What’s in a bell curve?

Geographers like statistics; they allow us to test for significant differences between samples and to look for empirical relationships between different variables.  Most statistics rely on probability and a definition of what ‘normal looks like’.  We exclude the outliers – data that appears to be anomalous or outside the bounds of probability (Fig. 1).  As such the focus is always on comparing normal with normal.  Was the Boxing Day Tsunami of 2004 normal?  Was it predictable?  If so why did so many people die?  The answer to this lies largely on the time-scale with which one views Earth processes.  In an influential book The Black Swan Nassim Taleb describes such events as outliers and goes on to argue that they are more significant than conventional statistics and science gives them credit.  In geographical terms this speaks to an early debate between those that favour uniformitarianism and those that believed in catastrophism. 


Figure 1: Normal probability curve showing confidence limits and potential outliers.

Early Geographical-Geological Ideas

Early geological thought was plagued by the idea of trying to reconcile religious beliefs (i.e. the account in genesis) with emerging ideas on the antiquity of the Earth.  Many early scientists were profoundly religious yet found their faith was challenged by the observations and inferences they made.

This was particularly true of James Hutton (1726-1797) who many would describe as the founder of geology.  He gave us the concept of uniformitarianism – the key to the past is the present.  Basically that the processes that we observe today however slow, must have always operated.  Since the Earth process he could observe on his estate in the Southern Uplands of Scotland were slow; the earth must be much older than suggested in the bible.  He challenged head on the idea that forces in the past must have been different from the present.  Catastrophism invoked exactly that – processes must have been more active, powerful or different in the past than in the present.  The latter requires much less time than the former and is easier to reconcile with religious estimates of the age of the Earth and such events as the biblical deluge.

Charles Lyell (1797-1875) a friend and contemporary of Charles Darwin codified uniformitarianism in the late nineteenth century.  He envisaged both the uniformity of process but also the uniformity of rate.  The fundamental physics of our universe have not changed through earth history; gravity, light, heat transfer and motion are guided by fundamental physical laws.  The processes by which land erodes and sediment is transported and deposited should not have changed therefore.  The uniformity of process is therefore built on a sound premise, but what about the uniformity of rate?

Let us image rain falling on a slope devoid of vegetation because it had yet to evolve.  Vegetation is good at binding soil and sediment together and resisting erosion.  So erosion rates may have been faster before terrestrial vegetation evolved?  Is this not an example of varying rate potential?  The past may not have been quite the same as the present.  What about catastrophic events like the occasional tsunami?

Humans tend to see things on human rather than geological timescales and set the temporal window accordingly.  Tsunamis occur quite frequently on geological timescales hundreds of thousands to millions of years, but not on human scales of one or two generations (living memory).  So a Black Swan event is only one if we restrict the timescales over which we view things.  We call this the return period for events.  For example, an observed flood event can be described as the one in 20 year event.  It means that in the normal course of events an event of this magnitude occurs once in 20 years; once in a generation or with a probability of one in five each year.  A larger event may have a return period of one in a hundred years; this does not mean that it will happen next year if it has not happened in the last 99 years.  It is just a probability statement.  The point here is that the longer the time period sampled the larger the events that may occur within it.  So in the case of the Boxing Day Tsunami the fact that such an event has a relatively high frequency when sampled over a 1000 years – may four or five – your average person works on ‘living memory’.  Since most humans can’t conceive a 1000 years we tend to work on generational time; the largest event in living memory.  It blinds us in some ways to the hazard that has not happened in living memory but is waiting; that is a Black Swan event.  The importance of temporal sampling is shown in Figure 2.  Only time sample B or E would capture the anomalously large event.  It is one of the reasons why good historical and geological records of event frequency are so important in assessing true risk.

bwell2Figure 2: The effect on temporal sampling on magnitude assessments.

Magnitude and Frequency

This leads us to the question of the relative impact of catastrophic versus non-catastrophic geomorphological events.  Does a large infrequent event have more geomorphological impact than a small but frequent event?

Image a large pile of sand and two students.  One has a t-spoon but no smart phone and diligently moves a spoonful once every minute.  The other student has a spade and a smart phone.  While the spade moves more per shovel full than the spoon, the student is too busy texting and looking for Pokémon Go creatures to move more than one shovelful an hour.  Who will shift most sand in 24 hours?  The chances are the student with the spoon will win out; each spoonful is a low magnitude event, but it is occurring with a high frequency.


Figure 3: The concept of magnitude and frequency in geomorphology.

This is the concept of Magnitude and Frequency in geomorphology discussed by Wolman and Millar (1960).  Low magnitude, but high frequency processes have the potential to do more geomorphological work than high magnitude but low frequency events.  Essentially there is a trade-off between frequency and magnitude as illustrated in Figure 3.  The classic example is soil creep – the down slope movement of individual soil particles as the ground expands and contracts with temperature and moisture changes.  Because soil creep occurs continuously some researchers have argued that its impact on slope morphology is greater than large mass movements.  There are a number of good illustrations of this type of balance to be had.

The concept is also nicely illustrated by coastal erosion rates.  The classic example is one from a coastal town called Walton on the Naze in Essex, just north of London.  Figure 4 shows the erosion rates along a stretch of coast below the Naze Tower.  The rates of recession are more variable in the south and accelerate toward the north as the cliff height falls.  This feels a little counter intuitive; surely bigger cliffs should erode faster?  The answer to this lies with the magnitude and frequency of the events doing the erosion.  In the south the cliffs are taller and fail periodically by large rotational mass movements founded in the London Clay at the base of the cliff.  The debris acts as a natural sea defence and must be eroded before the cliff can be undercut and steepened again by the sea, ready to fail again.  So the events are large 20 to 30 metres of cliff being lost each time, but the events only occur once every 30 to 40 years so on average the recession rate is modest (Fig. 5).  Contrast this with the cliffs in the north (Fig. 4).  Here the cliffs are only a metre or so high.  The sea is able to erode the base of these cliffs almost continually and when they fail the mass movements are small with little debris involved.  These low magnitude events occur frequently (in fact almost continuously during a high sea) and consequently erosion is fast.  Figure 6 shows this conceptually and is a model that holds for lots of coastal areas around the UK and elsewhere.    There are some good papers based on this case study the original is by Murray Gray and published in the Proceedings of the Geologists’ Association.


Figure 4: Coastal recession at Walton on the Naze.  Based on Bennett and Doyle (1997).

Figure 5: Periodic coastal erosion via mass movements.


Figure 6: The concept of magnitude and frequency applied to coastal erosion rates in Britain.  Based on Bennett and Doyle (1997) and Cosgrove et al. (1998).

To this we must add the concept of landscape relaxation.  That is not kicking-back in a nice landscape, but the time it takes to erase the evidence of a geomorphological event.

The last glacial cycle in Britain was a high impact event lasting about 100,000 years.  Its impact on the British landscape was profound and we can still see the evidence of its impact in the landscape ten thousand or more years later.  (As an aside the impact of the Anglian Glaciation (antepenultimate glaciation) was so profound it is still visible today in the form of the north-south UK river pattern and the existence of the Wash.)  The relaxation time from this high magnitude event is considerable.  Now let us consider the beach cusps on Bournemouth beach which are often present.  They form over a single tidal cycle, and can be destroyed over another.  As such they are a lower magnitude event, with a correspondingly lower relaxation time.

So while low magnitude, high frequency events, might do more geomorphological work the legacy of such events may be much more transitory.  Catastrophic events may occur infrequently, but their impact on the landscape may be correspondingly greater.

Plate tectonics and driver-less cars?

If driver-less cars are the future then we had better take account of plate tectonics!  You have no doubt heard the stories “driver follows satnav instructions and ends up in a field!”  Well what about driver-less cars?  Their success is crucially dependent on Global Positioning Systems (GPS).  If you have a smart phone you have a GPS; by linking with one or more satellites a GPS can triangulate its position with varying levels of accuracy.  In a driver-less car you want to be sure that the car’s GPS is both accurate and linked to the road map, half a metre out and you could be facing the oncoming traffic! 

Maps and their datum’s

In the UK the Ordnance Survey has been making maps since 1747.  These maps are based on the British National Grid, a system of rectilinear lines (northings and easting) superimposed on the curved surface of the earth.  This is typical of what are known as country, or local-co-ordinate, systems.

Australia is no exception.  In the summer of 2016 it was reported (BBC, 29 July 2016) that Geoscience Australia was moving Australia so that the gap between its local co-ordinate system and that of global navigation satellite systems (GNSS) were in closer agreement.  When we way ‘moving Australia’ what they mean is moving the official longitude and latitude of the origin (zero point) of their local co-ordinate system.  The Geocentric Datum of Australia, the origin for the country’s local co-ordinate system, was last updated in 1994 since when Australia has moved about 1.5 m north due to plate tectonics.  Driver-less tractors are already a feature of some Australian farms so the problem is very real irrespective of what may or may not happen with respect to driver-less cars.

Plate tectonics is constantly and subtly re-arranging the World’s geography.  For example, the distance between London and New York is growing by about 5 cm each year, in a decade that is 50 cm and in a hundred years 5 m.  Plate tectonics is a big deal and is also an essential paradigm to understanding the Earth’s geological past.

Paradigms and gladiatorial science

A paradigm is a model or conceptual framework of ideas with which to organise and interpret observations and data.  It is bigger than a hypothesis, but less definitive than fact or theory.  Scientists make arguments; they advance explanations, models and ideas by reasoned and evidenced argument.  They articulate their ideas, garner supporting evidence and/or test them against that evidence.  In natural science there are few absolute rights and wrongs; it’s not like a maths problem in which you can look the answer up in the back of the book!

Inductive science involves observing and noting everything around you; in our case observing the natural world.  From that body of data you look for patterns, make logical inferences and deductions developing ideas which as they gather support become irrefutable and take on the status of fact or theory.  It is a philosophy of investigation that was first formalised by Francis Bacon (1561-1626) in 1620: one observes nature, proposes a modest law to generalize an observed pattern, confirms it by many observations, ventures a modestly broader law, and confirms that, too, by many more observations, while discarding disconfirmed laws.  In this way a laws grow ever broader but never exceeds the observations on which it is founded.  As a philosophy of science it is not without its problems.  Take the case of the hypothesis ‘do black swans exist?’  Any number of observations of white swans will not address the question, but find one black swan and the hypothesis is proven.

This alternative method of science is called ‘falsification’ – rather than gather supporting evidence for an idea how can you formulate a test that will disprove it?  In this view of science one is constantly working to disprove the ideas and models you propose.  It is a view of science proposed by Karl Popper (1902-1994) amongst others.

Thomas Kuhn (1922-1996) proposed in his famous book The Structure of Scientific Revolutions, influential in both academic and popular communities, that periods of normal science dominated by paradigms are overturned by periods of revolutionary science establishing new paradigms and renewed stasis.

Little did Alfred Wegener (1880-1930), a leading explorer and meteorologist of his time, know that he was laying the foundation for one of the biggest paradigm shifts in earth science when he proposed his idea of continental drift on the 6 January 1912.  Amassing palaeontological, lithological and structural evidence he proposed that continents had moved over the Earth’s surface in the past.  He famously pointed to the ‘jigsaw’ like fit of Africa and South America, something that Francis Bacon had noted previously.  He coined the term Pangea for a giant supercontinent that had once existed.

Science can be brutal, often gladiatorial; propose an idea that is too radical for the scientific establishment and they will turn and savage you.  That is what happened to Wegener and his ideas of continental drift were neglected until geophysical exploration of the ocean following the Second World War began to throw up new data.  On the basis of this data the paradigm of plate tectonics emerged in the late 1960s and early 1970s revolutionising our understanding of our planet both past and present .  One of the greatest scientific paradigm shifts of the twentieth century.  A number of popular reviews were published to mark the centennial anniversary the best of these is by Romano and Cifelli (2015) published in Science.

Finally I came across this wonderful song on YouTube the other day which celebrates Wegener’s contribution; I have no idea what he would make of it!

What is the point of physical geography?

Imagine that you are a first year student sitting there fresh-faced in your first physical geography lecture.  Some of you will be excited having done physical geography at A-level and enjoyed it, others will be saying ‘I only like human geography, the physical stuff is boring!”  Perhaps others will be saying “I am an ecologist, why do I have to learn this geology stuff?”  All are valid viewpoints.  I have spent my life as a geographer, come geologist, working in the high arctic on glacial processes, reconstructing our Ice Age past, studying the geography of human evolution in Africa and applying geomorphological expertise to the study of forensic footprints at crime scenes. 

 It is hardly surprising therefore that I believe the world’s leaders and decision-makers all need to be both scientifically and geographically literate.  We must overcome huge challenges in the coming years as the Earth’s climate changes.  What ever happens about greenhouse gas emissions climate will change, in fact change is normal!  Geographers can help decision-makers face these challenges and inform the solutions.   Let me try and show you why geographers matter. 

Roll the clock forward and imagine that you are now working for an aid organisation coordinating humanitarian relief.  The news breaks of a major earthquake in northern Pakistan.  You have to mobilise people, resources and get them to the epicentre fast.  The questions flood in: what is the terrain like, what is the vegetation like, what is the climate and weather doing and where are the transport lines most vulnerable to after-shocks? These are just a small selection – Google Earth and the internet has its limits.  Later you may be asked to advise on rebuilding lost infrastructure or improving disaster/emergency planning.  All these questions are underpinned by physical geography.

If you don’t like this scenario image yourself as a conservation worker in Africa saving the white rhino.  The rhino is a product of its environment, the distribution of soil and food resources and its movements limited by the local terrain.  Climate change and local weather patterns all play apart in its survival even before we consider the social and cultural aspects that lead to is predation by poachers.

I could go on.  Understanding the Earth’s surface terrain its shape, composition and the processes that formed it in the past and that shape it now and will in the future is fundamental to almost all human interaction with the planet we live on.  That is what physical geography is about.  It is the foundation of environmental and ecological science a key component of geology and therefore to our understanding of Earth history and our past.  That is why all those interested in ecology, geology and the environment need to be versed in the fundamental Earth systems.

Definition and history

Physical geography is the study of the processes that shape the Earth’s surface, the animals and plants that inhabit it, and their spatial distribution.  This surface lies at the interface between the lithosphere and the atmosphere and is shaped by both.  Its study is by definition multi-disciplinary therefore drawing on geology and meteorology, and is fundamental to understanding the ecology and biogeography our planet.

As a discipline it emerged in the mid- to late 1800s with geomorphologists dominating the discipline at first (Table 1).  The emphasis was on the description of landscapes, climates and biomes. Ideas of environmental determinism dominated in which landscapes in particular were seen as part of development trajectories.  For example, William Morris Davis (1850-1934) saw fluvial landscape in a series of age related cycles.

Geographical sub-disciplines
Geomorphology – shape of the Earth’s surface and processes by which it is shaped, both at the present as well as in the past. It is closely linked to Geology.
Hydrology – the distribution, movement and quality of water on the land surface and in the soils and rocks near the surface.  Ground water hydrology is known as geo-hydrology.
Glaciology – study of the Earth’s current glaciers and ice sheets (cryosphere).  It is closely associated with Quaternary Science.
Biogeography – study of the geographic patterns of species distribution and the processes that result in these patterns.
Climatology – study of the Earth’s climate or weather patterns that predominate at a location, distinct from meteorology which is the study of day-to-day weather.
Pedology or Soil Science – the study of soils in their natural environment.
Oceanography – the study of the Earth’s oceans and seas, many people would recognise this as a discipline in its own right.
Quaternary Science – is the inter-disciplinary study of the Quaternary period, which encompasses the last 2.6 million years. This includes understanding past climates, landscape changes, ice sheets and the mechanisms of both climate and environmental change.
Geomatics – is the collection and process of geographically relevant ‘big-data’ from satellites and Earth observation systems.
Environmental Geography – this focuses on the interaction of humans and the natural world. In some respects it lies at the interface between human and physical geography.

Table 1: Some of the main sub-disciplines in Physical Geography.

Physical geography along with human geography underwent radical period of quantification in the late 1950s and early 1960s known as the Quantitative Revolution.  In geomorphology there was a radical shift from the description of landforms to process based experimentation on the mechanism by which landforms were formed.  What followed was massive growth in research and intense disciplinary specialisation around five broad themes: geomorphology, climatology, biogeography, soil science, and Quaternary environmental change

Today Physical Geographers remains an intrinsically inter-disciplinary subject of ever growing relevance as the pace of global environmental change accelerates. Geographers grapple with the inter-connected nature of the Earth’s fundamental geodynamic systems – lithosphere, hydrosphere, biosphere and atmosphere – and their impact on, and interaction with, different scales on the human use system.  It is by definition both local and global in scale and geographers’ bring their unique spatial and analytical skills to bear on these interactions.  Many geographers now recognise the Anthropocene as a new geological era; the era shaped by human activity.  It is an era in which geographical and scientific literacy are likely to be key to the survival of our species.

Succeeding in Physical Geography?

So you are still sitting there and now wondering how do I succeed in this unit?  How do I gain a fundamental knowledge of Physical Geography.  At this stage you probably want me to give you the answers to the exam or direct you to the magic ‘know it all geographical potions’.  Sadly the latter does not exist and the former would have William Davis turning in his grave.

The key is pro-active engagement in three vital areas, these are:

  • Preparation. Go on to Brightspace and engage with the material there.
  • Attend and engage. You will quickly find out that the lecture slides consist mainly of line diagrams and pictures.  Unless you note down the spoken words and explanations that go with them you won’t stand much chance of understanding the material.  If you could get everything from Brightspace why would we bother giving lectures?  You need to annotate a set of printed slides and write detailed notes during the lectures.  Without a good set of notes you will struggle and perhaps fail the unit – it’s that simple!  Take part in the discussions on the perspective pieces and use this as an opportunity to ask questions and seek greater understanding.
  • Reflect and read. So you leave the lecture and you are on to the next thing; your lecture notes end up at worse as a crumpled set of pages or at best get filed in a nice shiny, new binder.  You may even go as far as to buy a copy of one of the core texts and display it proudly on your shelves.  Have you ever heard of the ‘psychological value of unused information?’  People buy self-help books but never read them but feel better for having them – well that’s the concept.  It applies here – having that new shiny binder and copy of the core text makes you feel better, but in truth won’t improve your grade.  You need to engage with those notes and read the textbook!  You have to engage.  As soon as you get a chance after a lecture get the notes out and review them, don’t waste time copying them out and making them look pretty read and reflect them while making sure they are legible.  What do you understand and what don’t you?  What interests you and what left you feeling cold?  Look at the suggested reading list for the lecture provided each week and draw up a prioritised list of things to follow up on.  May be its to read a section of the core text and makes notes, may be it is to read suggested paper, or may be its to simply spend half an hour on the internet to get some specific examples, facts and illustrations.  Whatever it is augment your lecture notes by further research.  If you don’t understand stuff then be pro-active don’t sit there worrying about it – seek help.  You can get help from your Peer Assisted Learning (PAL) tutor if you have one or directly from the lecturer by attending one of the practical drop sessions.

Engage with the lectures as outline above and build a good body of notes and the assessment and exam will take care of themselves.

Goldilocks and the daisy?

Unfortunately scientist can slip into using cliches.  This is despite the fact that imagination and creativity are probably the key skills to successful science.  A quick Google Scholar search on the term ‘Goldilocks’ reveals over 24,000 scientific papers and books that use the term and only a small number of them involve three bears!  It has become shorthand for ‘just right’.  Understanding planetary temperatures is no exception.  Planet Earth is said to be located in the Goldilocks Zone of the solar system – not too far from the sun to be too cold, not to close to be too hot.  In practice, ‘just right’ conditions have prevailed throughout much of the 4.6 billion years of Earth history, the big question is why?

Just right: not too hot, not to cold

The probability of the Earth’s temperature remaining at suitable level for life to survive during much of the 4.6 billion years of Earth history has been described as a kin ‘to crossing a motorway blindfold during rush hour’.  When the Earth was first formed the solar constant, that is the amount of radiation received by the Earth from the sun, was lower than it is today.  The sun was dimmer and has been slowly warming ever since.  The solar constant has risen throughout Earth history but the average planetary temperature has remained broadly the same give or take a few wobbles.  The climate and the chemical properties of the Earth now and throughout its history seem always to have been optimal for life The question is why and how?

This observation led Professor James Lovelock to the radical conclusion that the Earth could regulate its own temperature just like any warm blooded animal.  He postulating thermo-regulation and coined the term Gaia – Goddess Earth – and suggested that the Earth was living.  In this context a living being is something able to regulate and optimise its environmental conditions.  He knew how to sell a story and the Gaia hypothesis was born.

Whether the Earth can truly be described as ‘living’ is largely irrelevant, the crux of the issue is that positive and negative feedback loops within planetary systems may operate against extremes just like the human body.  To hot and you begin to sweat, to cold and your body develops gooseflesh and a shiver.  The search was on to find self-regulating planetary systems and a number of such systems have emerged over the years.

One of the first of these was the role of dimethyl sulphide in cloud formation.  For condensation to occur you need condensation nuclei, tiny particles of dust or particulate matter around which water vapour can condense.  It is where ideas of cloud seeding in drought stricken areas come from.  Lovelock and his colleagues were able to demonstrate that a key condensation-nuclei was sulphur dioxide derived from dimethyl sulphide from plankton on the ocean surface.  As a result they found a large planetary feedback mechanism (Fig. 1).  The so called CLAW hypothesis was named after the authors involved in its discovery Charlson, Lovelock, Andrea and Warren (1987).

clawFigure 1: The CLAW hypothesis. (Source: Plumbago-Own work, CC BY 2.5, en.wikipedia.org/w/index.php?curid=12987086)

The sun acts to increase the growth rates of phytoplankton due either to increased surface temperature and/or increased availability of sunlight. Certain phytoplankton, such as coccolithophorids, synthesise dimethylsulphoniopropionate (DMSP), and their enhanced growth increases production.  As this breaks down dimethyl sulphide (DMS) is produced first in seawater, and then in the atmosphere. DMS is oxidised in the atmosphere to form sulphur dioxide an important condensation nuclei.  The water content of clouds increases as a result increasing their reflectivity to incoming solar radiation (enhanced albedo).  The result is less solar heating of the ocean surface and a decrease in phytoplankton production of DMS resulting in a self-regulatory system.

Scientific modelling

Geographical research is not all about direct observations and fieldwork; there is more to geography than standing in the rain measuring things.  Physical and laboratory based experiments all play a part as does modelling.  Modelling involves the conceptual or numerical simulation of a series of processes that give rise to something like a landform or ecological species distribution.  Modelling is a central to our understanding of many geographical phenomena; it lies at the heart of the weather forecast on your smart phone or TV.

There is a range of different opinions as to what actually constitutes a ‘model’ but the simplest way of looking at it as any construct which generates a prediction. It follows that modelling, like experimentation and observation, is simply an activity that enables theories to be tested and examined critically.  Here are a just a few different types of geographical model:

  • Conceptual models. These are theoretic expressions, often in cartoon form, of the key variables involved in a particular process.  They are designed to scope, inform, communicate and help the reader frame ideas for testing.  Figure 2 is a conceptual model.
  • Physical models. Flumes and wave machines are used sometimes to test coastal or flood defences.  Here reality is scaled down and observed in microcosm.
  • Empirical/statistical models. These are based on observed data and attempt to look for causal relationships between several variables.  Lots of data is collected and statistics are used to pull out salient points or relationships.
  • Numerical models. We can use physics and maths to create a set of equations which describe elements of a process.  For example, we can describe soil strength or the rate of glacier flow via simple equations.  These types of models can vary along a continuum from those that are specifically grounded in a time and place to those that are more abstract.  If we ground and parameterise these equations with data from a specific place or time we can attempt to model and thereby reconstruct (or predict) phenomena at a different time.  For example, we might try to create a model of an ice sheet that once covered Britain and vary the input parameters to see how it behaves.  The experiment here is to find the input parameters that best recreate what we think happened in the past.  Alternatively we may create a predictive model such as a general circulation model (GCM) of the Earth’s atmospheric systems and their interaction with land and sea in order to forecast the weather.  The predictions made by the model become the test of its accuracy.  At the other end of the continuum are more abstract models that are not grounded in a specific place and time but allow us to explore more theoretical relationships between the variables involved.
  • Agent based models. This type of model is a form of numerical model which empower individual elements with the power to make decisions.  Imaging a large crowded stadium and someone needlessly shouts ‘bomb’.  Each member of the crowd is capable of make its own decision; to you run left, right or duck.  Perhaps less dramatic is to think of a herd of wildebeest migrating on the Serengeti; in theory each animal can make a choice as to the route it will follow, although it may be highly influenced by other members of the herd.  We can simulate this numerically by giving each agent in the model an individual decision making capacity according to a set of pre-set rules and step back and observe.  There are lots of ecological and geographical systems that lend themselves to this type of modelling.


Modelling has played an important role in the debate about Gaia.

Lovelock and colleagues developed a conceptual model of a self-regulating world, known as Daisy world.  They were not modelling the Earth specifically simply illustrating how self-regulation might work.

diamodelFigure 2: Conceptual illustration of daisy world.  Think about the distribution of daises and why they occur where they do.


Daisy world is a very elegant model (Fig. 2).  To start with they imagine a world inhabited by just two types of daisy – black and white ones – warmed by a sun similar to our own that starts cold and gradually warms up.  Now at first the planet is cold and black daisies thrive since their black pigmentation allows them to absorb more radiation than their white counter parts which tends to reflect solar radiation (i.e. white daisies have a higher surface albedo or reflectance).  The planet is warmer than one might expect, but as the solar constant rises the black daisies begin to be scorched by the sun.  Progressively white daisies thrive, since they can reflect some of the excess heat and keep the planet cooler.  Planetary temperature is moderated in this way as the solar constant changes over time.  It is an elegant illustration of a thermal regulation without any teleological need (i.e. need for sentient thought).  The model can be made more complex by the introduction of grey daisies which thrive in intermediate conditions and by introducing grazing animals and then predators (Fig. 3).

reguFigure 3: Output from Daisy World model showing how temperature is regulated.

The model does not recreate reality or simulate a process that actually took place on Earth, but provides a conceptual illustration or demonstration of the principles of self-regulation and as such it is a supremely elegant piece of science communication.

Interestingly you can run a daisy world simulation for yourself using an agent based model (http://ccl.northwestern.edu/netlogo/models/Daisyworld).  While the original daisy world model was not an ABM the idea lends itself to individual based modelling and is a nice way of demonstrating the potential of such modelling approaches.

Composing computer programs to solve scientific problems is like writing poetry. You must choose every word with care and link it with the other words in perfect syntax. There is no place for verbosity or carelessness. To become fluent in a computer language demands almost the antithesis of modern loose thinking.

James Lovelock

Temperature regulation

So we know – or at least I hope you do – that daisies are not the answer just the illustration.  This still leaves the question of how the earth has moderated its temperature.  The simple answer is through plate tectonics.

One of the most important volcanic product is carbon dioxide; in terms of volume and impact it is far greater than anything else produced by an eruption.  The longest chains of volcanoes are those found at divergent plate boundaries, such as the mid-Atlantic ridge.  Large volumes of carbon dioxide are produced by sea floor spreading and at subduction zones.  So periods of geological time like the Mesozoic which are known to have been particularly warm are often associated with periods of active plate tectonics in this case the opening of the South and North Atlantic.

So if volcanism increases the carbon content and drives greenhouse conditions, what removes carbon dioxide?  Oceans absorb carbon dioxide but by far the most significant process is weathering.  Most rock forming minerals are silicate based and are easily attached by weak acid rain.  As rain fall it combines with carbon dioxide in the atmosphere and critically in the soil to form a weak carbonic acid (CO2 + H2O – H2CO3).  This weak acid attacks the silicates to give a carbonate which is removed in solution and is ultimately deposited in the oceans.  In the equation below X stands for any cation like sodium, potassium, or magnesium which are common rock forming elements.


So weathering removes, or scrubs, carbon dioxide from the atmosphere.  The more weathering the more carbon dioxide is removed.  No weathering is favoured by warm and damp conditions and by surface area.  Think of a flat hill top, exposed rock is weathered and a weathered regolith is produced (add organic matter and you would have a soil).  As the regolith builds up it begins to slow the weathering rate since the fresh rock is increasingly at depth.  Now contrast this with a steep slope under similar conditions.  Here the regolith as it forms is removed by gravity and fresh bedrock remains at the surface to be weathered.  More weathering will result than on the fat surface.  So uplift and mountain building, both a consequence of plate tectonics, favour weathering.  So if we have lots of plate convergence creating fold mountains and uplift we tend to draw down atmospheric carbon dioxide.

thermFigure 4: The feedback between carbon dioxide, global temperature and weathering.

We are not sure what came before plate tectonics when the earth was very molten and very young, perhaps the first 0.5 billion years or so.  But with the advent of plate tectonics came movement and with it the creation of continental crust.  This was probably formed by the accretion of volcanoes along continental edges and their erosion to create sediments.  Slowly by the processes of subduction at convergent boundaries continental rocks were formed.  We call these ancient pieces of crust cratons or shield areas.  The effect was to create land surfaces for weathering drawing down carbon dioxide rapidly.  Not only was the process creating land for future life but also terra forming the atmosphere.

The faint sun in the early days of the Earth was countered by a thick greenhouse atmosphere, helping to retain heat.  As the crust and weathering kicked in the carbon dioxide levels began to fall rapidly.  Terra forming by bacteria and algae boosted oxygen levels at the expense of carbon dioxide.  As the solar constant increased the earth needed less greenhouse gases.  The complex interaction between volcanoes, plate tectonics, weathering and the evolution of life is responsible for maintaining global temperature in a equable state.  It is perhaps odd to think that life of earth was dependent on greenhouses gases.

Final word: to live or not?

Lovelock and his colleagues were brilliant agents of ‘spin’.  The choice of Gaia and the implication that the Earth was something to be revered as a living system was inspired.  Unfortunately it also caused many more staid scientists to shake their head and shrouded the scientific value of their core ideas in a lot of mystic metaphysical nonsense.  It all depends on how you define whether something is living.

They never set out to create a teleological model or to imply that Earth processes were driven by a higher ‘purpose’ simply to demonstrate how a complex system could self-organise.  Complex systems whether living or not can result in emergent behaviours in which the sum is greater than the component parts.

What distinguishes a Complex Adaptive System (CAS) from a pure multi-agent system (MAS) is the focus on system survival and characteristics such as self-similarity, complexity, emergence and self-organization. A MAS is defined as a system composed of multiple interacting agents but in a CAS, the agents as well as the system are adaptive. Complex Adaptive Systems are therefore characterised by a high degree of adaptive capacity, giving them resilience in the face of perturbation.  The Earth is an example of a CAS, resilient in the face of perturbation, a collection of self-similar agents adapting in ways that the system behaviour emerges beyond the behaviours of the individual agents.  The behaviour of the collective is not predicted by the behaviour of the components.


Charlson, R. J., Lovelock, J. E., Andreae, M. O. and Warren, S. G. (1987). “Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate”. Nature. 326 (6114): 655–661. Bibcode:1987Natur.326..655C. doi:10.1038/326655a0.

Lovelock, J.E. (2000) [1979]. Gaia: A New Look at Life on Earth (3rd ed.). Oxford University Press. ISBN 0-19-286218-9.

Lovelock, James (2007). The Revenge of Gaia. Penguin. ISBN 0-14-102597-2.