I used to debate extensively with climate change contrarians on Twitter and on the blogs, but I'm more selective recently, realising that many of them are just time wasters.
The contrarians' hypothesis is that our CO2 will not have a serious effect on Earth's climate.
Some claim that CO2 has NO effect on climate. There are the ones who talk instead about CO2 being a plant fertiliser, being a trace gas and other red herrings. They are total time wasters, not worth debating since they deny physical science in the way creationists deny biological science.
Others accept the physics of the situation, and accept that doubling CO2 levels will cause a 1.2C rise in global temperature eventually, when the globe comes to equilibrium (that is, when the ocean has finished taking in extra heat. Oceans are slower to heat than land).
This second group - who call themselves lukewarmers - are worth debating time, since they do at least accept basic physics. The debate with them is over climate sensitivity - the extent to which feed backs will amplify this initial 1.2C rise.
In fact, there is an overlap between the sensitivity figure accepted by climatologists and that accepted by "lukewarmer" contrarians. It is in the region of 2C.
Therefore the debate has moved on, to the nature and stability of the climate changes that will occur about 2C. Naturally the lukewarmers say that these changes will be minor. Climatologists have a more serious view of a 2C rise.
But that's where the debate is at these days.