That global warming exists, there is no doubt. But whether the cause is human consumption of fossil fuels or a natural cycle, has not been adequately answered, in my opinion.
The drivers of climate change, both natural and anthropogenic, can be summed and compared to the observed rise in global temperature. When all of the natural drivers are summed, and yes there are error bars associated with these, the result is a rise in temperature that is less than what is observed. When the natural drivers are added along with the radiative forcing of anthropogenic CO2, the predicted temperature matches, within error, the observed record. What this means is that there are natural drivers, but anthropogenic CO2 is contributing to global warming. The issue should not be polarized yet we often hear one side argue that the warming is entirely a natural cycle and others that it is entirely our fault. The climate system is more complex than that.
Glaciers formed and covered most of North America and receeded without any human intervention several times in the last millinum. We need to understand that fact in relation to what is happening now with human intervention. The answer is not yet clear to me.
The first sentence is wrong. The vast ice sheets that covered North America advance and retreat on orbital time-scales and not over the last 1000 years. In particular, there is a 100,000 year cyclicity. There are also ~21ka and ~41ka cycles. The driver of these cycles is orbital dynamics (precession of the equinoxes, obliquity (tilt) of the axis, and orbital eccentricity. The last ice sheet that covered North America reached a maximum ice volume around ~17,000-21,000 years ago and retreated around ~10,000 years ago. This period since ~10 ka is called the Holocene and is a period of relative climate stability that allowed human civilization to flourish. During the Holocene, particularly the late Holocene, smaller mountain glaciers have shown episodes of advance and retreat, but nothing like the great ice sheets. The cause of these shorter period fluctuations is not as well known, but there are both celestial and terrestrial drivers that contributed.
Perhaps the cycle of volcanic activity was responsible for the formation and recession of the glaciers. Is the volcano of human consumption equivalent to that of mother earth? Dunno.
NO NO NO!!! I knew we would be talking volcanoes again. The USGS showed that annual CO2 emissions from all volcanic and geologic CO2 sources is 130-230 million tonnes. Humans emit a staggering 27 billion tonnes. That means we emit ~150X more than volcanoes. Here is a USGS website to read: http://volcanoes.usgs.gov/Hazards/What/VolGas/volgas.html