Greenhouse gases and climate change

Weather over the last 800 millennia

Richard McIntyre MB BS (Monash), D Phil (Oxon), FRACS, owner and winemaker of Moorooduc Estate in the Mornington Peninsula, contributes the following to the debate on climate-change effects on Australian wine. It includes extracts from 'Postlude: the amplifier metaphor for climate change', the last section of Lucidity and Science: The deepest connections, an e-book by his brother Michael E McIntyre, a Cambridge academic.

My initial incentive to write this document was a feeling of frustration about the public discussion in Australia concerning climate change. Not only do we have a federal government led by, it would seem, deniers of climate change caused by human activity but we have otherwise excellent journalists who seem unable to ask appropriate questions of these politicians. There is also political pressure on media to adopt ‘balanced’ reporting, even if this means giving equal consideration and air time to opposing views when one of them is absurd, as with the views that ‘the earth is round’ v ‘the earth is flat’.

It is difficult for a lay person to have informed opinions on complex matters such as climate change. Having read Michael’s book, I now have a much clearer understanding of these crucial matters.

(Michael is my brother. He is an Emeritus Professor of Applied Mathematics at the University of Cambridge. Although his research group has never received funding for climate research as such, his work on atmospheric fluid dynamics, including the dynamics of jet streams, has made him a close observer of developments in climate science over several decades. He was awarded the Carl-Gustaf Rossby Research Medal in 1987 by the American Meteorological Society for ‘his original and innovative works furthering our theoretical and conceptual understanding of the stratosphere’. The Rossby Medal is the highest award the Society can bestow upon an atmospheric scientist. Michael is also a Fellow of the Royal Society of London, and was closely involved in sorting out the cause of the hole in the ozone layer.)

The following is presented in the form of questions on climate change that a lay person might ask together with answers.

My questions and comments are in standard script and represent the interested lay person. Key questions are in bold. Quotes from Michael’s e-book are indented, and represent informed, expert opinion.

What should interested lay people (which include many of the voting public, our politicians and other influential people such as journalists) know about the science of climate change? How can we sort out the truths from the myths? We might be facing an existential crisis and surely, if we are to minimise the consequences, the more of us who have some understanding of what is happening and why, the greater the chance that concerted action will be taken.

Here is Michael’s introductory paragraph in the last chapter (‘Postlude’) of his e-book Lucidity and Science.

Journalist to scientist during a firestorm, flash flood, or other weather extreme such as Cyclone Idai, Hurricane Dorian or Typhoon Hagibis: ‘Tell me, Professor So-and-So, is this a one-off extreme – pure chance – or is it due to climate change?’ Well – once again – dichotomisation* does make us stupid, doesn't it? The professor needs to say ‘Hey, this isn't an either-or. It's both of course. Climate change produces long-term upward trends in the probabilities of extreme weather events, and in their intensities.’ This point is, at long, long last, gaining traction as devastating weather extremes become more frequent and more intense.’

The climate system is – with certain qualifications … a powerful but slowly-responding amplifier with sensitive inputs.

One of these sensitive inputs is carbon dioxide (CO2) which is especially important.

What are greenhouse gases and what is the ‘greenhouse effect’?

Greenhouse gases are good. They help make the earth more hospitable to life. They keep some of the heat energy from the sun from being lost into space. This is the ‘greenhouse effect’ which we learnt about at school (many years ago).

The physical and chemical properties of so-called greenhouse gases are well established and uncontentious, with very many cross-checks. Greenhouse gases in the atmosphere make the earth's surface roughly 30 °C [54 °F] warmer than it would otherwise be. For reasons connected with the properties of heat radiation, any gas whose molecules have three or more atoms can act as a greenhouse gas. Examples include carbon dioxide, water vapour, methane, and nitrous oxide. By contrast, the atmosphere's oxygen and nitrogen molecules have only two atoms and are very nearly transparent to heat radiation.

Why is carbon dioxide (CO2) so important?

One reason for the special importance of carbon dioxide is its great chemical stability as a gas. Other carbon-containing, non-condensing greenhouse gases such as methane tend to be converted fairly quickly into carbon dioxide. Fairly quickly means within a decade or two, for methane. And of all the non-condensing greenhouse gases, carbon dioxide has always had the most important long-term heating effect because of its chemical stability, not only today but also during the glacial cycles. That's clear from ice-core data, to be discussed below, along with the well-established heat-radiation physics.

What about water vapour? There’s so much more of it. So surely it’s far more important than carbon dioxide?

Water vapour has a central but entirely different role. It too is chemically stable but, unlike carbon dioxide, can and does condense or freeze, in vast amounts, as well as being copiously supplied by evaporation from the tropical oceans. This solar-powered supply of water vapour – sometimes called ‘weather fuel’ because of the ‘latent’ thermal energy released on condensing or freezing** – makes it part of the climate amplifier's power-supply or power-output circuitry rather than its input circuitry. Global warming is also global fuelling, because air can hold just over 6% more weather fuel for every degree Celsius rise in temperature. The power output includes tropical and extratropical thunderstorms and cyclonic storms, including those that produce the most extreme rainfall, flooding, and wind damage. The energy released dwarfs the energies of thermonuclear bombs. Cyclone Idai, which caused such devastation in Mozambique, and Hurricane Dorian, which flattened large areas of the Bahamas, and other recent examples, remind us what those huge energies mean in reality. Extremes go in both directions because more weather fuel makes the whole climate system more active and vigorous, and its fluctuations larger, including fluctuations coming from the amplified meandering of jetstreams, such as the ‘Beast from the East’ that brought extreme winter conditions to the UK in late February 2018. Extremes become more extreme because a storm powered by more weather fuel can suck the fuel up faster – a positive feedback in the run-up to peak intensity.

Extreme weather events in the early stages of global warming have long been predicted. (For example, in Tim Flannery’s We Are The Weather Makers: The story of global warming, The Text Publishing Company, 2006.) We are seeing these all the time but the climate-change deniers seem not to have noticed.

‘Climates have been changing for thousands of years’. So I have heard, as if this dismisses the issue. If the greenhouse effect is good and the climate is changing all the time, what’s all the fuss about?

The fuss is about the speed of change. Most climate change in the past has been much more gradual than the current one threatens to be, generally allowing evolutionary adaptations to take place.

Atmospheric CO2 levels have fluctuated between roughly 180 and 280 ppmv (parts per million by volume) over the last 400,000 years until the industrial revolution. (We have an 800,000-year record from Antarctic ice core samples which indicate global temperatures and atmospheric CO2 levels, but only the most recent half of it shows the full 100 ppmv range – see the diagram below.) CO2 levels have now risen past 400 ppmv and continue to rise. This much faster time scale (since the industrial revolution) corresponds with humans burning more and more fossil fuels. Coincidence? I don’t think so.

Weather over the last 800,000 years
Antarctic ice-core data from Lüthi et al (2008) (bibliographical reference at the end of this article) showing estimated temperature (upper graph) and measured atmospheric carbon dioxide (lower graph). Time, in millennia, runs from right to left up to the present day. The significance of the lower graph is discussed in the Postlude. The upper graph estimates air temperature changes over Antarctica, indicative of worldwide changes. The temperature changes are estimated from the amount of deuterium (hydrogen-2 isotope) in the ice, which is temperature-sensitive because of fractionation effects as water evaporates, transpires, precipitates, and redistributes itself between oceans, atmosphere, and ice sheets. The MIS numbers denote the ‘marine isotope stages’ whose signatures are recognised in many deep-ocean mud cores, and T means ‘termination’ or ‘major deglaciation’. The thin vertical line at around 70 millennia marks the time of the Lake Toba supervolcanic eruption.

The first point to note is that human activities are increasing the carbon dioxide in the atmosphere by amounts that will be large.

They will be large in the only relevant sense, that is, large by comparison with the natural range of variation of atmospheric carbon dioxide with the Earth system close to its present state. The natural range is well determined from ice-core data, recording the extremes of the hundred-millennium glacial cycles. That's one of the hardest, clearest, most unequivocal pieces of evidence we have. It comes from the ability of ice to trap air, beginning with compacted snowfall, giving us clean air samples from the past 800 millennia from which carbon dioxide concentrations can be reliably measured.

In round numbers the natural range of variation of atmospheric carbon dioxide is close to 100 ppmv, 100 parts per million by volume. The increase since pre-industrial times now exceeds 120 ppmv. In round numbers we have gone from a glacial 180 ppmv through a pre-industrial 280 ppmv up to today's values, just over 400 ppmv. And on current trends the 400 ppmv will have increased to 800 ppmv or more by the end of this century. An increase from 180 to 800 ppmv is an increase of the order of six times the natural range of variation. Whatever happens, therefore, the climate system will be like a sensitive amplifier subject to a large new input signal, the only question being just how large – just how many times larger than the natural range.

Why do so many of our leaders dismiss the concept of human-induced climate change linked to increasing levels of CO2 in the atmosphere? The vast majority of scientists accept the concept. Why would any sane person not at least take out some ‘insurance’ against the possibility that it is true? Would you not insure your house against fire until you were 100% convinced that it would burn down? Why do we not embrace moving to existing and rapidly developing technologies that do not involve burning fossil fuels, especially now that the new technologies are becoming cheaper than fossil fuels? After all, ‘the Stone Age did not end because we ran out of stone’ – not my original idea, but well worth repeating.

Michael believes that vested interests have been running a disinformation campaign similar to campaigns associated with the links between tobacco and lung cancer and the cause of the hole in the ozone layer.

In recent decades there's been a powerful disinformation campaign that’s created yet more confusion about climate … For me it's a case of déjà vu, because the earlier ozone-hole disinformation campaign, which I encountered at close quarters, was strikingly similar.

We now know that that similarity was no accident. According to extensive documentation cited in Oreskes and Conway (2010) (bibliographical reference at the end of this article) – including formerly secret documents now exposed through anti-tobacco litigation – the current climate-disinformation campaign was seeded, originally, by the same few professional disinformers who masterminded the ozone-hole disinformation campaign and, before that, the tobacco companies' lung-cancer campaigns. The secret documents describe how to manipulate the news media and sow confusion in place of understanding. For climate the confusion has spread into some parts of the scientific community, including a few influential senior scientists who are not, to my knowledge, among the professional disinformers and their political allies but who have tended to focus too narrowly on the shortcomings of the big climate prediction models, ignoring the many other lines of evidence.

The scientific method has been an extraordinarily powerful tool that is widely misunderstood. Non-scientists often think that scientists think they know everything when in fact, the opposite is the case. Not only is the recognition of ignorance fundamental to science, but every new understanding in science generally opens up many more questions. The more I know, the more I don’t know. Michael's e-book is careful to distinguish between what is practically certain about climate change, and what is less certain, such as how close we are to a tipping point.

When the term ‘tipping points’ is used in the context of climate change, what does it mean?

Here we are talking about ‘feedback mechanisms’.

Biological systems, for example, need to have relatively stable conditions to function. For these stable conditions to be achieved, ‘negative feedback’ is required. One illustration is when conditions are very warm, we mammals are able to counteract this by cooling our bodies – dogs pant, humans sweat.

‘Tipping points’ are conditions where a fast ‘positive feedback’ mechanism is initiated. One example here relates to methane gas trapped in frozen ground, ‘permafrost’. As temperatures rise due to rising levels of greenhouse gases, the frozen ground melts, releasing increasing amounts of methane which increases the amount of greenhouse gases in the atmosphere. This further increases greenhouse warming, causing more melting and the release of more methane and so on. This is one of the ways in which the climate system behaves like an amplifier. A ‘tipping point’ involves a relatively sudden increase in input sensitivity.

Are there other factors that contribute to changes in the climate?

What about changes in the Earth's orbit around the Sun and the tilt angle of the Earth's axis of rotation?

These become important when we look at the climate system on far longer timescales and, as we will see, even though small, they can initiate significant changes in the climate under certain circumstances.

… small orbital changes are well known and can be calculated very precisely over far greater, multi-million-year timespans, thanks to the remarkable stability of the solar system's planetary motions. The orbital changes include a 2° oscillation in the tilt of the Earth's axis (between about 22° and 24°)…

The orbital changes together cause fluctuations in the solar heating at high northern latitudes, with peaks and troughs following each other every 11,000 years or so. One gets a peak when the Earth is closest to the Sun in northern midsummer. About one in five of the peaks, over the last 400,000 years, corresponds to the start of a period of deglaciation from an ice age, in which sea levels rise by well over 100 metres. Why only one in five? The reason is that the system needs to be fully ‘primed’ for deglaciation to be the result. Two priming mechanisms occur in the coldest conditions.

One is that the system became more sensitive when it was fully primed for the next big carbon dioxide injection. To become fully primed it needed to store enough extra carbon dioxide in the deep oceans. Extra storage was favoured in the coldest conditions, which tended to prevail during the millennia preceding full deglaciations…

Also important was a different priming mechanism, the slow buildup and areal expansion of the northern land-based ice sheets. The ice sheets slowly became more vulnerable to melting in two ways, first by expanding equatorward into warmer latitudes, and second by bearing down on the earth's crust, taking the upper surface of the ice down to warmer altitudes. This ice-sheet-mediated priming mechanism would have made the system more sensitive still.

What about the Sun?

I have heard it said with great confidence that the warming climate is caused by sunspot activity (on the Sun). Like most people I have no knowledge of my own to refer to on this topic so I will defer to an informed scientist, my brother Michael, who as it happens has worked on solar physics as well as on phenomena in the earth's atmosphere. Prior to the following passage, he writes about the increasing power output of the Sun, increasing by about 1% every one hundred million years. And on shorter timescales:

Variability in the Sun's output on timescales much less than millions of years comes from variability in sunspots and other magnetic phenomena. These phenomena are by-products of the turbulent fluid motion caused by thermal convection in the Sun's outer layers. That variability is now known to have climatic effects distinctly smaller than the effects of carbon dioxide injections to date, and very much smaller than those to come. The climatic effects from solar magnetism include not only the direct response to a slight variability in the Sun's total power output, but also some small and subtle effects from a greater variability in the Sun's ultraviolet radiation, which is absorbed mainly at stratospheric and higher altitudes... Controversially, there might be an even more subtle effect from cloud modulation by cosmic-ray shielding. But to propose that any of these effects predominate over greenhouse-gas heating and even more that their timings should coincide with, for instance, the timings of full deglaciations … would be to propose something that's … overwhelmingly improbable.

In response to the question ‘What’s all the fuss about?’ I suggested that the fuss was about the speed of climate change.

That was an incomplete answer because, at whatever speed we warm the climate, we could be creating highly undesirable conditions for human civilisation, with sea levels for instance ending up about 70 m [230 ft] higher than today, as they were in the early Eocene.

The Eocene epoch (56 to 34 million years ago) was a time when a large amount of volcanic activity was likely to have resulted in high atmospheric CO2 levels, maybe in the thousands of ppmv. (Unlike the most recent 800,000 years, we have no direct data to confirm CO2 levels as such, though there are plenty of data to show that temperatures were much higher than now, with no great ice sheets at all and sea levels much higher, in the order of 70 metres.) If CO2 levels continue to rise, it is a real possibility that conditions similar to those that existed in the Eocene epoch could happen again.

Going into a new early Eocene or extreme ‘hothouse Earth’ would mean first that there'd be no great ice sheets at all, even in Antarctica, second that sea levels would be about 70 metres higher than today ... and third that cyclonic storms would be far more powerful than today... It's therefore no surprise that some land-based mammals found it useful to migrate into the oceans around the time of the early Eocene...

Within ... several million years, some of them had evolved into fully aquatic mammals like today's whales and dolphins. Selective pressures from extreme surface storminess – storms far more extreme, far more frequent, and far more widespread than anything seen today – can make sense of those extraordinary evolutionary events.


If you have found the above interesting, informative and/or thought-provoking, I encourage you to read some or all of Michael’s book, especially the postlude, and references in Chapter 2 to Dansgaard-Oeschger cycles. (These are transient, abrupt warming events recorded over the last 50,000 years in Greenland ice cores.)

The book covers these and other subjects in great detail as well as having an extensive bibliography.

An unusual aspect of the climate-change conversation is that, unlike other complex, specialised subjects, a wide range of people with little or no credentials to do so have strongly held views which they wish to impose on the rest of us. (Who would, for example, seek advice from a lawyer or an accountant on how to manage a brain tumour?)

Perhaps, contributing to this state of affairs has been the brief to the IPCC (Intergovernmental Panel on Climate Change) to come up with ‘definite predictions’ of the incredibly complex system, using climate (computer) models that, according to Michael, are useful in some ways but cannot, for instance, simulate the full complexity and, in particular, extreme storminess – which requires far more than the available computer power.

Michael believes that these big climate models are getting better but he thinks we should pay more attention to the longer-time-scale patterns as exemplified by the 800,000-year records from Antarctic ice cores, and many other palaeoclimatic lines of evidence.

We need our political leaders, our journalists and, most of all, our voting citizens to have a better understanding of the high risk to the future of our species, and many other plants and animals on this planet, by continuing to harvest and burn fossil fuels, especially when much better, safer, and cheaper technologies are here or on the near horizon.

Remember, the Stone Age did not end because we ran out of stone.

Notes and references

Oreskes, N, and Conway, E M, 2010: Merchants of Doubt: How a Handful of Scientists Obscured the Truth on Issues from Tobacco Smoke to Global Warming, Bloomsbury, 2010

Lüthi, D, et al, 2008: 'High-resolution carbon dioxide concentration record 650,000–800,000 years before present', Nature 453, 379–382

*Michael is referring here to 'the dichotomisation instinct, the primitive binary button that journalists and politicians keep on pressing to get us polarised [a phenomenon that may be familiar to anyone living in Brexit Britain – JR]. Dichotomisation, in this sense, is the visceral urge to see only two opposing choices and to decide between them without stopping to think, an ancient survival instinct – as with fight or flight, edible or inedible, male or female, friend or foe – but today an obstacle to clear thinking.’

**The technical term for this thermal energy is ‘latent heat’, the heat released when water vapour condenses to make water, or when liquid water freezes to make ice. During such processes, called ‘phase changes’, the heat comes out even when the temperature stays exactly the same. Conversely, when the phase change is in the other direction (liquid to vapour or solid to liquid), heat must be supplied, as when boiling away the water in a kettle (the temperature staying at about 100 °C) or when evaporating water from a warm ocean (with the temperature changing only a little, thanks to the enormous heat capacity of the ocean).