Discussions of climate change distinguish climate from weather, usually early on, and usually without much analysis. Thus weather is what we are aware of locally, day to day, what we experience: hot wet, cold dry, sunny cloudy, windy calm, frost snow ice, and so on. Weather is tangible and perceptible, but climate is not, not directly at any rate. Climate, or the climate, is what the weather is like on the whole, or in general. Since the weather changes with the seasons, unless one lives on the equator, one obvious way to describe the climate is with reference to the annual weather. So the prevailing climate at a particular place is what the weather is like on the whole over the year. This could be characterised objectively in terms of average monthly maximum and minimum temperature, rainfall, etc., or in evaluative terms, like “Sydney has a very pleasant climate”.
Average weather is caclulated on the basis of available data, on measurements of temperature, rainfall, etc., with some such data being more reliable and accurate that others. And for some locations there will be much more data available than at others, so the averages will be computed over readings for many more years – in Oxford for instance, records are available for 250 years. It is possible that (after the first year of recordings) the actual weather is never coicident with the average weather, and that outcome becomes more likely the more accurate the readings. For instance, suppose temperature is measured to a thousanth of a degree – like 23.031 C - this could be the average maximum for July in Oxford, while no July day for 250 years has ever been just this hot. In fact it is possible, even likely with very accurate readings, that the actual weather for a given year, in terms of temperature and rainfall for instance, never coicides with average on any day or month. Does this means that climate change is going on all the time because the actual weather differs from ‘the climate’? It certainly means that the climate varies a lot, but we want to reserve the term “climate change” for something else.
If the annual climate varies but the average does not itself change significantly, then we should say that the climate does not change. It is only if the average begins to change over a number of years that we should say that climate change is taking place. For the average to change, the annual variations must be in the same direction, that is, it is getting hotter or colder, or wetter or drier, year by year, otherwise, the average weather will stay roughtly the same. So climate change is long term change in the weather, but how long is ‘long term’? Droughts, periods of below average rainfall, are often accompanined by higher than average temperatures, and can last for long periods, sometimes decades. But if the annual rainfall returns to average and stayed there, and if droughts are periodic and never get worse, again I think we should not identify these as manifestations of climate change. The important qualification here is “periodic”; climate variation on a ten or so year time scale looks a lot more like climate change than annual variation, but if the climate does return to the average, I think it could be a mistake to say that this is an instance of climate change.
There are radical cyclical climate variations. Over the past million years, there have been ice ages about every eighty thousand years – at present we are twenty thousand years into an interglacial period. Ice ages are caused by changes in the Earth’s orbit which reduces the amount of solar radiation reaching the Earth (for more on this, see next two posts), particularly affecting the northern hemisphere, which results in ice sheets covering Canada, half of the United States, Northern Europe, etc. Do ice ages counts as genuine climate change, or are they mere climate variation, given that they are cyclical? It would be strange to say that such a radical change in the weather as an ice age is mere climate variation. Moreover, at the end of an ice age, when the glacier’s retreat, the climate does not return exactly to the way it was – as far as can be inferred from the available. Also, there is an explanation for the change. As a first attempt at a definition of climate change, we might therefore suggest:
(E) When variation in the climate has an underlying explanation, this is climate change.
The periodic droughts in Eastern and Northern Australia have been explained with reference to the heating of the Western Pacific near the equator. This is known as the El Niño Southern Oscillation (ENSO), which is defined as a temperature rise of 0.8 degrees above average (a more recent, and increasing more preferred, alternative explanation refers to warming of the Indian Ocean and is known as the Indian Ocean Dipole, but I will use the more familiar ENSO explanation as my example). This causes a change in the prevailing rain-bearing winds and so results in reduced rainfall in Australia. If the water temperature drops back to average in the Western Pacific, then according to this account, the prevailing winds will return and so will the rain. However, (as far as I know) there is no account of any long term ENSO trends, nor of the precise mechanisms which explain their occurrence. If these events were known to be essentially periodic, then we should say, as above, that this is climate variation. But if ENSO events are getting more severe and longer lasting, with the average temperature of the Western Pacific getting warmer, then even in the absence of a detailed mechanism as to why this is the case, then we should say that it is climate change. Again, the criterion being applied here is that if the weather is not returning to what is had been, then there is a case for climate change, and what we are trying to do is to make the nature of this case more precise. If droughts are becoming more severe owing to deeper ENSO events, then this would then be an instance of what I will call local climate change.
The present (I am writing this in December 2019) drought in Austrlia is the worst on record. Weather events in other parts of the world this year have also been unprecented. For instance, there were two Category 5 Altanic hurricanes, one of which, Dorian, was the worst natural disaster in history to befall the Bahamas, and Lorenzo, which was the Easternmost Cateory 5 storm on record. This year is the fourth in a row of above-average hurricane seasons in the Atlantic – and of course with more above average years, the average is pushed up. This is also local climate change, which prompts the questions: “Are these local climate change phenomena related”, and, “Is this because of global climate change?”, and,”Is global climate change like local climate change, but on a global scale?” To answer the last question first, if the local climate is a function of rainfall, temperature, etc., in the locale in question, then global climate, one supposes would also be given with reference to these quantities: changes in average temperature, rainfall, and so on. Terms like “drought” and ‘flood” have evaluative overtones and signify the impact of the local climate. The first two questions are hard to answer and I shall not try to do so yet.
Temperature is the variable that is used when it comes to tracking global climate change, and for the following reason: what ultimately determines the climate, both local and global, is the interaction of Earth as a whole with radiant energy from the Sun. How much radiant energy is absorbed by the planet, and how this happens, determines the way our climate changes. The average global temperature is the critical value. Average rainfall is obviously important and places where it rains a lot will tend to be cooler than places where it does not. And, as we shall see, water (vapour) is a geenhouse gas, so the more clouds, the greater greenhouse effect due to water molecules. The amount of water on the planet is basically stable, in the sense that there is no external source of water, although where the water is and what form it takes changes all the time. But heat is different, because there is an external source of heat. Temperature is a measure of the level or amount of heat contained in an object or system, and, as everyone knows, the hotter something is, the higher its temperature. Objects which are hotter than their surroundings give up heat and become cooler. As we shall see, the only way the Earth as a whole can give up heat is to radiate it back into space, space being ‘the surroundings’. But, as we shall also see, increasing concentrations of greenhouse gases inhibit this process, and the planet heats up.
According to NASA, since 1880 the global average temperature of the Earth has risen by 0.8 ⁰ C, with two thirds of this increase occurring since 1975. Leaving aside the significance of what seems on the face of it a small increase, let us ask again whether this is actually evidence of climate change, or if it is simply a variation and the global temperature will fall by the same (or a greater) amount. This is genuine climate change. It is genunine because global warming is caused by increasing and seemingly irreversible concentrations of greenhouse gases, and why that is is well-understood – as I will show in the next two posts. So, we can reformulate our definition as:
Climate Change (CC): Climate change occurs when there is a persistent change in the weather which is caused by corresponding (unidirectional) change in the underlying determinants of the weather, such as temperature.
The essential difference between E and CC is that the later refers to unidirectional change. If there is an underlying explanation of periodic ENSO events, then these events would not count as climate change unless they were seen to be becoming more severe. CC does not state that climate change is global, and so it allows for local climate change, such as ENSO events becoming more severe or North Atlantic hurricanes becoming more frequent, stronger, etc. With regard to global climate change, the consensus is that temperature is the relevant measure, and the challenge is to show how increased averaged temperature causes local climate change.
This definition does not make any mention of climate change being bad for us, which is a good idea because it is not always bad for us. A period known as the Classical Optimum began in 300 BCE and lasted for about 600 years and was characterised by warm and stable weather in the Mediteranean and Europe, which enabled regular and plentiful harvests. This was due to a persistent and postive phase of the North Atlantic Oscillation, and while this appears to be an instance of local climate change, some have suggested it had global effects. In contrast, the Little Ice Age, which began about 1300 and was due to variation in solar activity, was clearly global in reach, and was characterised by below average temperatures. This caused periodic famines from Europe to China. There is no a priori reason to think that climate change must necessarily be bad for us, and it is therefore a point in favour of CC that it does not have that connotation. CC may not be the final version of our characterisation of climate change, but it will do for the time being.