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 60, 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, September 2015.

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

“. . . it is likely that consumption of the bulk of the world’s known fossil fuel reserves would plunge the planet into a “super-interglacial age,” unlike anything experienced in the last million years. . . Assuming that the shock of the super-interglacial does not bring about a fundamental change in the earth’s climate system, the atmosphere would eventually rid itself of the excess carbon dioxide. Then the long-term cooling cycles . . . would reassert themselves . . . and the longest Pleistocene interglacial on record would come to an end. Global climate would then start a long downward slide until, 23,000 years from now, the earth would once more find itself in the depths of a new 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.

Early humans in Britain.
Jonathan Leake, Sunday Times, 14 August 2016.

The Earth has cooled since . . . the Eocene era, between 56m and 34m years ago, when . . . the world had no ice . . . and . . . global temperatures averaged 30C, about 15C warmer than now . . . Cooling meant that subtle changes in the Earth’s orbit could periodically push it below the temperature threshold at which . . . ice sheets could form over Britain – a cycle that began up to 800,000 years ago.

. . . Professor Chris Stringer, lead researcher of human origins at the Natural History Museum in London, said ice sheets repeatedly drove out early humans trying to colonise Britain. “Humans first arrived in Britain around 900,000 years ago . . . but they were forced out by the ice.”

Several more colonisations followed . . . but were also repeatedly driven out.

Stringer suggests that modern humans might have faced a similar fate but for the . . . artificial warming of the planet caused by greenhouse gases. . . “We have had a long interglacial and should be heading into a colder phase but are putting that off with greenhouse gases. The problem is that we are now at risk of overshooting and making the planet warmer than for 50m years. That kind of rapid climate change could be devastating.”

Arctic warming.
Matt Ridley, The Times, 29 August 2016.

A Swedish team, in a paper published in 2014 . . . concluded . . . that . . . between 10,000 . . . and 6,000 years ago, the Arctic experienced a “regime dominated by seasonal ice, i.e. ice-free summers.”

This was a period known as the “early Holocene insolation maximum” (EHIM). Because the Earth’s axis was tilted away from the vertical more than today (known as obliquity), and because we were then closer to the Sun in July than in January (known as precession), the amount of the Sun’s energy hitting the far north in summer was much greater than today. This . . . was the chief reason the Earth . . . emerged from an ice age . . . The effect was huge: about an extra 50 watts per square metre 80 degrees north in June. By contrast, the total effect of man-made global warming will reach 3.5 watts per square metre (but globally) only by the end of this century.

. . . To put it in context, the EHIM was the period during which agriculture was invented in about seven different parts of the globe at once.

. . . temperatures gradually but erratically cooled over the next few thousand years as the obliquity of the axis and the precession of the equinoxes changed. Sunlight is now weaker in July than January again (on global average).

Barring one especially cold snap 8,200 years ago, the coldest spell of the past ten millennia was the . . . “little ice age” of AD 1300-1850.

Today’s melting may be man-made but the EHIM precedent is still relevant . . . today’s carbon dioxide-induced man-made warming is happening more at night . . . during winter . . . and in the far north . . . the reduction in Arctic ice is the most visible, but least harmful, effect of global warming.

Further reading:

Green taxing and spending,