The Earth is currently in an interglacial period of an ice age that started about two and a half million years ago. The Earth's current ice age is primarily caused by Antarctica drifting over the South Pole 30 million years ago. This meant that a large area of the Earth's surface changed from being very low-albedo ocean to highly reflective ice and snow. The first small glaciers were formed in Antarctica perhaps as long ago as 40 million years. They expanded gradually until, about 20 million years ago, a permanent ice sheet covered the whole Antarctic continent. About 10 million years later, glaciers appeared on the high mountains of Alaska, and about 3 million years ago, ice sheets developed on lower ground in high northerly latitudes.

Pacific Ocean bottom water temperatures started declining 40 million years ago, falling 10° C to the current 3° C. The band of high ocean temperatures (above 25° C) also contracted towards the equator, from 45° latitude to 20°. Eventually the oceans lost enough heat that the Earth's orbital parameters started causing surges in ice formation. There are three orbital parameters: eccentricity, precession and obliquity, shown in Figure 1.
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Figure 1: Orbital Parameters: Eccentricity, Precession and Obliquity.

This figure is developed from A.L.Berger, 1978, Long Term Variations of Daily Insolation and Quaternary Climatic Changes, Journal of the Atmospheric Sciences, volume 35 (12), 2362-2367.

Eccentricity is caused by changes in the shape of the Earth's orbit due to the gravitational attraction of other planets. Precession is the change of direction of rotation. Obliquity is the tilt of the axis. When these effects aligned, their effect is reinforced. From three million years ago to about 800,000 years ago, the dominant pattern of glaciation corresponded to the 41,000 year period of changes in the Earth's obliquity. Since then, a 100,000 year cycle has been dominant.

Ice ages occur because the summer sun in the northern hemisphere does not get hot enough to melt all the ice that accumulates over winter. Ice has a much higher reflectivity than rocks or vegetation, and so reflects more sunlight into space and the cooling is reinforced. Eventually the orbital parameters change back and warming occurs. Glacial periods tend to cool slowly and warm abruptly. Because the Earth's orbital parameters can be calculated, the amount of sunlight in high northern latitudes can be calculated.
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Figure 2: June Mid-Month Insolation at 65° North

This figure is derived from M.F.Loutre and A.Berger, 2000, Future Climate Changes: Are we entering an exceptionally long interglacial?, Climatic Change 46, 61-90

Figure 2 shows how that translates to insolation (sunshine) at 65° North. The recent peak in insolation was 11,000 years ago at the end of the last glacial period. It has since declined by about 10% to 476 watts per square metre. Insolation will rise from here for the next 30,000 years, but it will still be low enough for the next glaciation to form. This is shown by Figure 3 of Northern Hemisphere ice volume for the last 200,000 years and a projection for the next 130,000 years. According to these calculations, the Earth is at the beginning of a 20,000 year plunge into the next ice age.

The reason why the Earth doesn't respond more rapidly to changes in insolation is due to the retained heat in the oceans, which smooths the whole process over thousands of years. Over the short term, the oceans are very responsive to changes in solar activity. Figure 4 shows the very strong correlation between the annual rate of sea level rise and solar cycles over the 20th century. The sea level rise of the 20th century can largely be attributed to a more active Sun relative to the 19th century. About 70% of the sea level rise of the 20th century was due to thermal expansion of the oceans, with the rest due to melting glaciers.
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Figure 3: Future Glaciation

This figure is derived from M.F.Loutre and A.Berger, 2000, Future Climate Changes: Are we entering an exceptionally long interglacial?, Climatic Change 46, 61-90
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Figure 4: The Correlation between Sea Level Rise and Solar Cycles over the 20th Century.

The sea level data is derived from S.Holgate, Decadal rates of sea level change during the twentieth century, Proudman Oceanic Laboratory, Liverpool, UK.