Posts Tagged ‘Climatology’

“Carbon dioxide involved but not determinative in 100,000 year glacial cycles”

August 10, 2013

We are still struggling to explain what initiates an ice age (glaciation) and what causes them to end and the ice sheets to withdraw giving the interglacials.

interglacials

That the Milankovitch cycles and variations of insolation are involved in the onset and retreat of glacial periods is clear but the “how” is still elusive. Now a new paper describes a model where the ice sheets and the mutual feedbacks with climate are considered.

“Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles”.

Abe-Ouchi A, Saito F, Kawamura K, Raymo ME, Okuno J, Takahashi K, Blatter H: Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume. Nature, 2013, 500: 190-193, doi: 10.1038/nature12374

ETH Press Release: 

Ice ages and warm periods have alternated fairly regularly in the Earth’s history: the Earth’s climate cools roughly every 100,000 years, with vast areas of North America, Europe and Asia being buried under thick ice sheets. Eventually, the pendulum swings back: it gets warmer and the ice masses melt. While geologists and climate physicists found solid evidence of this 100,000-year cycle in glacial moraines, marine sediments and arctic ice, until now they were unable to find a plausible explanation for it.

Using computer simulations, a Japanese, Swiss and American team including Heinz Blatter, an emeritus professor of physical climatology at ETH Zurich, has now managed to demonstrate that the ice-age/warm-period interchange depends heavily on the alternating influence of continental ice sheets and climate.

“If an entire continent is covered in a layer of ice that is 2,000 to 3,000 metres thick, the topography is completely different,” says Blatter, explaining this feedback effect. “This and the different albedo of glacial ice compared to ice-free earth lead to considerable changes in the surface temperature and the air circulation in the atmosphere.” Moreover, large-scale glaciation also alters the sea level and therefore the ocean currents, which also affects the climate. 

As the scientists from Tokyo University, ETH Zurich and Columbia University demonstrated in their paper published in the journal Nature, these feedback effects between the Earth and the climate occur on top of other known mechanisms. It has long been clear that the climate is greatly influenced by insolation on long-term time scales. Because the Earth’s rotation and its orbit around the sun periodically change slightly, the insolation also varies. If you examine this variation in detail, different overlapping cycles of around 20,000, 40,000 and 100,000 years are recognisable (see box).

Given the fact that the 100,000-year insolation cycle is comparatively weak, scientists could not easily explain the prominent 100,000-year-cycle of the ice ages with this information alone. With the aid of the feedback effects, however, this is now possible.

The researchers obtained their results from a comprehensive computer model, where they combined an ice-sheet simulation with an existing climate model, which enabled them to calculate the glaciation of the northern hemisphere for the last 400,000 years. The model not only takes the astronomical parameter values, ground topography and the physical flow properties of glacial ice into account but also especially the climate and feedback effects. “It’s the first time that the glaciation of the entire northern hemisphere has been simulated with a climate model that includes all the major aspects,” says Blatter.

Using the model, the researchers were also able to explain why ice ages always begin slowly and end relatively quickly. The ice-age ice masses accumulate over tens of thousands of years and recede within the space of a few thousand years. Now we know why: it is not only the surface temperature and precipitation that determine whether an ice sheet grows or shrinks. Due to the aforementioned feedback effects, its fate also depends on its size. “The larger the ice sheet, the colder the climate has to be to preserve it,” says Blatter. In the case of smaller continental ice sheets that are still forming, periods with a warmer climate are less likely to melt them. It is a different story with a large ice sheet that stretches into lower geographic latitudes: a comparatively brief warm spell of a few thousand years can be enough to cause an ice sheet to melt and herald the end of an ice age.

The Milankovitch cycles

The explanation for the cyclical alternation of ice and warm periods stems from Serbian mathematician Milutin Milankovitch (1879-1958), who calculated the changes in the Earth’s orbit and the resulting insolation on Earth, thus becoming the first to describe that the cyclical changes in insolation are the result of an overlapping of a whole series of cycles: the tilt of the Earth’s axis fluctuates by around two degrees in a 41,000-year cycle. Moreover, the Earth’s axis gyrates in a cycle of 26,000 years, much like a spinning top. Finally, the Earth’s elliptical orbit around the sun changes in a cycle of around 100,000 years in two respects: on the one hand, it changes from a weaker elliptical (circular) form into a stronger one. On the other hand, the axis of this ellipsis turns in the plane of the Earth’s orbit. The spinning of the Earth’s axis and the elliptical rotation of the axes cause the day on which the Earth is closest to the sun (perihelion) to migrate through the calendar year in a cycle of around 20,000 years: currently, it is at the beginning of January; in around 10,000 years, however, it will be at the beginning of July.

Based on his calculations, in 1941 Milankovitch postulated that insolation in the summer characterises the ice and warm periods at sixty-five degrees north, a theory that was rejected by the science community during his lifetime. From the 1970s, however, it gradually became clearer that it essentially coincides with the climate archives in marine sediments and ice cores. Nowadays, Milankovitch’s theory is widely accepted. “Milankovitch’s idea that insolation determines the ice ages was right in principle,” says Blatter. “However, science soon recognised that additional feedback effects in the climate system were necessary to explain ice ages. We are now able to name and identify these effects accurately.”

Download video: 

Simulated ice sheet change during the last glacial cycle (mov file, video: Abe-Ouchi et al. 2013)

Abstract: The growth and reduction of Northern Hemisphere ice sheets over the past million years is dominated by an approximately 100,000-year periodicity and a sawtooth pattern (gradual growth and fast termination). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles, and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles. Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal climatic feedbacks may also be at work. Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere ‘excess ice’, but the physical mechanisms underpinning the 100,000-year cycle remain unclear. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere–asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis, with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that after inception of the ice sheet, its mass balance remains mostly positive through several precession cycles, whose amplitudes decrease towards an eccentricity minimum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to make the mass balance negative. Therefore, once a large ice sheet is established, a moderate increase in insolation is sufficient to trigger a negative mass balance, leading to an almost complete retreat of the ice sheet within several thousand years. This fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound, which is the lithosphere–asthenosphere response. Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles.

How “Retrospective Prediction” works

May 14, 2013

I have posted earlier about Climate Science being reduced to “Retrospective Predictions”.

This is how it works:

I missed a flight today. I had predicted that I would arrive  30 minutes before check-in closed. But I was wrong. I arrived 37 minutes after check-in had closed. My prediction model was just not good enough.

My prediction model assumed a certain average velocity for my car, an empirically determined period for parking the car and getting to the check-in desk, an allowance of 15 minutes to stop for coffee along the way and an allowance of 10 minutes for inaccuracy of calculation. This gave me my starting time which – in the event – led to my being 37 minutes late for check-in.

I now applied “Retrospection” to my “Prediction”. Effectively this meant choosing which of my assumptions was wrong and where I would apply a “fudge factor”. I realised that I had not accounted for road works along the way. I therefore added in a period for “delays due to road-works” such that my total transit time was increased by precisely 67 minutes.

I then redid my calculation. Lo and Behold! My “Retrospective Prediction” was now spot on. It confirmed for me that I had been late for check-in.

(My addition of time for “delays due to road works” is a very simple but powerful factor and is given by the following equation:

delays due to road works (minutes) = 0.5 x number of days elapsed from 1st January to day of travel

In the present case, today being 14th of May it is the 134th day of the year and it is obvious that

delays due to road works = 0.5 x 134 = 67 minutes).

I am travelling tomorrow. So I shall be testing my new “Retrospective Prediction” in retrospect the day-after-tomorrow.

Tomorrow I will also try not to use the wrong departure time.

We learn about climate only when the models are wrong!

March 29, 2013

When a forecast based on a mathematical model is correct, we learn nothing.

A mathematical model is merely a theory, a simplification of reality or an approximation to the real world. By definition a mathematical model is a hypothesis.  When forecasts are incorrect, we can return to our model and improve it and make a new hypothesis. A forecast is then a test of the model but in just one particular set of circumstances. Being correct does not prove the theory behind the model. It does of course add to the body of evidence that the model may be a satisfactory representation of reality and it does allow further forecasts to be made without tweaking the model. For learning to take place the mathematical model must be the falsifiable hypothesis of the scientific method.

It seems to me that Solar Science has a much healthier (scientifically) attitude to models and forecasts than “Climate Science”. When observations don’t match a climate forecast, the observations are impugned rather than the models being improved. This is, I think, because the forecast climate results have been used to establish huge revenue flows in the political arena (whether as taxes or carbon credits or just as research funding). There has been a vested interest in denying the observations and calling the science “settled”. Once the science is “settled”  the climate forecast and its underlying model become sacrosanct and take on the certainty of prophecy. Instead of being falsifiable hypotheses, climate model forecasts have taken on the character of unfalsifiable prophecies!

No scientist would presume to claim that we know or understand all solar effects. Or that we know and understand the role of the oceans or of the water vapour and dust and aerosols in the atmosphere. “Climate” is contained in the thin, chaotic layer of atmosphere which surrounds us. Yet “Climate Science” makes the arrogant assumption that the effect of trace amounts of carbon dioxide on climate is known definitively. Filling a real greenhouse with higher concentrations of carbon dioxide does not make that greenhouse any warmer than one filled with normal air – but the plants do grow faster with access to the additional CO2!! But – claim the climate priesthood –  in the real atmosphere, carbon dioxide causes other forcings (clouds? aerosols? precipitation effects?) which maximise warming which means that our model is still valid. Why not just admit that we don’t know what we don’t know?

The behavioural issue of course is whether it is worth trying to control something as poorly understood as climate rather than ensuring that we have the wherewithal to adapt to whatever changes may come. Another ice age will surely come whether in 10 years or a 100 years or 2,000. It will then be our ability to harness all available energy sources around us which will determine our capacity to adapt.

Learning from forecasts when they are wrong – not just in science but also in business and project management and technology development – has long been a hobby-horse of mine and is why forecasts need to be wrong.

When there is no difference there is no learning.

  • I take prophecies to be a promise about the future  based primarily on faith and made by prophets , witchdoctors, soothsayers and politicians such as ”You will be doomed to eternal damnation if you don’t do as I say”,
  • I take “forecasts” to be an estimate of future conditions based on known data with the use of calculations, logic, judgement, some intuition and even some faith. They are extrapolations of historical conditions to anticipate – and thereby plan for -future conditions.

……. Over the last 30 years I have spent of a lot of time conducting and participating in reviews. Reviews of research projects, of construction projects, of organisations and processes, of designs, of strategies and action plans, of businesses and of companies. The common features  in all these different reviews, that I have found the most penetrating, have been the comparisons not only between forecast values  and actual values, (which may be any values indicating performance and capable of being extrapolated), but also between past forecasts and current forecasts.

Whether considering construction progress or costs or sales figures or cash flow or profit or number of patents applied for, it is the differences between forecast and actual values, or values forecast before and values forecast later which have led to learning. In all these fields we are in the area of the behaviour of complex systems; and where people and their behaviour is involved any system is inevitably a complex system.

When a forecast is fulfilled there is usually an air of congratulation, satisfaction and self-adulation and this leads to a deadly complacency that everything is “settled science” and well understood. In any enterprise of any kind, that kind of complacency is the kiss of death. It is the differences which lead to questioning, to proper scientific scepticism, to further investigation and ultimately to an increase of understanding and – perhaps – a better forecast. (Of course, ignoring all such differences  and to merely “continue as before” can be equally fatal).

Which brings me to climate (which is not a science by any stretch of the imagination) and solar cycles. They are both in the realm not only of where “what we know is a great deal less than what we don’t know” but they are also both in the region where “we don’t even know what we don’t know”. We do not even know all the questions to be asked. They are both complex systems where – by definition – the complexity lies in the multitude of the processes involved and their interactions.

When climate – which is contained in the 100 m of ocean and 20 or so km thick, turbulent and chaotic atmospheric layer (and which is dimensionally miniscule in relation to the 140 million km of the earth-to-sun system) – is so complacently considered to be “settled science” then we have shifted into the area of faith and soothsaying and prophecies. When climate modellers are smug enough to believe they have understood the climate system and believe that their models are complete, then the models produce outputs which are not forecasts but prophecies. (No doubt soothsayers and shamans have sometimes made accurate prophecies but I still would not buy a used car from one of them)! Weather is in the realm of forecast (though you could argue that the most accurate forecast is still that “the weather tomorrow will be like today”) but climate is not yet there.

This kind of “arrogance” which pervades some of the climate “scientists” is not so prevalent when it comes to the study of Solar Cycles. There is a clear understanding that “we don’t know what we don’t know”. In addition to the 11 year and 22 year cycles, other cycles are hypothesised for 87 years, 210 years, 2300 years (or maybe 2241 or 2500 years) and 6000 years. We have no idea what causes these cycles. Even the 11 year cycle which has been most studied produces  surprises every day but is properly in the area of “forecast” (and hopefully never again will be in the area of prophecy). ….

…… We seem to be in a solar minimum. We may be seeing a 210 year cycle – or maybe not. There are changes to the forecasts not only regarding the maximum level of sunspot activity but also about when it will occur and what the length of cycle 24 might be. There is speculation as to what effect the length of the solar cycle may have on climate – but we haven’t a clue as to what mechanisms may be involved.  This is not to say that there isn’t much speculation and hypothesising. There is a great deal of comment about the effect these changing forecasts may have on global warming or cooling or climate disruption.  In some quarters there is much glee that the forecasts have been “wrong”. Some comments question the intelligence of the forecasters.

But of course the forecasts themselves say nothing about how the behaviour of the sun may impact our climate. They do not pretend to be prophecies or to be statements of inevitable outcomes. All they do say is that we don’t know very much – yet – about the sun. But we do know enough to make some tentative forecasts.

But I am very glad that people continue to be brave enough to make forecasts and I am quite relieved that the forecasts are not spot on. That at least ensures we will continue learning.


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