I asked him to make his account as simple as possible, but no simpler. So it still takes a wee bit of effort.
As someone explained an aspect of this to me in another context:
"Understanding the sensitivity of the earth's climate to man made greenhouse gas emissions is difficult. We don't yet know how big the effect will be, but we can be sure that there will be an effect, and that this could be quite large.
Think of the extra greenhouse gas as analogous to additional weight on the accelerator pedal of a car. If one is not familiar with the car, one cannot be sure how quickly it will respond and how fast it may go. Climate modelling helps to limit the range of uncertainty.
The thing is, we don't know whether we're driving a Honda Civic or an S Class Mercedes, and we're not sure what happens when someone tunes up the engine or other parts of the car's system relevant to acceleration".
"Greenhouse gas emissions from human activities are likely to have a much larger effect on the climate during the 21st century than any natural factors, short of very low likelihood events such as a collision with a large meteor" (full text here).
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