A recent model study of some hypothetical effects of biological feedbacks of climate sensitivity to doubled CO2 came into the comments here. The paper was referenced in an article on the Register and is being misused as a revelation that the world is not going to warm so much after all.
mandas had a good go at it, identifying the assumptions and qualifications that the paper explicitly enumerated and the Register article completely ignored. I thought readers might also be interested in this video debunking from "potholer54" on youtube.
Just an aside: it is funny how worthless models are when the match the historical record beautifully and predict dramatic warming in the future but are not even on the discussion radar when they produce any excuse for downplaying the severity of our situation.
One of the arguments used by your average denier is that CO2 is in such small quantities, and the rise is so small, that it could not possibly make any difference to temperature or climate.
It is a stupid argument, but one which is used often nonetheless. The reason CO2 is a GHG is that it is effectively transparent to incoming light, but is effectively opaque to outgoing infra-red radiation (ie heat). Whilst it is more complex than that, it is a pretty robust analogy to explain the GH effect.
Unfortunately, just saying that NEVER convinces your average denialist. But what would happen if you could show convincingly how a small amount of an 'opaque' material, and small changes to the quantities of that material, can make a huge difference to how it influences its environment?
Because CO2 is optically transparent we can't see its effects. But what if we were to substitute it with an optically opaque material, so we can actually see the differences? This video provides an excellent example of how even very small quantities (such as the change in concentration of CO2 in the atmosphere) can substantially change its transparency to radiation (in this case visible light), and provides an excellent analogy of changing effects of increased CO2 (with acknowledgement to Climatecrocks.com and dannyastroYT):