There is a remarkable similarity in the variations in atmospheric CO2 around the 1998 super El Nino and the 1974 event that corresponds to a major shift in North Pacific SST.
Both events and the following stabilisation periods are almost identical.
The offsets required to align the two provide an estimation of the acceleration of CO2 concentration:
( 0.5 ppm/year ) / 25 year = 2 ppm/year/century
This represents the acceleration in CO2 in the last quarter of 20th. c.
Compared to the mean acceleration of the whole record from the second derivative that gives 2.8 ppm/a/ca