Climate change.

Histories of climate change and climate change predictions.

A short history of climate change,
with comments on the TV documentary “The Great Global Warming Swindle”.

Alison Marshall, March 2007 [with later modifications in square brackets].

Explanations for climate change include atmospheric CO2, other greenhouse gases, atmospheric aerosols, Milankovitch cycles in the Earth’s orbit and axis, and continental drift. There is respectable scientific evidence for all of these influences.

The 3-to-8-year El Nino Southern Oscillation and of course the 365-day annual cycle also affect the weather. Other possible causes of climate change are human agricultural activity, [. . .] sunspot cycles, and galactic cosmic rays. As Professor Wunsch tried to say in “The Great Global Warming Swindle”, climate change is complicated.

But the temperature increase in the last 3 decades is probably due to man-made atmospheric CO2. The average global surface temperature is now close to the maximum reached in each 80,000-to-125,000-year Milankovitch glacial cycle, and its likely that some time this century it will get higher than at any other time in more than a million years, since before humans existed.

Recently it was announced that the 2006-7 northern hemisphere winter was the warmest ever recorded, and that this was not due to global warming, but was caused by an El Nino [event]. As there is an El Nino [event] on average twice every decade, it seems to me that this is yet another example of climate change denial. The warm winter was probably the result of the CO2 increase as well as El Nino.

The absence of warming between 1940 and 1975 was probably due to global dimming caused by man-made sulphate pollution in the atmosphere.

The Little Ice Age between 1400 and 1700, with ice fairs on the Thames in London, may have been related to the decline in agricultural activity after the Black Death, or to a decline in sunspot activity, including the Maunder minimum.

The warmth of the last 3000 years may have been caused by the increase in human agricultural activity.

The warmth of the 5000 years before that, including the Holocene maximum, and the coldness of the 100,000-year ice age before that, were due to variations in solar radiation received on the Earth due to Milankovitch variations in the Earth’s orbit and axis.

The 800-year lag of CO2 behind temperature change is due to the response of CO2 to the temperature changes of the Milankovitch cycles. The strength and length of these glacial cycles may be influenced by CO2 feedback as well as by variations in the Earth’s orbit and axis.

In “The Great Global Warming Swindle” six of the Milankovitch glacial cycles were shown in a graph spanning thousands of years.  It related temperature to CO2 and was copied from Al Gore’s documentary “An Inconvenient Truth”.  In another graph which related temperature to galactic cosmic rays, there were four cycles spanning millions of years.

In the galactic cosmic ray theory, the flux of cosmic rays increases when the Earth passes through a spiral arm of the galaxy, once every 140 million years approximately. Ice age epochs and biological extinctions may also occur when galactic arms are passed.

The cosmic ray graph in “The Great Global Warming Swindle” covers nearly all of the Phanerozoic eon, which hasn’t finished yet but began long before the time of the dinosaurs. For such distant times there isn’t a scientific consensus about estimates of cosmic rays, temperature, and CO2 levels, and continental drift (plate tectonics) is important. Ocean currents and the amount of polar land are changed by continental drift, and so the climate also is changed.

[. . . For a graph of 20th century temperatures and their modelled response to greenhouse gases and solar, ozone, volcanic and sulphate influences, see

For graphs of atmospheric CO2 over the past 800 thousand and 65 million years, see “Long-term perspective” at]

For more detailed technical discussions see, which is run by a group of climate scientists.

Climate change predictions.
Alison Marshall, 2015 – 2017.

Some climate change sceptics think that scientists can’t be taken seriously because in the 1970s they were predicting an ice age.

What the scientists were actually saying was that the climate was likely to become warmer than at any other time in the last million years, if most of the known fossil fuel reserves were used. Then, when the excess carbon dioxide had gone from the atmosphere, the long-term cooling cycle would return, followed by the next ice age. (Imbrie, J. and Imbrie, K.P. (1979) Ice Ages: Solving the Mystery. London and Basingstoke, Macmillan Press.)

So probably we will get both warming and cooling, first one and then the other. What is least likely is that the man-made effect will be exactly the right amount to balance the ice age cycle, and keep the climate stable. To make adjustment easier, a much smaller population would be a good idea.

Milankovitch cycles.
May 2022.

Variations occur in the direction of the earth’s axis. The positions of equinox and solstice shift slowly around the earth’s elliptical orbit, and complete one full precession cycle about every 22,000 years. Independently of this cycle of axial precession, the tilt of the axis away from the vertical to the plane of the orbit varies about 1.5 degrees on either side of its average angle of 23.5 degrees.

In 1969, when Broecker and Kukla met at an international scientific congress, they found that their separate lines of research had led them to conclude that the major Pleistocene ice ages were spaced about 100,000 years apart, developed slowly, and terminated abruptly.

In 1973 Hays and Shackleton found that two Indian Ocean cores contained a detailed and undisturbed record of climate extending back 450,000 years. When the radiolarian and isotopic data had been graphed, frequencies higher than the 100,000 year cycle were clearly visible.

In 1974 Imbrie’s spectral analysis found that, as expected, the dominant climatic pulse was the 100,000 year ice age cycle. But three other smaller peaks occurred, and they matched the recently recalculated astronomical predictions. The tilt curve showed a single cycle of 41 to 43 thousand years. The precession curve contained two distinct cycles, a major cycle of 23 to 24 thousand years and a minor one of 18 to 19 thousand years.

(“Ice Ages. Solving the Mystery.” J. Imbrie and K.P. Imbrie, 1979 )

The Earth’s axial rotation is perturbed by gravitational interactions with the moon and the more massive planets that together induce periodic changes in the Earth’s orbit, including a 100,000 year cycle in the shape of the orbit (eccentricity), a 41,000 year cycle in the tilt of the Earth’s axis (obliquity) and a 20,000-year cycle in the “wobble” of the Earth’s axis (precession). The main climatic effect of precession, the so-called precession of the equinoxes, is to shift the season when the Earth has its closest pass to the Sun (perihelion). Today, perihelion occurs in northern hemisphere winter but 10,000 years ago (half of a precession cycle) it occurred in northern hemisphere summer, and summer radiation over North Africa was about 7% higher than it is today.

deMenocal, P. B. & Tierney, J. E. (2012) Green Sahara: African Humid Periods Paced by Earth’s Orbital Changes. Nature Education Knowledge 3(10):12 ).

Data from Nevada, sea level records, and an Antarctic ice core suggest that in each of the last four ice age cycles, the interglacial part lasted for about 20,000 years, with the warmest part being relatively stable for 10,000 to 15,000 years. The current interglacial temperature conditions may already have persisted for 17,000 years. Other workers have suggested that the current interglacial might last tens of thousands of years.

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Further reading.
Green taxing and spending,

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