Over geologic time, within a fluctuation of a few degrees, the Earth's climate has been moderate and stable.  The climate has long been similar to today's despite the fact that, in the past, carbon dioxide in the atmosphere has typically been very much greater than it is today.  Why is this so?  No one knows.  That is what scientists should be trying to discover.

The technical community is familiar with the concept of "black boxes."  The contents of a black box are, by definition, unknown.  What is known are the signals that go into the box and the results, or signals, that come out.  Over the years, some very sophisticated mathematical techniques have been developed to emulate the performance of various black boxes.  Really good emulation can easily fool people into believing that they understand what is taking place inside the box.  Physics gives examples.  We went to the Moon with Newton's black box model of gravity, but theoretical physicists will tell you that we really do not understand gravity.  Even though Einstein made an even better model of gravity, his model is still just a black box surrogate of the real thing.  Quantum mechanics is another example of a very good black box emulation.

Much of the controversy about the question of man-caused global warming really has to do with the black box emulations that climatologists have created.  It is important to recognize that such computer models are tautologies.  They tell us the consequences of the assumptions that are built into them -- nothing more.  If a model's prediction diverges from measured reality, we know that something is wrong with one, or more, of its assumptions.  This divergence does not tell us which assumptions are wrong -- simply that at least one of them is wrong.  That is all one needs to know to judge climate models.

As it happens, these models have failed to predict today's temperature hiatus.  They also fail to account for past temperature oscillations in the climate.  We surmise, then, that at least one of the assumptions of today's climate models is wrong.  Therefore, the models are not to be trusted, and policy should not be based on what the models are saying.

The Earth is a physical system with heat inputs and outputs.  (That is a black box concept; it is not the real Earth.)  In order to be stable, the Earth system output must always equal the input.  For the surface region of the Earth -- the physical surface and the atmosphere -- there are three principal energy inputs: solar irradiation, solar wind, and heat rising from the interior of the Earth.

Heating from the interior appears to be both minor and relatively constant, except for geologically rare catastrophic events.  Solar wind, interacting with the Earth's magnetic field, may be a cause of the Little Ice Ages, or so some people hypothesize.  In any case, solar wind effects are not built into the climate models -- so their absence may be one of the faulty assumptions.

Accurate measurements of the sun's irradiance have been available only during the last few decades, when satellite measurements, independent of the atmosphere, became available.  The solar variation during this short period has been about a tenth of a percent and is correlated with sunspots.  Climatologists assume that this relatively constant solar irradiance is true for all time, but it may not be so.

The amount of solar irradiance that the Earth can absorb is determined by the Earth's albedo.  Albedo is a measure of the reflectance of an object.  Clouds, snow, oceans, soil, and vegetation all contribute to the average albedo of the Earth.  In general, the Earth's albedo varies with time and place.

What about the energy output from the Earth?  The Earth is a Gray Body Radiator.  It emits a vast amount of electromagnetic radiation out to space, because its temperature is greater than absolute zero and its area is large.  The amount of radiation emitted is proportional to the fourth power of an object's temperature, so it is very sensitive to small temperature changes.  The departure of the Earth from a strict black body is because its surface emissivities are different from unity and vary with time and place.  Nevertheless, the time average radiated output must exactly equal the energy absorbed, or we have runaway thermal instability.

All this boils down to the key issue: what keeps the climate stable?  The fundamental mistake made consistently by almost all global warming advocates is that carbon dioxide induces net positive feedback.  The postulated mechanism is that increased carbon dioxide warms the atmosphere, which, in turn, causes increased evaporation of water, and water vapor is a much stronger greenhouse gas than carbon dioxide.  Arguments are also made that increased water vapor increases cloud cover, and clouds act to warm the Earth.  I argue in favor of the first part of this: increased carbon dioxide increases water vapor and clouds.  However, I come to the opposite conclusion about positive feedback.

What does positive feedback mean?  Let's start with the Earth at a given temperature.  If there is a slight increase in temperature, positive feedback will increase the temperature still more.  This increase induces a still greater increase in temperature, and the temperature runs away until the oceans vaporize and the Earth becomes a hot hell like Venus.  Conversely, starting at the same given temperature, if there is a slight decrease in temperature, positive feedback decreases the temperature still more until hell freezes over. 

Obviously, at some point in this positive feedback cycle, a very powerful negative feedback must come into play.  The negative feedback must more than cancel the positive feedback and thereby return the climate to stability.  What might that negative feedback mechanism be?

The most plausible mechanism for negative feedback is cloud cover.  On the other hand, if we exclude cloud cover from consideration, then nothing that we know of is available to give us the necessary negative feedback.  It is as simple as that!  Without negative feedback from clouds, the burden is placed squarely on climatologists to find a sufficiently potent negative feedback mechanism.

How might clouds provide the needed negative feedback?  Clouds are known to affect both albedo and emissivity.  Low-altitude clouds have high albedo -- nearly 100%.  High-altitude cirrus clouds, on the other hand, tend to trap and reflect infrared radiation coming up from lower altitudes and are therefore considered to have a low net albedo -- but their albedo is still positive.  Since most of the sun's radiative input is in visible wavelengths, the best way to estimate the reflective properties of clouds, compared to the surface of the Earth, is to look at ordinary photographs of the Earth taken from space.  Clouds obviously have a major negative impact on how much of the sun's energy reaches the Earth's surface.  The more clouds, the less energy gets through.  In terms of heat absorption, then, clouds produce negative feedback.

Emissivity is a different story.  Here clouds act to reduce the infrared emission from the Earth to space.  The emissivity of clouds is about 50%, whereas the emissivities of most of the Earth's surface, including the ocean, is in the 90% range.  Moreover, clouds are cooler than the surface.  Thus, as cloud cover increases, the total emission from the Earth back into space decreases.  In terms of energy shedding, an increase in cloud cover produces positive feedback in the heat balance of the Earth. 

Which of these competing cloud effects is dominant?  Negative feedback, obviously.  Let me explain: consider that if the effects are balanced -- or have been balanced by other factors -- then the average temperature would randomly drift.  The resulting random walk process would eventually push the Earth's temperature to a hot or cold extreme, and the Earth would be dead.  On the other hand, if the net effect of clouds is positive feedback, without a countervailing more powerful, but unknown, negative feedback, the climate would, long ago, have been propelled to a hot or cold extreme, and the Earth would be dead. 

I note that the Earth is alive.