Cổ khí hậu học nghiên cứu và miêu tả khí hậu trong quá khứ bằng cách sử dụng thông tin từ cả hai yếu tố không thuộc sinh vật như trầm tích trong các hồ nước lạnh hoặc trong lõi băng đá. | Green Energy and Global Warming Research 129 providing greater subsidies than the United States currently does. The United States is currently in a position to learn by the examples of several foreign countries that already understand the importance of conservation and environmental protection. For years other countries have not had access to inexpensive fuels for their cars and homes and have had to adjust accordingly. The United States is in a position now where they have an opportunity to learn from their neighbors and must use that opportunity about fuel efficiency and sustainable energy practices if the problem of global warming is to be successfully addressed. One major lesson to be learned is that by increasing renewables there are many associated benefits. Prior to the 1980s the only widely used renewable electricity technology used in the United States was hydropower. It is still the most significant source of renewable energy producing 20 percent of the world s electricity and 10 percent of that of the United States. The 1973 oil crisis grabbed the nation s attention as to its vulnerability because of its dependence on foreign oil. It was the resulting subsequent changes in federal policy that spurred the development of renewable technologies other than hydro. In 1978 Congress passed the Public Utility Regulatory Policies Act PURPA which required utilities to purchase electricity from renewable generators and from cogenerators which produce combined heat and power usually natural gas when it was less expensive than electric utilities could generate themselves. Some states especially California and those in the Northeast required utilities to sign contracts for renewables whenever electricity from those sources was expected to be cheaper over the long term than electricity from traditional sources. It was these states that had the largest growth of renewables development under PURPA. However because oil price projections were high and because utilities .
