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Steve Case
That's right, CH4 is not as scary as portrayed. But watts per
square meter doesn't mean anything to most people including me.
I'm not picking on you, I always run into the watts per square
meter lingo when I start looking for this answer. But it's an
answer that will mean something. After all Climate Change is all
about temperature. Just look at all the arguments over hottest
year ever etc. Temperature is what people want to know.

So your answer didn't tell me what the temperature would be for a
given increase in methane. Didn't say how long it would take either.
But you did show your work (-:

I should have asked, if you double methane how much and how long?

The how long part is easy. This web page
tells us that CH4 is in the air at about 1850 ppb and is
increasing at an average rate of about 6 ppb which works
out to around 300 years to double its concentration.

The temperature part should be easy because just like CO2
the climate sensitivity for CH4 should be well known. But
it's not. I'm talking the absolute value without feed backs.
For CO2 it's 1.2 K per doubling. I'm pretty sure the value
for doubling methane is somewhere between 0.1 and 0.3 K.
I've gotten to those answers by asking and some crude
arithmetic. The crude arithmetic got me to 0.18 K.

Here's the crude arithmetic:

Methane's concentration is about 2 ppm and an equal mass
of CO2 is about 0.7 ppm. Adding 2 ppm to the concentration
of methane is a 100% increase, and adding 0.7 ppm to CO2
is a 0.2% increase.

It doesn’t take a genius to figure out that a 0.2% increase
in CO2 won't run the temperature up very much and 84 times
that isn't very much either.

0.7 ppm/400 ppm x 1.2 K x 84 = 0.18 K

So in general terms, how much will a given rise in methane
run up the temperature? The answer is not much and it will
take a long time.

If you read all this, thanks. 


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