Paleoclimate, Global Change and the Future

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Preface WE LIVE IN UNUSUAL TIMES. Greenhouse gas concentrations are increasing rapidly and are now much higher than they have been for at least 420,000 years. Global average temperatures exceed anything seen in the last thousand years. The evidence is now overwhelming that such changes are a consequence of human activities, but these are superimposed on underlying natural variations. Climate on Earth naturally undergoes changes driven by external factors such as variations in solar output and internal factors like volcanic eruptions. How can we distinguish the human from the natural impacts? And what might the changes herald for the future of human societies as population pressure grows, as fossil fuel consumption increases and as land cover is altered? Such questions are compelling, and the need for answers urgent. But the search for answers will only be successful when we have developed insight into the full range of natural variability of the climate system. That range is illustrated by the events of the past, and it is only by unravelling those events that we will be able to predict the future, and our place in it, with confidence. This book stands as a progress report in the search for the past. It highlights a number of the extraordinary discoveries about the operation of the Earth System through time that have been made by natural scientists around the world over the last few decades. The great gains described in these pages have been wrought through exploration across the face of the planet and beyond: on land, sea, lakes, ice caps, via satellite observations and through simulations run on silicon chips. But that is only one dimension of the search, for critical to the future of human society is an improved understanding of the sensitivity of civilizations to climate change. Increasingly, paleoclimatologists are working with social scientists to disentangle the impacts of evolving social pressures and cultural practices from those induced by past climate change. The scientific findings in these pages give cause for both exhilaration and concern. The exhilaration lies in appreciating the remarkable increase in our understanding of the complexity and elegance of the Earth System. The concern is rooted in recognizing that we are now pushing the planet beyond anything experienced naturally for many thousands of years. The records of the past show that climate shifts can appear abruptly and be global in extent, while archaeological and other data emphasize that such shifts have had devastating consequences for human societies. In the past, therefore, lies a lesson. And as this book illustrates, we should heed it. Keith Alverson, Ray Bradley, Tom Pedersen, September 2002.

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Table of Contents The Societal Relevance of Paleoenvironmental Research 1.1 Introduction 1.2 A paleo-perspective on earth system function 1.3 Past climate variability, human societies and human impacts 1.3.1 The Anthropocene 1.3.2 Societal responses to past climatic change 1.3.3 Decadal-centennial modulation of modes of climate variability 1.3.4 Vulnerability to extreme events 1.4 Hydrological variability 1.5 Ecosystem processes 1.6 Landcover change 1.7 Biodiversity 1.8 Testing climate models with paleodata 1.9 A paleo-perspective on future global change

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The Late Quaternary History of Atmospheric Trace Gases and Aerosols: Interactions Between Climate and Biogeochemical Cycles 2.1 Introduction: anthropogenic and natural changes 2.1.1 Greenhouse gases 2.1.2 Aerosols 2.2 The significance of past atmospheric records 2.2.1 Aerosol incorporation and gas occlusion in ice 2.2.2 How reliable are the climate records obtained from ice cores? 2.3 Glacial-interglacial cycles 2.3.1 Greenhouse gases 2.3.2 Aerosols and DMS 2.4 Abrupt climatic changes during the last ice age 2.4.1 CH4 variations 2.4.2 N2O variations 2.4.3 CO2 variations 2.4.4 Dust 2.5 The Holocene 2.5.1 CH4 variation over the Holocene 2.5.2 CO2 increase over the Holocene 2.5.3 The Holocene N2O level 2.6 The last millennium 2.6.1 Greenhouse gases 2.6.2 Aerosols Pre-industrial Anthropogenic increase 2.7 Conclusions, a view in the context of future changes

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The History of Climate Dynamics in the Late Quaternary 3.1 Introduction 3.2 Climate change under orbital forcing 3.2.1 Developing a chronology of past climatic change 3.2.2 Understanding glacial cycles 3.2.3 Glacial inception 3.2.4 The Last Glacial Maximum 3.2.5 Glacial Termination 3.3 Interaction among climate system components on millennial time scales 3.3.1 Millennial scale variability in proxy data: high latitude signals 3.3.2 Millennial variability of climate at low latitudes 3.3.3 Modeling millennial scale climate variability 3.4 Climate modes on interannual to centennial scales 3.4.1 The tropical Pacific: El Niño/Southern Oscillation ENSO in recent centuries

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VIII ENSO in the late Quaternary ENSO in the mid-Holocene 3.4.2 Decadal variability in the extratropical Pacific 3.4.3 North Atlantic Oscillation 3.4.4 Tropical Atlantic: the dipole and extratropical links 3.4.5 Global teleconnectivity The Late Quaternary History of Biogeochemical Cycling of Carbon 4.1. Introduction 4.2. Continental processes and their impact on atmospheric CO2 4.2.1 Biospheric carbon 4.2.2 Soil carbonate 4.2.3 Weathering and river transport 4.3 Marine processes that affect atmospheric CO2 4.3.1 Air-sea flux 4.3.2 SST and SSS control (the solubility pump) 4.3.3 Removal of ΣCO2 from surface waters by sinking Corg 4.3.4 Supply of carbonate ions to surface waters (the alkalinity pump) 4.3.5 The export ratio (biological versus alkalinity pumps) 4.4 Impact of marine processes on atmospheric CO2 4.4.1 Contribution from the solubility pump 4.4.2 Global export of ΣCO2 from surface waters Changes in N and P supply in oligotrophic regions Changes in Fe supply in HNLC regions Changes in Si supply 4.4.3 Global rate of supply of ΣCO2 to surface waters 4.4.4 Contributions from the alkalinity pump 4.4.5 Contributors to transient excursions in atmospheric CO 2 4.5 Summary and critical areas for future research Terrestrial Biosphere Dynamics in the Climate System: Past and Future 5.1 Introduction 5.2 The roles of the terrestrial biosphere in the climate system 5.2.1 Biogeochemical roles 5.2.2 Biophysical roles 5.3 Terrestrial biosphere changes in the past 5.3.1 Response of the biosphere Growth and/or death Species migration Changes in community composition Changes through evolution Extinction 5.3.2 The temporal hierarchy of climate change and biospheric response The tectonic frequency band The “Orbital” frequency band (1 million to 10,000 years) The millennial frequency band (10,000 to 1,000 years) Sub-millennial frequency bands (