Weather is local and short-term. If it snows in the town where you live next Tuesday, that's weather. Climate is long-term and doesn't relate to one small location. The climate of an area is the average weather conditions in a region over a long period of time. If the part of the world you live in has cold winters with lots of snow, that would be part of the climate for the region you live in. The winters there have been cold and snowy for as long as weather has been recorded, so we know generally what to expect.
It's important to understand that when we talk about climate being long-term, we mean really long-term. Even a few hundred years is pretty short-term when it comes to climate. In fact, changes in climate sometimes take tens of thousands of years. That means if you happen to have a winter that isn't as cold as usual, with not very much snow -- or even two or three such winters in a row -- that isn't a change in climate. That's just an anomaly -- an event that falls outside of the usual statistical range but doesn't represent any permanent, long-term change.
It's also important to understand that even small changes in climate can have major effects. When scientists talk about "the Ice Age," you probably envision the world frozen, covered with snow and suffering from frigid temperatures. In fact, during the last ice age (ice ages recur roughly every 50,000 to 100,000 years), the earth's average temperature was only 5 Celsius degrees cooler than modern temperature averages [Source: NASA].
Global warming is a significant increase in the Earth's climatic temperature over a relatively short period of time as a result of the activities of humans.
In specific terms, an increase of 1 or more Celsius degrees in a period of one hundred to two hundred years would be considered global warming. Over the course of a single century, an increase of even 0.4 degrees Celsius would be significant. The Intergovernmental Panel on Climate Change (IPCC), a group of over 2,500 scientists from countries across the world, convened in Paris in February, 2007 to compare and advance climate research. The scientists determined that the Earth has warmed .6 degrees Celsius between 1901 and 2000. When the timeframe is advanced by five years, from 1906 to 2006, the scientists found that the temperature increase was .74 degrees Celsius.
Other observations from the IPCC include:
- Of the last 12 years, 11 have ranked among the warmest years since 1850.
- The warming trend of the last 50 years is nearly double that of the last 100 years, meaning that the rate of warming is increasing.
- The ocean's temperature has increased at least to depths of 3,000 meters (over 9,800 feet); the ocean absorbs more than 80 percent of all heat added to the climate system.
- Glaciers and snow cover have decreased in regions both in the Northern and Southern hemispheres, which has contributed to the rise of sea levels.
- Average Arctic temperatures increased by nearly twice the global average rate over the last 100 years (the IPCC also noted that Arctic temperatures have are highly variable from decade to decade).
- The area covered by frozen ground in the Arctic has decreased by approximately 7 percent since 1900, with seasonal decreases of up to 15 percent.
- Precipitation has increased in eastern regions of the Americas, northern Europe and parts of Asia; other regions such as the Mediterranean and southern Africa have experienced drying trends.
- Westerly winds have been growing stronger.
- Droughts are more intense, have lasted longer and covered larger areas than in the past.
- There have been significant changes in extreme temperatures -- hot days and heat waves have become more frequent while cold days and nights have become less frequent.
- While scientists have not observed an increase in the number of tropical storms, they have observed an increase in the intensity of such storms in the Atlantic correlated with a rise in ocean surface temperatures.