Climate cycles

Climate cycles are natural trends in which temperatures fluctuate on large time-scales, ranging from minor fluctuations over a few centuries to massive shifts in climate over thousands of years.

Using proxy data from various natural sources (including deuterium content in ice cores, calcium content and thickness of sea shells and tree ring growth data) we can establish that these cycles do exist, and that they occur for several reasons, perhaps the most prominent of which is the Milkanovitch cycle.

However, whilst the variation over large time-scales is extreme (deviating from the long-term average by as much as ten degrees Celsius at a time), the changes are usually very gradual and take place over many millennia.

Argument against the theory of man-made global warming

The existence of these climate cycles is often used as a contradiction of the theory of anthropogenic global warming, or man-made climate change.

However, this argument does not stand up to scrutiny. It can be seen that the Earth is currently in the middle of a time of peak temperatures during which little or no warming should be expected.

This contradicts the observed facts, which indicate ~0.76 degrees Celsius of warming (with error bounds of 0.2 degrees Celsius either way) over the past century and a half or so.

Changes of such magnitude, when natural, normally take place over much longer time-scales.

The theory of anthropogenic global warming states that greenhouse gas emissions since the Industrial Revolution have influenced global mean temperatures in a way that is outside the norm, particularly the rate of change of temperature.

This is not to say that the current warming period could not be natural.

However, such a theory is unsupported by the evidence.