If you want to know what is changing climate now, it's helpful to understand what has changed it in the past. The record of the Earth's past climate comes from many different data sources
Other techniques that the general public is more familiar with include:
Ice cores: Ice cores drilled from deep within ice sheets or glaciers give us our longest look back in time—as far back as 800,000 years, so far. Analyses of the water molecules, air bubbles, and material such as ash and dust can provide information on local temperatures, greenhouse gases, volcanic eruptions, and other factors that influence climate.
Tree rings: Tree ring analysis (dendrochronology) looks at changes in the thickness of annual growth rings that are determined by seasonal and climatic variations of temperature, precipitation, or both. Dendrochronology is a very useful tool for looking at climate over the last 7,000 years or so. For example, tree rings have shown poor growth in years when ice cores indicate large volcanic eruptions, which shroud the sun and cool the earth.
Coral growth patterns: The growth rate and density of coral skeletons also vary with temperature and other environmental conditions, so their growth patterns can be analyzed much like tree rings. The chemical composition of the coral in a particular ring provides information about sea surface temperatures, salinity, runoff, and upwelling.
Climate models: Climate models are used to test explanations for past and future climate changes. For example, a model run by NCAR looked at four periods— a warm period 250 million years ago, one of abrupt warming 55 million years ago, a glacial period about 21,000 years ago, and the Little Ice Age (about 500 years ago). Scientists found that the models produced changes similar to what actually happened. This gives scientists more confidence about using models to make future estimates.