It's too late for reduction, in other words.  Given that this argument is coming most loudly from the same folks who for the last 20 years have been denying that global warming is happening at all, then denying that it is anthropogenic...

Not so fast... look at the precious little facts first:

• There is a greenhouse effect.  It is caused by carbon dioxide, methane, nitrous oxides, but mostly by water vapor.  It's a good thing.
  
• The effect of any atmospheric gas is roughly logarithmic.  Increasing CO2 concentration from 0ppm to 80ppm has a lot more effect than from 300ppm to 380ppm.
  
• Atmospheric composition is not the only variable that affects average temperatures, variations in Earth's orbit are another, and I'm sure there are still others.
  
• From about 1940 to 1970, average temperature on Earth was falling.  Now it is rising, but is not (yet?) unusally high.
  
• Currently Earth is picking up excess warmth (2W/m² or so).  That means, even if we reduce the greenhouse effect, Earth will not cool, it will just stop heating at a lower peak temperature.

Considering all this, your choice of the word "deny" is wrong, because you can only "deny" a fact you are convinced of.  There is quite some reason to doubt that significant global warming occurs, that it is human made if it occurs and that it is catastrophic if it occurs.  Don't insult the few people who still approach the topic scientifically.

Assuming there are only two options...

Who said that?  There are always at least two options: 1) do $WHATEVER (nothing, reduce CO2 emission, fertilize the ocean, pray, ...) and 2) do $WHATEVER and put up the sunscreen.  The point being made is that option 2 is always better, no matter what you chose to put in for $WHATEVER.

I have a friend who's dying of cancer.

My consolations.  But regarding the metaphor: do you know a cure for the cancer?  Likely not, for killing the patient (civilization) is out of the question.  But then again, more medication might actually be an improvement:

http://www-formal.stanford.edu/jmc/progress/