climate change: Milankovich cycles

Some people have heard that the earth naturally warms and cools, and that we are in a natural warming period right now. According to paleoclimate records this is true. The Earth has gone through glacial (icy/cold) and interglacial (warm) cycles (1 cycle every ~10,000 years) for the past 400,000-650,000 years approximately.

We can tell that this has been happening because of greenhouse gases preserved in ice cores, and oxygen isotopes (as mentioned in a previous post). Here is a figure of the glacial/interglacial cycles (this shows methane in red and carbon dioxide in blue - present day is on the left and 650,000 years ago to the right.) This is from an ice core in Antarctica (Vostok). You can see that we are indeed in a warming cycle currently.

So, what causes these cycles? The theory behind this is called the Milankovitch Theory of Glaciation. This Serbian dude Milutin Milankovitch proposed it in the early 20th century. He said that there are three factors regarding the earth's orbit that cause glacial and interglacial cycles:

1. Eccentricity - the shape of the Earth's orbit around the sun is not a perfect circle, and the 'oblong-ness' of this orbit changes every ~100,000 years.

2. Obliquity - the tilt of the earth's axis. This affects the seasons on earth, and changes every 41,000 years.

3. Precession of the equinoxes - wobble of the earth - the earth spins, almost like a top. just as a top wobbles, so does the earth. This wobble changes every 23,000 years.

When the eccentricity is more oblong, and the tilt is less, and the Northern Hemisphere is tilted away from the sun the earth will enter a glacial period. This lineup causes summers to be less warm on average, so that ice does not melt away as much each year, causing an ice buildup. The Northern hemisphere must be pointed away from the sun, because ice builds up easier in the North since there is land surrounding the Arctic. The Antarctic is surrounded by water, and since ice builds up more easily on land, ice will form more easily in the north (Arctic).

As more and more of this ice builds up, more light is reflected off of the ice. (Ice reflects light/heat since it is white, whereas open water/land absorbs light/heat). Since more light and heat are being reflected off of the earth's surface, the earth cools and enters an ice age. These cycles of the earth entering an ice age (and the opposite happens for entering a warm period or 'interglacial cycle') are what are reflected in the first graph in this post. These cycles of the Milankovitch Theory have been confirmed by marine cores.