Once again, Ian Wishart is working himself up into a fine frenzy over at his blog, responding to a perceptive post by Bomber Bradbury at Tumeke! In the comments there he claimed to have “pointed out numerous mistakes in Gareth’s snide and out of context ‘review'”, and — funnily enough — I didn’t feel inclined to let that pass. So I suggested a little wager, and drew this furious response. So, knowing it will make precious little difference in the strange version of reality that Wishart occupies, here’s my (final) response…
The legacy of our release of fossil fuel CO2 to the atmosphere will be long-lasting. It will affect the Earth’s climate for millenia. We are becoming players in geologic time. That is the conclusion that climatologist David Archer shares with a general audience in his newly published book The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth’s Climate.
The author is a professor in the Department of The Geophysical Sciences at the University of Chicago and a contributing editor at Real Climate. His book is relaxed in style, almost conversational sometimes, repetitive on occasion, but nevertheless closely focused and packed with instructive detail. It was a pleasure for a non-scientist like me to read. He seems to understand how to illuminate processes for the general reader. For example, his chapter on the distribution of carbon in the atmosphere, the land and the ocean, and his explanation of the interactions between them in the carbon cycle, provided angles and information that pulled together satisfyingly the bits and pieces of my hesitant understanding. Similarly what he writes about the acidifying of the ocean by CO2 and the part calcium carbonate plays in slowly neutralising its effect is a model of lucidity.
The book’s structure is simple. There are three sections. The first describes the situation we are in right now – meaning the 20th and 21st centuries. The second section is about the past, investigated as a forecast for the future. The final section looks into the deep future.
Archer produces no surprises about our current situation. The basic physics of the greenhouse effect – that gases in the atmosphere that absorb infrared radiation could eventually warm up the surface of the earth – was described in 1827 by the French mathematician Fourier. Then in 1896 Swedish chemist Arrhenius estimated the amount of warming that the Earth would undergo on average from a doubling of the atmospheric CO2 concentration – what we now call the climate sensitivity. Such work sets the scene for the climate science which has exploded in the past few decades as global warming grew from a prediction into an observation. He describes many aspects of our current understanding of global warming, with several particularly helpful sequences, such as that on the relative strengths of four external agents of climate change called climate forcings – greenhouse gases, sulfur from burning coal, volcanic eruptions, changes in intensity of the sun. The warming that is occurring cannot be explained by natural forcings. Looking ahead in the present century he is very aware that sea level rise by 2100 may well be higher than predicted by the IPCC, as it begins to appear that the ice models used to forecast may be too sluggish to predict the behaviour of real ice.
In the second section he moves steadily back in time, starting with the last 100,000 years where the abruptness of some of the changes detected leads him to reflect that the IPCC forecast of a smooth rise in temperature from 0.5 degrees excess warmth today to about 3.0 degrees excess warmth in 2100 represents a best-case scenario in that it contains no unfortunate surprises. He then treats the longer-term glacial climate cycles through the last 650,000 years, paying attention to orbital forcing and to the ups and downs of atmospheric CO2 through the cycles. He envisages the ice sheets and CO2“entwined in a feedback loop of cause and effect, like two figure skaters twirling and throwing each other around on the rink.” His final step back is to the hothouse world of 50 million years ago and beyond that to transitions between hothouse and ice age climates over 500 million years. He selects the Paleocene Eocene Thermal Maximum event (recently discussed on Hot Topic) as an analogue for the global warming future.
The third section looks at that future. In discussing the land’s and ocean’s ability to take up carbon being released from fossil fuels he considers it likely that there are limits to that process which will mean that a significant fraction of fossil fuel CO2 will remain in the atmosphere for millenia into the future. There are calming effects from the carbon cycle, but there can also be opposite effects as seems likely to have been the case at times in the past. Hopefully large scale methane hydrate release won’t be a large part of such feedbacks, but if the ocean gets warm enough it is possible and could double the long-term climate impact of global warming.
For now the carbon cycle is responding to the CO2 increase by inhaling the gas into the ocean and high-latitude land surface, damping down the warming effect. But on the timescale of centuries and longer the lesson from the past is that this situation could reverse itself, and the warming planet could cause the natural carbon cycle to exhale CO2, amplifying the human-induced climate changes.
The clearest long-term impact of fossil-fuel CO2 release is on sea level rise. The book has a restrained chapter on this, but there is no escaping what will happen if the ice sheets melt. “We have the capacity to ultimately sacrifice the land under our feet.“
Have we averted an ice age? Archer discusses this possibility, but finds the evidence uncertain. He would in any case not put such a possibility forward as an argument in favour of CO2 emissions. All it means is that natural cooling driven by orbital variation is unlikely to save us from global warming – at this stage the much greater danger. Incidentally he mentions Ruddiman’s book Plows, Plagues, and Petroleum briefly and appreciatively in this section, but gives reasons for doubting its conclusions. (The book was reviewed on Hot Topic recently.)
In his epilogue on economics and ethics, where he ponders whether we are likely to turn away from the path we are currently on, he offers a comparison with slavery, another ethical issue: “Ultimately it didn’t matter whether it was economically beneficial or costly to give up. It was simply wrong.”
James Hansen describes the book as the best about carbon dioxide and climate change that he has read. “David Archer knows what he is talking about.” To which I would add that he also knows how to explain it clearly to anyone prepared to give him reasonable attention.
William Ruddiman’s book Plows, Plagues, and Petroleum: How Humans Took Control of Climate has attracted a good deal of interest in the climate history world since it was published in 2005.
His major thesis is that even before the industrial revolution, human activity over a period of 8000 years was responsible for a significant rise in carbon dioxide and methane levels in the atmosphere. After explaining orbital changes and their effect on ice-age cycles and monsoon cycles he turns to what he considers an anomalous rise in methane concentration in the atmosphere which began 5000 years ago – anomalous in that the naturally declining solar radiation would be expected to result in gradually lowering methane levels as wetlands diminished in size. He patiently works his way through what can be inferred of early agriculture to a hypothesis that increasing rice irrigation was mainly responsible for the methane increase, with lesser contributions also from domestic grazing animals and biomass burning. His next step is to look at CO2 changes, where he detects a similar anomaly beginning 8000 years ago when the natural downward trend was interrupted at around 260 parts per million to rise to around 280 ppm by the start of the industrial revolution. In the case of CO2 he attributes the rise to deforestation for agriculture, aided perhaps by some peat and coal burning. This attribution is tested against the population levels and likely amount of deforestation resulting from agricultural activities; he explains the calculations which he undertook to establish its credibility.
If he is correct this would mean that the warm and stable climate of the last 8000 years has been due to unwitting human intervention which offset a natural cooling that would otherwise have gradually developed. He sets out reasons for the conclusion that a degree of glaciation may by now have been occurring in Canada were it not for human farming activities.
His hypothesis has been challenged, as he expects and, as a scientist, welcomes. He responds in his book to two of the challenges, both of which he acknowledges to have merit. One is that he did not go far enough back in the sequence of ice-age cycles when looking at the pattern of previous interglaciation periods for a comparison of what might be expected from natural processes today. The other is that humans could not possibly have cleared and burned enough forest to account for such a large CO2 anomaly. He considers that with some adjustment his hypothesis survives these challenges.
He then considers the “wiggles” that have occurred in the CO2 increases over the 2000 years prior to the industrial revolution, particularly the so-called Little Ice Age, when the CO2 level dropped somewhat. He doubts natural causes and looks instead for processes that might have reversed the slow deforestation which he has suggested responsible for the gradual CO2 increase. He rules out war and famine as not disastrous enough on a large enough scale and settles on disease, especially the plague. He theorises that as epidemics and pandemics caused major drops in population, reforestation occurred in abandoned farmland, there was a slowdown in new deforestation, and in China a decrease in the amount of coal burning.
The question Ruddiman then proceeds to address is why the relatively modest rise in greenhouse gases caused by humans before the Industrial Revolution led to so relatively large an increase in temperature – 0.8 degrees – while the relatively large rise in greenhouse gases since the Industrial Revolution has been accompanied by a still relatively small increase in temperature. His answer lies in the response time required for the full effects to be felt and he explores this in some detail in relation to both land and ocean, showing how response times vary in different environments.
Ruddiman’s hypotheses continue to be debated by climate scientists. In fact Wallace Broecker, one of the world’s leading experts on climate history describes them as “total and utter nonsense”, according to a September 2008 New Scientist article. The article reports Broecker reckoning there’s a natural explanation for the CO2 rise. Deep-sea sediments record a drop in carbonate concentrations that could account for the rise in atmospheric CO2. But others find Ruddiman at least worthy of further consideration. His book is certainly a fascinating detective science story, readily accessible to a general reader.
But I enter a caveat in relation to his closing chapters where he looks at future warming prospects as the full effect of the large post-industrial revolution rise in greenhouse gases begins to be felt. His prognostications are more optimistic than many climatologists would be willing to offer today. He gives us a century or two to melt much of the world’s sea ice and mountain glaciers and push back the seasonal limits of snow cover, but considers the two great ice sheets will be largely intact. The book was published over three years ago. One wonders whether he would be as sanguine now.
When he wrote the book he was certainly disinclined to see climate change as an overwhelming challenge. In fact he is quite testy about what he describes as the alarmism of extreme environmentalists and their organisations. He doesn’t name them, but he accuses them of oversimplifying the complexities of the global warming issue. He should have identified the organisations he refers to. Maybe he has noticed more alarmist predictions than I have. Major environmental advocacy groups such as Greenpeace and WWF seem to me to stay well within the bounds of responsible science on climate change.
He finally states that the depletion of precious resources – naming water, topsoil and fossil fuel – poses a greater threat to the human future than the threat of global warming does. There are certainly many serious threats to the continuance of human society, but he provided me with no grounds for relegating global warming to a secondary rank.