How Plants Make the Global Environment

(Second Edition)

Spri ringer

Published in association with

Praxis Publishing

Chichester, UK

Dr Jonathan Adams

Assistant Professor in Biological Sciences Seoul National University Gwanak-Ro Seoul

Republic of Korea


ISBN 978-3-642-00880-1 Springer Berlin Heidelberg New York Springer is part of Springer-Science + Business Media ( Library of Congress Control Number: 2009933599

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Preface to the Second Edition xi

Preface to the First Edition xiii

Foreword xv

List of figures xvii

List of tables xxiii

List of abbreviations and acronyms xxv

About the author xxvii

1 The climate system 1

1.1 Why does climate vary from one place to another? 2

1.1.1 Why mountains are colder 4

1.2 Winds and currents: the atmosphere and oceans 6

1.3 The ocean circulation 9

1.3.1 Ocean gyres and the "Roaring Forties" (or Furious Fifties) 9

1.3.2 Winds and ocean currents push against one another ... 10

1.4 The thermohaline circulation 10

1.5 The great heat-transporting machine 15

1.5.1 The "continental" climate 17

1.5.2 Patterns of precipitation 18

2 From climate to vegetation 27

2.1 Biomes: the broad vegetation types of the world 27

2.2 An example of a biome or broad-scale vegetation type: tropical rainforest 28

2.3 The world's major vegetation types 31

2.4 Understanding the patterns 37

2.5 What favors forest vegetation 39

2.5.1 Why trees need more warmth 39

2.5.2 Why trees need more water 40

2.6 Deciduous or evergreen: the adaptive choices that plants make. . 43

2.7 Cold-climate evergreenness 48

2.8 The latitudinal bands of evergreen and deciduous forest 50

2.9 Nutrients and evergreenness 50

2.10 Other trends in forest with climate 52

2.11 Non-forest biomes 53

2.12 Scrub biomes 53

2.13 Grasslands 53

2.14 Deserts 54

2.15 Biomes are to some extent subjective 54

2.16 Humans altering the natural vegetation, shifting biomes 55

2.17 "Predicting" where vegetation types will occur 55

2.18 Species distributions and climate 59

2.18.1 Patterns in species richness 60

3 Plants on the move 67

3.1 Vegetation can move as the climate shifts 67

3.2 The Quaternary: the last 2.4 million years 67

3.3 Biomes in the distant past 75

3.3.1 Sudden changes in climate, and how vegetation responds 75

3.4 The increasing greenhouse effect, and future vegetation change. . 81

3.5 Response of vegetation to the present warming of climate 81

3.6 Seasons as well as vegetation distribution are changing 86

3.7 What will happen as the warming continues? 88

3.7.1 Movement of biomes under greenhouse effect warming . 92

4 Microclimates and vegetation 97

4.1 What causes microclimates? 97

4.1.1 At the soil surface and below 98

4.1.2 Above the surface: the boundary layer and wind speed . 99

4.1.3 Roughness and turbulence 102

4.1.4 Microclimates of a forest canopy 103

4.1.5 Under the canopy 106

4.1.6 Big plants "make" the microclimates of smaller plants. . 108

4.1.7 The importance of sun angle 110

4.1.8 Bumps and hollows in the landscape have their own microclimate 112

4.1.9 Life within rocks: endolithic lichens and algae 114

4.1.10 Plants creating their own microclimate 115

4.1.11 Dark colors 115

4.1.12 Protection against freezing 115

4.1.13 Internal heating 115

4.1.14 Volátiles from leaves 116

4.1.15 Utilization of microclimates in agriculture 116

4.2 From microclimates to macroclimates 117

5 The desert makes the desert: Climate feedbacks from the vegetation of arid zones 121

5.1 Geography makes deserts 121

5.2 But deserts make themselves 122

5.2.1 The Sahel and vegetation feedbacks 127

5.2.2 Have humans really caused the Sahelian droughts? 132

5.3 Could the Sahara be made green? 132

5.4 A human effect on climate? The grasslands of the Great Plains in the USA 136

5.5 The Green Sahara of the past 139

5.6 Could other arid regions show the same amplification of change by vegetation cover? 143

5.7 Dust 145

5.7.1 Sudden climate switches and dust 149

5.8 The future 150

6 Forests 153

6.1 Finding out what forests really do to climate 155

6.2 What deforestation does to climate within a region 161

6.3 Re-afforestation 169

6.4 The remote effects of deforestation 169

6.5 The role of forest feedback in broad swings in climate 170

6.5.1 Deforestation and the Little Ice Age 170

6.5.2 Deforestation around the Mediterranean and drying in North Africa 173

6.5.3 Forest feedbacks during the Quaternary 173

6.6 Volatile organic compounds and climate 176

6.7 Forest-climate feedbacks in the greenhouse world 177

7 Plants and the carbon cycle 181

7.1 The ocean 183

7.2 Plants as a control on C02 and 02 185

7.3 Methane: the other carbon gas 187

7.3.1 Carbon and the history of the earth's temperature 188

7.3.2 Plants, weathering and C02 189

7.3.3 Plants, C02 and ice ages 193

7.4 Humans and the carbon store of plants 198

7.5 The present increase in C02 201

7.5.1 The oceans as a carbon sink 204

7.5.2 Seasonal and year-to-year wiggles in C02 level 205

7.6 The signal in the atmosphere 210

7.7 The strength of the seasonal "wiggle" in C02 212

7.8 Accounting errors: the missing sink 213

7.9 Watching forests take up carbon 215

7.9.1 Predicting changes in global carbon balance under global warming 217

8 The direct carbon dioxide effect on plants 221

8.1 The two direct effects of C02 on plants: photosynthesis and water balance 221

8.2 Increased C02 effects at the scale of a leaf 222

8.3 Modeling direct C02 effects 223

8.4 What models predict for increasing C02 and global vegetation. . 224

8.5 Adding climate change to the C02 fertilization effect 225

8.6 Experiments with raised C02 and whole plants 227

8.6.1 The sort of results that are found in C02 enrichment experiments 230

8.6.2 A decline in response with time 233

8.7 Temperature and C02 responses interacting 233

8.8 A few examples of what is found in FACE experiments 234

8.8.1 Forests 234

8.8.2 Semi-desert and dry grassland vegetation 236

8.8.3 Will C4 plants lose out in an increased C02 world?. . . . 237

8.9 0ther FACE experiments 242

8.9.1 FACE studies on agricultural systems 242

8.10 Some conclusions about FACE experiments 244

8.10.1 Will a high C02 world favor C3 species over C4 species? 244

8.10.2 What factors tend to decrease plant responses to C02 fertilization? 245

8.11 There are other effects of enhanced C02 on plants apart from growth rate 245

8.12 C02 fertilization and soils 246

8.13 C02 fertilization effects across trophic levels 247

8.13.1 Looking for signs of a C02 fertilization effect in agriculture 248

8.13.2 Looking for signs of a C02 fertilization effect in natural plant communities 249

8.13.3 The changing seasonal amplitude of C02 252

8.14 C02 levels and stomata out in nature 253

8.15 Direct C02 effects and the ecology of the past 253

8.15.1 Direct C02 effects on longer geological timescales 256

8.15.2 Ancient moist climates or high C02 effects? 257

8.16 Other direct C02 effects: in the oceans 258

8.17 The future direct C02 effect: a good or a bad thing for the natural world? 259

8.18 Conclusion: The limits to what we can know 260

Bibliography 261

Index 265

Preface to the Second Edition

In the two years since the First Edition of this book, the study of climate and vegetation has continued to advance rapidly, with many new and interesting things to write about. I have also been able to benefit from the comments and suggestions of colleagues who have read the book. And, furthermore, I have thought up some new— and hopefully better—ways to explain things, including various new diagrams and photographs. All these seem like good reasons to attempt a new edition, which I hope will continue to serve as an up-to-date review of the complex role of vegetation in our planet's environment. Once again, my wife Mei Ling and my daughters have been a great help in encouraging my efforts, trying to improve my limited photographic skills, and politely listening as I try to explain earth system science to them.

Jonathan Adams Seoul, Republic of Korea, 2009

Preface to the First Edition

I had wanted to write something like this book for many years, but would probably never have dared to attempt it unless I had been asked to by Clive Horwood at Praxis Publishing. As it is, this has been a rewarding experience for me personally, something which has forced me to read literature that I would not otherwise have read, and to clarify things in my head that would have remained muddled.

What I have set out to do here is provide an accessible textbook for university students, and a generalized source of current scientific information and opinion for both academics and the interested lay reader. I have myself often found it frustrating that there have been no accessible textbooks on most of the subjects dealt with here, and I hope that this book will fill the gap.

My friends and colleagues have provided valuable comment, amongst them David Schwartzman, Axel Kleidon, Alex Guenther, Ellen Thomas, Tyler Volk, Ning Zeng, Hans Renssen, Mary Killilea, Charlie Zender, Rich Norby, Christian Koerner and Roger Pielke Sr. I could not stop myself from adding to the manuscript even after they had sent me their careful advice, and any embarrassing errors that have slipped through are of course a result of my doing this. I am also very grateful to everyone who has generously given me permission to use their own photographs as illustrations in this book, and I have named each one in the photo caption. Lastly but very importantly, Mei Ling Lee has provided the encouragement to show that what I have been writing is of interest to somebody, somewhere.

Thanks in particular to Neil Cobb for providing the photo of a mountain scene, used on the cover of this book.

Jonathan Adams Newark, New Jersey, 2007


This book has been written with the aim of providing an accessible introduction to the many ways in which plants respond to and form the environment of our planet. As an academic scientist, and yet as a teacher, I have tried to balance conflicting needs between something which can be trusted and useful to my colleagues, and something which can enthuse newcomers to the subject. For too long, I feel, earth system science has been a closed door to students because of its jargon, its mathematics and its emphasis on meticulous but rather tedious explanations of concepts. I hate to think how many good potential scientists we have lost because of all this, and how many students who could have understood how the living earth worked have gone away bored or baffled. At a time when we may be facing one of the greatest challenges to our well-being in recent history, from global warming, it is essential that we recruit all the good researchers that we can. Ifwe want the public, business people and politicians to understand the problems they are facing, we need to disseminate knowledge of earth system processes as widely as possibly.

In line with the aims of Praxis—and with my own aims too—I have not attempted a complete referenced literature review in this book. Instead, selected papers of authors named in the text are listed in a bibliography, to provide the reader with some useful leads into the literature. Many important studies are not directly referenced even if their findings are mentioned in the text, and I hope that the authors of these studies will not feel snubbed (because my selection of papers to reference was often fairly arbitrary). The text is written in an informal way, reflecting my own dislike of pomposity in academia. Jargon in science gives precision, but it also takes away understanding if newcomers to the subject are driven away by it. As part of my balancing act, I have tried to keep jargon to a minimum. I have also used some homey and traditional categories such as "plants" to apply to all photosynthesizers, bacterial or eukaryotic (I regard being a plant as a lifestyle, not a birthright), and somehow I could not bear to keep throwing the word "archaea" around when I could just call them "bacteria".

Dedicated to the irreverent and brilliant Hugues Faure (1928-2003)


(asterisks indicate color images)

1.1 Why the tropics are colder than the poles 3

1.2 How the tilt of the earth's axis affects the angle of the sun, giving the seasons 4

1.3 Why the upper parts of mountains are colder 5

1.4 How mid-altitude warm belts form 5

1.5 The general position of the ITCZ along the equator 6

1.6 The intertropical convergence zone, a belt of rising air heated by the equatorial sun 7

1.7 The Coriolis effect, and the Ekman spiral 8

1.8 Thermohaline circulation in the Atlantic 11

1.9 Ocean gyres 13

1.10 How the greenhouse effect works 14

1.11 Antarctica is cut off by a continuous belt of winds and currents 16

1.12 The annual temperature cycle of an oceanic and a continental location compared 17

1.13 How the rain-making machine of the tropics works 18

1.14 How the monsoon rains move north then south of the equator during the year, following the zone where the sun is directly overhead 19

1.15 Two seasonal rainfall peaks at the equator 20

1.16 The passage of peak rains from south to north of the equator with the seasons 20 1.17* A satellite image of the density of vegetation across northernmost South

America (upper part of image can also be found in the colour section) 22

1.18 The Mediterranean climate. In winter, moist westerly winds cross southern Europe and bring rain. In summer, rain-bearing winds are pushed away by descending air from the "desert belt'' of North Africa 23

1.19 Where cold seawater wells up off the coast, air cools and then is warmed as it passes over land; and the way in which coastal deserts form 24

1.20* A view off the coast of Peru 25

1.21 The rainshadow effect 25

2.1* (a) Map of major biome distributions 28

2.1* (b) Areas of the most intense human alteration of vegetation 29

2.2* Buttress roots in a tropical rainforest tree 30

2.3* Drip tips on leaves of a rainforest tree shortly after a thunderstorm 31

2.4* An epiphyte growing on a tropical rainforest tree 32

2.5 General form of vegetation 33

2.6* Tropical rainforest, Malaysia 34

2.7* Cold climate conifer forest, mountains of California 34

2.8* Semi-deciduous forest during the dry season, with some trees leaf-less,

Malaysia, near Thai border 35

2.9* Tropical montane forest with a cloud layer blanketing the upper parts of the forest, Sabah, Borneo 35

2.10* Deciduous oak scrub, western Iran 36

2.11* Grassland, California 36

2.12* Tundra, above tree line in the Andes, Chile 37

2.13* Semi-desert, Mohave Desert, Arizona 38

2.14* Semi-desert, Iran 38

2.15 A tree's large area of leaves places a high demand in terms of evaporation, and a shrub loses less water by evaporation 41

2.16* Tree line on a mountain 43

2.17* Autumn leaves in a northern temperate deciduous tree, Norway maple (Acer platanoides) 45

2.18 The relationship between January temperature and leafing out date in a range of

North American trees 47

2.19* Toothed or lobed leaves are far more prevalent in cooler climate forests ... 48

2.20 Typical leaf of red maple (Acer rubrum) population in Canada compared with one from Florida 49

2.21 The proportion of species of trees with "entire" leaves depends closely on the warmth of the climate 49

2.22 Latitudinal bands of alternating evergreen and deciduous forest 51

2.23 Map of eastern Asia with zones of evergreen vs. deciduous forest 51

2.24 Holdridge's predictive scheme for relating biomes to climate 58

2.25a* The cross-leaved heath (Erica tetralix), a plant with an oceanic distribution . 61 2.25b The cross-leaved heath (Erica tetralix) shows a typical "oceanic" range, along the western side of Europe next to the Atlantic Ocean 61

2.26* Rhododendron ponticum 62

2.27 Tree species richness map of parts of eastern Asia 63

2.28 Wild tree species richness for North America 64

3.1 The global temperate history of the last 450,000 years showing a sawtooth pattern which appeared by 700,000 years ago 68

3.2 Distribution of forest vs desert 70

3.3 Biome distributions of Europe, North America at the present day and last glacial maximum 72

3.4* Temperature zones in the USA for the last glacial maximum 20,000 years ago and present day compared 74

3.5 Maps of migration rate of trees 76

3.6 Temperature history of the late glacial 77

3.7* The hazel tree (Corylus avelana) 79

3.8 Increase in pollen abundance of Scots pine (Pinus sylestris) in eastern England after the end of the ice age 80

3.9* The greening trend around the Arctic from satellite data 82

3.10 Instrumental temperature record of the last 120 years 83

3.11 Arctic shrub cover change in northern Canada 84

3.12 The heat-trapping effect of large cities alters the time of arrival of spring... 88

3.13 Sugar maple extends from southeastern Canada to the south-central USA. . 89

3.14* A bluebell (Hyacinthoides non-scripta) woodland in spring, southwestern

England 95

4.1 The boundary layer over a surface 99

4.2 Shrubs trap more heat amongst their branches than trees do, because the wind cannot blow between the tightly packed branches of a shrub 101

4.3* An alpine cushion plant 102

4.4* The lumpy, uneven tree crowns of tropical forest create turbulence in the air that flows over them 103

4.5* The eery gloom of tropical montane forest shrouded by cloud 105

4.6 During the day, heating of the canopy leaves gives an "inversion layer" of warm air floating within it; after the sun goes down, the canopy loses heat to the night sky, and the air floating just above it is left relatively warm, as it fails to cool down as fast 107

4.7* This species of Begonia lives in the understory of mountain rainforests in SouthEast Asia 109

4.8 Distribution of temperatures on a sunny summer's day on a hill in Massachusetts, USA 111

4.9 Temperature profile against height on a cold spring morning in a Pennsylvania valley that acts as a frost hollow 113

4.10 The daisyworld model of Lovelock illustrates how the microclimate effects of plants could scale up to global climates 117

5.1 If we take two equally moist surfaces with different albedo values, the darker low-albedo one is more likely to give rain 124

5.2 How positive feedback affects the slope of a response 126

5.3 A metastable system has multiple states 127

5.4 The Sahel, at the southern border of the Sahara desert 128

5.5 Record of rainfall in the Sahel since 1900 129

5.6 How a GCM works and how a mesoscale model slots into it 134

5.7 Temperature map for a warm day in northeastern Colorado 137

5.8* The distribution of vegetation zones of the present-day and the Holocene

"Green Sahara'' 141

5.9 In the Sahara, during the last 9,000 years, the summer solar energy input changes slowly 144

5.10 The opposing effects of dust in the atmosphere on temperature at ground level. 147

6.1 Forest loss in tropical lands 156-8

6.2 Some of the ways in which forests modify temperature 159

6.3 As the leaves come out, the progressive warming into spring halts for a few days because of the latent heat taken up by evaporation from the leaves 160

6.4 In tropical rainforest, loss of latent heat uptake and roughness dominates and deforestation is predicted to result in a regional temperature increase; in boreal forest the albedo effect dominates upon forest clearance, producing cooling . 162

6.5 In Georgia, USA, models suggest that after the lowlands were deforested, the maximum rainfall area shifted away from the mountains towards the boundary between forested land and cropland 165

6.6 In Costa Rica, before deforestation, evaporation from lowland forests supplied abundant clouds shrouding mountaintops and supporting cloud forest; now, lowland deforestation has resulted in less moisture supply to the atmosphere from the lowlands, so clouds are sparser and higher in the mountains 167

6.7 Having small open areas in a mainly forested landscape can actually increase rainfall by providing focal points for rising air which can be carried high and condense out rain clouds 168

6.8* Global temperature history of the last 2,000 years from several sources of tree ring data 171

6.9 Scene from a frozen river in Holland, 1608 172

7.1 Some basic components of the carbon cycle 182

7.2 A huge amount of C02 is stored in the form of both bicarbonate and dissolved

C02 in the ocean 184

7.3 Estimated C02 concentrations in the atmosphere over the last several hundred million years 186

7.4 0ne of the thousands of species of lichens—symbiotic combinations of a fungus and alga 190

7.5 Results of an experiment that compared the amounts of salts turning up in rainwater that had run off lichen-covered rocks vs. bare rock surfaces 191

7.6 History of temperature and atmospheric C02, deduced from polar ice cores 194

7.7 How plankton activity may have decreased the C02 concentration during glacials 195

7.8 The distribution of forest and desert in the present natural world and the LGM 197

7.9 How the land reservoir of carbon may help keep up C02 concentrations in the atmosphere when the oceans are dragging carbon down 198

7.10 Ice core record of atmospheric C02 since 1000 ad, Law Dome, Antarctica ice cores 201

7.11* Annual net flux of carbon to the atmosphere from land use change: 1850-2000 203

7.12 When an area of land is allowed to return to forest, there is an initial accumulation of carbon 203

7.13 The record of atmospheric C02 increase since the 1950s, measured directly at the Mauna Loa 0bservatory in Hawaii 204

7.14 The seasonal cycle in C02 concentration varies with latitude 206

7.15 "Lightening" of the isotope composition of atmospheric C02 over time. . . . 208

7.16 A carbon isotope shift around 7 million years ago indicates that C4 plants suddenly became much more common 209

7.17* Map showing correlation between temperature and global C02 increment each year, and map showing correlation between the amount of rainfall and the size of the global C02 increment around the world 211

7.18 The strength of the seasonal C02 wiggle is strongly related to the state of the NA0, and variation in the carbon isotope composition of C02 213

7.19 The eddy flux covariance method picks up C02 215

7.20 Model results with and without the "gushing out" of carbon that would result from warming affecting the carbon balance of forests 218

8.1 Key steps in photosynthesis which are altered by C02 concentrations 223

8.2 The three types of increased C02 experiment: closed-chamber, open-top chamber, free air release 228

8.3 The Tennessee FACE site showing the towers used to release C02 into the forest 230

8.4 Aerial view of the Tennessee FACE experiment showing rings of towers ... 231

8.5 The Swiss FACE site on mature mixed temperate forest uses a network of tubes twisting up the branches to deliver CO2 in the right places and right quantities to simulate a higher CO2 atmosphere 231

8.6* Scientists at the Swiss FACE site inspect the forest canopy for direct CO2 effects using a crane 232

8.7 The sequence of reactions in a C4 leaf 239

8.8* Thermal infrared image taken looking down on a wheat field fertilized by a ring of pipes releasing CO2 243

8.9 Stomatal index change 246

8.10 The shift in 13C in sediments in North America, South Asia, and in the global ocean indicating a "take-over" by C4 plants 258


5.1 Typical values for various land surface types 123

5.2 Climate history of northwestern China over the last 10,000 years 145

Abbreviations and acronyms


Crassulacean Acid Metabolism


Carbon Dioxide Information and Analysis Center


Commonwealth Scientific and Industrial Research



Free Air CO2 Experiment


General Circulation Model


Intergovernmental Panel on Climate Change


Inter-Tropical Convergence Zone


Leaf Area Index


Last Glacial Maximum


National Center for Atmospheric Research


National Centers for Environmental Prediction


National Oceanic and Aerospace Administration


Net Primary Production




Volatile Organic Compound

About the author

Jonathan Adams was born in England and studied Botany at St Catherine's College of the University of Oxford. His PhD was in Geology from the University of Aix-Marseilles II, France, where his mentor was the distinguished Quaternary geologist Hugues Faure.

After postdoctoral studies at Cambridge University and at Oak Ridge National Laboratory (Tennessee), Jonathan Adams has taught at the University of Adelaide (Australia), Rutgers University (New Jersey), and latterly at Seoul National University (South Korea).

The climate system

Though few people stop to think of it, much of the character of a place comes from its covering of plants. Southern France, with scented hard-leaved scrublands, has an entirely different feel about it from the tropical rainforest of Brazil, or the conifer forests of Canada. Vegetation is as important a part of the landscape as topography and the architecture of buildings, and yet it is an accepted and almost subconscious part of the order of things.

Even fewer people ever ask themselves "why" vegetation should be any different from one place to another. Why do conifers dominate in some parts of the world, but not others? Why are there broadleaved trees that drop their leaves in winter some places, while elsewhere they keep them all year round? Why are some places covered in grasslands and not forest? As with almost everything in nature, there is a combination of reasons why things are the way they are. Most important in the case of vegetation are two factors: humans, and climate.

In some cases, the landscape we see is almost completely a product of what mankind is currently doing. Humans have cleared away much of the world's natural plant cover, and replaced it with fields and buildings, or forest plantations of trees from other parts of the world. Yet, even in such heavily modified areas, fragments of the original vegetation often survive. In other instances the vegetation is a sort of hybrid of human influence and nature; battered by fires or by grazing animals, and yet still distinctive to its region. Most of the landscapes of Europe (including, for example, southern France) are like this, produced by the combination of climate, local flora and rural land use patterns.

However, over large areas the vegetation is still much as it was before humans dominated the planet. This original cover tends to survive in the areas where the landscape is too mountainous to farm, or the climate or soils are in other ways unsuitable for cultivation. Most of Siberia, Canada, the Himalayan Plateau and the Amazon Basin are like this, and scattered areas of protected wilderness survive in hilly or marginal areas in most countries. If we concentrate on these most natural areas in particular, there are clear trends in the look of vegetation which tend to correlate with climate. Such relationships between vegetation and climate first became apparent when explorers, traders and colonialists began to voyage around the world during the last few centuries. The tradition of natural history that grew out of these early explorations has tried to make sense of it all. Vegetation takes on a myriad of forms, which can be difficult to push into orderly boxes for classification. Yet there is no doubt that there is a lot of predictability about it.

Variation in climate, then, is a major factor that determines the way vegetation varies around the world. But why does the climate itself vary so much between different regions? The basic processes that make climate are important not just in understanding why vegetation types occur where they do, but also in understanding the complex feedbacks explored in the later chapters of this book. As we shall see, not only is the vegetation made by the climate, but the climate itself is also made by vegetation!


Essentially, there are two main reasons that climate varies from place to place; first, the amount of energy arriving from the sun, and second the circulation of the atmosphere and oceans which carry heat and moisture from one place to another.

One of the major factors determining the relative warmth of a climate is the angle of the sun in the sky. The sun shines almost straight at the earth's equator, because the equator sits in the direct plane of the sun within the solar system. So, if you stand on the equator during the middle part of the day, the sun passes straight overhead. At higher latitudes, such as in Europe or North America, you would be standing a little way around the curve of the earth and so the sun always stays lower in the sky. The farther away from the equator you go, the lower the sun stays until at the poles it is really only barely above the horizon during the day.

Having the sun directly overhead gives a lot more energy to the surface than if the sun is at an angle. It is rather like shining a flashlight down onto a table. Hold the flashlight pointing straight down at the table and you have an intense beam on the surface. But hold it at an angle and the light is spread out across the table top and much weaker. If the sun is high in the sky, a lot of light energy hits each square kilometer of the earth's surface and warms the air above. If the sun is low in the sky, the energy is splurged out across the land; so there is less energy falling on the same unit area (Figure 1.1a). This tends to make the poles colder than the tropics, because they are getting less heat from sunlight.

A second factor relating to sun angle, which helps make the high latitudes cooler, is the depth of atmosphere that the sun's rays must pass through on the way to the earth's surface (Figure 1.1b). Because at high latitudes the sun is lower in the sky, it shines through the atmosphere on a slanting path. At this angle, the light must pass a longer distance through more gases, dust and haze. This keeps more of the sun's energy away from the surface, and what is absorbed high in the atmosphere is quickly lost again up into space. Think how weak the sun is around sunset just before it sinks

Why does climate vary from one place to another? 3 Sun's beam from above

Sun's beam from the side

Light spread across large area

Sun's beam spread across surface

Light concentrated onto small area

Sun's beam concentrated on smaller area

Top of atmosphere

Figure 1.1. Why the tropics are colder than the poles. (a) A direct beam gives more energy than an angled beam. (b) Passing through greater depth of atmosphere absorbs more energy before it can hit the earth.

Shorter path through atmosphere

Figure 1.1. Why the tropics are colder than the poles. (a) A direct beam gives more energy than an angled beam. (b) Passing through greater depth of atmosphere absorbs more energy before it can hit the earth.

Summer at point A

More concentrated beam of sun

Summer at point A

More concentrated beam of sun

Figure 1.2. How the tilt of the earth's axis affects the angle of the sun, giving the seasons.

below the horizon—so weak that you can stare straight into it. The dimness of the setting sun is an example of the effect of it having to shine through a longer path of atmosphere, which absorbs and scatters the sun's light before it can reach the surface. So, the lower in the sky the sun is, the longer is its path through the atmosphere, and the less energy reaches the ground.

Only in the tropics is the sun right overhead throughout the year, giving the maximum amount of energy. This then is the key to why the poles are cooler than the tropics.

The seasons of the year are also basically the result of the same sun angle effects (Figure 1.2). The earth is rotating on its axis at a slight angle to the sun, and at one part of its yearly orbit the northern hemisphere is tilted so the sun is higher in the sky; it gets more energy. This time of year will be the northern summer. At the same time, the southern hemisphere is getting less energy due to the sun being lower. During the other half of the year, the southern hemisphere gets favored and this is the southern summer. Adding to these effects of sun angle is day length; the "winter" hemisphere is in night more of the time because the lower sun spends more time below the horizon. This adds to the coldness—the warming effect of the sun during the day lasts less time, because the days are shorter.

1.1.1 Why mountains are colder

If you climb up a mountain, the air usually gets colder. The temperature tends to decline by about 0.5°C for every hundred meters ascended, although this does vary. The rate of decrease of temperature with altitude is called the "lapse rate''. Lapse rate tends to be less if the air is moist, and more if the air is dry. Generally, every 10 meters higher up a mountain is the climatic equivalent of traveling about 15 km towards the poles. Unlike the decline in temperature with latitude, sun angle does not explain why higher altitudes are generally colder. The relative coldness of mountains is a byproduct of the way that the atmosphere acts as a blanket, letting the sun's light in but preventing heat from being lost into space (see Box Section 1.1 on the greenhouse effect). Because they protrude up into the atmosphere, mountain tops have less of this blanket above them, so they are colder (Figure 1.3).

Figure 1.3. Why the upper parts of mountains are colder. A thinner layer of greenhouse gases causes them to lose heat rapidly.

There are however some exceptions to this pattern of temperature decline with altitude: places where the mid-altitudes of a mountain are warmer on average than the lowest altitudes. This occurs where there are enclosed valleys between mountains, where there is not much wind. At night, cold air from the upper mountain slopes tends to drain as a fluid into the valley below, and accumulate. Just above the level that this cold draining air tops up to, there is a warm mid-altitude belt that can have warmer-climate plants than the valley below (Figure 1.4). Mid-altitude warm belts like this often occur in the Austrian Alps, for example.

The general pattern of cooler temperatures at higher altitudes occurs not only on mountains, but through the atmosphere in general, essentially because of the same factor—a thinner blanket of greenhouse gases higher up. If air is rising up from the surface due to the sun's heating, it will tend to cool as it rises due to this same factor. Another thing that will tend to make it cool is that it expands as it rises into the thinner upper atmosphere—an expanding gas always takes up heat. If the rising air is moist, the cooling may cause it to condense out water droplets as cloud, and then perhaps rain drops which will fall back down to earth.

Figure 1.4. How mid-altitude warm belts form. Cold air drains down as "rivers" from the upper slopes of the mountain, and fills up the valley below. Just above the top of the accumulated cold air, temperatures are warmer.

Figure 1.4. How mid-altitude warm belts form. Cold air drains down as "rivers" from the upper slopes of the mountain, and fills up the valley below. Just above the top of the accumulated cold air, temperatures are warmer.

Why does climate vary from one place to another? 5 Top of atmosphere

Shorter distance

Figure 1.3. Why the upper parts of mountains are colder. A thinner layer of greenhouse gases causes them to lose heat rapidly.

Longer distance

Why does climate vary from one place to another? 5 Top of atmosphere

Shorter distance

Longer distance


Differences in the amount of the sun's energy received by the surface drive a powerful global circulation pattern of winds and water currents. The most basic feature of this circulation, and a major driving force for almost everything else, is a broad belt of rising air along the equator (Figures 1.5 and 1.6). This is known as the intertropical convergence zone, or ITCZ for short. The air within the ITCZ is rising by a process known as convection; intense tropical sunlight heats the land and ocean surface and the air above it warms and expands. Along most of this long belt, the expanding air rises up into the atmosphere as a plume, sucking in air sideways from near ground level to replace the air that has already risen up. Essentially the same process of convection occurs within a saucepan full of soup heated on a hot plate, or air warmed by a heater within a room; any fluid whether air or water can show convection if it is heated from below. The difference with the ITCZ, though, is that it is convection occurring on an enormous scale. Because air is being sucked away upwards, this means that the air pressure at ground level is reduced—so the ITCZ is a zone of low air pressure in the sense that it would be measured by a barometer at ground level.

What goes up has to come down, and the air that rises along the equator ends up cooling and sinking several hundred kilometers to the north or south of the equator. These two belts of sinking air press down on the ground from above, imposing higher pressure at the surface as they push downwards.

The air that sinks down in these outer tropical high-pressure belts gets sucked back at ground level towards the equator, to replace the air that is rising up from being heated by the sun. It would be easiest for these winds blowing back to the equator to take a simple north-south path; this after all is the shortest distance. But heating is strongest


Figure 1.5. The general position of the ITCZ along the equator.

The intertropical convergence zone (ITCZ)

forms along the equator where the sun s heating is strongest


Figure 1.5. The general position of the ITCZ along the equator.

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