Introduction
Many diverse techniques are used to measure the worlds changing climate, and some do so more efficiently than others. In the run-up to the 2009 United Nations Climate Change Conference (commonly known as the Copenhagen Summit), the study of past climate change came under severe attack as there were numerous issues with current measuring techniques and their use for climate reconstruction. This essay will explain how several climate-measuring techniques are used, as well as their advantages and disadvantages. The use of these techniques will also be demonstrated in several case studies. The techniques that will be examined are tree ring data, ice core records, and corals.
Tree ring data
Dendroclimatology is described by Grissino-Mayer (2016) as “the science that uses tree rings to study present climate change and reconstruct past climate”. In temperate areas, trees have a yearly growing season where new wood grows and tree rings are created. During this time, tree growth is greatly influenced by variations in the climate (US Environmental Protection Agency 2016). The boundary between each tree ring is visible as at the beginning of the growing season, trees grow quickly and produce large cells and thus less-dense, paler wood is created. Darker, denser wood is formed near the end of the growing season as the cells grown are smaller due to drought stress. The width of a tree ring varies depending on whether the climate is favourable (wet and warm) or unfavourable (dry and cold) (Fritts 1976), conveyed in figure 1. Thus dendroclimatology can offer an accurate form of palaeoclimate reconstruction for most of the late Holocene.
Figure 1. An illustration of varied tree ring width and the seasons they were formed in (Paugh 2012)
This type of dendroecological application is useful in examining past climate conditions as the existing climatological records are too short to determine long-term climatic variability and changes (Fritts and Swetnam 1989). It is possible to know more about the changing climate in an even longer period of time as the rings of a dead, well-preserved tree can be correlated to the rings of a living tree, demonstrated in figure 2. The bristlecone pines of North America are the oldest trees on earth; they can survive for up to 5,000 years and are often well preserved (Salzer et al 2009).
Figure 2. Dendrochronological age determination by overlapping trees of different ages (Hammarlund 2013).
The growing period of a tree can also be affected by complex, biochemical variables such as competition, soil nutrients, and defoliating insect species (Global Climate Change 2016). The Minnesota Department of Natural Resources (2016) describes how trees are not able to make food and thus grow very little in years where trees have lost most or all of their leaves due to an insect attack. Therefore, using tree ring width to determine past climates can lead to inaccuracies. Dendroclimatology has been considered impractical in tropical regions as tropical trees grow all year round, and thus do not have distinctive light and dark rings that allow scientists to reconstruct past climates (Jones 2009). Additionally, tropical forest resources are under constant pressure from logging and disturbances, which restricts the availability of older trees.
Villalba (1994) studied tree ring data from northern Patagonia, South America, where distinct episodes of warm and cold periods were discovered. The warm period, from A.D. 900 to 1070, corresponds with the Medieval Warm Period that occurred in the North Atlantic region. Furthermore, the dendrochronologist discovered from the tree rings that a long, cold period followed the Medieval Warm Period from A.D. 1270 to 1660, peaking around 1340 and 1640. Again, this is consistent with the Little Ice Age events that occurred in the Northern Hemisphere. This case study illustrates how tree ring data can be used to uncover major climatic events that have occurred in the past.
Ice core records
The British Antarctic Survey (2015) describes ice cores as cylinders of ice drilled out of an ice sheet or glacier, typically located in Greenland or Antarctica. Ice cores allow scientists to reconstruct the climate for the past 800,000 years (Davies 2015). Layers of snow fall over ice sheets each year; each layer is different as summer snow differs from winter snow (The British Antarctic Survey 2015). Light can be filtered through the ice cores to distinguish the annual layers of snow that have been deposited, which allows scientists to date the ice being analysed. The layering of an ice core is visible in figure 3. Additionally, the thickness of each layer determines the precipitation rate of each year, which is important as this rate is often correlated to climate change.
Figure 3. “A 19cm long ice core from 1855m depth shows annual layers in the ice. This section contains 11 annual layers with summer layers (arrowed) sandwiched between darer winter layers” (Davies 2015).
As snow accumulates, small bubbles of air are trapped within the ice. From the samples of air, scientists are able to obtain information about the atmosphere from the time the ice was formed and thus can make a prediction of the previous air temperatures of the earth (Jones et al 2009). The different concentrations of carbon dioxide, methane and other greenhouse gases are studied, as a higher concentration of these gases in the atmosphere would suggest a warmer climate than an atmosphere with a lower concentration. Figure 4 illustrates deuterium levels, which is a proxy for the local temperature, and CO2 from the ice core air. This graph is consistent with the idea that temperature and CO2 are closely linked.
Figure 4. “Ice core data from the EPICA Dome C (Antarctica) ice core: deuterium (D) is a proxy for local temperature; CO2 from the ice core air” (British Antarctic Survey 2015).
Not only is the proportion of trapped gas measured in ice cores, but also the specific oxygen isotopes. Assessing the fluctuating concentrations of these isotopes provides a thorough record of temperature change. For example, in 2001 and 2002, a deep ice core was drilled from Antarctica and the oxygen isotopes it contained were analysed and used to reconstruct changes in summer temperature over the last 900 years (Davies 2009). The data indicate that there was a distinct cool period where surface temperatures were around 2°C cooler. This cooler period found by the ice core correlates with the Little Ice Age (1288 to 1807 AD), conveying how significant ice core data can be.
A disadvantage of ice cores is that, evidently, ice core data are restricted to areas covered in ice. However, this is often areas where other sources of climatic data are not available (e.g. tree ring data), so this can also be viewed as an advantage. Furthermore, a limit on high-resolution records can exist when the thinning of annual layers occurs due to ice movement. Ice flow can reduce layer thickness to such an extent that layers cannot be analysed (Jones et al 2009).
Corals
Corals can provide a high-resolution reconstruction of past ocean climates in tropical regions. As they grow, corals record information about the temperature and composition of the water in which they live. This information is recorded in the chemistry and structure of their skeletons; “hard” (scleractinian) corals deposit a skeleton of calcium carbonate (CaCO3). Alternating light and dark density layers exist (due to seasonal variations in growth rates) in the skeleton that can be examined under an x-ray to determine the age of the coral, similar to tree rings (Jones et al 2009).
However, annual banding only occurs in coral skeletons in areas of strong seasonality; banding may be subdued in areas of little or no seasonality, leaving uncertainty (Cobb et al 2008). Further uncertainties exist regarding corals with complex growth forms and corals that have experienced partial mortality.
Coral skeletons can be analysed geochemically to measure oxygen and carbon isotopic composition. The oxygen isotopes present in the skeleton are used to determine past sea surface temperature (SST) and seawater salinity (Jones et al 2009). The relationship between SST and the oxygen isotope δ18O found in coral is illustrated in figure 5. Additionally, carbon isotopes (predominantly carbon-14) can be used to investigate changes in deep-ocean circulation.
Figure 5. “coral oxygen isotopes compared to sea surface temperature (SST)” (Hetzinger 2016)
Although skeletal δ18O can be an accurate climate proxy, uncertainties surround the use of this oxygen isotope for SST reconstruction as potential changes in seawater δ18O composition exist (Jones et al 2009). These potential changes decrease the reliability of results if they are not calibrated against instrumental-based records.
Corals used for climatic reconstruction are located in shallow-water tropical ocean regions, and therefore provide information about past climates that are poorly represented by other measuring techniques, such as tree rings or ice cores (Jones et al 2009). However, these shallow, tropical areas (for example lagoons) may have restricted access to the open ocean. This means that the conditions recorded by the corals may not be representative of the regional climate (Cobb et al 2008).
Coral δ18O records from the western equatorial Pacific reflect δ18O seawater variability driven by changes in precipitation. Strong coral δ18O anomalies are created during El Niño