Climatic Data, Proxy Records

CLIMATE SCIENTISTS WOULD prefer to have all climate change information recorded by weather instruments, but suitable instrumentation was practically nonexistent before the 19th century. As a result, investigators of longer-term (paleoclimatic) climate changes rely on datable noninstrumental information. Climate scientists refer to noninstrumental records as proxy records, because they are substitutes for direct measurements taken by instruments. There are four principle proxy record sources: human/historical, glaciological, geological, and biological. The timespan and level of detailed information among the sources vary, but getting the right climate change signal is important, because proxy data play a central role in developing accurate climate prediction models.

Human (or historical) proxy evidence for climate change is taken from a diversity of sources, ranging from clues in prehistoric cave paintings, to remarks about the weather in old letters, diaries, and newspapers. Historical records of changes in crop production [Page 243]also provide valuable proxy information about changes in climate. The other three categories of proxy records are products of climate-related environmental processes. The proxy records can be hundreds, thousands, millions, or even billions of years old. Due to their long timespans, such records are the bases of most paleoclimatic studies.

Glaciological proxy records appear in glacial ice; as the climate changes, so do the physical and chemical characteristics of ice. Geological proxy data rely on the climate-related weathering of rocks and the transport, deposition, and chemistry of the weathered products (sediments) by streams, lakes, wind, and oceans. Plants keep biological proxy data, due to their dependence on climate, incorporated into their structure or reproductive capability. For instance, flowering plants of various climatic tolerances record their location, duration, and abundance by means of production of pollen, so that pollen becomes the plants' climate proxy record.

Climate scientists can infer climatic conditions from analyses of natural phenomena, but without an ability to date the material it is not possible to construct a chronology or calibrate a record. Fortunately, natural proxy records lend themselves to temporal analyses because they accumulate in datable layers, such as growth rings in trees, and layers of sediment and ice laid down in an undisturbed succession over long time intervals. Dating methods include the correlation of proxy records with radiometrie (isotopie) dating of igneous rocks, radiocarbon dating of carbon-bearing sediments, and other proxy records, counting annual layers (such as in trees, ice, corals, and lake beds), and correlating proxy records with periods of Earths orbital cycles.

The timespans of proxy records vary widely. Historical climate records are reliable for a few thousand years. The resolution of the data depends on the person recording the information. Cross-dating tree rings from various sources provide reliable records for tens of thousands of years. Ice cores, lake sediments, and coral reefs yield records for hundreds of thousands of years, and deep ocean sediments for hundreds of millions of years. The oldest climate proxy records are in marine sedimentary rocks that tectonic uplift has preserved on land. Such records are billions of years old, but they are not widely distributed. In general, the reliability, geographical range, and amount of useful climate information, increases rapidly toward the younger part of the record. The resolution of most proxy records that are in layered deposits also depends on the processes that disturb the climate after depositions and the rate of accumulation of the record, which determines how fast it gains protection from disturbances.

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