One of the more interesting results from recent research into climate is that for the past million years or so, the earth’s climate has shown a distinct 100,000 cycle of long ice ages punctuated by a brief, warm intergalacials. We are in one of those brief warm periods, an interglacial period that is called the Holocene.
This cycle can be seen in the temperature graph below taken from publically available Vostok ice core data. The ice cores are very long sections of ice in which ancient climates can be worked out by careful analysis of atomic isotopes of layers of ice, where each wafer thin layer represents a year. Although there are other ways to determine past climatic conditions the Vostok cores (the site is in Antarctica but run by Russian scientists) are remarkable in being a connected track of temperatures of several hundred thousand years.
As you can see climate changes by about 14 degrees C from the top of the interglacial to the bottom of the ice age, but even the 420,000 or so years covered by the Vostok cores is just a moment in earth’s geological history. As noted, the 100,000 year cycle has been a feature of climate for about a million years. For two million years before that the cycle was the same except that it was 40,000 years long and temperatures were warmer. The whole three million years, in turn, is part of an ice age phase of the earth in which temperatures have generally been falling. There have also been hot house phases.
Temperatures over the most recent 420,000 years of earth’s history. Note that the graph time line is from left to right. All of recorded human history is in a couple of squiggles on the far right. Source: Vostok cores and Petit et al, 1999. Also note that the temperature scale is relative and the zero line is just a handy reference point. It has nothing to do with the freezing point of water.
Scientists have no real idea why the earth has been cooling over the past three million years and, in particular, are puzzled over the shift from 40,000 to 100,000 cycles. But until recently they thought they had a mechanism for explaining the current ice age cycle. This to be found in the Milankovitch cycles which are known, slight changes in the earth’s orbit and the angle of the earth’s pole. The theory was first proposed by James Croll, a self-taught 19th century physicist and greatly extended by a Serbian civil engineer and geophysicist Milutin Milankovitch who died in 1958. The theory, which has fallen in and out of favour a couple of times since it was first proposed, supposes that the orbital variations changes the strength of the seasons. Summers may become hotter or colder, winters may become colder or milder. That very slight change then becomes amplified in some way, perhaps by melting ice exposing rock which absorbs more heat.
As is generally accepted once the warming is underway it warms the oceans, and warmer oceans hold less carbon dioxide so there is more in the atmosphere, and those increased concentrations should cause more warming. Global warmers say that the CO2 increases then drive the warming. One problem with that supposition is the long time lag between warming and increases in CO2 levels. This point has been explored through a series of scientific papers with the latest published in the journal Science last year by a team led by Lowell Scott of the Department of Earth Sciences, University of Southern California. Entitled Southern Hemisphere and Deep-Sea Warming Led Deglacial Atmospheric CO2 Rise and Tropic Warming, the paper puts the lag at 1,000 years.
Another glance at the graph indicates that temperatures in the previous two interglacials seem to spike to well above modern temperatures, but the ice core readings for the time show that CO2 levels were at about 300 parts per million or far less than present levels. The Arctic Climate Impact Assessment Report 2005, compiled by a group called the International Arctic Science Committee and used as a source for the 2007 IPCC report says that during the inter-glacial preceding this one, known as the Eemian, conditions were generally warmer than they are now. The 2007 IPCC report says that polar temperatures at the time were 3 to 5 degrees higher than now, but attributes the difference to Milankovitch cycles.
Conditions were also warmer just a few thousand years ago in what is usually called the mid-Holocene maximum, and again the IPCC blames this on orbital cycles reinforced by natural CO2 emissions. Perhaps. The orbital cycles theory has come under major attack of late with recent work suggesting that the Milankovitch cycles peak thousands of years after the effect they are supposed to cause. That work is all subject to argument but there are other puzzles.
As you can see from the graph the Holocene is already longer than the preceding three interglacial periods at least. Estimating from graphs that are better than mine, the Eemian lasted less than 10,000 years and the previous two interglacials lasted perhaps just 5,000 years. The Holocene, in contrast, has clocked up 10,000 years plus. Further, those 10,000 years have remained comparatively warm, in contrast to the previous interglacials in which temperatures spiked then fell away again very quickly. In all cases the final collapse into an ice age has been abrupt - perhaps just a few generations. The atmosphere-oceanic system just seems to trip over itself to fall 10 degrees and more, and with climate theory in its present state there is no way to predict when it will occur. Orbital cycles theory suggests the Holocene has another 14,000 or so, although the theory doesn’t have much to say about how interglacials end. Then there is the comparative newcomer, solar magnetic theory, which is set to sweep orbital cycles off the board. For the evidence that the sun’s magnetic field affects climate is overwhelming.
The sun has an 11 year cycle marked by sun spots that astronomers can observe on the face of the sun. They have been counting and tracking these spots for more than 300 years. At the height of the solar cycle there are lots of these sunspots and the sun is said to be very active, generating plenty of flares and solar storms which affect satellites, and its magnetic field is stronger. At the bottom of the cycle there are few or no spots, and distinctly less solar magnetic activity. The sun is quiet. But that cycle is just part of the changes in solar magnetism which scientists have now being able to track through thousands of years by measuring very fine changes in isotopes of carbon in living and fossilised trees.
Just how solar magnetic activity affects climate is still a matter of argument but, scientists suppose, a stronger field helps shield the earth from solar radiation which is supposed to help in forming clouds. The stronger the solar magnetic field (which is different from the overall amount of energy the sun emits) the fewer clouds, and the warmer the earth. When the field weakens there are more clouds which cool the earth – or so the theory goes.