Understanding Earth's Deep Past Lessons for Our Climate Future (2011) / Chapter Skim
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4 Deciphering Past Climates - Reconciling Models and Observations
4 Deciphering Past Climates - Reconciling Models and Observations
Pages 81-105
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From page 81... ...
offered by near-time paleoclimate archives, which are capable of identifying the typically nonlinear components of the climate system -- characterized by rapid response times -- that are relevant to human society. A critical prerequisite for accurate forecasts of future regional and global climate changes based on GCMs, however, is the requirement that these models use parameters that are relevant to the future we seek to better understand.
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From page 82... ...
Furthermore, discrepancies between model outputs and paleoclimate observations may indicate the existence of additional processes, feedbacks, and/or sensitivities that are not present in the model or expressed in the modern climate system. For example, the exceptionally warm high latitudes during all past warm periods -- whether transient or long term -- cannot be reproduced by models without invoking unreasonable CO2 levels, revealing the inability of current models to fully capture the processes and feedbacks governing heat transport and retention or the processes that might generate heat in the polar regions under elevated atmospheric greenhouse gas levels (Covey and Barron, 1988; Rind and Chandler, 1991; Covey, 1991; Sloan and Pollard, 1998; Bice et al., 2006; Huber, 2008; Kump and Pollard, 2008; Spicer et al., 2008; Zachos et al., 2008)
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From page 83... ...
Examples include laminated sediments from the Pleistocene Santa Barbara Basin (Figure 4.1a) , Eocene sediments of the North Sea (Figure 4.1b)
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From page 84... ...
from a drill core in the Paleocene of the western North Atlantic (from ODP Site 1051; Norris and Röhl, 1999)
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From page 85... ...
To maximize this potential, the community is presented with three primary challenges: • To determine precise chronologies for existing and new geo logical archives of paleoclimate interest -- where feasible at the temporal resolutions that are possible in the Pleistocene and Holocene -- through continued improvements in the precision and accuracy of geochronological techniques applicable to the sedimentary record (Ar-Ar [argon-argon]
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From page 86... ...
For many years, these models included only the physical aspects of the atmosphere, dynamic ocean, land, and sea ice components of the climate system. More recently, however, these models have begun to include coupling to a dynamic ice sheet model and prognostic components for biogeochemistry, atmospheric chemistry, dynamic vegetation, and ecology.
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From page 87... ...
. Geochemical box models are used to study the temporal evolution of quantities such as atmospheric CO2 and oxygen, ocean stable isotopes, and other geochemical variables.
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From page 88... ...
Information from the GCMs or EMICs can also be used to better represent climate feedbacks in geochemical box models. Spatial Resolution (kilometers)
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From page 89... ...
Key boundary conditions that must be prescribed for deep-time or warm world models include the paleogeography of land areas, past vegetation distributions, paleotopography, and ice sheet extent. Coupling to ocean models additionally requires knowledge of paleoeustasy in order to specify the distribution of shallow seas and the paleobathymetry for the deep oceans: • Global paleogeography is an important boundary condition for constraining elevation models of continental topography and oceanic bathymetry, the geography of oceanic gateways and shallow continental (epeiric)
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From page 90... ...
. Therefore, ultimately, the accurate reconstruction of paleotopography requires some degree of iteration between modeling and proxy methods since topographic relief strongly affects regional climate patterns, influencing all of the proxy-based estimates (Ehlers and Poulsen, 2009)
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From page 91... ...
A far more coordinated effort is needed to expand scientific understanding of fossil plant physiological mechanisms and to synthesize disparate paleobotanical data into more comprehen sive and temporally constrained compilations that can be used to refine dynamic vegetation models for climate modeling. Further improvements in scientific knowledge of these physical and ecological boundary conditions will require more detailed analysis of paleomagnetic, paleoclimatic, paleotectonic, and paleontologic data (Van der Voo, 1993; Parrish, 1998; Kiessling et al., 1999)
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From page 92... ...
In addition to the need to better constrain the physical boundary conditions for global climate models applied to deep-time climates, the concentration of greenhouse gases (in particular CO2) in ancient atmospheres and the solar and spectral irradiance must be determined given their fundamental contribution to radiative forcing of the climate sys tem.
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From page 93... ...
warming has been constrained using paleo-CO2 and paleotemperature estimates inferred from stable isotopic values of marine carbonates and biomarkers -- these place the climate sensitivity during the greenhouse gas-forced event between a lower bound of 2.4-3.0°C and as high as ~4°C (Pagani et al., 2006; Panchuk et al., 2008)
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From page 94... ...
. The highest-precision paleoatmospheric pCO2 estimates, in particular for the Cenozoic and Cretaceous, are based on proxy methods that utilize fossil marine and terrestrial photosynthetic flora: • The alkenone paleobarometer, which uses the δ13C of lipid biomarkers derived from haptophyte algae in marine sediments (Freeman and Hayes, 1992; Pagani et al., 1999)
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From page 95... ...
. Recent coupled ice sheet-climate model simulations suggest that the Miocene climate transition was largely CO2 induced, involving the crossing of a threshold pCO2 of 400 parts per million by volume (ppmv)
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From page 96... ...
-- and the lack of extant plant calibrations for stomatal index-based estimates in pre-Cretaceous intervals. Marine Temperatures A major challenge in determining ancient climate sensitivity is the need for robust estimates of ocean temperatures and latitudinal ocean temperature gradients.
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From page 97... ...
have led to increased focus on independent temperature proxy methods. The Mg/Ca ratios of planktonic foraminiferal calcite have been shown to be sensitive to temperature, increasing exponentially with increasing SSTs, providing an SST paleothermometer (e.g., Elderfield and Ganssen, 2000; Lear et al., 2000)
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From page 98... ...
Several additional proxy methods show promise for paleothermometry but are still in the development and testing phases: • The calcium isotope (δ44Ca) composition of well-preserved foraminifera may provide an independent paleotemperature proxy and a test of the reliability of the Mg/Ca proxy (e.g., Nägler et al., 2000)
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From page 99... ...
FIGURE 4.5 Tropical paleotemperature estimates based on δ18O data from unaltered benthic and planktonic foraminifera (red circles and squares, respectively) and from TEX86 data (yellow circles)
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From page 100... ...
For example, the Mg/Ca proxy is least sensitive at low temperatures, whereas the Uk'37' method is least sensitive at high temperatures. INDICATORS OF REGIONAL CLIMATES Climate models and paleoclimate archives indicate that one of the larger impacts of global warming is likely to be regional changes in continental temperatures and precipitation.
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From page 101... ...
, although this approach requires assumptions regarding the stable isotope composition of meteoric water. This limitation can be overcome for soil carbonates by application of the carbonate clumped isotope thermometer to paleosol carbonates (Passey et al., 2010)
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From page 102... ...
Reconstructing regional patterns in relative humidity and precipitation is far more challenging in deep-time records because of the overall lower levels of temporal and spatial resolution, stratigraphic continuity, and geochemical susceptibility to diagenesis, although several new approaches are being evaluated. For much of the pre-Neogene, scientific understanding of climate regimes is based on low spatial and temporal resolution global syntheses of published databases (Ziegler et al., 2003; Boucot et al., 2004)
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From page 103... ...
. Given the control of relative humidity on these processes, fossil leaf wax n-alkanes are being explored as a paleoaridity proxy (e.g., Liu and Huang, 2005; Pagani et al., 2006; Smith and Freeman, 2006)
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From page 104... ...
. For example, the U/Ca ratios of certain planktonic foraminifera genera have been used to determine variations in seawater carbonate ion content over the last glacial cycle that track atmospheric pCO2 variations archived in polar ice cores (Russell et al., 1996, 2004)
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From page 105... ...
Here, continued improvement and development of new innovative observational techniques have strengthened the conclusion that models are challenged to simulate such high polar surface temperatures. This disparity has led to active model exploration of feedback processes that may operate in warm greenhouse climates but are not revealed by data-model studies of Earth's more recent glacial state.
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