Clues to Past Climate Change

A sampling of paleo-climate reconstruction studies

How can we measure past climate? Because it is not possible to go back in time to see what climates were like, past climate cannot be estimated or measured using ordinary methods. In response to these challenges scientists have developed a number of methods, which are known as climate proxies.

What is a climate proxy? Climate proxies are objects that can be measured to create approximations of past climate patterns. A climate proxy must possess a “climate” component and a “date” component, which together allow the reconstruction of the past climate for that locale. There are many climate proxies that can be used but the four most commonly studied are; ice cores, tree rings, coral, lake and ocean floor sediments.

World Data Center for Paleoclimatology - Data Sets Listing This website contains the databases (collection of data from various studies) a wide variety of climate proxies http://www.ncdc.noaa.gov/paleo/datalist.html

Ice Cores Ice cores have been one of the most valuable records of past climate since they can provide information on temperature, atmospheric characteristics, precipitation, and even volcanic activity from the past!

For example; The thickness of each ice layer tells us how much snow accumulated during the year , indicating the general climate.

More information can be extracted from chemistry-based (CO2, SO4, even dust) ice core studies which indicate higher and lower air temperatures. Although ice core records are limited to areas where ice could have existed at the time of formation they still gives us a general pattern of past climate. http://www.iceandclimate.nbi.ku.dk/research/past_atmos/ice_core_impurities/dust_in_ice_cores/

Tree ring studies (Dendrochronology) Many tree species can live for several hundreds of years. However, the Bristlecone Pine can live for up to five thousand years and so is particularly useful as records of past climate.

Tree rings indicate not only past air temperature but moisture and cloudiness. A narrow ring implies a cold and dry season while… a wide ring indicates a warm and wet season.

Coral studies Corals also form annual growth rings. As corals grow, layers of calcium carbonate (CaCO3)are deposited in its skeleton.

Thicker growth recorded by the “coral rings” indicates a warmer water temperature at that time. Chemistry-based studies are possible because the calcium carbonate skeleton absorbs 18O when the oceans are warm but 16O when they are cooler.

Lake and ocean sediments studies Lake and ocean bottom sediments, formed by seasonal deposits, can also act as a climate proxy thanks to their layered structure. The composition of each layer of sediment changes according to the climate conditions at the time.

Fossils of sea animals may reflect the temperature of the ocean at the time as many sea animals live in a narrow range of temperatures. Sediment layers formed in warmer areas will contain fossils with a higher growing temperature.

Putting it all together Glaciologists, dendrochronologists, oceanographers, and sedimentologists assemble these pieces of evidence and reconstruct ancient climate (paleo-climates) in order to test climate models which are then used to make predictions about future climate.