a near-zero pre-industrial background concentration,
primarily due to human activities. {WGI 2.3, SPM; SROC SPM}
There is very high confidence that the global average
net
effect of human activities since 1750 has been one of warm-
ing, with a radiative forcing of +1.6 [+0.6 to +2.4] W/m2
(Figure 2.4). {WGI 2.3, 6.5, 2.9, SPM}
The combined radiative forcing due to increases in CO2, CH4
and N2O
/media/loftslag/IPPC-2007-ar4_syr.pdf