Gallia omnia est divisa in partes tres. This phrase from Julius Gaius Caesar about the division of Gaul nicely illustrates the universe of climate scientistsalso divided into three parts. On the one side are the warmistas, with fixed views about apocalyptic man-made global warming; at the other extreme are the deniers. Somewhere in the middle are climate skeptics.

In principle, every true scientist must be a skeptic. Thats how were trained; we question experiments, and we question theories. We try to repeat or independently derive what we read in publicationsjust to make sure that no mistakes have been made.

In my view, warmistas and deniers are very similar in some respectsat least their extremists are. They have fixed ideas about climate, its change, and its cause. They both ignore inconvenient truths and select data and facts that support their preconceived views. Many of them are also quite intolerant and unwilling to discuss or debate these viewsand quite willing to think the worst of their opponents.

Of course, these three categories do not have sharp boundaries; there are gradations. For example, many skeptics go along with the general conclusion of the warmistas but simply claim that the human contribution is not as large as indicated by climate models. But at the same time, they join with deniers in opposing drastic efforts to mitigate greenhouse (GH) gas emissions.

I am going to resist the temptation to name names. But everyone working in the field knows who is a warmista, skeptic, or denier. The warmistas, generally speaking, populate the U.N.s IPCC (Intergovernmental Panel on Climate Change) and subscribe to its conclusion that most of the temperature increase of the last century is due to carbon-dioxide emissions produced by the use of fossil fuels. At any rate, this is the conclusion of the most recent IPCC report, the fourth in a series, published in 2007. Since I am an Expert Reviewer of IPCC, Ive had an opportunity to review part of the 5th Assessment Report, due in 2013. Without revealing deep secrets, I can say that the AR5 uses essentially the same argument and evidence as AR4so let me discuss this "evidence" in some detail.

IPCC-AR4 uses only the global surface temperature (GST) record (shown in fig. 9.5 on page 648). It exhibits a rapid rise in 1910-1940, a slight decline in 1940-1975, a sharp jump around 1976-77and then a steady increase up to 2000 (except for the temperature spike of the 1998 Super-El Niño). No increase is seen after about 2001.

Most everyone seems to agree that this earlier increase (1910-1940) is caused by natural forces whose nature the IPCC does not specify. Clearly, the decline of 1940-1975 does not fit the picture of an increasing level of carbon dioxide, nor do the jump and spike. So the IPCC uses the increase between 1978 and 2000 as evidence for human (anthropogenic) global warming (AGW).

Their argument is somewhat strained, and their evidence is questionable. They claim that their models simulating the temperature history of the 20th century show no warming between 1970 and 2000when they omit the warming effect of the steady, slow CO2 increase. But once they add the CO2 increase into the models, they claim good agreement with the reported global surface temperature record. Ergo evidence for AGW.

There are three things wrong with the IPCC argument. It depends very much on detailed and somewhat arbitrary choices of model inputse.g., the properties and effects of atmospheric aerosols, and their temporal and geographic distribution. It also makes arbitrary assumptions about clouds and water vapor, which produce the most important greenhouse forcings. One might therefore say that the IPCCs evidence is nothing more than an exercise in curve-fitting. According to physicist Freeman Dyson, the famous mathematician John von Neumann stated: Give me four adjustable parameters and I can fit an elephant. Give me one more, and I can make his trunk wiggle.

The second question: can the IPCC fit other climate records of importance besides the reported global surface record? For example, can they fit northern and southern hemisphere temperatures using the same assumptions in their models about aerosols, clouds, and water vapor? Can they fit the atmospheric temperature record as obtained from satellites, and also from radiosondes carried in weather balloons? The IPCC report does not show such results, and one therefore suspects that their curve-fitting exercise may not work, except with the global surface record.

The third problem may be the most important and likely also the most contested one. But first let me parse the IPCC conclusion, which depends crucially on the reported global surface warming between 1978 and 2000. As stated in their Summary for Policymakers (IPCC-AR4, vol. 1, page 10): Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.

But what if there is little to no warming between 1978 and 2000? What if the data from thousands of poorly distributed weather stations do not represent a true global warming? The atmospheric temperature record between 1978 and 2000 (both from satellites and, independently, from radiosondes) doesnt show a warming. Neither does the ocean. And even the so-called proxy recordfrom tree rings, ice cores, ocean sediments, corals, stalagmites, etc.shows mostly no warming during the same period.

Now let me turn to the deniers. One of their favorite arguments is that the greenhouse effect does not exist at all because it violates the Second Law of Thermodynamicsi.e., one cannot transfer energy from a cold atmosphere to a warmer surface. It is surprising that this simplistic argument is used by physicists, and even by professors who teach thermodynamics. One can show them data of downwelling infrared radiation from CO2, water vapor, and clouds, which clearly impinge on the surface. But their minds are closed to any such evidence.

Then there is another group of deniers who accept the existence of the greenhouse effect but argue about the cause and effect of the observed increase of atmospheric carbon dioxide. One subgroup holds that CO2 levels were much higher in the 19th century, so there really hasnt been a long-term increase from human activities. They even believe in a conspiracy to suppress these facts. Another subgroup accepts that CO2 levels are increasing in the 20th century but claims that the source is release of dissolved CO2 from the warming ocean. In other words, they argue that oceans warm first, which then causes the CO2 increase. In fact, such a phenomenon is observed in the ice-core record, where sudden temperature increases precede increases in CO2. While this fact is a good argument against the story put forth by Al Gore, it does not apply to the 20th century: isotopic and other evidence destroys their case.

Another subgroup simply says that the concentration of atmospheric CO2 is so small that they cant see how it could possibly change global temperature. But laboratory data show that CO2 absorbs IR radiation very strongly. Another subgroup says that natural annual additions to atmospheric CO2 are many times greater than any human source; they ignore the natural sinks that have kept CO2 reasonably constant before humans started burning fossil fuels. Finally, there are the claims that major volcanic eruptions produce the equivalent of many years of human emission from fossil-fuel burning. To which I reply: OK, but show me a step increase in measured atmospheric CO2 related to a volcanic eruption.

I have concluded that we can accomplish very little with convinced warmistas and probably even less with true deniers. So we just make our measurements, perfect our theories, publish our work, and hope that in time the truth will out.