Modtran is not a climate model, it is an atmospheric radiative transfer model used in the study of climate. Gary F's answer is very informative and Ottawa Mike's answer is insightful into the minds of deniers. Clearly if you assume a value of climate sensitivity that is close to zero you can make the warming due to CO2 as low as you want. Ottawa Mike would have you believe this is an honest thing to do.

This technique by deniers reminds me of the classic Sidney Harris cartoon:

http://www.google.com/imgres?imgurl=http...

Climate sensitivity with a doubling of CO2 is very low. CO2 levels have gone up 40% since the late 1800s, yet average temperatures have only gone up 1%. AGW climate scientists are trying to tell us that most all of that warming is CO2 induced. Past temperature records show how variable our average temperature really is and the temperature fluctuation is normal.

Why do I need a climate model to tell me that? ... or even a scientific formula to understand the climate? Simple math and observation has always worked as long as you can trust what others around the world tell you.

One only has to consider that the IPCC is being paid to prove that CO2 is causing the planet to warm and "that" evidence is stacked up against them. The benefits of added CO2 in our atmosphere far outweigh its detriments. It's essential and very beneficial to all of life.

Double the current CO2 index from 400 ppm to 800ppm. Will the atmospheric temperature double? Big no. The atmospheric temperature will advance possibly only several degrees. Why? Because heat moves rapidly from 'hot' to cold. Where is the 'cold' as opposed to the 'hot' in this equation? Sea water and ice. Sea water, because there's so much of it can absorb considerable heat without raising its overall temperature by much, so see only a slight increase there. Ice is a different story. What we do observe in the northern hemisphere is a geologically and historically rapid melting of both floating sea ice and glacier ice... that's where most of the atmospheric heating goes.

In the southern hemisphere we see melting of sea ice, but as the Antarctic is mostly high mountains most of the ice exists well above the 32.1 degree line. Oddly, because of increased moisture due to even the slight atmospheric warming we see there tends to be greater snow fall in these high mountains so the ice load there may even be increasing as the evidence shows.

Conclusion: As long as there significant ice to absorb atmospheric heat and to melt, as a response atmospheric heating will be moderate. When the ice is mostly gone, and should the CO2 levels continue to rise the resulting greenhouse effect will be considerable

The fact I can use words does not make me an novelist.

The fact that you can run a piece of software does not mean the outputs are meaningful. This is what skeptics and deniers tend to forget. Science isn't just collecting data or running models. It's having years and years worth of experience not to draw the wrong conclusions from that data or those models. Someone drew the wrong conclusion. Skeptics and deniers liked those conclusions so now defend them. Fortunately, professional scientists are not as biased as they are.

I think it shows how important assumptions are regarding climate feedbacks. So this is, as shown by Gary F's answer, another "my assumptions are better than your assumptions" exercise with the added intellectual superiority shot (from Gary F) of your assumptions being not incorrect or miscalculated but "dishonest".

And evidence that Archer really believes his assumptions (and other alarmist scientists) are the best is perhaps shown by a cartoon on his web page (another intellectual superiority shot): http://geosci.uchicago.edu/~archer/scien...

Edit: For reference, I assume you mean an analysis such as this one: http://tallbloke.wordpress.com/2012/09/1...

Edit: This is the study that Gary F's realclimate.org post is talking about: http://www.davidarchibald.info/papers/Pa...

There may be climate change and many oyher problem related to climate

According to David Archer at the University of Chicago who developed the web interface for MODTRAN, the reason for the difference is Denier incompetence/dishonesty in running the program.

http://www.realclimate.org/index.php/arc...

http://geosci.uchicago.edu/people/archer...

are run on the US air force program MODTRAN why are the results less than the IPCC 3.7w M2, using the worst configuration tropics clear sky no clouds, the highest they could achieve is 3.2w