Although detailed climate records exist for only 150 years or so, several Earth sensors keep records that allow us to infer past climate information. These proxy climate data sources substitute for actual weather instruments.

Proxy climate data have built-in limitations because the relationship between climate-forcing mechanisms and environmental response is complex. As a result the older the period under study the more challenging and less reliable are the correlated climatic events.

Annual tree growth rings record seasonal differences in density and color of wood. The width of the rings changes in response to stressful growing conditions that are the result of climatic factors such as temperature and rainfall. Detailed tree ring chronologies date back 700 years but can be extended back in time thousands of years by matching growth rings from living trees with timbers in prehistoric dwellings.

Ponds, bogs, marshes and swamps preserve pollen grains. Borne by wind, the tiny dust-like fertilizing components of seed plants mix with clay, silt and other organic particles in sedimentary basins. Up to 20,000 pollen grains are often mixed in a single cubic centimeter of pond mud. When climate changes so does the vegetation. Changes in pollen species at various depths in pond sediment provides a record of past climatic regimes and changes over the past 15,000 years, encompassing the late ice age and modern Holocene.

Oxygen isotope analysis is useful in analyzing any structures that contain oxygen, including organic remains in deep-sea sediments, glacial ice cores, coral reefs and dripstones in caves. Data from hundreds of thousands or even millions of years can be extracted from the isotopes in these structures.

Oxygen occurs in nature in the form of two isotopes with slightly different atomic weights. Oxygen 16 atoms contain eight protons and eight neutrons, while oxygen 18 atoms have eight protons and 10 neutrons. The weight of the extra neutrons makes oxygen 18 atoms just a little more sluggish and slower to react. Although oxygen 18 is much less abundant than oxygen 16, small variations in the ratio of light to heavy oxygen exist in all aspects of the global water cycle.

Water molecules containing the lighter oxygen 16 atoms move faster at any given temperature than those containing the heavier oxygen 18 atoms. Because of this thermodynamic property, different processes cause the enrichment or depletion of one or the other of the isotopes. Melting and evaporation enrich the liquid and vapor in oxygen 16

while condensation or freezing enrich the liquid or solid in oxygen 18.

Evaporation and precipitation balances affect the isotope distribution in seawater. Moisture plumes moving from the tropics to high latitudes lose heavy oxygen through condensation so snow at high latitudes contains a higher ratio of oxygen 16 than tropical rainfall. Therefore, growing ice sheets sequester oxygen 16 while seawater has less and less and becomes enriched in oxygen 18.

Organisms such as foraminifera that extract oxygen from seawater use a higher ratio of oxygen 16 to oxygen 18 at higher water temperatures. These are preserved when the organisms die and sink to the bottom and become part of deep-sea sediments.

When pieced together and correlated from various sites around the globe, these and other methods afford us the only method of knowing about the history of climate change on planet Earth.