Excerpt 1 From 2003 to 2050, the worlds population is projected to grow from 6．4 billion to 9.1 billion, a 42% increase．If energy use per person and technology remain the same, total energy use and greenhouse gas emissions(mainly, CO2)will be 42% higher in 2050．But thats too low, because societies that grow richer use more energy．We need economic growth unless we condemn the worlds poor to their present poverty and freeze everyone elses living standards．With modest growth, energy use and greenhouse emissions more than double by 2050．Excerpt 2 Although the threat of global warming has been known to the world for decades and all countries and leaders agree that we need to deal with the problem, we also know that the effects of measures, especially harsh measures taken in some countries, would be nullified(抵消)if other countries do not control their emissions．Whereas the UN team on climate change has found that the emissions of carbon dioxide would have to be cut globally by 60% to stabilize the content of CO2 in the atmosphere, this path is not feasible for several reasons．Such deep cuts would cause a breakdown of the world economy．Excerpt 3 Climate change is one of the most important environmental issues facing humankind．Climate change may affect natural ecosystems in a variety of ways．In the short term, climate change can alter the mix of plant species in land ecosystems such as grasslands．In the long term, climate change has the potential to dramatically alter the geographic distribution of major vegetation types—savannas, forests, and tundra．Climate change can also potentially alter global ecosystem processes, including the cycling of carbon, nitrogen, phosphorus, and sulfur．Moreover, changes in these ecosystem processes can affect and be affected by changes in the plant species of the ecosystem and vegetation type．All of the climate change-induced alterations of natural ecosystems affect the services that these ecosystems provide to humans．Excerpt 4 Plants and animals adapt to climate change over centuries．At the current estimate of half a degree centigrade of warming per decade, vegetation may not keep up．Climatologist James Hansen predicts climate zones will shift toward the poles by 50 to 75 kilometers a year—faster than trees can naturally migrate．Species that find themselves in an unfamiliar environment will die．Excerpt 5 Scientists have long warned that some level of global warming is a done deal—due in large part to heat-trapping greenhouse gases humans already have pumped skyward．Now, however, researchers are fleshing out how much future warming and sea-level rise the world has triggered．Excerpt 6 The practical conclusion is that if global warming is a potential disaster, the only solution is new technology．Only an aggressive research and development program might find ways of breaking our dependence on fossil fuels or dealing with it.
Greenhouse emissions will more than double by 2050 because of______.