Lida Teneva
Tristan Landot
December 9, 2006
Survival Guide for Climate Conversation
Paleoclimate
“The further back that you can look, the further forward you are likely to see.”
Winston Churchill
Paleoclimate is the study of ancient climates; paleoclimatological and paleoenvironmental research can be done on many different time scales, using a variety of proxies. Scientists need to understand the ways that climate has changed in the past in order to be able to predict better what will happen to the Earth System globally in the future. Paleoclimate studies not only help us reveal how the Earth System evolves, but also quantify the forces that drive climate change in the first place. Since instrumental data about the Earth’s climate did not exist prior to the middle of the 19th century, we need to use proxy data from several types of materials: ice cores, corals, tree rings, ocean sediments (foraminiferal testsshells (tests is technically correct, but rarely used, diatoms), lake sediments (pollen, dynoflagellate cysts), cave stalagmites, sedimentary rocks, etc. (Bradley, 1999).
Scientists do not just try to reconstruct past temperatures, but also track changes in greenhouse-gas levels in the atmosphere, precipitation patterns, changes in ocean currents, changes in terrestrial vegetation patterns, etc. (Bradley, 1999). Past climates can also be determined by mapping the distribution of ancient coals, desert deposits, tropical soils, salt deposits, glacial material, etc. (Scotese, 2006). Some of the most important controversies in paleoclimate studies have to do with two main issues: 1) decrease in data dating precision the further we try to look in the past, and 2) clear understanding of all the mechanisms that influence climate signatures in the different proxies.
In this survival guide we will discuss the main “tools of the trade” in paleoclimate: corals, ice cores, foraminifera in ocean sediments, tree rings, and sedimentary rocks. Different proxies have different geologic time ranges, variable resolution, and different levels of uncertainty. Ice cores, boreholes, cave stalagmites, and lake and ocean sediments over data resolution at centennial scales; however, they are based on less precise dating. On the other hand, tree rings and corals are usually more precisely dated and offer information on multidecadal variability (Moberg et al. 2005).
Corals
Corals grow much like tree rings, forming layers in the skeletons every year; they can live up to 300 years or more (Schrag and Linsley, 2002) (Fig.1). Coral reefs are found mostly in the tropical and subtropical regions of the world’s oceans, with greatest concentration in the Indian-Western Pacific region and the Caribbean (Fig.2). The geochemistry of corals is very useful for the reconstruction of temperature history of the tropical regions since coral physiology is quite sensitive to small changes in their local environmental conditions (Grottoli, 2001). Corals can also be used to improve our understanding of climate oscillations, such as El Nino, as well as paleocenographic trends and changes in sea level through time. Most studies of coral paleothermometry focus on Sr/Ca ratios in conjunction with stable oxygen isotopes, which with the aid of a temperature transfer function, are claimed to produce temperature reconstructions with precision better than 0.5 oC (Beck et al., 1992). However, such reconstructions are still considered somewhat controversial since it is possible that algal symbionts within corals may affect the chemistry of the skeleton (de Villiers et al., 1995). Cohen et al. (2002) propose that as much as 65% of the Sr/Ca variation in corals is governed by algal symbiont activity and may not be related to sea surface temperatures.
Furthermore, let’s consider paleotemperature reconstructions from corals based on stable oxygen isotopes (18O) (Fig.3). The isotopic signature of corals depends on sea surface temperature variations, seawater 18O composition, as well as biological fractionation effects in corals (Cole, 2003). This technique remains the most commonly used in coral paleoclimatology. Interestingly though, intercomparisons of coral oxygen isotope data and historical sea surface temperatures show that coral time series seem to record interannual SST variability quite well, while decadal isotopic variability does not seem to show a linear relationship to local SST anomalies (Evans et al., 2000). Furthermore, we need to consider that differences in salinity and general local hydrography of the reef community may significantly influence the local seawater 18O value, which would get translated into the coral skeleton, and may not necessary represent a regional climatic signal (Swart et al., 2001). Thus, scientists have to be careful when combining coral time series from different coral colonies from different locations. However, corals still provide a great deal of useful information about many factors of climate change, as well as the carbon cycle (Druffel, 1995).
Ice cores
While corals have the potential to provide highly detailed information about the relatively recent past, ice cores can take us as far back as at least 650,000 years, albeit with less precision and resolution the further we go back in time (Chandler, 2005). The sites for ice core field sites include Greenland, Antarctica, Himalayas, Andes, Kilimanjaro, and the Alps (Fig.4). The basic principle in ice core paleoclimatology is that ice cores contain annual layers of snow/ice, which have trapped air bubbles from the time of deposition. Isotopic (oxygen and hydrogen) studies of the ice (via melting) can show scientists temperature changes through time since the isotopic composition of the ice will reflect changes in the temperature-dependent vapor pressure [and hence, extent of isotopic fractionation] at the time of precipitation (Mayewski and Bender, 1995). The isotopic information in the ice cores may also be used to reconstruct past changes in sea level (Stuiver and Grootes, 2000). In addition, studies of the air bubbles are useful in reconstructing changes in the concentrations of greenhouse gases in the atmosphere through time (Siegenthaler et al., 2005).
The greatest uncertainties will ice cores come from dating individual layers and estimating the age of the air trapped in the bubbles. One of the ways to date a core is to count visible layers; however, the further we go back in time, the more indistinguishable the layer boundaries become (Fig.5). Thus, the resolution at a certain site may depend on snow accumulation rate and amount of compaction (Mayewski and Bender, 1995). Also, the air trapped in bubbles in the ice is essentially younger than the ice surrounding it because the consolidation of snow to ice takes a while and it takes place at depth. Climatic conditions govern trapping depth, and the air-ice age difference may vary between 2500 and 6000 years (Barnola et al., 1991). While there are some limitations to the precision of data from ice cores, the amount of information scientists can gain from studying them still has incredible relevance to our understanding of past climate. Scientists are constantly trying to extend the ice core record, aiming to go as far back as 800,000 years or more and learn more about the dynamics of Earth’s glacial-interglacial cycles (Kettlewell, 2004).
Tree rings
Tree growth is generally controlled by climate conditions during the year prior to and including the growing season. Variations in tree growth closely reflect the amount of soil moisture at the onset of the growing season, which is controlled by variations in precipitation, and, to some degree, temperature, humidity, and wind (Fig.6).
A tree-ring reconstruction is a time series of tree ring-width data that have been calibrated with an instrumental or gauged record of a hydrologic or climatic variable (e.g. annual streamflow, precipitation, snow water equivalent, drought index). They are developped from tree-ring chronologies. A tree-ring chronology is a time-series of annual values derived from the ring-width measurements of 10 or more trees of the same species at a single site. The reconstruction, based on a statistical model that describes the relationship between tree growth and the gage record, extends that record back hundreds of years into the past. The bristlecone pine (Pinus longaeva) of the great basin region of western North America are the oldest known living trees, up to 5,000 years old. Therefore, they allow records back to this time.
Trees that provide the best information about hydroclimatic variability are those particularly sensitive to variations in moisture. These include species such as ponderosa pine, pinyon pine, and Douglas-fir, growing in open stands on dry and rocky sites where soil moisture storage is minimal. Trees growing in these types of sites are also less likely to be subject to non-climatic disturbances (such as fires and insect infestation) and the effects of competition from nearby trees. In addition, the oldest trees of these species tend to be found on these sites (Fig.7).
Because the reconstruction models explain most (60-75%), but not all, of the variance in the gauge record, there are uncertainties in the reconstructions. Estimates of uncertainty can be described by confidence intervals around the reconstruction. Furthermore, this technique can be only used for places and times corresponding to the existing trees. (It can be used also on fossilized trees that are dated using radioactivity but then the temporal uncertainty is bigger).
Foraminifera
The analysis of oceanic cores gave important results. These cores are extracted using boat with special equipment. Oceanic sediments contain many rests shells of organisms, more speciallyespecially plenty of foraminifera testsshells, which are around 1mm (Fig.8). These Fforaminifera are present in all the oceans. and there are plenty of their tests in the sediments. Paleoclimatologists are interested in them because they are very sensible sensitive to their environment.
Foraminifera build their tests from elements they pick from the water, specially the oxygen. A test composition reflects the water composition at the time when this test has been built. As stated above for corals, the concentrations in different stable oxygen isotopes reflects temperature of water. Another indicator of temperature is the kind of species of foraminifera found in the core, since some species are known to live in colder water while other are know to live in warmer water.
Studying these tests permits to access information about more than 100,000 years ago but of course with many uncertainties. Most of these are due to uncertainties on the tested sample.
Sedimentary rocks
We are more specifically going to speak about Loess. Loess are sediments carried by winds and that cover large plains like the ones in central China, in Germany or in the North of France (Fig.9). At some places, these sediments are accumulated on to depths of hundreds of meters and they allow scientists to access information from more than 600,000 years ago.
Scientists are interested in these Loess because their composition vary with the atmospheric circulation and they can only be produced if some conditions allow it. Loess deposit has long been regarded a Pleistocene sediment and its date of formation has been put to ca 600 ka BP. Recently, the beginning of the Pleistocene is identified in the official concord reached — at 1.6 Ma BP, while not infrequently others propose 2.4 Ma BP.WHY GET INTO THIS DEBATE HERE? Some hold that the beginning of loess formation can be put as far back as 1.6 – 2.4 Ma BP. Loess formation intervals are usually correlated with the cold stages, while soil formation is believed to correspond to the warm stages. The typical loess, i. e. loess proper, are not older than 0.96 Ma BP. Previous to this date climatic conditions had not generally favoured loess formation and paleosols formed one above the other with clay, loam or carbonate intercalations.
Conclusion
Paleoclimate studies are an incredibly powerful tool, not only helping us decipher the evolution of earth systems in the past, but also enabling us to improve our capacity to predict future climate change. We need to keep the limitations of paleoclimate research in mind. There are two main caveats. Firstly, precision of climate reconstructions decreases the further scientists try to go in the past due to increasing inability of precisely dating the proxies as they get older. In addition, the reliability of many proxies is still largely debated since the geochemistry in some materials may be quite complex and influenced by climate in more ways than was previously thought, as well as by non-climatic (e.g. biological) factors. However, if scientists had never tried to use geochemical proxies without completely understanding the origin of the signal, then we would not know much about Earth’s climate history. Today, fragile global environmental conditions demand urgency of the Earth Science community in improving their understanding of both past climate change and the mechanisms that influence geochemical data preservation in the proxies.
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