Why do things stay in orbit?
To understand this you must know Newton's first law of motion: an object in motion stays in motion unless otherwise acted upon. Basically, when you supply force to any type of matter it will continue to do what you forced it to do. The initial speed gives the object tangential velocity (The instantaneous linear velocity of a body moving in a circular path), and it will stay in motion because of its inertia (Inertia is the resistance an object has to a change in its motion). It will continue to move until something else happens, such as collisions or an opposing force. With objects like satellites, gravity acts as an unseen string tying the objects to earth. This is an orbit.
So if things are never pulled down to earth, why should we be concerned?
We should be concerned because opposing forces do occur. Collisions between fragments of space junk often happen. These collisions break apart existing orbiters producing more hazardous material in space and knock some pieces to earth where they then become a problem. Additionally, the bits and pieces that orbit earth are not perfect spheres. Since they are not perfect spheres the gravity doesn't pull on them evenly therefore making them unstable orbiter.
How will global warming effect the space problem?
Global warming is caused by an increase in green house gasses. These gases cause a cooling in upper altitudes decreasing the density of the earth's atmosphere. This reduces density keeping objects in space longer this would be great except space junk stays up longer too. With more objects orbiting longer, more collisions occur. Because of the increased amount of collisions more fragments are produced, and with more fragments comes more problems, more damage and more destruction.
Why can't we just shoot our junk into the sun to dispose of it?
Though theoretically a good idea, this can not be compensated financially. Imagine the amount of money necessary to project materials 11.2Km/sec (necessary to escape the earth's pull of gravity) and expect to never see the ship again because it would be incinerated. Not to mention the incalculable catastrophes that would occur if any of the calculations were incorrect.
