Global warming is the concept that the world is being heated by human activity and emissions from human industry and vehicles. This concept has been studied for decades, and has generated a lot of controversy, but is widely held to be true. Models of global warming predict major polar ice melts, rising sea levels, severe weather, and serious deterioration of water resources, agriculture, and other essential human resources. Many governments are currently putting into place local policies to deal with global warming, and attempting to curb emissions. Alternative energy sources are also being widely deployed commercially to combat emissions and pollution.