Tilting at windmills

James and Gavin?


April 5th, 2009 by Roy W. Spencer, Ph. D.

Circular Reasoning in the Theory of Manmade Global Warming

As regular readers here are already aware, I am increasingly convinced that the greatest mistake that the IPCC and mainstream climate researchers have made is their assumption that cloud cover on the Earth remains the same on climate time scales, say over 30 years or longer. The issue is critical because clouds determine how much solar energy is absorbed by the Earth, which then largely determines average global temperatures.

Assuming that cloud cover remains the same is part of the climate modelers’ worldview in which nature has been in balance until humans came along.

But there is no way to support this worldview with data…it is a matter of faith. It takes only a 1% change in global average cloud cover to cause substantial climate change, either warming or cooling. It has only been since 2000 that have we had sufficiently stable satellite data (from NASA’s Terra and Aqua satellites) to determine whether there are any long-term, natural changes in global cloudiness, and unfortunately eight years of data is not much in the context of climate change.

Even our latest, state-of-the-art radiation budget instruments on those same satellites are still not sufficient to measure whether an imbalance in the Earth’s radiative budget exists to better than about 1% (about 4 Watts per sq. meter). That’s five times the ‘manmade imbalance’ that has been theorized to exist today from anthropogenic greenhouse gas emissions.

But there’s an interesting consequence of this assumption that the climate system has always been in balance: The assumption ends up leading to a tautology – a process of circular reasoning – regarding the role of mankind in climate change. Let me explain.

As we demonstrated in our 2008 J. Climate paper, chaotic fluctuations in cloud cover can give the illusion of positive feedback – that is, of a sensitive climate system. Climate researchers who have previously tried to determine how sensitive the climate system is from satellite data have ignored the biasing effect that natural, chaotic cloud fluctuations have on their estimates of feedback. And as a result, they have overestimated climate sensitivity. (As I have mentioned before, this can also be explained as getting cause and effect mixed up when observing cloud and temperature variations.)

This, in turn, has influenced how the climate modelers build their climate models. All of the IPCC climate models now produce a sensitive climate system, which amplifies the small amount of warming from the extra carbon dioxide in the atmosphere by a sufficient amount to explain most, if not all, of the warming we have seen in the last 50 years.

So, if the climate system is sensitive, then there is no need for a natural explanation for global warming (such as clouds) because increasing CO2 concentrations are sufficient to explain the warming.

But do you see what has happened here? By ignoring natural, chaotic fluctuations in clouds, researchers have come to the (mistaken) conclusion that there is no need to look for clouds as a cause of climate change. They ended up concluding only what they had assumed to begin with!

But I will admit that circular reasoning has one advantage: It always results in a self-consistent explanation.

So, when you hear that climate researchers have determined that the only way they can explain warming in the last 50 years is with human influences, you need to be very wary. The reason they can explain the warming only when they include humanity’s greenhouse gas emissions is because we do not have sufficiently accurate global observations to say otherwise.

So, since they don’t have the data to determine whether there are natural sources of climate change, they simply ASSUME it does not happen. This makes the theory of manmade global warming to large extent a matter of faith. From talking to a few of them, I think that many of these researchers are not even aware they have made this implicit assumption.

And the truth is there is no unique human fingerprint of global warming to identify our present warmth as manmade, either. For instance, a slight natural decrease in cloud cover would let more sunlight in and cause the oceans to warm, thus causing an increase in water vapor (our main greenhouse gas), which would then look like manmade warming. Even the greater warming over land than the ocean we have experienced can be explained by warming oceans…because warmer oceans send more humid airmasses over land, which then causes a naturally-enhanced greenhouse effect over land. (This finding was published here.)

I find that meteorologists (by training) are perfectly comfortable with the idea of natural cloud fluctuations causing climate change. But climate modelers tend to look down on meteorologists, who as a group are generally distrustful of climate models.

I would argue that, until climate modelers begin to appreciate the complexities of weather – especially of clouds — as much as meteorologists do, then their efforts to predict climate change with computer programs will be as futile as tilting at windmills.