First, they have not come up with any plausible alternative culprit for the disruption of global climate that is being observed, for example, a culprit other than the greenhouse-gas buildups in the atmosphere that have been measured and tied beyond doubt to human activities. (The argument that variations in the sun's output might be responsible fails a number of elementary scientific tests.)It's an effective short opinion piece. But for copyright I'd paste the whole thing. There's more here; nothing surprising but very well put.
Second, having not succeeded in finding an alternative, they haven't even tried to do what would be logically necessary if they had one, which is to explain how it can be that everything modern science tells us about the interactions of greenhouse gases with energy flow in the atmosphere is wrong.
The science of climate change is telling us that we need to get going. Those who still think this is all a mistake or a hoax need to think again.
Update: In comments to this article, Richard Reiss sent along another excellent article by David Sington about the skeptics. An excellent point that I had about given up trying to make, made very well:
In fact, only three factors determine the planet's energy balance: the sun's output, the Earth's reflectivity, or albedo, and the thermal properties of the atmosphere, which are affected by the level of certain trace gases like carbon dioxide and water vapor. Reduced to its essentials, the greenhouse effect is a problem in 19th-century classical physics, and the basic theory was worked out with pencil and paper in the 1890s. To say that increasing CO2 levels leads to more heat trapped in the atmosphere is really no more scientifically controversial than saying you'll feel warmer if you put on a sweater.Update: See also the follow-up at Dot Earth.
The difficulty arises when you try to work out what this extra heat energy will do. Will it lead to increased rainfall, or more cloud, or higher winds? It will raise temperatures, but by how much? This is where the complex computer models and the (legitimate) scientific arguments come in—accompanied by the occasional science filmmaker!