Paleoclimatology in the 21st Century
Judith Totman Parrish and Paul Koch
Recent predictions of the magnitude of future climate change in response to anthropogenic emissions have been controversial, generating significant political heat. Indeed, there is still debate as to whether the warming documented in historical records over the past 150 years is an extraordinary result of human activity, or merely a typical fluctuation in a variable system. These controversies exist because of a lack of understanding about the global climate system, which permits alternate interpretations of global climate data, as well as a lack of understanding of the historical range of climate variability. Thus despite 20 years of intense research and numerous significant developments, the range of possible future climate-change scenarios has not changed. These controversies illustrate that the Earth's climate system is a subject of immediate concern to society and that the need for more research in this area is great.
The geologic and paleontologic record of climate change continues to
be the best source of information on the Earth's climate system. It provides
a record of trends in mean climate and variability in the recent past that
can extend beyond the paltry human time scales on which climate has been
measured directly. In addition, if we can understand climatic conditions
that have not existed during the history of recorded climate, we gain a
more fundamental understanding of the climate system than is attainable
through studies based solely on modern conditions. By the same token, understanding
climate change over the full variety of temporal and spatial scales recorded
in the geologic record contributes to knowledge of the fundamentals of
climate dynamics. This knowledge is essential if we are to model future
climate with confidence.
Paleoclimatology as a discipline has expanded exponentially (as measured, for example, by the output of literature) in the last 25 years. The impetus for this has only partly been societal concerns about global change. Knowledge of paleoclimatology is valuable in its own right, and is essential to progress in our understanding of the role of the physical environment as a force driving evolutionary and ecological change. In addition, developments in other fields have allowed paleoclimates to be studied at levels of resolution that were not possible before. These recent developments include:
In a real sense, then, although paleoclimates have been the subject
of study since Lyell's time, paleoclimatology is a new field of study.
Paleontology has been a key component in paleoclimatic research because
fossils, together with sedimentary rocks, are the repository of nearly
all paleoclimatic data, particularly quantitative data. Sometimes overlooked
in the excitement, for example, of stable isotope geochemical interpretations
of paleoclimate is the fact that the stable isotopes are most commonly
measured from fossils. Understanding the evolution, paleobiology, and paleoecology
of organisms is vital to paleoclimatic interpretations of stable isotope
records. For example, vertical or horizontal migrations during ontogeny,
as well as seasonality in reproduction, will have large consequences for
estimates of ocean temperatures from marine microfossils. The paleobiology
and paleoecology of organisms is also crucial to interpretation of shifts
in paleobiogeographic patterns in a term of paleoclimatic change. Biogeographic
patterns in organisms that are strongly influenced by interactions with
other species, or that are limited by physical parameters other than those
related to climate, may provide misleading estimates of climate change.
Finally, paleobiological information in and of itself can provide information
about paleoclimates. For example, when assumptions about crocodilian physiology
and the thermal tolerances of palms were examined closely, it was found
that these groups are, indeed, excellent indicators of climate, as had
been previously assumed.
Below, we review paleontological research that has proven fruitful in paleoclimate research and outline future research that is likely to further increase the utility of fossils as paleoclimatic indicators. We reemphasize that an increased understanding of paleoclimates is needed to understand the Earth's climate system and that understanding paleoclimates depends critically on paleontology. An improved understanding of the Earth's climate system depends utterly on advances in paleontology.
Paleobiology. - One of the most exciting advances in paleoclimatology in the last 30 years has been the development of quantitative, taxon-independent methods of interpreting climate from fossils. The most widely used method is based on variations in the oxygen isotope composition of biogenic marine carbonate, from which the broad pattern of change in marine temperatures has been reconstructed. Recent work suggesting low latitudinal and vertical marine temperature gradients in the Cretaceous and Eocene, if verified, will require a significant rethinking of climate dynamics. Isotopic reconstruction of continental climates is a less mature but rapidly expanding field. In most studies, the goal is estimation of the oxygen isotope composition of meteoric water, which is related to surface temperature and vapor transport. These analyses are performed on lacustrine macro- and microfossils, fossil wood, freshwater bivalves, and fossil vertebrates. The most accurate method for reconstruction of pre-Quaternary land climates is based on the fossil leaves of flowering plants, in which paleotemperature is estimated from the distribution of leaf morphologies within a sample. Research underway at present is investigating whether rainfall can be quantified using these data. Similar methodologies are being developed for other types of paleontological information, such as wood anatomy. Some animal groups also show potential for such methods.
Another promising area of research has been the attempt to correlate the stomatal density of fossil leaves with paleo-CO2 levels. If this method proves viable, it will provide a means for directly measuring those levels, which will then provide more information with which to interpret observed paleoclimatic changes. The method will also provide an independent check on determinations of paleo-CO2 from isotopic analysis of pedogenic carbonates and leaf organic matter.
Paleoecology. - The distribution of feeding strategies among the vertebrates in a fossil fauna provides information about the vegetation, which in turn provides information about climate. This is useful because vertebrates are commonly preserved in sediments that do not preserve plants or pollen. There have also been attempts to use the body-size distribution of fossil mammalian herbivores to estimate vegetation structure in Cenozoic continental ecosystems.
The isotope compositions of marine and continental organisms provide information on the ecosystems that may be related to climate. Carbon isotope analysis of planktic and benthic foraminifera, as well as mammalian tooth enamel, have demonstrated a dramatic, short-term drop in the carbon isotope composition of the surface carbon reservoir that is correlated to a brief pulse of warming in marine and continental settings. This carbon isotope signal may reflect the sudden release of methane from ocean floor sediments. Methane, an extremely efficient greenhouse gas, may have contributed to the short-term warming.
Because of their selectivity in feeding and relatively short life spans, animals provide isotopic records of vegetation structure at a finer-scale than paleosol minerals and organic matter. For example, carbon isotope analysis of Neogene paleosol carbonates demonstrate that grasses using the C4 photosynthetic pathway, which today occur in regions with warm and dry climates, came to dominate in low latitude grasslands only at the end of the Miocene. Analysis of individual mammals suggests that C4 grasses persisted at low levels in some of these ecosystems for a much greater period of time.
Paleobiogeography. - Paleobiogeographic information on fossils
is important for interpreting stable isotope data and for constraining
paleoclimatic models, as well as for providing direct information about
paleoclimates. The fact that modern and, for the most part, ancient species
of foraminifera are widespread makes them ideal for isotopic studies because
one can control for taxon-specific variations in isotopic fractionation.
In contrast, the fact that modern and ancient species of ostracodes are
very diverse and have very narrow biogeographic ranges is useful for paleoclimate
studies because a fossil fauna may permit a tight estimate of climatic
conditions based on the narrow tolerances of extant taxa. Quantitatively
documented and calibrated changes in the distributions of entire floras
involving taxa that are still extant have provided a great deal of information
on vegetational changes in the last few thousands of years. These changes
may prove critical to understanding the effects of future global climate
change on agriculture and native systems.
Recent climate simulations have demonstrated the sensitivity of the climate system to vegetation structure. Vegetation influences climate directly, through its impact on the albedo of the planetary surface, and indirectly, through its effect on the continental vapor budget. More realistic global climate simulations, particularly for warm intervals in the past, may only be possible once representative biomes are included in the calculations. Some outstanding questions that remain to be answered for fossils to serve their full potential as paleoclimatic indicators are the following:
Prof. Judy Parrish--Topic Coordinator
Dept. of Geosciences, University of Arizona
Tucson, AZ 85721
Prof. Paul Koch
Earth Sciences Board
University of California
Santa Cruz, CA 95064
Dr. Ruben Cúneo
Museu Paleontologico Egidio Feruglio
9 de Julio 655, 9100 Trelew, Chubut
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