A dimming of the sun to match conditions in the "Little Ice Age" of the 17th century would only slightly slow global warming, according to a study.

A weakening of solar activity in recent years, linked to fewer sunspots, would cut at most 0.3 C from a projected rise in temperatures by 2100 if it becomes a long-lasting "Grand Minimum" of brightness, they said.

"The notion that we are heading for a new Little Ice Age if the sun actually entered a Grand Minimum is wrong," Georg Feulner, lead author of the study at the Potsdam Institute for Climate Impact Research, said in a statement.

World temperatures are likely to rise by between 3.7 and 4.5 C by 2100 if greenhouse gas emissions keep building up - far more than the impact of known shifts in solar output, the study showed.

The sun has gone through four Grand Minima since the 13th century, including the Maunder Minimum from 1645-1715 that overlapped with the Little Ice Age. The Thames River froze in London, for instance, during a "Great Frost" of 1683-84.

World temperatures have risen 0.7 ºC since the Industrial Revolution led to increasing use of fossil fuels that release greenhouse gases when burned, according to the UN panel of climate scientists.

"Current temperature data also confirm that the effect of low solar activity on the climate is very small," said Stefan Rahmstorf, also of the Potsdam Institute, of the study published in the journal Geophysical Research Letters.

Despite a deep winter chill in parts of Europe and North America, January 2010 was the equal second warmest January worldwide since records began in the 19th century, according to Nasa data. The warmest January was in 2007.

Dr Feulner said that some who doubted that human activity was to blame for global warming had wrongly suggested that a prolonged solar slowdown "might rescue us from global warming."

Sunspots, dark dots caused by shifts in the sun's magnetic field, go through an 11-year cycle. Periods with few sunspots paradoxically indicate weaker solar output.

"We have experienced a low and long solar minimum on its current 11-year cycle. Some solar scientists have suggested that it might indicate a the start of a type of Maunder Minimum," Dr Feulner said.