Why a 2 degree max target for global warming?

[lenny]

So it seems many at Copenhagen will argue we should aim to limit the increase in global mean temperature below a "two degree target",
what exactly does that mean? and what is the scientific basis selecting 2 degrees?

[macdoc]

a) we won't get there on the targets scheduled.

b) +/- 2 degrees C is the maximum range that human civilization was established on.

c) it's considered achievable ( I doubt it ) and limits the forseeable consequences ( that's depending somewhat on methane not spiking ).

There is some good coverage here from Nature

http://blogs.nature.com/climatefeedback/

There needs to be some target....it's a long way to 2100 when BAU indicates 4 degrees as early as 2055.
That would be very bad news as we can some from the Arctic now and southern Europe even .6 degrees means major change.

The tropics have already expanded 200km and the desertification zone moved north....at .6

China will go ahead as fast as possible as they have to.

Europe is trundling along.

The US ?? Much talk...

[macdoc]

Came across this from Nature on the two degree target.

http://blogs.nature.com/climatefeedback/2009/10/the_twodegree_target_film_on_y.html

[Wangler]

b) +/- 2 degrees C is the maximum range that human civilization was established on.

Hey, Mac, what is this based upon?

[macdoc]

It's a pretty narrow range and you can image what this graph would look like extended out to +4 by 2055

http://en.wikipedia.org/wiki/File:Holocene_Temperature_Variations.png

This in no way rules out relatively large swings regionally and of course temperature is only one aspect of climate.

[maddog]

2 degrees puts France underwater -- no great loss there -- and Germany becomes oceanfront property!

[macdoc]

After a very very long while. :rolleyes: and lots of France would remain islands even then.