On December 11, 1997, delegates from more than 150 countries signed the Kyoto Protocol, an agreement to lower the amount of greenhouse gases released into the atmosphere. Human activities release greenhouse gases into the atmosphere, which have been proven to cause climate change. The Kyoto Protocol states that industrialized nations will reduce their greenhouse gas emissions to below what they were in 1990. The Kyoto Protocol was the first major international effort to slow global climate change. Since 1997, 191 countries have backed the agreement, though the United States has not. Nations in the developing world are not bound by targets in the Kyoto Protocol. Some of these nations, such as China and Brazil, support industrial economies. Some developed nations, including the U.S., Canada, and Russia, have not agreed to emissions targets. Perhaps because of its lack of worldwide support, the Kyoto Protocol has been limited in its success: greenhouse gas output has increased since 1997, not decreased. Despite not meeting its goals, Kyoto has been significant as a symbol. It was the first step in the process to combat global warming.