Since 1890, coal mining has also been a political and social issue. Coal miners' labour and trade unions became powerful in many countries in the 20th century, and often, the miners were leaders of the Left or Socialist movements (as in Britain, Germany, Poland, Japan, Chile, Canada and the U.S.) Since 1970, environmental issues have been increasingly important, including the health of miners, destruction of the landscape from strip mines and mountaintop removal, air pollution, and coal combustion's contribution to global warming.