Additional info from Skeptical Science
For someone to state that the global warming we’re experiencing is actually part of a 1,500-year natural cycle of global temperature variation is interesting for two reasons. First — in contradiction to the great majority of skeptic arguments that actually deny global warming — this argument requires that the person promoting this explanation must first agree that climate change is, indeed, happening.
Second, they must also refuse to accept the greenhouse effect, a theory first proposed more than 100 years ago and which even many skeptics of the human contribution to climate change readily accept.
The 1,500-year cycle in question has been observed mainly through ice core data as a warming in the Northern Hemisphere, matched at precisely the same time by a cooling in the Southern Hemisphere. So it’s a heat distribution issue: a global temperature "see-saw" effect. The total heat in the global system remains constant.
In contrast, human-produced global warming has been caused by the rapidly increasing CO2 concentrations in the atmosphere over the last 200 years — rising over 390 parts per million after remaining below 300 parts per million for the previous 800,000 years. And unlike natural heat variations, the current temperature increase caused by CO2 is occurring all around the globe — on the ground, in the air and in the oceans.
Adapted from © John Cook and Skeptical Science