last glacial maximum
This is little more than a “data dump.” I already knew about this, but Mithen does the best coverage (that is, of the little I have read) of a topic I think about on-again, off-again.
The Serbian scientist Milutin Milankovitch first appreciated the significance of such orbital change in the 1920s. By building on his theories, scientists have established that every 95,800 years the earth's orbit changes from being roughly circular to elliptical. As this happens, the Northern Hemisphere develops greater seasonality, while the converse happens in the south. This sparks off the growth of northern ice sheets. When a circular orbit returns, the north-south contrasts in seasonality are reduced, global warming occurs and the ice sheets melt.
Alterations in the earth's tilt during its orbit also have climatic implications. Every 41,000 years, the inclination of the earth changes from 21.39 to 24.36 degrees and back. As this angle increases, the seasons become more intense: hotter summers, colder winters. The earth also has a regular wobble on its axis of rotation, which has its own cycle of 21,700 years. This influences the point on its orbit around the sun at which the earth is tilted with its Northern Hemisphere directed towards the sun. If this happens when the earth is relatively close to the sun, the winters will be short and warm; conversely, if the earth is relatively distant from the sun when tilted in this fashion, winters will be longer and colder.
While these changes in the shape, the tilt and the wobble of the earth's orbit will alter the earth's climate, scientists think that they are insufficient in themselves to account for the immense magnitude and speed of past climate change. Processes happening on the planet itself must have substantially amplified the slight changes they induced. Several of these are known: changes in ocean and atmospheric currents, the build-up of greenhouse gases (principally carbon dioxide) and the growth of the ice sheets themselves (which reflect increasing amounts of solar radiation as they increase in size). The combined impact of orbital change and amplifying mechanisms has been the see-sawing of climate from glacial to interglacial and back every 100,000 years, often with an extraordinarily rapid switch from one state to another. One of the most dramatic of these switches came about in 9600 BC, following on from l0,000 years of ups and downs of rainfall and temperature since the climatic extreme of the LGM.
Mithen, Steven. After the Ice: A Global Human History, 20,000-5000 BC. Cambridge: Harvard University Press, 2006. Print.
The Serbian scientist Milutin Milankovitch first appreciated the significance of such orbital change in the 1920s. By building on his theories, scientists have established that every 95,800 years the earth's orbit changes from being roughly circular to elliptical. As this happens, the Northern Hemisphere develops greater seasonality, while the converse happens in the south. This sparks off the growth of northern ice sheets. When a circular orbit returns, the north-south contrasts in seasonality are reduced, global warming occurs and the ice sheets melt.
Alterations in the earth's tilt during its orbit also have climatic implications. Every 41,000 years, the inclination of the earth changes from 21.39 to 24.36 degrees and back. As this angle increases, the seasons become more intense: hotter summers, colder winters. The earth also has a regular wobble on its axis of rotation, which has its own cycle of 21,700 years. This influences the point on its orbit around the sun at which the earth is tilted with its Northern Hemisphere directed towards the sun. If this happens when the earth is relatively close to the sun, the winters will be short and warm; conversely, if the earth is relatively distant from the sun when tilted in this fashion, winters will be longer and colder.
While these changes in the shape, the tilt and the wobble of the earth's orbit will alter the earth's climate, scientists think that they are insufficient in themselves to account for the immense magnitude and speed of past climate change. Processes happening on the planet itself must have substantially amplified the slight changes they induced. Several of these are known: changes in ocean and atmospheric currents, the build-up of greenhouse gases (principally carbon dioxide) and the growth of the ice sheets themselves (which reflect increasing amounts of solar radiation as they increase in size). The combined impact of orbital change and amplifying mechanisms has been the see-sawing of climate from glacial to interglacial and back every 100,000 years, often with an extraordinarily rapid switch from one state to another. One of the most dramatic of these switches came about in 9600 BC, following on from l0,000 years of ups and downs of rainfall and temperature since the climatic extreme of the LGM.
Mithen, Steven. After the Ice: A Global Human History, 20,000-5000 BC. Cambridge: Harvard University Press, 2006. Print.