A new study says that lakes and rivers in North America,Asia and Europe are freezing later and melting earlier than they did many years ago.
Some scientists say the findings provide more evidence that the Earth is getting warmer.
The findings were reported by John Magnuson and a research team from the University of Wisconsin,in Madison.
The researchers collected historical information from Eighteen-Forty-Six to Nineteen-Ninety-Five about twenty-six lakes and rivers.
The lakes and rivers are in the United States,Canada,Russia,Japan,Finland and Switzerland.
The researchers found that many lakes and rivers were freezing about nine days later and thawing about ten days sooner than they did one-hundred-fifty years ago.
They say those changes would mean a rise in air temperatures around the world of almost two degrees Celsius every one-hundred-fifty years.
Scientists say even small changes in the Earth's temperature can have serious effects on the environment.
Most scientists think the Earth is getting warmer and that people are causing the problem.
They say people are burning more fuels that trap heat in the atmosphere,such as coal and oil.
But some scientists say global warming is not real.
They say it is part of the natural system unaffected by human action.
The new results alone do not prove that the Earth's temperature is rising or that people are causing it.
But scientists say their findings support the idea that the Earth is getting warmer.
The researchers collected information from historical records.
They got the information from newspaper reports,fur traders' records,ship navigation records and notes of religious events.
They discovered the dates of ice formation and breakup on three continents since Eighteen-Forty-Six.
Some of the information was recorded because rivers and lakes were important for trade or transportation.
People also took note of freezing or thawing as a sign of the seasons.
The researchers note that the changes in ice formation on the lakes and rivers appear to have begun before Eighteen-Forty-Six.
But they say the changes were happening at a slower rate then.
They say most warming during the past one-hundred-fifty years is probably the result of human activity.