Is global warming a threat or not and if so why?
Scientists have said that events such as deforestation, the use of automobiles, burning of fossil fuels, etc. will intensify the greenhouse effect. The greenhouse effect occurs within the Earth's atmosphere and acts a "heat blanket". Over time, it slowly warms up the surface of the Earth causing global warming.
There are people who believe that global warming is a threat. Those who think it is a threat will argue that global warming is placing the planet in danger. Ice caps are melting, droughts and heat waves will occur, economic issues will surface, there will more hurricanes, and there will be an increase in disease.
There are also people who believe that global warming is a natural occurring event and will happen regardless of what we do.
Let me explain a bit more about why global warming would be a threat (assuming that it is, in fact, happening the way most scientists think it is).
As the first answer mentioned, ice caps would melt. This would raise the sea level and have huge impacts on low-lying areas of the world. This makes global warming a serious threat to the very existence of small countries made up mostly of atolls (like the Marshall Islands or the Maldives).
Global warming could also change climate patterns. This might make it so that areas that are now great for growing crops will no longer be able to sustain them.
Both of these effects would be quite serious. That is why global warming, if real, is a threat.