With sun's activity set to diminish, is global cooling coming?

The debate over global warming may be heating up again amid new scientific evidence that the sun's activity is cooling down -- which will cause temperatures to fall on planet Earth, scientists say.

A recent surge in scorching solar flares millions of miles long comes from a peak in the sun's activity cycle. Yet "Cycle 25," the next 11-year activity phase, will be one of the weakest in centuries, NASA predicts -- a decrease that will mean fewer flares and more fleece sweaters.

“If solar output reduced below that seen [in the late 1600s] the global temperature reduction would be 0.13 C,” the U.K. Met Office said.

That's not a big change, of course. But since global air temperatures have remained more or less flat over the past 12 years, according to the newest climate data, the coming lull in the sun’s activity may mean a decrease in world temperatures.

Indeed, a chart of global temperatures released last week by British climate scientists at the Met Office and the University of East Anglia's Climatic Research Unit shows average temperatures on planet Earth reverting in 2011 to the same levels seen in 1997 -- still elevated, but unmistakably down from the peaks of last decade.

The average global temperature in 2011 was 0.36 C above average, the exact same average as in 1997. In the previous decade, temperatures held fairly steady at around 0.44 C degrees above average.

An article in The Daily Mail goes so far as to say those numbers reveal a cooling trend -- and even foreseeing “a mini ice age” in the future.

“The figures suggest that we could even be heading for a mini ice age to rival the 70-year temperature drop that saw frost fairs held on the Thames in the 17th Century,” the paper said.

Yet the Met's researchers insist the planet is still warming.

“What is absolutely clear is that we have continued to see a trend of warming,” said a spokesman for the Met Office. “The decade of 2000-2009 [is] clearly the warmest in the instrumental record going back to 1850.”

The newly published numbers from East Anglia -- numbers largely confirmed by U.S. climate scientists at the National Oceanic and Atmospheric Administration (NOAA) and the National Air and Space Administration (NASA) -- show little temperature variations over the past 15 years, with averages ranging from half a degree Centigrade above average in 1998 to as little as 1/3 of a degree Centigrade over average in 2011.

Yet The Met Office called the Daily Mail’s conclusions that the planet has not warmed for the past 15 years “entirely misleading.”

Phil Jones, director of the Climatic Research Unit, said that due to natural variability we do not expect to see each year warmer than the last, but the long-term trend is clear.

"The HadCRUT3 record, supported by the other records, is one indicator amongst several which provide overwhelming evidence that the climate has warmed," he said.

The Met was quick to clarify its own conclusions about Solar Cycle 25 as well.

"This research shows that the most likely change in the Sun's output will not have a big impact on global temperatures or do much to slow the warming we expect from greenhouse gases,” said Gareth Jones, a climate change detection scientist with the Met Office.

Indeed, brightness will play a larger role in temperature change anyway, said Mike Lockwood, an expert in solar studies at the University of Reading.

"The 11-year solar cycle of waxing and waning sunspot numbers is perhaps the best known way the Sun changes, but longer term changes in its brightness are more important for possible influences on climate.”