If we cut past the hyperbole, the hype, and the political nitpicking, we can get to the heart of the matter: the Earth's average temperature is rising. I highlight "average," because the Earth is not a uniform body, the atmosphere is not a uniform covering (the composition and thickness varying from place to place), and the planet is not a closed system. If we go back as far as we have solid records for local temperatures, chart them, and compare the values over time, we see local variations, rises and falls, and overall, like the stock market, a continuous rise.
The planet is slowly getting warmer.
The mechanism by which this occurs has been known since the 1950's. Solar radiation bathes the Earth. Some is reflected back into space, some of it is absorbed by the upper layers of the atmosphere, and much of it penetrates to reach the surface, to be absorbed by land and water. That absorbed energy, mainly in the form of heat, is radiated back into the atmosphere, to worm its way back to space. Much of it escapes from the top of the atmosphere, but some of it is reabsorbed by certain molecules found there, which are greedy for heat energy. These molecules, the most well-known being carbon dioxide, trap the heat and re-radiate it back toward the surface of hold it in the atmosphere. In any event, the result is simple: a certain amount of these molecules holds/maintains a certain amount of heat; fewer molecules, less heat retained -- more molecules, more heat retained.
The amount of these greedy, heat-loving molecules is altered by global processes that we have an incomplete but general understanding of. For carbon dioxide, it is absorbed by plant life and converted into oxygen, or sinks into the deep ocean, or becomes locked up in rocks through many processes. It can also be released, through volcanism, large scale fires, deforestation, and of course, the burning of fossil fuels. The Earth's system for regulating the ecosphere is well-established, being as old as the planet itself. It evolved over millions of years, has survived global catastrophes and alterations, and operates on its own, subject only to the changes in the amounts of molecules in the atmosphere, the amount of solar radiation being intercepted, the amount of vegetation covering the surface, the albedo (shininess) of the surface, and myriad smaller-scale factors, which we are only now beginning to understand. While not a closed system, it is a system nonetheless, operating automatically, behind the scenes, as we go about our daily lives.
Therein lies the crux of the problem, for the system that is our ecosphere, a system governed by the large-scale effects of its constituents and the small-scale effects of the laws of physics and chemistry, is going about its business, day and night, unconcerned with our existence. Natural forces continue to shape and mold the world we live on, oblivious to our wants, desires, or preconceptions.
Belief is not required.
So the question should really be, are the actions of humanity having a measurable effect on the changes we are seeing in the global climate? And the answer must be: unequivocally. How much of an effect, and how quickly that effect is being felt, should be the object of the debate. The constant, fractious, and puerile arguments over "global warming" suffice only to waste time and effort that could be better spent determining a baseline for global climate change that would allow us to measure the significance of our impact, beyond the use of an average. We must study the Earth's systems in finer detail, to try and determine how the shifts in various factors shape the responses of the systems to our machinations. We must also find ways to mitigate our effects on the planet, for even if we determine that we are causing potentially catastrophic harm, it is better to have started to make attempts to reduce our impact on the ecosphere, than to wait until we are at the edge of the precipice.
Whatever choices we make from this moment on, the Earth will continue spinning through the cold void, sweeping through the tendrils of solar particles and waves of energy emitted by the Sun, and its systems will keep on acting and reacting to the changes that occur, a minuet of chemistry and physics. Should we fail to heed our own warnings, should we delay and deny, should we choose to put less than our best efforts into working with our home world -- as opposed to merely existing upon it, rapacious in our desire for resources -- then the Earth will not even shrug. It will simply continue on, carrying on its surface the burnt, collapsed, and abandoned remains of the only intelligent civilization that, so far as we know, ever existed.