"It is the largest increase we have ever seen in the 30 years we have had this network," Dr Oksana Tarasova, chief of WMO's global atmosphere watch programme, told BBC News.
"The largest increase was in the previous El NiƱo, in 1997-1998 and it was 2.7ppm and now it is 3.3ppm, it is also 50% higher than the average of the last ten years."