Or as I prefer to be called “A skeptic”
The issue of difficulty is key. Let me tell you in a few short words why I am a skeptic. First of all, if one examines the complete geological record of global temperature variation on planet Earth (as best as we can reconstruct it) not just over the last 200 years but over the last 25 million years, over the last billion years — one learns that there is absolutely nothing remarkable about today’s temperatures! Seriously. Not one human being on the planet would look at that complete record — or even the complete record of temperatures during the Holocene, or the Pliestocene — and stab down their finger at the present and go “Oh no!”. Quite the contrary. It isn’t the warmest. It isn’t close to the warmest. It isn’t the warmest in the last 2 or 3 thousand years. It isn’t warming the fastest. It isn’t doing anything that can be resolved from the natural statistical variation of the data. Indeed, now that Mann’s utterly fallacious hockey stick reconstruction has been re-reconstructed with the LIA and MWP restored, it isn’t even remarkable in the last thousand years!
Furthermore, examination of this record over the last 5 million years reveals a sobering fact. We are in an ice age, where the Earth spends 80 to 90% of its geological time in the grip of vast ice sheets that cover the polar latitudes well down into what is currently the temperate zone. We are at the (probable) end of the Holocene, the interglacial in which humans emerged all the way from tribal hunter-gatherers to modern civilization. The Earth’s climate is manifestly, empirically bistable, with a warm phase and cold phase, and the cold phase is both more likely and more stable. As a physicist who has extensively studied bistable open systems, this empirical result clearly visible in the data has profound implications. The fact that the LIA was the coldest point in the entire Holocene (which has been systematically cooling from the Holocene Optimum on) is also worrisome. Decades are irrelevant on the scale of these changes. Centuries are barely relevant. We are nowhere near the warmest, but the coldest century in the last 10,000 years ended a mere 300 years ago, and corresponded almost perfectly with the Maunder minimum in solar activity.
There is absolutely no evidence in this historical record of a third stable warm phase that might be associated with a “tipping point” and hence “catastrophe” (in the specific mathematical sense of catastrophe, a first order phase transition to a new stable phase). It has been far warmer in the past without tipping into this phase. If anything, we are geologically approaching the point where the Earth is likely to tip the other way, into the phase that we know is there — the cold phase. A cold phase transition, which the historical record indicates can occur quite rapidly with large secular temperature changes on a decadal time scale, would truly be a catastrophe. Even if “catastrophic” AGW is correct and we do warm another 3 C over the next century, if it stabilized the Earth in warm phase and prevented or delayed the Earth’s transition into cold phase it would be worth it because the cold phase transition would kill billions of people, quite rapidly, as crops failed throughout the temperate breadbasket of the world.
Three paragraphs from a longer and well written post at Watts Up With That. Read the whole post, it is well worth it.