Some people argue that global warming (or, more accurately, global climate change) is simply a result of cyclical climate changes that take place naturally throughout time. However, in the past 30+ years, scientists have accumulated lots of data that points to human activity as the main contributor to the big worldwide climate changes.
The earth's atmosphere is now the warmest that it has ever been in recorded history. And it continues to get warmer with each passing year. This warming just happens to coincide directly with the increasing levels of carbon dioxide (CO2), methane, and nitrous oxide released into the atmosphere every year. (These are also known as "greenhouse gasses.") Some levels of greenhouse gasses in the earth's atmosphere are natural, but natural sources are tiny compared to the huge amounts that come from man-made sources.
The greatest amount of man-made greenhouse gasses is generated by the burning of fossil fuels such as coal, oil, and gas. Think power plants, automobiles, manufacturing, land development, and industrial farming. All of these human activities result in tons of CO2 and other greenhouse gasses being released into the atmosphere every day.
Since the dawn of the Industrial Age (just under 300 years ago), when humans first began relying on fossil fuels as a main source of energy, the world's population has managed to alter the earth's atmosphere significantly. As we humans continue to learn about the impact of climate change on our planet, we will also figure out new ways to counter its effects.