               Global Warming
 
    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 the 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 1846. 
    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 1846. 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.   
