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By Roger Harrabin
BBC News environment analyst
Global temperatures will drop slightly this year as a result of the cooling
effect of the La Nina current in the Pacific, UN meteorologists have said.
The World Meteorological Organization's secretary-general, Michel Jarraud, told
the BBC it was likely that La Nina would continue into the summer.
This would mean global temperatures have not risen since 1998, prompting some
to question climate change theory.
But experts say we are still clearly in a long-term warming trend - and they
forecast a new record high temperature within five years.
The WMO points out that the decade from 1998 to 2007 was the warmest on record.
Since the beginning of the 20th Century, the global average surface temperature
has risen by 0.74C.
Rises 'stalled'
LA NINA KEY FACTS
La Nina translates from the Spanish as "The Child Girl"
Refers to the extensive cooling of the central and eastern Pacific
Increased sea temperatures on the western side of the Pacific mean the
atmosphere has more energy and frequency of heavy rain and thunderstorms is
increased
Typically lasts for up to 12 months and generally less damaging event than the
stronger El Nino
La Nina and El Nino are two great natural Pacific currents whose effects are so
huge they resonate round the world.
El Nino warms the planet when it happens; La Nina cools it. This year, the
Pacific is in the grip of a powerful La Nina.
It has contributed to torrential rains in Australia and to some of the coldest
temperatures in memory in snow-bound parts of China.
Mr Jarraud told the BBC that the effect was likely to continue into the summer,
depressing temperatures globally by a fraction of a degree.
This would mean that temperatures have not risen globally since 1998 when El
Nino warmed the world.
Watching trends
A minority of scientists question whether this means global warming has peaked
and argue the Earth has proved more resilient to greenhouse gases than
predicted.
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Animation of El Nino and La Nina effects
But Mr Jarraud insisted this was not the case and noted that 2008 temperatures
would still be well above average for the century.
"When you look at climate change you should not look at any particular year,"
he said. "You should look at trends over a pretty long period and the trend of
temperature globally is still very much indicative of warming.
"La Nina is part of what we call 'variability'. There has always been and there
will always be cooler and warmer years, but what is important for climate
change is that the trend is up; the climate on average is warming even if there
is a temporary cooling because of La Nina."
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China suffered from heavy snow in January
Adam Scaife, lead scientist for Modelling Climate Variability at the Hadley
Centre in Exeter, UK, said their best estimate for 2008 was about 0.4C above
the 1961-1990 average, and higher than this if you compared it with further
back in the 20th Century.
Mr Scaife told the BBC: "What's happened now is that La Nina has come along and
depressed temperatures slightly but these changes are very small compared to
the long-term climate change signal, and in a few years time we are confident
that the current record temperature of 1998 will be beaten when the La Nina has
ended."