A Convenient Little Number

Here is a nice rule of thumb: the carbon-climate response function (CCR) is, from Mathews et al (2009), 1.5°C of average global surface warming per TtC (TtC = trillion tons of carbon emitted; t="ton" = metric ton = 1,000 kg).

That is, each unit of carbon emitted causes (approximately) the same amount of warming, regardless of the carbon concentration of the atmosphere or how fast it's changing, and the cumulative carbon emitted determines the total temperature rise. regardless of exactly when it occurs (that is, assuming in occurs on century time scales and not 10s of millennia, where carbon is removed by natural process).

So CDIAC says the total carbon emitted as of 2005, from burning fossil fuels and cement production and gas flaring is 321 GtC, and carbon emissions from land use changes are 156 Tg GtC in 2005, for a total of 477 GtC (=0.477 TtC). So that would imply a warming of 0.71°C, which is pretty close.

So...if you're an American who only understands English units -- and believe it or not, some people understand even less than that, like the QVC hosts who think the moon is a planet, or perhaps even a sun, maybe with people on it -- and if, like most nonscientists, you think in terms of CO2 and not carbon...then this rule of thumb has a nice and tidy translation:

1.5°C/Tt C = 2/3 °F/T st CO2

where "st" is a short ton (=2000 lbs).

And since the range for CCR is 1.0–2.1 °C per Tt C (5th to 95th percentiles), which I'll approximate as 1.0-2.0°C, the uncertainty of the CCR is

(1.5 ± 0.5)°C/TtC = (1.5 ± 33%)°C/TtC = (2/3 ± 2/9)°F/T st CO2 

That's about as tidy as it gets, even in a system of units opposed to tidiness and clarity.

from Mathews et al, Nature (2009)