Declines in snowfall because of global warming are shortening the winter ski season in the United States and harming the economy, according to a report from two environmental groups.

In poor snow seasons such as last winter, visits drop to ski areas in states such as Colorado and New York, costing ski resorts $1 billion in revenue over the past decade, the Natural Resources Defense Council and Protect Our Winters said in the report released Thursday. The winter sports industry contributes more than $12 billion a year to the U.S. economy, the study said.

"Climate change spells trouble for all businesses dependent on winter weather, from snowmobiling, snowboarding, and ice fishing to snowshoeing and skiing," the groups said in the report.

In the U.S., average temperatures from December through February have increased by 2.2 degrees Fahrenheit (1.2 Celsius) since 1970, according to the report. The National Oceanic and Atmospheric Administration said Thursday the first 11 months of 2012 were the warmest start to any year in the contiguous 48 states since the U.S. began keeping records in 1895.

Carbon-dioxide emissions since the Industrial Revolution have led to a warming of the Earth's temperature, which threatens to cause extreme weather, drought and coastal flooding, according to the U.S. Global Change Research Program.