[thedoc (OP)]

Lee asked the Naked Scientists:
   
A friend recently stated to me that climate changes are cyclic, measured in millennial time scale... in fact there's some good evidence that the so-called 'global warming' the planet's going through at present is actually DELAYED by that very man created result [i.e. pollution, et al.] and we're in fact long overdue for the 'global warming'' and the subsequent 'freeze' period but it's been postponed/delayed - not triggered - by the very man made/created pollution....

What do you think?

[alancalverd]

The influence of human activity on climate is negligible.

It is certainly true that parts of the world for which we have reliable temperature data are on average warmer than they were 50 years ago, but (a) we do not have any reliable temperature data much more than 100 years old (b) the discovery of 500-year-old bryophytes under a retreating glacier shows that the world was warmer, not merely millions of years ago but within recorded history than it is now (c) we seem to be still climbing out of a cold snap that radically altered European architecture in the 11th century....and so it goes on.

If you look at the annual cycle of carbon dioxide levels recorded at Mauna Loa observatory, it is obvious that CO2 is dependent on  temperature, not the other way around, and non-anthropogenic mechanisms dominate the cycle.

Climate change is a natural and inevitable phenomenon, and is essentially chaotic. There is an underlying cycle of about 500,000 years but within that, very rapid rises and slow falls of temperature are entirely normal, and the CO2 level follows suit.   

[CliffordK]

There is a very strong cycle of mostly cool for about 100,000 years, then a brief warm period of about 10,000 years, of which we may be late for the next cool period.

There is a lot more data that we need to truly determine the cause of the cycles.  There is some evidence for feedback systems with colder weather causing a reduction of the partial pressure of CO₂ from the oceans, and decreased atmospheric CO₂, and in turn colder weather. 

Unfortunately the warming/cooling triggers aren't fully known.

As Alan mentioned, there is some evidence for lower solar activity in the 1600's, and a period of global cooling.  Perhaps other periods too.  While we only have about 500 years of detailed solar records, the last 100 years seems to have higher solar activity than previous (recent) periods.

Anyway, if the sun gets cooler again (not happening yet), perhaps we'll be able to block the feedback cycle that plunged us int last glacial period, but it could be a dangerous edge to be walking as too much warming plus excess CO₂ would also be bad.

[evan_au]

It is thought that Milankovitch cycles might be one of the triggers of climate change, on longer timescales (>10,000 years). This is caused by periodic exchanges of angular momentum between the Earth, Moon and other planets (a planetary extension of the dreaded "3-body problem").

These cycles cause changes in the eccentricity of the Earth's orbit, and the angle of the Earth's axis (and hence the severity of Summer and Winter). There are several such cycles with different periods; sometimes they tend to add, and sometimes they tend to cancel. But once you tip the Earth into an ice age, the ice reflects more of the sunlight back into space, tending to reinforce the ice age. Similarly, when the ice retreats, the exposed dirt absorbs more heat from the Sun, melting small snowfalls and reinforcing the interglacial.

The influence of human activity on climate is negligible.

Activities like agriculture, industrial revolution, mass production of chlorofluorocarbons and growth in per-capita energy consumption are just some of man's contributions to this mix of climate factors.

• With 1 million humans on Earth , man's contribution would have been negligible.
  
• With 7 billion humans it can no longer be ignored
  
• In the USA, the per-capita power consumption is around 10kW, which is as much power as wielded by 50 of your stereotypical cave-men 
• ...and many other countries aspire to this as a target. 

In the 21st century, "I'm Irresponsible" is no longer a defense.