While the weather can change in just a few hours, climate changes take place over longer timeframes. Climate “events”, like El Nino, happen over several years, small-scale “fluctuations” happen over decades, and larger climate changes happen over hundreds and thousands of years. Climates have always changed. According to climate scientists, today the Earth is warming more quickly than it has in the past. Hot summer days may be quite typical of climates in many regions of the world, but global warming is causing Earth’s average global temperature to increase. The amount of solar radiation, the chemistry of the atmosphere, clouds, and the biosphere all affect Earth’s climate.
According to the current findings of Intergovernmental Panel on Climate Change (IPCC), the world’s surface air temperature has increased an average of 0.6°C (1.0 °F) during the 20th Century. That may not sound like very much, but even one degree can cause changes around the world. Additionally, given the accelerating rate of temperature rise, the IPCC projects that during the 21st century, temperatures will rise much more than they did during the past century. Since temperatures will likely continue to climb, it is important to understand how the Earth has responded to climate change during the past century and to be able to better predict how it may respond in the future.
Read more about "Climate and Global Change" at Windows to the Universe.