answer: #Person2# tells #Person1# about the reasons for developing alternative energy sources. One is that human beings have to find other fuel sources eventually, the other one is that there will be some long-term negative effects, like global warming and the greenhouse effect, if not.
documents: #Person1#: What does the term 'alternative energy source' mean?
  #Person2#: When we think of energy or fuel for our homes and cars, we think of petroleum, a fossil fuel processed from oil removed from the ground, of which there is a limited supply. But alternative fuels can be many things. Wind, sun and water can all be used to create fuel.
  #Person1#: Is the threat of running out of petroleum real?
  #Person2#: It has taken thousands of years to create the natural stores of petroleum we have available at a much faster rate than it is being produced over time. The real controversy surrounding the amount of petroleum we have is how much we need to keep in reserve for future use. Most experts agree that by around 2025, the amount of petroleum we use will reach a peak. Then production and availability will begin to seriously decline. This is not to say there will be no petroleum at this point. But it'll become very difficult and therefore expensive to extract.
  #Person1#: Is that the most important reason to develop alternative fuel and energy sources?
  #Person2#: There are two very clear reasons to do so. One is that whether we have 60 or 600 years of fossil fuels left, we have to find other fuel sources eventually. So the sooner we start, the better off we will be. The other big argument is that when we use fossil fuels for a long time, which leads to some long-term negative effects, like global warming and the greenhouse effect, there could be extinctions of many creatures, such as polarbears. It will be human beings that pay the price.