I find this EPA graph interesting:

Figure 1. This graph shows fluctuations in temperature and in the atmospheric concentration of carbon dioxide over the past 400,000 years as inferred from Antarctic ice-core records. Temperature and carbon dioxide concentrations vary roughly in tandem over the period, with corresponding peaks and troughs. At the end of the period there is a very sharp increase in atmospheric carbon dioxide levels, representing the rapid increase over the past two centuries.

Figure 1: Fluctuations in temperature (blue) and in the atmospheric concentration of carbon dioxide (red) over the past 400,000 years as inferred from Antarctic ice-core records. The vertical red bar is the increase in atmospheric carbon dioxide levels over the past two centuries and before 2006. From A. V. Fedorov et al. Science 312, 1485 (2006)17. Exit EPA Disclaimer18. Reprinted with permission of AAAS19* Exit EPA Disclaimer18.


Hmm...  looks periodic.

I'm not saying there's not a warming trend right now.  There does appear to be one.  However, we can't get any conclusive evidence that it's caused by human factors (because no one was around 200-300 thousand years ago to make temperature measurements).  As far as anyone can tell, we could be in a natural warming trend.  It's happened several times before.

What happens if we act on an "unprecedented global scale" and do something drastic to defeat global warming?  What if it doesn't work?  What would be the economic, social, and ecological impact of a failed attempt, even if the climate change is caused by humans?  What if global warming is a natural phenomenon?  Could our attempt to reverse a natural phenomenon have adverse consequences on the world's climate?