closely linked to climate and its change. The usual definition of climate is that it includes the slowly varying aspects of the atmosphere hydrosphere surface system. Let's look at the climate change over longer (millions and thousands of years) time scales. It has been established from the paleoclimate and paleo-oceanographic data (Zachos et al., 2001) that during the past 100 million of years in the Cretaceous epoch, which ended 65 Myr BP (Figure 1), surface air temperatures were higher than they are at present. Since about 65 million years ago, Earth s climate has undergone a significant cooling and complex evolution. About 55 million years ago, at the end of the Paleocene, there was a sudden warming event in which temperatures rose by about 6ºC globally and by 10-20ºC at the poles (Zachos et al., 2008). Hansen et al. (2011) showed that the deep-sea temperature is closely related to the global average temperature and they roughly estimated that the change in global average temperature was perhaps ~ 12 o C over the past 50 million years. The Earth s climate has cooled over the last 5 million years. Today, mankind is living in an interglacial period that began about 11 ky ago. In the light of discussion about global warming observed in recent decades, which advocates an anthropogenic impact associated with the emission of greenhouse gases due to combustion of fossil fuel, the question concerning the duration of the current interglacial arises. The available data on climate change and solar radiation on a time scale of the last millions of years are critically analyzed and the problem of the length of the current interglacial is discussed. 245

2 Figure 1. Climate during the Cenozoic Era (~10 8 foraminifers (Zachos et al., 2001). years). Oxygen isotope measurements of benthic Climate changes over the past and orbital cycles One of the highest resolution records of global proxy temperature data over the last 5.3 million years was published by Lisiecki and Raymo [2005] (Figure 2a). This set consists of a stack of 57 globally distributed benthic δ 18 O records. The data are based on samples of deep sea sediments consisting of calcium carbonate from plankton. As one can see from Figure 2, the Earth s climate has cooled down over the last 5 million years. The data provide overwhelming evidence for a major cooling trend. The cooling culminated in the Pleistocene glaciation, which began about 2.5 Myr ago. As a result, the Earth s climate has been marked by temperature swings between extended glacial periods, leading to a series of major glaciations over the last 900,000 years, which were characterized by thick ice sheets covering large parts of North America, Northern Europe and Siberia, and interglacial times characterized by the ice coverage only in Antarctica and sometimes Greenland, as at the present time. Figure 2. a - δ 18 O benthic stack over the past 5 Myr [Lisiecki and Raymo, 2005]. b - Local wavelet power spectra of δ 18 O data derived using a modified version of the WTC-16 code [Grinsted et al., 2004]. Shaded areas show the cone-of-interference, within which edge effects become significant. The data were smoothed to 5 kyr resolution before wavelet decomposition [Russon et al., 2011], which limits the capacity of the method to resolved periodicities <20 kyr. The causes of glacial interglacial climate changes can be linked to solar activity or to processes occurring on earth. Since major climate changes have a cyclic character, the astronomical theory, proposed by 246

3 Milankovitch [1948], relates glacial interglacial changes to changes in the Earth s orbit and its rotation axis in time. Theory suggests that a primary driver of ice ages is the total summer radiation received in northern latitude zones where major ice sheets have being formed in the past, near 65 degrees of Northern latitudes. A spectral analysis (Figure 2b) of the data supports this relationship up to a certain degree. These climatic data clearly show a certain kind of periodic variability. The main orbital periodicities are the precession (~20 kyr), eccentricity (~100 kyr and ~400 kyr) and obliquity (~40 kyr) cycles (Laskar et al., 2004). The combined effect of these orbital cycles causes long term changes in the amount of sunlight hitting the Earth in different seasons, particularly at high latitudes. The data of δ 18 O exhibit the cycles 20, 41 and 100 kyr (Figure 2b), which coincide with the orbital periodicities mentioned above. Climate variability during the last three million years, on the basis of Lisiecki and Raymo data, was investigated by Dergachev and Dmitriev (2014) to reveal the hidden periodicities. The authors have found in this sample five periodicities of about 19, 22.4, 23.7, 41 and 98 kyrs which are similar to the Earth orbital cycles. It is clear that the climatic system does not respond linearly to the insolation variations though astronomical frequencies corresponding to orbital cycles are found in almost all paleoclimatic records. Ice cores from Greenland and Antarctica provide at present the most detailed information about climate change in the past. Figure 3 shows a comparison of changes in climate parameters in the ice core from the Antarctic Dome 2 station (concentration D) (Jouzel et al., 2007) and from ocean sediments (concentration 18 O) (Lisiecki and Raymo, 2005) during the last kyr. It is seen that the curves reflecting alteration of warm and cold periods nicely match each other. Over the last 800 kyr, the orbital cycles were the dominant causes and pace-makers for climate variability. Global temperatures cooled at irregular rates during the extended glacial epochs and rose much faster at the beginning of the interglacials (MIS-1 MIS-19). These interglacials recurred during the recorded period at intervals of roughly 100,000 years and had durations typically of about 12,000 years. Documenting natural interglacial climate variability in the past provides a deeper understanding of the physical climate responses to orbital forcing. Figure 3. Comparison between climate changes reconstructed from Antarctic ice cores for various glacial interglacial intervals ( D) (Jouzel et al., 2007) and δ 18 O in ocean sediment cores (Lisiecki and Raymo, 2005) during the last 900 kyr. Interglacials are indicated MIS (Marine Isotope Stage). MIS-1 is the current interglacial. Detailed records from ice cores show that three previous interglacials (Figure 3) differ from the fourth which, as well as Holocene, coincides with the period when orbital eccentricity approaches to the minimum of a 400-thousand-year orbital cycle. The duration of the interglacial which took place ~ 400 kyr ago, is estimated to be much longer, than of the subsequent interglacial. New data with the updated age models, have provided an expanded view on temperature patterns during interglacials since 800 kyr (e.g., Rohling et al., 2012). However, there is currently no consensus on whether the interglacial periods have changed their intensity after ~430 kyr. 247

4 Interglacials Interglacials can be defined as time intervals, during which the global climate was incompatible with the wide extent of glacier conditions. In the context of future global warming induced by human activities, it is essential to assess the role of natural climatic variations. Precise knowledge of the duration of past interglacial periods is fundamental to the understanding of the potential future evolution of the Holocene. Past interglacials are characterized by different combinations of orbital forcing, atmospheric composition and climate responses. Palaeorecords from ice, land and oceans extend over the last 800 kyr, revealing eight glacial-interglacial cycles, with a range of insolation and greenhouse gas influences. To understand better our current interglacial (the Holocene, MIS-1 [Marine Isotope Stage]) and its future, it is necessary to investigate the response of the climate system to the peaks of interglacials in the past. Yin and Beger (2010) used the Earth system model of intermediate complexity to assess the contributions of insolation and greenhouse-gas concentrations to the climate associated with the peaks of all the interglacials over the past 800,000 years. The effect of boreal winters and of the Southern Hemisphere, which is also warmer during austral winters, on the carbon cycle should be assessed when investigating the underlying causes of the higher CO 2 concentrations during the later interglacials. The interglacials are compared in terms of their forcings and responses of surface air temperature, vegetation and sea ice. The results show that the relative magnitude of the simulated interglacials is in reasonable agreement with proxy data. The interglacials (Figure 4) display a correspondingly large variety of intensity and duration, thus providing an opportunity for major insights into the mechanisms involved in the behaviour of interglacial climates. A comparison of the duration of these interglacials, however, is often difficult, as the definition of the interglacial depends on the archive that is considered. Therefore, to compare interglacial length and climate conditions from different archives, a consistent definition of the interglacial conditions is required. The phasing and strengths of the precessional parameter and obliquity varied over past interglacials, influencing their timing, duration, and intensity (Tzedakis et al., 2012; Yin and Berger, 2012). Figure 4. Marine δ 18 O (Lisiecki and Raymo, 2005), precession and obliquity around the past 10 interglacial peaks ((Laskar et al., 2004)). The black bars localize δ 18 O minima, precession minima and obliquity maxima. The dates of δ 18 O minima and corresponding MISs (Tzedakis et al., 2012) are indicated. 248

5 Lang, and Wolff (2011) have compiled the data from 37 glacial, terrestrial and marine archives to obtain a data set covering years and allowing to study the features of glacial and interglacial periods. The comparison of interglacials concentrates on the peaks immediately before and after the terminations; particularly strong and weak interglacials have been identified. This confirms the idea that strong interglacials are limited to the last 450 kyr, and that this is a globally robust pattern. Orbital parameters as an analogue for the future current interglacial duration In order to understand better our current interglacial and its future, it is necessary to investigate the response of the climate system to insolation at the peaks of the interglacials with similar latitudinal and seasonal distribution of the incoming solar radiation over the past 800,000 years, which can lead to the similar climate response to insolation in the Holocene. Figure 5 shows three astronomical analogs of our Holocene interglacial in the vicinity of MIS-1, MIS-11 and MIS-19 (red arrows). The previous (in respect to the current one) interglacial period is marked by the green curve. Figure 5. Comparison of the orbital parameters (a) and various interglacial intervals (b). The red arrows show the analogs of the future current duration (Yin and Berger, 2012) of the current interglacial (blue). The green curve shows the last interglacial. Figure 6 compares similar latitudinal and seasonal distribution of the incoming solar radiation (precession and obliquity) and marine δ 18 O, around the past 3 interglacial peaks (MIS-1, MIS-11, MIS-19). MIS-1, MIS-11 and MIS-19 show as well similar latitudinal and seasonal distributions of the incoming solar radiation, which lead to similar climate response to insolation, however, differences exist. Recent research has focused on MIS 11 as a possible analog for the current interglacial, but the estimated length of MIS-11 seems to vary from 20 to 33 kyrs. Besides of this, it has been shown that MIS-11 consists of at least two insolation peaks. At the same time, MIS19 exhibits a very similar insolation minimum in the Holocene as MIS11, but it lasts for only kyr, similar to the current interglacial. Berger and Yin (2014) have shown that the equivalent of CO2 concentration is basically the same for MIS-1 and MIS-19 but larger for MIS-11. The evidences from MIS19 help to confine the debates about the real length of the current interglacial. 249

6 Figure 6. Marine δ 18 O, precession and obliquity around the past 3 interglacial peaks (MIS-1, MIS-11, MIS- 19). The black bars localize δ 18 O minima, precession minima and obliquity maxima (see Figure 4). The dates of δ 18 O minima and corresponding MISs are indicated. Conclusion The most recent researches have focused on MIS 11 as a possible analog for the current interglacial. However, MIS 11 spans two precession cycles, when the Holocene contains one insolation peak so far. Besides, the warm period around 400 kyr ago remains a contradiction. MIS-19 appears to be the best analogue for MIS-1. MIS19 represent a very similar insolation minimum to the Holocene and MIS11, but lasted for only kyr. It is interesting that CO2 concentration at that time were approximately 240 ppm. Thus with respect to the Holocene, MIS19 may act as a good analogue for climatic change under natural conditions, i.e. CO 2 levels are not influenced by mankind. Assuming the Holocene followed the pattern seen during MIS19, we would have expected that the inception of the next glacial period will start soon, if it is not already underway. References Berger, A. and Q. Yin (2014), About past interglacials as analogues to the Holocene and Anthropocene. Geophysical Research Abstracts Vol. 16, EGU , Dergachev, V.A., P.B. Dmitriev (2014), The orbital cycles of climate variability during the last three million years. XVIII Russian annual international conference Solar and Sun-Earth physics-2014» (20-24 October 2014, St. Petersburg, Pulkovo Observatory RAS). Grinsted, A., J.C. Moore, and S. Jevrejeva (2004), Application of the cross wavelet transform and wavelet coherence to geophysical time series. Nonlin. Processes Geophys., 11, Jouzel, J., V. Masson-Delmotte, O. Cattani, et al. (2007), Orbital and millennial Antarctic climate variability over the past Years, Science, 2007, 317, Lang, N.and E. W. Wolff (2011), Interglacial and glacial variability from the last 800 ka in marine, ice and terrestrial archives. Clim. Past, 7, , doi: /cp Laskar, J., P. Robutel, F. Joutel, M. Gastineau, A. C. M. Correia, and B. Levrard (2004), A long-term numerical solution for the insolation quantities of the Earth. Astron. Astrophys., 428(1), , doi: / : Milankovitch, M. (1948), Ausbau und gegenwartiger Stand der astronomischen Theorie der erdgeschichtlichen Klimate, Experientia, 4(11), , doi: /bf Rohling, E.J., A. Sluijs, H.A. Dijkstra, P. Köhler, R.S.W. van de Wal, et al. (2012), Making sense of palaeoclimate sensitivity. Nature, 491, , doi: /nature

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