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 shells (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
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
Figure 1.
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
Figure 2.
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
18
O) (Fig.3). The isotopic signature of corals depends on sea
18
O 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
18
O value, which would get translated
into the coral skeleton, and may not
Figure 3.
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
Figure 4.
since the isotopic composition of the ice
will reflect changes in the temperaturedependent 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
Figure 6.
Figure
.
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
developed 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
Figure 7.
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 shells of organisms, especially foraminifera
shells, which are around 1mm (Fig.8). Foraminifera are
present in all the oceans. Paleoclimatologists are
interested in them because they are very 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
Figure 8.
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 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 BPWHY 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
Figure 9.
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.
References:
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Beck, J.W., Edwards, R.L., Ito, E., Taylor, F.W., Recy, J., Rougerie, F., Joannot, P.,
Henin, C. 1992. Sea-surface temperature from coral skeletal strontium/calcium
ratios. Science 257: 644-647
Bradley, R.S. 1999. Paleoclimatology: Reconstructing climates of the Quaternary.
Interational Geophysics Series, vol. 64
Chandler, D.L. 2005. Record ice core reveals Earth’s ancient atmosphere. New Scientist.
Nov. 24th, 2005. Available on-line:
http://www.newscientist.com/article.ns?id=dn8369
Cohen, A.L., Owens, K.E., Layne, G.D., Shimizu, N. 2002. The effect of algal symbionts
on the accuracy of Sr/Ca paleotemperatures from coral. Science, 296: 331-333
Cole, J.E. 2003. Holocene coral records: windows on tropical climate variability. In:
Global Change in the Holocene. Mackay, A., Battarbee, R., Birks, J., and
Oldfield, F. (eds). Oxford University Press, p. 168-184
de Villiers, S., Nelson, B.K., and Chivas, A.R. 1995. Biological controls on coral Sr/Ca
and d18O reconstructions of sea surface temperatures. Science, 269: 1247-1249
Druffel, E.R.M. 1997. Geochemistry of corals: Proxies of past ocean chemistry, ocean
circulation, and climate. Colloquium Paper. In: Proceedings of the National
Academy of Sciences, USA, 94: 8354-8361. Also available on-line:
http://www.pnas.org/cgi/reprint/94/16/8354
Evans, M.N., Kaplan, A., and Cane, M.A. 2000. Intercomparison of coral oxygen isotope
data and historical sea surface temperature (SST): Potential for coral-based SST
field reconstructions. Paleocenography, 15: 551-563
Grottoli, A. 2001. Past climate from corals. In: Encyclopedia of Ocean Sciences.
Academic Press London, p. 2098-2107.
Kettlewell, J. Ice cores unlock climate secrets. BBC News Online. 9 June 2004. Available
on-line: http://news.bbc.co.uk/2/hi/science/nature/3792209.stm
Mahowald,N.M., D. R. Muhs, S. Levis, Philip J. Rasch, MasaruYoshioka, Charles S.
Zender, Chao Luo, Change in atmospheric mineral aerosols in response to
climate: last glacial period, pre-industrial, modern and doubled carbon dioxide
climates.
Mayewski, R.A. and Bender, M. 1995. The GISP2 ice core record—paleoclimate
highlights. Reviews in Geophysics, v.33 supplement. American Geophysical
Union. Available on-line:
http://www.agu.org/revgeophys/mayews01/mayews01.html
Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M, and Karlen, W. 2005.
Low- and High-resolution proxy data over the last 2000 years. Nature, 433: 613617
Pecsi, M. 1991. Problems of Loess Chronology. GeoJournal Vol.24, no.2
Schrag, D.P. and Linsley, B.K. 2002. Paleoclimate: Corals, Chemistry, and Climate.
Science, 296: 277-278
Scotese, C.R. 2002. Climate History. PALEOMAP Project. Available on-line:
http://www.scotese.com/climate.htm
Siegenthaler, U., Stocker, T.F., Monnin, E., Luthi, D., Schwander, J., Stauffer, B.,
Raynaud, D., Barnola, J.-M., Fischer, H., Masson-Delmotte, V., Jouzel, J. 2005.
Stable carbon cycle-climate relationship during the Late Pleistocene. Science,
310: 1313-1317
Stuiver, M. and Grootes, M. 2000. GISP2 oxygen-isotope ratios. Quaternary Research,
53: 277-284
Swart, P.K., Price, R.M., and Greer, L. 2001. The relationship between isotopic
variations (O, H, and C) and salinity in waters and corals from environments in
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Image references:
Figure 1: http://www.personal.psu.edu/faculty/s/b/sbj4/aquarium/reef_pictures/corals.jpg
Figure 2 : http://www.aims.gov.au/pages/research/organicgeochemistry/cccgsag/images01/coral-density-bands-dated-480.jpg
Figure 3: http://wwwrses.anu.edu.au/admin/images/content/HendyCoralData.jpg
Figure 4:
http://earthobservatory.nasa.gov/Study/Paleoclimatology_IceCores/Images/greenland_dri
lling.jpg
Figure 5:
http://upload.wikimedia.org/wikipedia/en/thumb/7/7f/GISP2_1855m_ice_core_layers.gif/
384px-GISP2_1855m_ice_core_layers.gif
Figure 6: http://web.utk.edu/~grissino/images/small%20fir.jpg
Figure 7: http://www.geo.arizona.edu/palynology/geos462/treeringbob.gif
Figure 8: http://www.u-picardie.fr/~beaucham/Paleoclim/31-foram.jpg
Figure 9: http://www.cis.umassd.edu/~gleung/sumfo/gully14a.jpg
Suggestions for further reading and data:
“Sentinels of the Sea: fossil corals help predict severity of global warming”. Airhart, M.
Jackson School of Geosciences News Releases and Features. October 30th, 2006.
Available on-line: http://www.jsg.utexas.edu/news/feats/2006/quinn06.html
“Corals and Sclerosponges”. NOAA Satellite and Information Service. Available on-line:
http://www.ncdc.noaa.gov/paleo/corals.html
“Ice cores”. NOAA Satellite and Information Service. Available on-line:
http://www.ncdc.noaa.gov/paleo/icecore.html
“Ice cores”. Wikipedia, the free encyclopedia. Available on-line:
http://en.wikipedia.org/wiki/Ice_core
Climate change and paleoclimatology. Archives from peer-reviewed journals on the latest
climate studies. Available on-line: http://www.earth-pages.com/archive/climate.asp
“Brief overview of the history of 20th century climate”. Available on-line:
http://muller.lbl.gov/pages/IceAgeBook/history_of_climate.html