The earth's climate has changed throughout earth's history. There have been periods, such as the glacial periods (ice ages) where earth has been covered by ice or interglacial periods where ice has retreated entirely to the poles or has melted completely. Samples left by ancient animals and plants can give clues about past climate change. Scientists are able to measure the rates at which climate has changed by analysing data from natural recorders of climate variability found in ice cores, tree rings, fossil pollen, corals, ocean sediments and historical data. these resources can give clues at how global climate has been changing for millions of years.
Every day, weather stations, ships, climate reference stations, buoys, and autonomous gliders in the oceans are recording temperature around the globe. Satellites orbiting the Earth are collecting climate data, as well. All this data is processed, analyzed and combined to give a global average temperature. Global warming can be confirmed by observations, including the melting of glaciers at the poles, rising sea levels, ocean heat, reduced arctic sea ice and the earlier blooming of plants in spring. Global average temperatures show an increase of 1.4 °F since the beginning of the 20th century.
Twenty of the warmest years have occurred in the last thirty years. The global sea level has increased at an average rate of 1.7 mm per year over the last century, and this increase has doubled since 1993 at a rate of approximately 3.5 mm per year. Much of sea level rise is due to thermal expansion caused by increasing heat of the ocean. Snow cover in the Northern Hemisphere has decreased in recent decades. Glaciers have retreated significantly in the last century due to climate change.
The way in which people know that global climate has been changing for millions of years is through the data collected from the remains of fossils, corals, tree rings, ice and rocks. The information contained within these sources offer evidence that the climate has changed many times throughout history. There have been periods of time when most of the Earth's surface has been covered with ice, and periods when the Earth has become warmer.
Paleoclimatology is the science that deals with the climatic variations occurred throughout earth's history. Through data gathered from rocks, sediments, microfossils and other sources, scientists determine the climate that was prevalent in various regions of the world. Bubbles of air trapped in layers of snow give clues to the composition of the air millions of years ago. Through the analysis of ice cores, paleoclimatologists can determine data, including solar activity, temperature change, volcanic ash and amount of CO2 in the atmosphere.
The difference in thickness of the layers in the earth's mantle can be used to identify temperature variations and rainfall levels. Pollen found in the ice cores can reveal the kind of plants in a region, which, as a consequence, leads to an understanding of the temperature, precipitation, flora and fauna. The temperature patterns observed in the ice cores are used to infer the climatic changes in a region.
Sedimentary deposits of tropical plants and animals during the Holocene Maximum, Medieval Warm Period and Little Ice Age can reveal the climatic conditions and an understanding of the reason of animal migration and their subsequent extinction. Migration patterns followed by certain animal species can tell us about the weather conditions which made them displace from a region to another.
The study of data collected from microfossils, ocean sediments, tree rings, pollen samples and sedimentary rocks can tell people how climate change has affected the ecosystems of the world in the past. The traces deposited by plants and animals on the snow, rocks, seas, corals, etc., give an idea on how global climate has been changing for millions of years.