Niwa scientist Dr Jim Salinger said temperatures were about 0.7 degrees higher than at the beginning of last century.
He said one explanation was a return to warmer water in the central and eastern equatorial Pacific Ocean following the end of the La Nina climate pattern last year. Another was global warming brought on by a build-up of greenhouse gases in the atmosphere.
The warmest year on record, 1998, was also the year of the strongest El Nino for 100 years. Scientists have identified conditions which could produce a new El Nino this year.
Dr Salinger said: "Should an El Nino occur, it's likely to be a fairly warm year, well above average globally."
nzherald.co.nz/climate
Intergovernmental Panel on Climate Change (IPCC)
United Nations Environment Program
World Meteorological Organisation
Framework Convention on Climate Change
Executive summary: Climate change impacts on NZ
IPCC Summary: Climate Change 2001