Reconstructing Past Climate Change

Numerous data sets of temperature anomalies from various proxies, dating back to A.D. 1000. Each of the colored lines represents a different study, the authors of which are noted in the legend. The black line from 1850 to the present represents direct observations of temperature from thermometers.
Instrumental Records of Climate
Thermometers have only been in widespread use since around 1850. Thus, the instrumental record for earlier times is quite poor and full of gaps. Essentially nothing is available in the way of quantitative measurements of weather conditions for the time before 1800 A.D. To reconstruct climate change, therefore, we need to use indirect indicators. One source of information is historical records: logs, dairies, lists on when the wine harvest began, reports on when the ice first broke up in a northern river, or when the cherry trees first blossomed. In some cases, such reports go back hundreds of years, although rarely in unbroken sequence. Logs and dairies are treasured finds, they do not exist for most regions of the planet.

Proxy Records of Climate
In many areas, Earth itself has kept a detailed log, albeit in a language that must first be deciphered. Paleoclimatologists (climatologists who study past – or paleo – climates) use the term "proxy" to describe a way that climate change is recorded in nature, within geological materials such as ocean or lake sediments, tree-rings, coral growth-bands, ice-cores, and cave deposits.

For a proxy to be useful it must first be established that the proxy (i.e. tree-ring width, stable isotope composition of ice, sediment composition) is in fact sensitive to changes in temperature (or some other environmental parameter). This phase of research is known as calibration of the proxy. Perhaps the most frequently used temperature proxy is the relative abundance of microfossils in sediments. That microfossils bear witness to temperature was recognized early in the history of oceanography. John Murray, naturalist of the Challenger Expedition (1872-1875), found that planktonic foraminifers not only make up much of the sediment on the sea floor –but that different species indicate the temperature of the waters wherein they live. The simplest way to estimate temperature from such an assemblage is to assign to each species a typical temperature and calculate the weighted average depending on abundance.

Tree-ring width is another reliable proxy of ambient environmental conditions. When a tree grows at high elevation, near the tree limit, its growth is limited by temperature, and the thickness of its growth rings contains clues about whether the growing season was warm or cold. An equation can then be written relating the changes in ring width to temperature change. Similarly, if the growth is limited by water (say, in a warm semi-arid setting) the ring width can be used to calculate changes in rainfall. Climate proxies have been utilized to provide a semi-quantitative record of average temperature in the Northern Hemisphere back to 1000 A.D.