Climate model sensitivity to atmospheric CO2 levels in the Early–Middle Paleogene
Introduction
The wide variation in carbon dioxide (CO2) estimates for the Early Paleogene (50–60 Ma), a period that proxies suggest was the warmest of the Cenozoic, have added considerable difficulty to the already challenging effort to understand the driving mechanisms of past climate states. Pearson and Palmer (2000) used boron isotope ratios to reconstruct the pH of seawater, from which they inferred very high atmospheric CO2 levels (>2000 ppm) for the Late Paleocene and Early Eocene, and values that may have been as high as 3000 ppm at the Paleocene–Eocene boundary. This is approximately ten times the pre-industrial level of atmospheric CO2 (280 ppm). Retallack (2001), Kürschner et al. (2001), and Royer et al. (2001a) all used stomatal abundances from fossil leaf cuticles to infer ancient CO2 levels, but their estimates for the Paleogene vary widely (∼2000 ppm, ∼500 ppm, and ∼300–400 ppm, respectively). Estimates from paleosol carbonates also vary considerably for the Early–Middle Paleogene, from 0 to ∼1000 ppm (Cerling, 1991, Ekart et al., 1999). In addition, mass balance models of CO2 suggest that CO2 has been decreasing steadily over the past 170 m.y. (with a slight rise at ∼50 Ma), but was anywhere from 300 ppm to 1000 ppm during the Early Paleogene (Tajika, 1998, Berner and Kothavala, 2001).
Fossil leaf assemblages support the picture of a warm, wet climate at middle and high latitudes during the Late Paleocene and Early Eocene (e.g. Wing and Greenwood, 1993, Greenwood and Wing, 1995), and data from oxygen isotope paleothermometry suggest a fairly shallow latitudinal sea surface temperature (SST) gradient (Zachos et al., 1994, Tripati et al., 2001).
Paleoclimate modeling studies generally have had difficulty reproducing the shallow gradients of the greenhouse world depicted by the proxy climate data no matter what forcing mechanisms are invoked (e.g. Sloan and Rea, 1995, Sloan and Pollard, 1998, Huber and Sloan, 2001). Understanding the mechanisms that kept the planet warm during the Early Paleogene is particularly important because, superimposed on a long-term warming through the Late Paleocene, there is a period of abrupt warming (∼56 Ma) that lasted ∼100 k.y. (Zachos et al., 1994, Bralower et al., 1995, Zachos et al., 2001). The most plausible mechanism for this abrupt warming, known as the Paleocene–Eocene Thermal Maximum (PETM), is the release of a large amount of methane gas (CH4) from clathrates in ocean sediments (Dickens et al., 1995, Katz et al., 1999). The rate at which carbon was added to the atmosphere at the PETM approximates the rate at which carbon is presently being added to the atmosphere. We do not know how sensitive a warm climate, such as that of the Late Paleocene or Early Eocene, would be to subsequent additions of CO2 or CH4.
In this study we examine the sensitivity of the slab ocean version of the National Center for Atmospheric Research (NCAR) Climate System Model (CSM) (v.1.2) with Eocene geography to a range of CO2 values, from 500 to 2000 ppm. Our primary goal is to determine whether the newer NCAR CSM can produce a better match between the model results and the inferred proxy climate of the Early Paleogene than the older GENESIS model used by Sloan and Rea (1995). The CSM is a more recently developed model that includes improvements in cloud parameterizations, long wave radiation, and land surface processes. Of particular importance for this study, the atmosphere component of CSM, the NCAR Community Climate Model (v.3.6.6) (CCM3), includes long wave radiative effects of CH4, and two additional weak CO2 bands at 9.4 and 10.4 μm, which become important at very high levels of atmospheric CO2 (Kiehl et al., 1998, Kothavala et al., 1999). This new atmospheric physics in the model may provide us with new information about the role of greenhouse gases in past climate states.
This is the first study to examine the sensitivity of Paleogene climate to a CO2 level of 2000 ppm and a CH4 level of 3.5 ppm. Additionally, we have updated Eocene geography and vegetation with that of Sewall et al. (2000), and in order to account for a slight reduction in solar luminosity at 50–60 Ma, we have reduced the present-day solar constant by 6 Wm−2 to 1361 Wm−2.
We take both a quantitative and a qualitative approach to comparing model results with proxy data. While we recognize that there are other mechanisms affecting surface temperature, for this study we choose to focus only on CO2.
Section snippets
Experimental design
We use the NCAR CSM (v.1.2) as described by Boville and Gent (1998). Our boundary and initial conditions are from recent modeling studies of Early Eocene climate (Sewall et al., 2000, Huber and Sloan, 2001). This is the first study to use the CSM with Eocene boundary conditions coupled to land surface, sea ice, and slab ocean model components to examine the sensitivity of the global ocean response to massive increases in CO2.
The CSM divides the climate into component models for the atmosphere,
Surface temperature
As expected, global mean surface temperature rose with increasing CO2. From 20°C in the LoCO2, it increased by 2°C in the MidCO2 scenario and by 4°C in the HiCO2 scenario relative to LoCO2. Fig. 1 shows the mean annual, zonally averaged combined ocean and land surface temperature for each case plotted with Eocene mean annual temperature (MAT) estimates from marine and terrestrial proxy data. The most dramatic increases in surface temperatures with increasing CO2 were in the high latitudes
Discussion
Overall, our results for the higher CO2 experiments (1000–2000 ppm) are in stronger agreement with proxy data for the Late Paleocene and Early Eocene than the results for the lower pCO2 experiments. Of the three experiments, the HiCO2 scenario is the only one that produces mean annual and CMM temperatures in mid-latitudes and high latitude coastal regions compatible with climate interpretations from Eocene flora. Below we provide a more detailed comparison of our experimental output with proxy
Conclusions
The Mid- and HiCO2 experiments produce modeled climates more consistent with that depicted by proxy data than the LoCO2 scenario. The higher CO2 cases are in agreement with proxy data in the following key areas:
• Antarctic coastal temperatures in the Mid- and HiCO2 cases are warm enough to support observed fossil flora in coastal regions.
• CMM land temperatures in the Mid- and HiCO2 cases fall within the mean error of CMM estimates from proxy data in the Southern Hemisphere.
• Fossil plants
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