The tragic thing about the thoughtless use of a stereotype (denier) is that it reveals that you really think of people in terms of its projected meaning. In particular, even in your response you seem to equate the term “skeptic” with “denier of AGW”.
This is silly. On WUWT most of the skeptics do not “deny” AGW, certainly not the scientists or professional weather people (I myself am a physicist) and honestly, most of the non-scientist skeptics have learned better than that. What they challenge is the catastrophic label and the alleged magnitude of the projected warming on a doubling of CO_2. They challenge this on rather solid empirical grounds and with physical arguments and data analysis that is every bit as scientifically valid as that used to support larger estimates, often obtaining numbers that are in better agreement with observation. For this honest doubt and skepticism that the highly complex global climate models are correct you have the temerity to socially stigmatize them in a scientific journal with a catch-all term that implies that they are as morally reprehensible as those that “deny” that the Nazi Holocaust of genocide against the Jews?
For shame.Lots of good stuff to read, but moving on to the science:
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.
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.
Now let us try to analyze the modern era bearing in mind the evidence of an utterly unremarkable present. To begin with, we need a model that predicts the swings of glaciation and interglacials. Lacking this, we cannot predict the temperature that we should have outside for any given baseline concentration of CO_2, nor can we resolve variations in this baseline due to things other than CO_2 from that due to CO_2. We don’t have any such thing. We don’t have anything close to this. We cannot predict, or explain after the fact, the huge (by comparison with the present) secular variations in temperature observed over the last 20,000 years, let alone the last 5 million or 25 million or billion. We do not understand the forces that set the baseline “thermostat” for the Earth before any modulation due to anthropogenic CO_2, and hence we have no idea if those forces are naturally warming or cooling the Earth as a trend that has to be accounted for before assigning the “anthropogenic” component of any warming.
This is a hard problem. Not settled science, not well understood, not understood.
One part of the difficulty is that the Earth is a highly multivariate and chaotic driven/open system with complex nonlinear coupling between all of its many drivers, and with anything but a regular surface. If one tried to actually write “the” partial differential equation for the global climate system, it would be a set of coupled Navier-Stokes equations with unbelievably nasty nonlinear coupling terms — if one can actually include the physics of the water and carbon cycles in the N-S equations at all. It is, quite literally, the most difficult problem in mathematical physics we have ever attempted to solve or understand! Global Climate Models are children’s toys in comparison to the actual underlying complexity, especially when (as noted) the major drivers setting the baseline behavior are not well understood or quantitatively available.
And later he sums it up:
Why am I a skeptic? Because I recognize the true degree of our ignorance in addressing this supremely difficult problem, while at the same time as a mere citizen I weigh civilization and its benefits against draconian energy austerity on the basis of no actual evidence that global climate is in any way behaving unusually on a geological time scale.