The first half of 2008 was the coolest for at least five years, the World Meteorological Organisation (WMO) said on Wednesday.
The whole year will almost certainly be cooler than recent years, although temperatures remain above the historical average.
Global temperatures vary annually according to natural cycles. For example, they are driven by shifting ocean currents, and dips do not undermine the case that man-made greenhouse gas emissions are causing long-term global warming, climate scientists say.
Chillier weather this year is partly because of a global weather pattern called La Nina that follows a periodic warming effect called El Nino.
"We can expect with high probability this year will be cooler than the previous five years," said Omar Baddour, responsible for climate data and monitoring at the WMO.
"Definitely the La Nina should have had an effect, how much we cannot say."
"Up to July 2008, this year has been cooler than the previous five years at least. It still looks like it's warmer than average," added Baddour.
The global mean temperature to end-July was 0.28 degrees Celsius above the 1961-1990 average, the UK-based MetOffice Hadley Centre for climate change research said on Wednesday. That would make the first half of 2008 the coolest since 2000.
"Of course at the beginning of the year there was La Nina, and that would have had the effect of suppressing temperatures somewhat as well," Met Office meteorologist John Hammond said.
"But actually La Nina is showing signs of moving towards a more neutral state."
The weakening of the La Nina effect over the last few months could see the global mean temperature creep up again in the latter part of the year, he added.
The past decade ending in 2007 was the hottest since reliable records began around 1850, according to the WMO. World temperatures are about 0.74 Celsius (1.2 F) higher than a century ago.
The U.N. Intergovernmental Panel on Climate Change (IPCC), a group of hundreds of scientists, last year said global warming was "unequivocal" and that manmade greenhouse gas emissions were very likely part of the problem.
The WMO releases its final figures for global temperature and ranking for 2008 in December.
Thomson Reuters 2008