Global warming is a gradual increase in the Earth's surface temperature (this also includes the temperature of the oceans). Some scientists believe that global warming is a natural occurring event that will happen regardless of what humans do to the environment.
If it is a threat then I do not believe it is an immediate one. Global warming occurs because of the Greenhouse Effect that traps warm air (by trapping the suns energy) around the Earth. This happens because gases are released into the air from engines, chemical plants, etc. and they combine.
I suppose you'd have to say that it depends on the person you ask. If you are among folks who believe that global warming is not caused by man, then you would be able to probably say that it is simply a cycle of nature, there have been warm periods before and cold periods and nature has always adjusted. Therefore the current trend of warming is just that, a trend, and will bring about adjustments in nature and is not a real threat to us as a species.
You could also take a view that humans may have even gotten us into this but thanks to our ingenuity, our resourcefulness, and the way that we generally respond with brilliance in times of real crisis, we will be able to handle anything, even if there is a significant and lasting rise in temperature.
I think that there are couple of ways to make this argument.
First, you can argue that it is not really happening. Some scientists try to make this argument.
Second, and I think that this is an easier argument to make, climate change is not necessarily a bad thing. Let's say that the world warms up a few degrees. This might mean that some crops can no longer grow where they now grow. But maybe those crops could be grown somewhere else and new crops that can deal with the warmer weather can be grown where they used to be.
In other words, so what if now we have to grow grain in Alaska and we can grow bananas in Kansas -- there will be somewhere for everything to grow even so.
Alternately, maybe the growing season will get longer -- you can grow more crops per year because it will get warm earlier in spring and stay warm later in fall.
In addition, as one of the links below argues, people enjoy warm weather more than cold and warm weather is healthier for people than cold