Proxies, proxies, proxiesMarch 2, 2010
Anyone viewing any of the discussion regarding past temperature reconstructions have come across the use of a “proxy” as a stand-in for a temperature measurement. A proxy is some form of determination or measurement that is shown to be dependent on another factor in which you are interested. In this case, the proxies are used as a stand-in to help make a determination of the relative measurement of past temperatures. Why do we need proxies to determine temperatures in the past? Because direct measurement of temperature has only been available since the invention of the thermometer with a standardized temperature scale which happened in the early 1700’s. Before the 1700’s there had been some attempts to construct some form of a thermometer, but no standardized temperature scale was adopted and the resulting information non-comparable between instruments. The new standardized scale (Fahrenheit) thermometer was a true state-of-the-art scientific instrument of its time, expensive and only found in the possession of early scientists and the wealthy (often the same), and not in widespread distribution or use. The further from “civilized” areas one went, the less likely that any thermometers were in use. As a result, early temperature measurements are concentrated primarily in England, parts of Europe, and in the major population centers in eastern America. Early temperature measurements in other areas of the world are rare or nonexistent. What you end up with is a limited aerial extent and quality of early thermometer measurements which only goes back slightly over 300 years with emphasis in the Northern Hemisphere. Worldwide coverage is a relatively recent occurrence, dating from the very late 1890’s and the early 20th century. Is 300 years a good enough time frame to look at possible world-wide climate changes? No! We need to know much more regarding world temperatures predating the 1700’s and some way to fill in the gaps in the temperature measurements in areas of the world that have no early records. Here is where the use of temperature proxies become very useful.
So what types of proxies are used? We need to select a proxy where the sensitivity to temperature is not swamped by other unrelated factors. We need a proxy that has a long-term presence (at least in the rage of up to several thousand, and perhaps greater years) and that is not far removed from the true temperature it will be measuring. There are several usable proxies from tree rings on long-lived trees, glacial ice from the polar parts of the world, and sediments in the deep ocean that meet some or most of these selection criteria. Unfortunately all of these proxies have compromising factors that make the certainty of the proxy measurements ability to deliver an unbiased temperature measurement a problem.
As most of us learned in elementary school botany class, trees grow by increasing the mass of the tree trunks and limbs by adding woody plant material in the form of a yearly growth rings. These rings make-up a year-by-year record that reflects the climate conditions in which the tree grew. A good year with adequate moisture would result in a larger growth ring than a drought year that results in little new growth and a smaller growth ring. Visual comparisons between rings are used to estimate these differences and a determination made concerning the overall temperature at the time of growth based on the “optimum” temperature needed for the tree growth. Dendrochronology (especially if used with C14 dating) is a great tool for determining a date for a log used in a prehistoric hut, but other environmental factors can result in quite a bit of variability in the absolute temperature measurement determination based on a tree ring growth. Drought or very wet conditions, or difference in soils from location to location, or insect infestations, fungal infections, forest fire damage, to name just a few examples of non-temperature related effects, will limit the accuracy of tree ring temperature determinations. Another limiting factor is that any long-term tree ring temperature record needs long-lived (300 yrs old or greater age) species of trees. Unfortunately most tree species are not very long lived (or harvested for lumber, etc. in the recent past) and those tree that do meet our requirements are not widely dispersed. This severely limits the usability of tree rings for historical temperature reconstructions. Another problem with tree rings as a temperature proxy is that most tropical trees lack well defined growth rings (when everyday of the year is a good growth day, ring definition is generally non-existent) giving us a real data “gap” over large portions of the globe. Scientist working with these types of proxy data use a variety of statistical methods to help to “normalize” all these type of temperature determinations and the level of “statistical certainty” varies dramatically from area to area. Tree rings analysis must be allied with other types of measurements as a check and remain a scientific iffy piece of data when used by itself as unassailable proof of the temperature determination. As proxies go, tree ring analyses have the most overall error on temperature determinations. The following graph show one of these temperature reconstructions (note the temp scale is expanded to emphasize the relative temperature changes).
Deep Ocean Sediments
Like tree rings, sediments deposited on the deep ocean floors also show a form of seasonality. Warm seasons are related to high marine production; cold seasons – lower productivity.
Core photo from IODP/USIO
There is a long history of deep ocean drilling as a vehicle for geological, biological, and oceanographic study starting with the Deep Sea Drilling Project (1963 through 1983), continuing through the Ocean Drilling Program (1985 through 2004) which has evolved into the present Integrated Ocean Drilling Program. Initially the DSDP and ODP focus was on basic earth and biological sciences research but has evolved over time (as the IODP) into one of the main collectors of proxy data on temperatures for climate reconstructions. There are a whole variety of sophisticated isotopic analyses conducted in these sediment samples although one analysis in particular is very useful as a proxy for past temperature. The ratio of the stable isotopes 18O to 16O of oxygen is particularly useful. The heaver isotope 18O condenses more readily a temperature decreases and falls as precipitation, while the lighter isotope 16O becomes more concentrated. Sediment (actually shells of a family microorganisms called foraminifera [aka diatoms]) enriched in 16O shows temperature cooling and sediment enriched in 18O shows temperature warming.
Coupled with 14C dating of organic material in the sediments allows for detailed temperature reconstructions over time ( to around 60,000 years before present) . Other isotope determinations have been used to extend the climate record to greater time intervals (back to the Cambrian (540 million years before present). Note how the following chart shows that overall climate is much cooler starting in the Paleogene (50 MYBP) to present when compared to most of the proceeding 500 mys, and as you can see the time scale, although lengthy, does not have the same level of resolution seen with tree ring data.
Here is a reconstruction of the past 5 million years from ocean sediments.
Like both tree rings and deep ocean sediments, glacial ice from both the arctic and antartic can also show layers seasonal layers created by the accumulation of yearly snowfall. As each successive years’ accumulation is buried by the following year the accumulated snow it caries with it the compounds in the air and dust. As the snow gets deeper buried by successive seasons (air temperature always below freezing), the snow crystals form firn, a granular icy material like sugar. With an accumulation of around 250 feet of thickness, the firn snow recrystallizes into ice trapping air bubbles from that time. There can be a time lag between the age of the ice and the entrapped air which may be on the order of several decades or more. Coring of this ice, results in an cylindrical ice plug (the core) with date of the ice increasing with the depth of the coring. The core is recovered and kept frozen (among other practices to eliminate unwanted “modern” contamination) , and eventually stored in a facility where scientific tests are conducted. Because there are very few areas of the work that have the geologically persistent icefields where ice cores can be collected, the important ice core temperature record are limited to parts of the Arctic and Antarctic. Some less important cores have been collected from mountain glaciers or mountain top snowfields outside the polar regions that have provided useful although age limited information. The two main analyses from ice cores that are directly related to climate and temperature studies involve the oxygen stable isotopes (18O/16O) and analysis of the air composition (CO2 and others such as methane). The two longest history ice cores are the Vostok (Antarctic) reaching back 420,000 years and the Greenland Ice Core Project/Greenland Ice Sheet Project (GRIP/GISP) core reaching back 100,000 to 123,000 years. The two following figures are time – temperature charts for these two ice cores.