Ran across this article from the Christian Science Monitor (which generally has responsible coverage about global warming/climate change that I have noticed...)
...does not believe that anthropogenic (human-caused) greenhouse gases – mainly from coal-fired power plants and motor vehicles spewing carbon dioxide – are the main culprits. In fact, he says, "It's my belief that in the last 100 years or so natural variations have played a bigger role."
Among the forces of nature he cites are changes in solar radiation, "very significant influences" of the tropical Pacific (El Nino and La Nina events in decades-long cycles), as well as changes in Earth's tilt and orbit over cycles lasting thousands of years.
This whole line of argument is tantamount to denying that CO2 acts as a radiative forcing. This is not controversial. The basic science has been known for quite some time, over 100 years. Carbon dioxide has increased by 35% since the beginning of the industrial revolution c.1850 or so. Humans are responsible. Not only is CO2 a climate forcing, but it is the largest climate forcing
As for solar radiation, yes, the Sun's output does vary. However, as shown recently in a well-publicized paper by Lockwood and Forhlich [PDF] , 'All the trends in the Sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures'. This is not speculation, this is taken from direct observation of the Sun.
There also variations of the Earth's orbit. These are known as Milankovitch cycles. As correctly noted in the quote, they occur over thousands of years. Given these long time scales, these forcings cannot be responsible for the recent increases in temperature, either. These cycles are responsible for the formation of glacial and interglacial periods and such. The tilt of the earth and other factors involved have not changed significantly over the Industrial era.
The tropical Pacific has also been behaving in something of an unprecedented fashion in recent decades. Since the mid-70s, El Nino, the warm phase of the El Nino-Southern Oscillation (ENSO) phenomenon, has been quite pronounced. There were 'monstrous' events in 82-83 and 97-98 and an extended multi-year episode in the early-90s. This century has seen El Nino in 2002-3 and 2006-7 (and almost in 2004-5). La Nina has quite quite weak since the late-80s. Even the ongoing La Nina is not particularly strong...It has really only 'limped across the line' to reach that status, and is not expected to persist.
Not much is certain about the interaction of climate change and ENSO. A highly uncertain outlook for the future suggests that it will remain about the same. But climate models do a pretty poor job at simulating ENSO, so that result is highly speculative. It is not hard to find credible papers in the literature which suggest that global warming has already affected ENSO. Further, a recent study has shown that the heat over the US in 2006 is attributed to greenhouse gases, not ENSO.
The evidence for climate change is strong. It is not just the amount of change, but the pace at which it is occurring. Without question, natural environmental variability plays a role in producing the observed changes. However, the evidence for CO2 forcing is also strong, and as it and other greenhouse gases accumulate at an ever-increasing rate, the effects are going to continue to get worse. The timing is too coincidental: a known climate forcing and a sharp temperature rise. It's not hard to connect the dots. Even if by some chance the majority of climate scientists were wrong, moving towards a greener lifestyle and society is not a bad thing. It has been estimated that up to 40% of deaths worldwide can be attributed to human pollution. It is time for humanity to clean up its act along with the planet.
We're all in this together, and together is the only way out.