Science Questions

Is global warming all made by humans?

I have been learning about environmental science in school (I just graduated), and have been confronted with an alternate view of global warming. My textbook tells me that global warming is entirely anthropogenic, but I have heard from some sources that the earth has cycles in which it heats and cools. These sources suggest that we are simply going through another one of these warm periods, and that the large amounts of carbon dioxide, methane, etc. are not significantly affecting the environment. How do you respond to this proposition? How have other scientists responded to this? Jeremy Conrad (via email)

[We put this question to Jonathan Shanklin, who discovered the hole in the ozone layer and was our guest on the Atmospheric Analysis show]

The Earth does go through regular cycles of warm and cold, and we are currently in one of the warm periods.  The cycles are clearly shown in Antarctic ice cores, which show several cycles over the last 750,000 years, each one lasting around 120,000 years.  Carbon dioxide levels and methane levels are high during the warm periods and low during the cold periods.  The main driver behind these changes are variations in the Earth's orbit, and these would suggest that we should now be heading towards a very slow cooling.
We have however changed our atmosphere so fundamentally that carbon dioxide levels are much higher than at any time in the last 750,000 years.  This much higher level of carbon dioxide (30%) is what is giving rise to the anthropogenic global warming.  We are probably committed to at least a 5 degree rise in average global temperature.  This will have dramatic effects on ice cover and sea level.  The need to cut carbon dioxide levels so urgently is to prevent things getting even worse.  The problem is actually even greater than this, as in the developed world we are consumming the resources of over three planets.  This is not sustainable as we only have one planet!

June 2007