OSLO, March 26 (Reuters) - The coldest winter days in Russia and Canada have become up to 4 Celsius (7 Fahrenheit) milder since the 1950s in an extreme sign of climate change, the British Meteorological Office said on Wednesday.

A study of daily minimum and maximum temperatures said that a trend towards warmer nights and hotter days was set to bring more heatwaves and shifts in crop growing seasons.

"Minimum temperatures have seen the biggest increases, most notably over Russia and Canada, where the coldest days are now up to 4 Celsius warmer than they were in the middle of the 20th century," the Met Office's Hadley Centre said.

A statement by the Centre also said that the largest changes in maximum temperatures were "found across Canada and Eurasia where they had typically warmed by 1 to 3 Celsius". In Britain, warming was between 0.5 and 2.0 Celsius.

The study, published in the Journal of Geophysical Research, showed that "some extreme events are already increasing," said Simon Brown, Met Office climate scientist.

"The trend is set to continue with our changing climate having a significant impact, with warmer nights and hotter days in future," he said.

Last year, the U.N. Climate Panel projected a "best estimate" that world temperatures would rise by between 1.8 and 4.0 Celsius by 2100 because of a build-up of greenhouse gases, after a rise of 0.7 Celsius in the 20th century.

A heatwave in Europe in summer 2003 caused between 22,000 and 35,000 heat-related deaths and almost $14 billion in agricultural losses, the Met Office said.

But some experts say that warmer winters will bring benefits such as falls in the number of deaths from extreme cold.

Musing about the possible advantages of a warmer world, Russian President Vladimir Putin said in 2002 that milder winters might at least cut Russians' spending on fur coats. -- For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/ (Editing by Tim Pearce)

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