Questions 47 to 56 are based on the following passage. Global warming refers to an increase in global surface temperature resulting from an increase in the amount of carbon dioxide, methane, and certain other trace gases in the atmosphere. These gases are (47) known as greenhouse gases because they (48) to the warming of the earth’s surface and lower atmosphere, a phenomenon called the greenhouse effect. Partly because the (49) of carbon dioxide is related to the essential use of the carbon based energy sources, the (50) of global warming incorporates a broad, scientific and political debate about its significance and consequences. Today, as the results of the efforts of thousands of scientists over several decades who have studied climate and its (51) with the global land-ocean-atmosphere system, the scientific understanding of global warming is both more clear and complex. Scientists have obtained strong (52) that climate does indeed change, on scales (53) from a few seasons to millennia. Scientists have further shown that human activities can indeed influence the climate. As a sign of change in the international responses to global warming, delegates at a meeting under the Framework Convention on Climate Change in 2002 in New Delhi issued a declaration concluding that "Actions are required to (54) energy supply by developing advanced, clearer, more efficient, affordable and cost-effective energy technologies". No matter how the science and politics (55) over the coming years, one fact remains (56) : global warming presents a challenge that will be with us for decades and perhaps centuries to come.