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«Late Quaternary changes in climate Karin Holmgren and Wibjorn Karien Department of Physical Geography Stockholm University December 1998 Svensk ...»

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Tecnmcai neport


Late Quaternary changes

in climate

Karin Holmgren and Wibjorn Karien

Department of Physical Geography

Stockholm University

December 1998

Svensk Kambranslehantering AB

Swedish Nuclear Fuel

and Waste Management Co

Box 5864

SE-102 40 Stockholm Sweden

Tel 08-459 84 00

+46 8 459 84 00

Fax 08-661 57 19

+46 8 661 57 19

3 0 - 07

Late Quaternary changes in climate

Karin Holmgren and Wibjorn Karlen

Department of Physical Geography, Stockholm University December 1998 Keywords: Pleistocene, Holocene, climate change, glaciation, inter-glacial, rapid fluctuations, synchrony, forcing factor, feed-back.

This report concerns a study which was conducted for SKB. The conclusions and viewpoints presented in the report are those of the author(s) and do not necessarily coincide with those of the client.

Information on SKB technical reports fromi 977-1978 {TR 121), 1979 (TR 79-28), 1980 (TR 80-26), 1981 (TR81-17), 1982 (TR 82-28), 1983 (TR 83-77), 1984 (TR 85-01), 1985 (TR 85-20), 1986 (TR 86-31), 1987 (TR 87-33), 1988 (TR 88-32), 1989 (TR 89-40), 1990 (TR 90-46), 1991 (TR 91-64), 1992 (TR 92-46), 1993 (TR 93-34), 1994 (TR 94-33), 1995 (TR 95-37) and 1996 (TR 96-25) is available through SKB.

Abstract This review concerns the Quaternary climate (last two million years) with an emphasis on the last 200 000 years. The present state of art in this field is described and evaluated.

The review builds on a thorough examination of classic and recent literature (up to October 1998) comprising more than 200 scientific papers. General as well as detailed patterns in climate are described and the forcing factors and feed-back effects are discussed.

Changes in climate occur on all time-scales. During more than 90% of the Quaternary period earth has experienced vast ice sheets, i.e. glaciations have been more normal for the period than the warm interglacial conditions we face today. Major changes in climate, such as the 100 000 years glacial/interglacial cycle, are forced by the Milankovitch three astronomical cycles. Because the cycles have different length climate changes on earth do not follow a simple pattern and it is not possible to find perfect analogues of a certain period in the geological record.

Recent discoveries include the observation that major changes in climate seem to occur at the same time on both hemispheres, although the astronomical theory implies a time-lag between latitudes. This probably reflects the influence of feed-back effects within the climate system. Another recent finding of importance is the rapid fluctuations that seem to be a normal process. When earth warmed after the last glaciation temperature jumps of up to 10°C occurred within less than a decade and precipitation more than doubled within the same time. The forcing factors behind these rapid fluctuations are not well understood but are believed to be a result of major re-organisations in the oceanic circulation. Realising that nature, on it's own, can cause rapid climate changes of this magnitude put some perspective on the antropogenic global warming debate, where it is believed that the release of greenhouse gases will result in a global warming of a few °C.

To understand the forcing behind natural rapid climate changes appears as important as to understand the role of man in changing climate, if accurate predictions of future climate changes are to be made.

Sammanfattning Denna litteratursammanställning behandlar det kvartära (senast 2 miljoner åren) klimatets variationer, med tonvikt på de senaste 200 000 åren. De senaste rönen inom området beskrivs och diskuteras. Sammanställningen bygger på en noggrann genomgång av klassisk såväl som ny litteratur omfattande mer än 200 vetenskapliga artiklar. Generella och specifika mönster i jordens klimat presenteras och bakomliggande orsaker till klimatvariationerna diskuteras.

Klimatet varierar naturligt och på alla tids-skalor. Under mer än 90% av kvartärtiden har stora områden på jorden varit nedisade; nedisningar har varit mer normalt än varma perioder, som den vi lever i idag. Stora förändringar i klimatet, som den 100 000 åriga nedisnings-cykeln, styrs av de tre Milankovitchska astronomiska cyklerna. På grund av att dessa cykler har olika tids-längd, så återupprepar sig klimatet aldrig exakt och det går alltså inte att hitta perfekta analoger av en specifik period i den geologiska historien.

Nya rön visar att stora klimatförändringar tycks ske samtidigt över jorden, trots att de astronomiska cyklerna förutbestämmer en breddgradsbunden tidsförskjutning. Det är troligt att samtidigheten är ett resultat av omfattande återkopplingsmekanismer inom klimatsystemet. Ett annat viktigt rön är upptäckten av snabba kraftiga klimatvariationer associerat till perioder av instabilt klimat. När jorden värmdes efter senaste istiden skedde detta i form av snabba temperaturökningar på uppemot 10°C under loppet av mindre än 10 år, samtidigt som nederbörden mer än fördubblades. Drivkraften bakom dessa snabba förändringar är inte klarlagd, men kan vara relaterad till förändringar i havscirkulationen.

Förekomsten av så snabba och stora naturliga förändringar ger perspektiv på rådande växthus-debatt, där det förmodas att en, av människan styrd, ökning av CO2 utsläpp, kan värma jorden med ett par grader. Att förstå orsakerna bakom de naturliga snabba klimatförändringarna förefaller lika viktigt som att förstå betydelsen av en antropogen påverkan på klimatet om realistiska prognoser för framtida klimatförändringar ska kunna göras.

Summary and conclusions Knowledge about past climate on all time-scales has improved considerably during last decades. Several new concepts, now being discussed, are likely to become general knowledge in a few years and some opinions may turn out to be wrong. We try to summarise what appear to be well-founded opinions below.

• Long term variations are determined by orbital forcing, which include a 100 ka cycle caused by variations in the distance to the sun during the year, a 41 ka long cycle in the tilt of the earth and an about 23 ka long cycle in the time of the year when the earth is located relatively close to the sun.

• Each 100 ka cycle in the climate includes a long cooling period abruptly ending with a distinct temperature increase. The warm periods (interglacials) only embrace about 8% of the whole glacial cycle.

• Short-term variations are superimposed on the long-term changes in climate. A dominating, quasi periodic variation recurs with intervals of between 1000 and 2500 years. The amplitude of this variation was during Pleistocene more than 10°C, but the amplitude during the Holocene has only been 1-2°C.

• Long-term climatic variations caused by the orbital cycles are modified and emphasised by internal feed-back mechanisms. Several such are discussed, but the effect of each is so far not known.

• Among such feedback mechanisms are the atmospheric concentration of water vapour, CO2, and other gases. The climate is also affected by forcing from atmospheric concentration of volcanic dust and variations in the cloud cover.

• Variations in the solar irradiation is another factor which appears to affect climate.

• The climate is further affected by changes in the atmospheric and oceanic circulation, which may or may not be forced by external forcing.

–  –  –

Several processes affect climate on earth. The sun, the atmosphere, the oceans; all of them play a role in causing changes in earth's climate on various time-scales. Some of the changes and climate cycles are well documented, others are less well understood. Some theories on forcing factors are broadly accepted, others are debated.

A rationale for studying past climate is the conviction that cyclic processes drive changes in climate. With a better documentation of past climates, these cycles and the driving forces behind them will be better understood and the accuracy of climate models predicting future climates can be improved.

The Swedish Nuclear Fuel and Waste Management Co., SKB, is responsible for the management and disposal of Swedish radioactive waste. Most of the waste originates from the nuclear power plants. The spent nuclear fuel is planned to be stored in deep geological repositories. The repositories shall keep the radiotoxic material separated from man and environment for hundreds of thousands of years. During this time span long cold periods with permafrost and ice sheets over Scandinavia are expected. These climatedriven changes will affect the repository. To investigate climate-driven changes and their impact on a deep geological repository SKB has carried through a paleogeological research program (Bolton et al., in progress). This report is a part of this program. Similar programs have been performed by other nuclear waste managing companies (e.g. Adcock etal, 1997).

The aim of this report is to review the present state of art of late Quaternary climatic changes, through an examination of classic and recent literature, published up to October

1998. The Quaternary period comprises the last 2 million years and is divided into the Pleistocene epoch, including repeated glacials and interglacials and the Holocene epoch, i.e. the postglacial time we live in today (Fig. 1-1).

During the last decades several new archives keeping records of the past have been discovered and the geographical coverage of paleoclimatic information has increased.

Improved sampling techniques permit fine interval sampling, yielding data of high time resolution. Although methods of dating also have been greatly improved, precise absolute dating is still a problem, especially when dating events older than the last glaciation. The emphasis of this report will be on the last ca 200 ka (200,000 years), for which a lot of relatively detailed information is available. The patterns of climate changes will be described, the latest findings and the most debated subjects will be high-lighted and the potential problems and doubts of the data will be evaluated.

KaBP no

–  –  –

When James Croll, in the late 1800s, predicted long term changes in climate, the existence of one major glaciation had been suggested by Louis Agassiz and his evidence was in the process of becoming accepted. In the early 1900s A. Pencks and E. Briickner had convincing evidence for at least 4 glaciations (Giinz, Mindel, Riss and Wiirm) on the northern foreland of the Alps, and evidence of three glaciations (Elster, Saale and Weichsel) in northern Germany (Magnusson et al., 1963). First with improved deep-sea coring technique and the use of high precision mass spectrometry analyses of long cores, physical evidence of a long series of ice ages was obtained. Today, it is widely accepted that during the last 2 million years, i.e. the Quaternary period, vast glaciations on the Northern Hemisphere have occurred at intervals of around 100 ka and that the glaciation is a more normal situation for the period than the relative warm climate earth experiences at present.

2.1 General patterns The pattern of glaciations are known for the last 1 million years from loess in China (Maher and Thompson, 1995) and at least for the last 1.2 million years from deep sea sediment cores (Raymo et al., 1997) (Fig. 2-1). The 818O variations of deep-sea sediment cores1 show that considerable amounts of water at intervals of around 100 ka have been removed from the oceans and that these long-term changes were modified by shorter superimposed variations. The pattern has been relatively regular for the last 400 ka, but beyond this time, the 100 ka cycle appears less dominant (Henderson-Sellers and Robinson, 1986; Crowley and North, 1991).

A major step was taken in the 1970s, when a chronology based on radiometric dates and magnetic reversals was constructed (Hays et al., 1976). The chronology for the last 450 ka Oxygen isotope record —i [— j - warmer

–  –  –

Figure 2-1. A deep-sea oxygen isotope record for the last 1.2 million years shows the cyclic pattern of glaciations and interglacials. From Raymo et al., 1997.

–  –  –

II was compared with the Milankovitch cycles (this report, 2.4.1) and a slightly "tuned up" chronology was proposed, which fits this astronomical theory (Imbrie et al., 1984). The justified chronology was largely accepted but has been challenged lately (Winograd et al., 1992; Imbrie et al., 1993; Muller and MacDonald, 1997).

A few other long records of changes in environment and climate are available. One of the real long records comes from lake sediments in Lake Baikal and yields environmental information based on changes in the composition of diatoms and biogenic silica for approximately the last 2.6 million years (Williams et al., 1997). The record is well correlated with the marine record. A pollen study from north-western Greece has revealed major changes in the occurrence of trees during the last 430 ka (Tzedakis, 1993).

Ice cores from Antarctica and Greenland have provided details about variations in climate during the last 100 to 200 ka (Jouzel et al., 1993; Johnsen et al., 1995). In addition, information about the long term climate has been obtained from other lake sediment (van der Hammen et al., 1971) and cave deposits, such as stalagmites and flow stones (BarMatthews et al., 1998a; 1998b). All these long records repeat the general pattern of glaciations and interglacials observed in the marine record and also demonstrate the occurrence of great fluctuations within the glaciations. However, the records have so far provided few quantitative estimates on absolute temperature and precipitation changes.

Absolute temperature figures of any reliability are only available for the last glaciation (120-11.5 ka BP) and the Holocene.

2.1.1 Temperature During the last two million years, climate has mainly been cold enough to support major ice sheets; only during about 8% of the time climate has been as warm, or warmer, than at present. The magnitude of the changes between glacial and interglacial climate appears to have been similar during at least the last 700 ka.

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