Staggering Through Time
The Earth’s climate history has been like a drunken stagger home after a party; each step almost random but with an underlying directionality. Over the last 3 billion years, the global climate system has frequently gone in unpredictable directions but there has also been a distinct trend towards gradual cooling. This chapter takes a closer look at the causes of the lurching and likely reasons for the underlying cooling.
By far the most significant environmental change during this 3 billion-year period was the rise of oxygen. The massive increase in the atmospheric concentration of this initially highly toxic gas had profound consequences for all living organisms, but it also had serious climatic consequences. At the beginning of the Proterozoic our planet was at least as warm as today, and possibly much hotter, even though the fainter Sun produced only about 85 per cent of the heat it does now. This unexpectedly high temperature can only have been because the Earth was less reflective or because it had a bigger greenhouse effect. The most likely explanation is higher greenhouse warming due partly to higher carbon dioxide levels and partly to higher levels of atmospheric methane, which would have been produced in vast quantities by the kinds of bacteria that dominated the Earth at that time. However, high concentrations of methane cannot co-exist in the atmosphere with oxygen since they react together to produce water and carbon dioxide. So, it’s quite likely that a falling greenhouse effect and rising oxygen levels are linked: as the oxygen concentration rose, methane reacted with it and levels of that gas fell. But where did the oxygen come from? That is a more complex story than you might at first imagine.
Oxygen is produced by plants during photosynthesis, the process by which they use the energy from sunlight to manufacture carbohydrates, the basic foodstuff of all life. However, plants evolved on Earth less than 500 million years ago and were therefore not the source of oxygen in the more distant past. The first photosynthetic organisms were bacteria, as we saw in Chapter 2, and they evolved even before the Proterozoic began 2.5 billion years ago. Carbohydrates, as the name implies, are made from carbon, hydrogen and oxygen. The photosynthetic microbes took carbon and oxygen from carbon dioxide dissolved in the sea but they also needed a source of hydrogen. Early photosynthetic bacteria got this from hydrogen sulphide gas emerging from volcanic vents on the sea floor. The sulphur atoms attached to this hydrogen were largely surplus to requirements and these organisms therefore excreted sulphur and sulphur compounds into the environment.
The shallow Archaean (the name given to this early Eon by geologists) seas must therefore have been painted in the vivid red, orange and yellow colours of sulphur and sulphur compounds, but 700 million years later, those seas suddenly turned green. Microbes had arisen that got their hydrogen fix from hydrogen oxide instead of hydrogen sulphide and they used chlorophyll, the green pigment found in all plants, to do so. Hydrogen oxide is better known as water, and although it is by far the most plentiful source of hydrogen on our planet, it takes a lot of energy to split this compound into its constituents. Water is a tough hydrogen source to use and it was a long time before any organisms appeared that could do it; but once the trick evolved, the bacteria using it thrived and started to dump the useless, to them, oxygen compounds into the environment. A little under 3 billion years ago the world became dominated by the first organisms to give off pure oxygen gas, cyanobacteria. These photosynthetic micro-organisms are very green and very successful; so successful that they are still common on our planet today.
The evolution of cyanobacteria led to the biggest and most deadly pollution incident in the whole of Earth history because free oxygen was lethal to all existing life. Indeed, this may be why micro-organisms initially evolved this chemical curiosity; it allowed them to kill off competitors. Evolution is full of examples of important innovations whose original purpose was quite different to their final one. Free oxygen emission may have originally evolved as a weapon in the constant chemical warfare between micro-organisms and only later became the basis of nearly all food manufacture on the planet. As we have seen, the free oxygen also had the potential to cause massive climate change. The climatic effects weren’t too dramatic to start with. The very reactive oxygen combined with volcanically produced minerals, such as iron, dissolved in the sea and this kept atmospheric levels of oxygen down. Nevertheless, local oxygen levels in the shallow seas rose significantly and the life-styles of many micro-organisms become untenable. Fortunately, microbes are tough, rapidly evolving organisms and forms appeared that were oxygen-tolerant. Some bacteria simply learned to neutralise the oxygen but one group of purple bacteria evolved that used oxygen and carbohydrates as a source of energy. In other words, they ate and breathed as we do. Indeed, it is more accurate to say that purple bacteria still do all the eating and breathing since mitochondria, the oxygen-consuming power stations of plant and animal cells, are almost certainly descended from purple bacteria captured and enslaved for this purpose by the single-celled common ancestor of all plants and animals.
The evolution of breathing organisms provided yet another mechanism that kept oxygen levels low. All the oxygen released by photosynthetic organisms when they made carbohydrates was turned back into carbon dioxide when the photosynthesisers were eaten. Non-photosynthetic organisms, such as humans, get the energy they need for life by doing the reverse of photosynthesis. They combine carbohydrates with oxygen from the air and then breathe out carbon dioxide. Photosynthesisers therefore turn carbon dioxide into oxygen and then oxygen-breathers turn it back into carbon dioxide. Once breathing organisms had evolved there could be no increase in atmospheric oxygen levels; that is, unless some carbohydrate avoided being consumed so that the oxygen released during its manufacture stayed in the atmosphere. Given this, it’s not surprising that the atmospheric oxygen level remained a tiny fraction of its modern value throughout most of Earth’s history. The real puzzle is, why did it ever rise? That brings me to the fascinating topic of mud.
Mud doesn’t sound fascinating to most people. I’ve heard the science of sedimentology described as ‘mud moves from here to there and then from there to here’, which is reminiscent of the classic description of history as ‘just one damned thing after another’. Not very flattering, especially to those of us who have spent careers largely devoted to understanding how mud (and other detritus) is moved around by rivers, currents, waves and tides. My eldest son recently used similar language to dismiss this as the most boring of all possible topics: ‘Mud gets washed into the sea, what’s interesting about that?’ Actually, with that simple statement he hit the nail on the head. Washing of mud into the deep ocean really is interesting! Without the settling of mud onto the deep sea floor, oxygen levels couldn’t have risen in the atmosphere and large oxygen-consuming animals, like us, wouldn’t exist. Mud contains organic material produced by photosynthesisers and some of it gets buried quickly enough in deep parts of the ocean to avoid being eaten. For each atom of carbon, extracted from carbon dioxide during photosynthesis and then buried at sea, there is a molecule of oxygen left behind in the atmosphere. The amount of carbon buried every year is a tiny fraction of the total biomass contained in plants, animals and other organisms, but over geological time it adds up, allowing oxygen levels to grow. Through the Proterozoic the burial rates started to rise, and as reactive gases and minerals were also used up, atmospheric oxygen levels increased. The resulting methane destruction then forced temperatures down. This trend seems to have continued into the Phanerozoic with events such as the rise of land plants acting to accelerate burial of organic-rich muds even further and push temperatures lower still. The world has never been colder, on average, than it has been during the last 30 million years (the snowball Earth periods were quite warm on average since the glaciations were relatively brief).
Declining greenhouse warming, countered by a steadily warming Sun, therefore explains the broad picture of long-term moderate climate change on our planet. But why did temperatures also fluctuate both in the Proterozoic and more recently through the Phanerozoic? Atmospheric composition and solar activity are not the only things that have changed significantly over the long history of our planet. Variations have also occurred in the Earth’s reflectivity because of changes in ice, clouds, seas, continents and plants. Cloud cover, for example, must have changed dramatically through the ages. Cloud formation is strongly encouraged by atmospheric dust, and this dust has probably increased in volume because the Earth’s continents have grown through time, and that’s where dust comes from. More recently, the amount of dust must have changed significantly when the continents were conquered by plants 400 million years ago. In addition, plants encourage cloud formation by increasing the efficiency of evaporation, and cloud cover is also affected by biological activity in the oceans. Some ocean-going microbes produce droplets of chemicals in sea-spray that migrate into the atmosphere and help clouds form. So, the amount of cloud cover has probably gone up and down dramatically over the eons in response to continental growth and a continuously evolving biosphere.
More subtly, the slowly growing continents must have directly changed the Earth’s reflectivity since land is more reflective than sea. Furthermore, the land’s reflectivity altered as it became colonised, first by bacteria, then by lichens, plants and, finally, animals. The position of the land also changed through time as a result of continental drift and this affected the reflectivity through its influence on ice build-up. The world tends to be colder during times, such as today, when there is a large continent at one of the poles onto which a thick ice cap can grow. The position of land also affects ocean currents, and if these are blocked from moving heat from the equator towards the poles, then once again there is a build-up of ice at the poles leading to an increase in reflectivity and a drop in the world’s average temperature. These changes in reflectivity due to varying clouds, varying ice cover, changing land-mass size and changing continental cover could theoretically have caused the Earth’s temperature to alter by as much as 80°C. Add that to the effects of the variation in solar heat output and the potential changes in temperature could have been as much as 100°C.
These calculations ignore the effects of changing greenhouse gases, and these are affected by geological processes as well as by biological ones. For example, when mountain building is intense, weathering also increases and this takes carbon dioxide out of the atmosphere. The rate at which organic carbon is buried is also affected by geological factors such as the location of the continents and the depth of the oceans, and this also causes carbon dioxide levels to fluctuate through time. If we include such greenhouse gas variations, the potential climatic variability of our world becomes enormous. Given this, it really shouldn’t be surprising that the Proterozoic and Phanerozoic experienced fluctuations in temperature. The real surprise is that the temperature changes were so small. Solar luminosity, planetary reflectivity and atmospheric composition have all changed dramatically during the Earth’s 4.5 billion-year history and, in principle, these factors could have conspired to create very inhospitable temperatures on our planet. A completely ice-covered Earth, illuminated by a faint young Sun, and with no greenhouse effect, would have had an average temperature of 90°C below freezing. On the other hand, if the Earth had similar properties to Venus and was warmed by our present Sun, it would have warmed to over 400°C.
However, none of these massive potential changes in temperature ever actually occurred. The coldest our world has been was during snowball Earth episodes when globally averaged temperatures were certainly below freezing and may have been as low as –50°C. At the other extreme it is unlikely that temperatures over the last 4 billion years were ever more than 60°C for long, since this would have led to a runaway greenhouse effect and the Earth would now resemble Venus. Temperatures may have changed by tens of degrees over the billions of years of Earth history but certainly not by the hundreds of degrees of fluctuation that could have occurred. The Earth has therefore had surprisingly stable temperatures for a very long time and the various geological, biological and astronomical influences on climate must therefore have somehow nearly cancelled each other out.
There are three possible explanations for why we happen to live on such a well regulated planet and, with a little poetic licence, these can be characterised as God, Gaia or Goldilocks. God seems like a very straightforward explanation; we live on a planet with good weather because God made it that way. However, I don’t believe that we should resort to transcendental explanations for what we see in the physical Universe around us. History shows science to be better equipped than religion when seeking that particular kind of enlightenment. If you’re looking for God, I wouldn’t start from here.
What of Gaia? Gaia has already been alluded to in this book, but to recap: the Gaia hypothesis postulates that life itself regulates our planet’s climate in a way that ensures the biosphere thrives. Gaian proposals can be thought of as an extreme form of the conventional scientific view discussed earlier that says that climate stability is the result of natural geological and biological feedback mechanisms that automatically moderate climate change. As I mentioned before, this hypothesis may have confused cause and effect but biological feedback control does need to be taken very seriously as an explanation for the continuous good weather our planet has enjoyed.
The final explanation, Goldilocks, is that, just as with the third bowl of porridge from Goldilocks and the Three Bears, the temperature is ‘just right’ purely by good fortune. That, of course, is what this book is about. Perhaps planets where frequent, severe climate catastrophes happen are much more common than worlds with long periods between disasters. Perhaps planets that cool too quickly, or too slowly, are also far more common than planets like the Earth where the effects of solar and Earth evolution happen to roughly cancel each other out. Intelligent observers couldn’t look out onto such worlds because observers would be unlikely to evolve in the impoverished biospheres that would result. Again, we can’t conclude anything about what is normal or natural when considering properties of our planet that were essential preconditions for our existence.
Distinguishing between Gaia and Goldilocks is a major theme in this book. I will come back to Gaia later for a more detailed look before suggesting ways we might tell Gaia and Goldilocks apart. But first, I want to look at one final climate-related topic: the Earth’s recent ice ages.