biogeochemical cycles and climate Biogeochemical Cycles and Climate Han Dolman Vrije Universiteit Amsterdam 1 1 Great Clarendon Street, Oxford, OX2 6DP, United Kingdom Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide. Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries © Han Dolman 2019 The moral rights of the author have been asserted First Edition published in 2019 Impression: 1 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, by licence or under terms agreed with the appropriate reprographics rights organization. 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Preface It was at the annual meeting of the European Geosciences Union in Vienna in 2014 that I was approached by Sonke Adlung from Oxford University Press with the question if any and, if so, what book was missing in my field of research. I think now that I did not immediately reply, but that it took me some time before I was able to clearly identify what was missing. The reasons for that lie in my experience of having taught Earth science students the basic physics of climate for some time, and in the fact that Earth science itself provides numerous beautiful examples of how that physics and biogeochemistry operated in the geological past, aspects which I did not teach. I needed some time to formulate the importance of presenting these together. Most university courses present our students with a rather incomplete picture of the climate of our planet. Meteorology students are made familiar with the physics of the inter- action of greenhouse gases and radiation, and the thermodynamics and transport of the atmosphere, but generally lack knowledge of the role of oceans and the interaction of the biosphere with climate. They also miss a palaeoclimatological perspective. Earth science students gain insight into long-term climate processes such as the geological thermostat, the role of plate tectonics in climate and various other aspects of palaeoclimate and bio- geochemistry but are hardly made familiar with the physics of atmosphere and ocean. The societal implications of this development are severe. We can only understand climate (change) as long as we can quantify the multitude of feedbacks between these important physical and biogeochemical and biological processes. This requires under- standing not only what processes change the rates and magnitudes of biogeochemical cycles such as the carbon cycle, the nitrogen cycle and the water cycle but also how the physics of motion, thermodynamics and radiation respond to those changes. This book follows from these considerations. It is about the science of the interactions and the exchange processes between reservoirs such as the ocean, the atmosphere, the geosphere and the biosphere. It is less about the human impact, as there is already a wealth of good textbooks that deal with issues of current climate change such as adapta- tion, mitigation and impacts. The book is primarily aimed at Earth science students who are at the advanced-bachelor’s or master’s level and have a basic understanding of alge- bra, chemistry and physics. Its purpose is not at all to be complete. It aims rather to provide a more integrated view of the climate system from an Earth science perspective. The choices of the subjects are, however, my own and based on personal and probably biased preferences. It is up to the reader to judge how well I succeeded in this compli- cated attempt at integration. The first three chapters offer a general introduction to the context of the book, out- lining the climate system as a complex interplay between biogeochemistry and physics vi Preface and describing the tools we have to understand climate: observations and models. They describe the basics of the system and the biogeochemical cycles. The second part con- sists of four chapters that describe the necessary physics of climate to understand the interaction of climate with biogeochemistry and change. The third part of the book deals with Earth’s (bio)geochemical cycles. These chapters treat the main reservoirs of Earth’s biogeochemical cycles—atmosphere, land and ocean—together with their role in the cycles of carbon, oxygen, nitrogen, iron, phosphorus, oxygen, sulphur and water and their interactions with climate. The final two chapters describe possible mitigation and adaptation actions, always with an emphasis on the biogeochemical aspects. I have tried to make these last six chapters as up to date as possible by providing more references in them than in the previous chapters. I am grateful to Sonke Adlung for posing that initial question to me in 2014. He and Ania Wronski at Oxford University Press have always been very supportive of the pro- ject, even when the start was somewhat difficult. John Gash, former colleague and life- long friend, edited the first draft. Various colleagues at the Department of Earth Science of the Vrije Universiteit have provided feedback, both on ideas and on chapters. Gerald Ganssen has always been a great stimulating force for this project and I very much appreciate his role as discussion partner providing the larger-scale geological perspective. I sincerely acknowledge the freedom I experienced in the Department of Earth Sciences to be able to write and finish this book as I wished. Further support was received from the Darwin Centre for Biogeology and the Netherlands Earth System Science Centre. Several people have commented on individual chapters: Antoon Meesters, Jan Willem Erisman, Joshua Dean, Sander Houweling Nick Schutgens, Appy Sluys and Jack Middelburg. Ingeborg Levin and Martin Heimann read the near-final draft. It is fully due to these people that several (embarrassing) errors have been corrected in time. I very much appreciate the opportunity provided by the president of the Nanjing University of Information Science and Technology and by Tiexi Chen and Guojie Wang to spend some time at Nanjing in the spring of 2018, that allowed me to finish the book. Kim Helmer was of great help in obtaining copyright permissions for the various figures used in this book. The writing of this book took quite some time, but was overall a very enjoyable experi- ence. I like to thank my lifelong partner Agnes and my two sons Jim and Wouter for their support. Words are simply not enough to express my gratitude to them. Han Dolman June 2018 Contents 1 Introduction 1 1.1 Biogeochemical cycles, rates and magnitudes 1 1.2 The geological cycle 2 1.3 The carbon cycle 3 1.4 Feedbacks and steady states 6 1.5 The greenhouse effect and the availability of water 10 1.6 The rise of oxygen 11 1.7 Non-linearity 13 2 Climate Variability, Climate Change and Earth System Sensitivity 16 2.1 Earth system sensitivity 16 2.2 Geological-scale variability 18 2.3 Glacial–interglacial variability 19 2.4 Centennial-scale variability 21 2.5 Earth system variability 23 3 Biogeochemistry and Climate: The Tools 29 3.1 Climate and biogeochemical observations and proxies 29 3.2 Physical climate observations 31 3.3 Climate records 33 3.4 Measuring the composition of the atmosphere 33 3.5 Isotopes 35 3.6 Ice cores 37 3.7 Ocean sediments 38 3.8 Earth system modelling 38 3.9 Inverse modelling 42 4 The Physics of Radiation 44 4.1 Radiation first principles 44 4.2 Scattering, absorption and emission 47 4.3 Two-layer radiation models 51 4.4 The greenhouse gas effect 52 5 Aerosols and Climate 58 5.1 Aerosols and climate 58 5.2 Sources and distribution of aerosols 60 5.3 Aerosol–climate interaction 62 viii Contents 5.4 Aerosol–radiation interaction 62 5.5 Aerosol–cloud interaction 65 5.6 Aerosol–surface interactions 68 5.7 Dust in the glacial–interglacial records 68 6 Physics and Dynamics of the Atmosphere 71 6.1 The atmosphere as a heat engine 71 6.2 Basic atmospheric thermodynamics 72 6.3 The tropospheric lapse rate and potential temperature 73 6.4 Moisture in the atmosphere 74 6.5 The equations of motion 77 6.6 The thermal wind equation 81 6.7 Weather systems and global climate 84 7 Physics and Dynamics of the Oceans 91 7.1 Earth’s oceans 91 7.2 Density, salinity and temperature 92 7.3 Ekman flow 95 7.4 Geostrophic flow in the ocean 98 7.5 The ocean circulation 100 7.6 Ocean and climate 103 8 The Hydrological Cycle and Climate 105 8.1 The global water cycle 105 8.2 Water vapour, lapse rate and cloud feedback 107 8.3 Transport of atmospheric moisture 112 8.4 Precipitation 114 8.5 Runoff and river discharge 115 8.6 Evaporation 119 8.7 Recycling of moisture 122 8.8 Frozen water 124 9 The Carbon Cycle 129 9.1 Carbon dioxide variability at geological timescales 129 9.2 The ocean carbonate system 132 9.3 The biological carbon pump 135 9.4 Ocean–air fluxes 136 9.5 Ocean carbon stocks 138 9.6 Terrestrial carbon 139 9.7 Terrestrial carbon fluxes 142 9.8 Terrestrial carbon stocks 143 9.9 Mean residence time of carbon on land 145 9.10 Geological carbon cycle 146 9.11 The Paleocene–Eocene Thermal Maximum 147 Contents ix 9.12 Carbon cycle in glacial–interglacial cycles 148 9.13 The modern anthropogenic perturbation 152 9.14 The future carbon cycle 155 10 Methane Cycling and Climate 159 10.1 Methane 159 10.2 The terrestrial methane budget 160 10.3 Decomposition 161 10.4 Methane sources 165 10.5 Methane in the geological perspective: the faint sun paradox 169 10.6 Methane in glacial cycles 171 10.7 The anthropogenic perturbation 172 10.8 Future methane emissions 173 11 The Nitrogen Cycle and Climate 176 11.1 The nitrogen cycle 176 11.2 Human intervention: the Haber–Bosch process 179 11.3 Atmospheric nitrogen 180 11.4 Terrestrial nitrogen cycle 181 11.5 Nitrous oxide 184 11.6 Nitrogen stimulation of plant growth 185 11.7 Oceanic nitrogen cycle 187 11.8 Nitrous oxide from oceans 189 11.9 The nitrogen cycle in geological times 190 11.10 Nitrogen in glacial–interglacial cycles 192 12 Phosphorus, Sulphur, Iron, Oxygen and Climate 194 12.1 Phosphorus, sulphur, iron and oxygen 194 12.2 The phosphorus cycle 195 12.3 Terrestrial phosphorus 196 12.4 Phosphorus at geological timescales 197 12.5 The iron cycle 200 12.6 The sulphur cycle 202 12.7 Iron, sulphur and oxygen in the geological past 203 12.8 Oxygen in the Pleistocene and the modern world 206 12.9 Oxygen in the modern ocean 208 13 The Future of Climate Change: Adaptation, Mitigation, Geoengineering and Decarbonization 210 13.1 Mitigation, adaptation, geoengineering 210 13.2 Planetary boundaries and adaptation 211 13.3 Carbon cycle 213 13.4 Ocean acidification 219 13.5 Mitigation and negative emissions 220