NephyrS wrote:
I think what we have are:
A generalized agreement that the climate is changing.
A generalized agreement that human activity is contributing to changes in levels of certain atmospheric gasses, and
A generalized agreement that certain atmospheric gasses can contribute to climate change.
The causal link between point 2 and 3 isn't one on which there's a good agreement, or a sense of magnitude. There's not a great link between the degree of climate change and amounts of gasses in atmosphere, nor is a directional link solid- ie, do levels increase with a changing climate, or do increasing levels cause a climate change. Without a solid directional link, you can't say that we are contributing to climate change- you can just say we may be.
I think the consensus view goes further than that, NephyrS. Following your and Taly's approach, I think we have:
1. A generalized agreement that the climate is changing;
2. A generalized agreement that human activity is adding
massive quantities of certain atmospheric gasses at a rapid pace;
and3. A generalized agreement that such atmospheric gasses do contribute to climate change in the direction of increased warming.
The causal link between point 2 and 3 is actually quite clear. I would even go so far as to say it's basically undisputed. There's simply no doubt that CO2, CH4, etc. are greenhouse gases that have a positive forcing effect on temperature. The magnitude of the forcing can even be calculated to a pretty precise degree. Likewise, there is no doubt that increased levels of atmospheric greenhouse gases will, over time, result in a higher equilibrium temperature for the planet. What
is unclear, however, is exactly how the interplay of various secondary effects and feedback mechanisms - e.g., increased cloud cover, changes in ocean currents, salinity and acidity, increased vegetation, loss of arctic sea ice, thawing of permafrost, etc. - will affect climate change in the near term (say, over the next century).
In short, there's no doubt that our having doubled or tripled the level of atmospheric carbon over the course of a century or two will result in a warmer global climate; we're just not sure exactly how long the warming will take or how smooth or choppy the transition will be.