The Late Quaternary History of Atmospheric Trace Gases and Aerosols: Interactions Between Climate and Biogeochemical Cycles D. Raynaud Laboratoire de Glaciologie et de Géophysique de l'Environnement, LGGE Centre National de la Recherche Scientifique, 54 Rue Molière, FR-38402 Saint-Martin-d'Hères, France T. Blunier Climate & Environmental Physics, Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland Y. Ono Laboratory of Geoecology, Graduate School for Environmental Earth Science, Hokkaido University Kita10 Nishi5, Kita-ku, Sapporo, Hokkaido 60, Japan R. J. Delmas Laboratoire de Glaciologie et Géophysique de l'Environnement, Domaine Universitaire B.P. 96, Saint Martin d'Hères Cedex, 38402 France Contributors: J-M. Barnola, F. Joos, J-R. Petit, R. Spahni 2.1 Introduction: anthropogenic and natural changes Human activities perturb the atmosphere and the processes involved. thereby influence the global climate. Prominent • The paleorecord offers a unique opportunity to examples are greenhouse trace gases and sulfate test the ability of climate models to simulate inter- aerosols which both affect the radiative balance at actions between atmospheric composition and cli- the surface of the Earth. This is also true for black mate, under different climatic conditions. carbon and organic carbon aerosols emitted by The Earth has experienced major changes in its burning of biomass and fossil fuel, as well as eolian atmospheric composition over the past several hun- mineral dust originating from changes in land use dred thousand years. During this time, atmospheric and land cover. composition and climate have interacted alongside Given these ongoing anthropogenic changes, un- external influences (or forcings), such as changes in derstanding the past record of atmospheric compo- the pattern of solar radiation incident on the Earth’s sition is important for several reasons: surface. The aim of this chapter is to review the • Over the past hundred years it is not always an history of atmospheric composition as it interacted easy task to separate atmospheric changes in- with climate during the last four major climatic duced by human activities from those related to cycles (roughly the last 400,000 years). We con- natural variability. Only the longer term past centrate on processes operating on time scales cov- record provides the context of natural variabil- ering the interval between 100,000 years and a ity within which recent anthropogenic pertur- century. There are two major reasons for choosing bation has taken place. this time period: (1) the atmospheric record further • The rate of anthropogenic perturbation is very back in time is generally not well documented and high. Since 1750 the atmospheric CO2 concen- (2) the last four glacial cycles encompass a wide tration has increased by 30% and CH4 by spectrum of climatic conditions. 150%. Did similar rates occur in the past and, if In this chapter, we begin with a brief introduction so, how did they affect climate and environ- on past variations of greenhouse gases and aerosols ment? during several different, climatically significant • Complex feedback mechanisms exist between periods of the past: glacial-interglacial cycles (sec- the climate system and the trace gas content of tion 2.3), abrupt climatic changes (section 2.4), the the atmosphere. Past changes can be used to Holocene (section 2.5), and the last millennium study such atmosphere-climate feedbacks and (section 2.6). In the last section (2.7) we highlight 14 Raynaud et al. significant conclusions that can be drawn in terms Secondary aerosols are formed by chemical reac- of climate and biogeochemical cycles. The follow- tions and condensation of atmospheric gases and ing section (2.1) is devoted to explaining the sig- vapors. The sulfur cycle dominates the tropospheric nificance of the ice core archive of atmospheric secondary aerosol budget. In pre-industrial condi- gases and aerosols. tions, it is mainly linked to marine biogenic activity, which produces large amounts of gaseous dimethyl- 2.1.1 Greenhouse gases sulfide (DMS). Once in the atmosphere, DMS oxi- dizes primarily into sulfuric acid, ultimately present Greenhouse gases are transparent to incoming solar in the atmosphere in the form of fine aerosol drop- radiation but opaque to the infrared radiation emit- lets. This fine aerosol is also partly of volcanic ted by the earth’s surface. The main greenhouse gas origin. Presently, anthropogenic SO2 emission is water vapor whose atmospheric content is domi- linked to fossil fuel, biomass burning and industrial nantly influenced by climatic conditions, primarily processes dominates over all natural sources of temperature. Water vapor concentration in the at- atmospheric sulfur. In addition, it has been sug- mosphere varies widely in space and time, though gested recently that organic matter, frequently asso- its long term temporal variability is not well con- ciated with sulfur species, may constitute a major strained by data. It is worth noting that the direct fraction of secondary aerosol particles. Organic emission of water vapor by human activities is carbon and "black carbon" (called soot) are the negligible compared to natural fluxes. Other green- main organic aerosol particles. They are primarily house gases exist in much smaller concentrations in produced by the emission of smoke from biomass the atmosphere, but their increase due to anthropo- burning. genic activities may drastically affect our future Primary and secondary particles may interact climate. Most of them, such as CO2, CH4, N2O and strongly in the atmosphere, turning atmospheric tropospheric ozone were present in the atmosphere aerosols into a very complex mixture. They are prior to the changes brought about by anthropogenic removed from the air by both dry and wet deposi- activities. Some trace-gases, like CFC’s, exist solely tion. due to humans. Atmospheric aerosols influence climate in two Some trace gases, which are not themselves ra- ways: directly through reflection and absorption of diatively active, nonetheless interact through at- solar radiation, and indirectly by modifying the mospheric chemistry processes with greenhouse optical properties and lifetimes of clouds. In addi- gases. CO, for example, although not itself a green- tion to ash, large explosive volcanic eruptions spo- house gas is involved in setting the oxidizing ca- radically inject (a few times per century) huge pacity (OH cycle) of the atmosphere, and therefore amounts of SO2 into the stratosphere. The sulfuric affects the atmospheric sink of CH4. acid veil formed after such eruptions may persist for several years at an elevation of about 20 km, mark- 2.1.2 Aerosols edly cooling the global climate. Dust, be it desert, volcanic or soot particles, when An aerosol is a suspended liquid or solid particle in deposited on the surface of snow and ice may de- a gas. Aerosols constitute a minor (∼ one ppb by crease its albedo and enhance surface melting. This mass) but important component of the atmosphere. process, along with high altitude temperature in- Indeed, aerosols play an active role in atmospheric creases, is contributing to the retreat of glaciers and chemistry. Natural aerosols may be divided into two snowfields in high mountain areas worldwide. It has classes, primary and secondary aerosols, arising even been suggested that bacteria living on the from two different basic processes: surface of glaciers could be nourished by these Primary aerosols derive from the dispersal of fine aerosol deposits, further changing the albedo and materials from the earth's surface. There are two melting rate. major categories of natural primary aerosols: sea Large amounts of mineral substances (e.g. iron, salt and soil dust. Most sea salt particles are pro- nitrate, phosphorous) are transported as aerosols. In duced by evaporation of spray from breaking waves some cases, these substances can act as nutrients, at the ocean surface whereas mineral dust particles enhancing marine biogenic activity and the rate of are mostly generated by winds in arid continental the atmospheric CO2 sequestration in the ocean. regions and, sporadically, by explosive volcanic Moreover, continental carbonate aerosol inputs may eruptions which emit huge amounts of ash particles. potentially change the alkalinity of shallow oceanic The continental sediments formed during glacial layers, modifying surface ocean chemistry and periods by the deposition of wind blown dust are consequently air/sea exchanges of CO2. Since at- called loess. mospheric dust concentrations were strongly en- Late Quaternary History of Trace Gases 15 hanced during glacial periods, these processes must up to 50% to the global mean aerosol optical depth. be considered in paleoclimatic studies. It is generally accepted that the net global radiative As previously mentioned, dimethylsulfide (DMS) forcing due to anthropogenic aerosols is significant is produced in the ocean by marine biogenic activ- and negative (i.e. it tends to cool the average global ity. This gas is the primary natural source of atmos- temperature) (Sato et al. 1993). However, quantifi- pheric sulfate. Non-sea-salt sulfate (or nss-SO4) and cation of their climatic effects remains difficult, in methanesulfonic acid (MSA) are the two most im- particular due to large uncertainties associated with portant compounds formed as DMS is oxidized in the indirect impact of aerosols on clouds. the atmosphere. MSA is of particular interest in that it is uniquely