Why should we use alternative energy?

Before answering this question, it would be good to discuss the terminology we use in any discussion of energy. The term "alternate energy" describes some source of energy we are not using now, but that is too vague a term for the discussion.  For example, if we live in an area in which coal is the primary energy source and we move to the use of natural gas, we are using an "alternative energy," but not one that addresses the world's climate problems.  The terms generally used are renewable energy sources and non-renewable or carbon-based energy sources or sometimes fossil fuels. 
The reason we want to use precise language is because these terms help us to explain what the point is in having this dichotomy.  In order to stave off any worse climate change and in order to have a means of providing energy in the future, we need to use the alternative of renewable energy sources as opposed to non-renewable energy sources.  The carbon-based sources of energy, coal, gas, and oil are finite sources.  Once we use them up, there are no more.  These are the sources of energy that have either created or contributed to global warming and climate change. Thus, if we use renewable sources of energy, such as sun, wind, and water, we are at the very least not making global warming any worse than it already is, and we need not worry about depleting our sources because each of these alternates is renewable. 
If we do not make the change to renewable energy sources and continue as we are, global warming and climate change will continue to get worse, with dreadful consequences for the earth.  Islands will disappear under rising seas, while coastal areas will be either under water or severely damaged.  The agricultural implications are staggering. Wheat is grown in a temperate climate.  Cows cannot probably not thrive in very hot areas.  Animal and plant species that cannot adapt will disappear, harming entire ecosystems.  Some parts of the world in which people live now will become uninhabitable.  As I write, Arizona is experiencing 120 temperatures.  Within the last year, India lost hundreds of people to an extreme heatwave.  Business activities will be severely disrupted as well.  Insurance companies that have to pay out for an entire east coast under water will not have the wherewithal to do so.  Land that is used now for the production of goods may not be viable for use in the future because of warming or extreme weather. 
All of these are reasons that we should be focusing our efforts on renewable energy sources.  Continuing to use the carbon-based energy sources will result in a devastated earth.