By Richard Black
Environment Correspondent, BBC News website
We're right, the sceptics are wrong
Dr David Viner
It is the second warmest globally since the 1860s, when reliable records began, they add.
Ocean temperatures recorded in the Northern Hemisphere Atlantic Ocean have also been the hottest on record.
The researchers, from the UK Met Office and the University of East Anglia, say this is more evidence for the reality of human-induced global warming.
Their data show that the average temperature during 2005 in the Northern Hemisphere is 0.65C above the average for 1961-1990, a conventional baseline against which scientists compare temperatures.
The global increase is 0.48C, making 2005 the second warmest year on record behind 1998, though the 1998 figure was inflated by strong El Nino conditions.
The Northern Hemisphere is warming faster than the south, scientists believe, because a greater proportion of it is land, which responds faster to atmospheric conditions than the ocean.
Northern Hemisphere temperatures are now about 0.4C higher than a decade ago.
"The data also show that the sea surface temperature in the northern hemisphere Atlantic is the highest since 1880," said Dr David Viner, from the Climatic Research Unit at the University of East Anglia (UEA).
Error bar
No measurements of average temperature can be completely accurate, and Dr Viner believes the team's calculations are subject to an error of about plus or minus 0.1C.
However, he says, the long-term trend is clearly upwards - rapidly over the last decade - indicating the reality of human-induced global warming.
"We're right, the sceptics are wrong," he told the BBC News website.
"It's simple physics; more greenhouse gases in the atmosphere, emissions growing on a global basis, and consequently increasing temperatures."
However, Fred Singer from the Science & Environmental Policy Project in Washington DC, a centre of the "climate sceptics" community, disputed this interpretation.
"If indeed 2005 is the warmest northern hemisphere year since 1860, all this proves is that 2005 is the warmest northern hemisphere year since 1860," he told the BBC News website.
"It doesn't prove anything else, and certainly cannot be used by itself to prove that the cause of warming is the emission of greenhouse gases.
"It requires a more subtle examination to know how much of warming is due to man-made causes - there must be some - and how much is down to natural causes."
Eight of the 10 warmest years since 1860 have occurred within the last decade.
Global warming

Scientists in the UK, have claimed that Summer 2005 has been
the warmest in the Northern Hempisphere since reliable recording
began in the 19th Century.

The claim comes from researchers at the University of East Anglia(UEA)
working in conjunction with those of the (UK) Meteorological Office.

Dr David Viner of the UEA's Climatic Reasearch Unit is
quoted (BBC News) as saying: "The data also show that the sea surface temperature in the northern hemisphere Atlantic is the highest since 1880,"