Global warming seems to have paused since the turn of the millennium, but climatologists believe this slowdown is not a reason for celebration.

The Pacific Ocean is cooling, which could be offsetting some of the warming caused by the release of carbon dioxide and other greenhouse gases into the atmosphere, researchers believe.

El Niño cycles in the Pacific Ocean warm the water, and the last of these cycles took place between 1997 and 1998. Since that time, the world's largest ocean has exhibited a cool phase. However, these conditions will not last, and once the cycle reverses its trend, global warming will once again start to heat the planet, researchers note.

"It's fair to say that over the next couple of decades, we would expect to see the trend reverse, and internal variability accelerating the warming,"  Byron Steinman, at the Large Lakes Observatory managed by the University of Minnesota, said.

Previous theories held that heat could be stored deep in oceans around the world, or that atmospheric aerosols reflecting heat back into space could be reducing warming.

"What this study addresses is what's better described as a false pause, or slowdown. There have been various explanations for why [the slowdown is happening], none of which involve climate models being fundamentally wrong," Michael Mann, a climatologist at Pennsylvania State University, said.

Some critics of the idea of man-made global warming insist the reduction in global warming seen in the last 15 years shows human activities are having minimal effect on climate change. On February 26, Senator Jim Inhofe of Oklahoma, chairman of the Senate Environment and Public Works Committee, brought a snowball into the Senate chambers, in order to illustrate that global warming is not taking place.

The largest ocean on the globe cycles through temperature changes over a period of 16 to 20 years. During this time, waters experienced El Niño, as well as cold La Niña periods, each lasting between three to seven years.

"The North Atlantic and North Pacific oceans appear to be drivers of substantial natural, internal climate variability on timescales of decades," Mann said.

Climate models were averaged in an effort to eliminate random variability present in the simulations (which is also seen in the real world). Researchers can then subtract the effects of human activity on the environment, as well as dramatic natural events, such as the eruption of volcanoes. Doing so provides  researchers with a model showing the effect of ocean cycles on global temperatures.

Investigation of the role of Pacific cooling on the global warming pause was published in the journal Science.