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centromere wrote:
The problem with global warming is that because of the enormous consequences, the science community is being asked to form an opinion based on incomplete data and preliminary models. The scientific "consensus" is that climatogy is poorly understood and the current data inadequate, but if forced to make a conclusion, most scientists believe that carbon levels in air and water is trending upward, that human activity contributes significantly to those levels, and that this has potentially severe climatic repercussions.

In comparison, there is far more physical evidence supporting "scientific consensus" such as relativity, quantum mechanices, and evolution. So much so that evolution has scientific credibilty at least as high as the other two theories.


With regards to "most scientists believe that carbon levels in air and water is trending upward", you didn't specifically say "most climate scientists" - I don't think there's much debate among them on this basic fact:
http://www.globalwarmingart.com/wiki/Image:Carbon_Dioxide_40...

In short, between 1000 and 1800, CO2 levels were relatively flat - but in the last 200 years, concentrations have increased by ~35%.


To make some more general observations (as a member of the public who started looking into this about 2 years ago):


From an increase in CO2 you get a direct and immediate effect due to atmospheric absorption:
http://www.globalwarmingart.com/wiki/Image:Atmospheric_Trans...

In short, CO2 absorbs infrared radiation that comes from the warm surface - it effectively reduces/slows heat loss (which is why CO2 makes a bigger difference at night than during the day). To overturn this would require overturning quantum physics and actual experimental results.


As a brief diversion, the reason CO2 is considered much more of a problem than methane is that CO2 lasts a lot longer than methane in the atmosphere (and also, methane levels seem to be stabilising):
http://www.globalwarmingart.com/wiki/Image:Carbon_Dioxide_Re...

The above graph doesn't include methane, which is mostly gone in 12 years IIRC. However, that's only at current atmospheric levels - at 4x current levels, methane will last a lot longer, significantly increasing the overall impact.


With human-induced global warming, CO2 is the main "forcing" (cause of a change in temperature from the initial conditions). But CO2 is also a "feedback" - an increase in temperature leads to more CO2 in the atmosphere. Fortunately, this cycle tails off - else the climate would be really chaotic.

If we look at this image from the IPCC 2007 Fourth Assessment Report Summary for Policymakers:
http://www.greenfacts.org/en/climate-change-ar4/images/figur...

What this shows is the difference in energy absorption (and hence temperature) between 1750 and 2000, the main underlying causes (with error bars) and the level of confidence / scientific understanding (on the right). Essentially, the immediate effects of CO2 and other hydrocarbons are considered to be understood to a high degree. Cloud / water vapour effects are the main remaining problem.

Note however, that this is more about global temperatures - it does not cover major weather events (eg hurricanes) or sea levels. Sea level is a combination of thermal expansion and ice melt... and seems to be causing a fair amount of angst among climate scientists - the predictions in the IPCC report last year are generally considered to be "wrong" (the basic model has consistently under-estimated sea level rise) but there isn't strong agreement on a replacement. If there is by the time of the next report, expect to see a significant rise in predicted 2100 sea levels - and the denialists to cry foul.


On a final note, the way in which the climate is modelled is from first principals - to try to create an approximation of how the climate of an Earth-like planet behaves and then run a simulation of that model, on an approximation of the Earth. More computer power allows for higher resolution models (less of an approximation), and also more complex models. Unfortunately, there's currently not enough historical data to check against - a somewhat cynical view would be that the more global warming we cause, the better we'll be able to model it.
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