What is climate change?From a human perspective, climate change is the departure from the expected average weather or climate normals (temperature and precipitation) for a given place and time of year. In contrast with extreme events, such as snowfall in Florida, climate change is the long-term shift in the expected or average weather. Climate change reflects significant shifts in the mean state of the atmosphere-ocean-land system that results in shifts in the atmosphere and ocean circulation patterns, which in turn impacts regional weather.
From the paleoclimate perspective, climate change is normal and part of the earth's natural variability related to interactions among the atmosphere, ocean, and land, as well as changes in the amount of solar radiation reaching the earth. The geologic record includes a plethora of evidence for large-scale climate changes. Massive terrestrial ice sheets throughout the Northern Hemisphere indicate cold conditions during the last glacial maximum (21,000 years ago). Warm climate vegetation, dinosaurs, and corals living at high latitudes during the mid Cretaceous (120-90 million years ago) indicate globally warm conditions. More recently during the Little Ice Age (roughly 1450 - 1890 AD) historic and instrumental records, predominantly around the North Atlantic, indicate colder than modern temperatures.
If global temperatures over the next decades to centuries continue to represent new highs (as seen for the 1990's), scientists will continue to conclude that climate change is occurring. Understanding the underlying processes of this change will allow us to determine whether this climate change is the result of greenhouse warming or part of longer cycle of natural climate change and variability.
The following links provide more information on Climate or Global Change:
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