Steven K. Baum

Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model

Ice sheets may have reached the Equator in the late Proterozoic era (600–800 Myr ago), according to geological and palaeomagnetic studies, possibly resulting in a ‘snowball Earth’. But this period was a critical time in the evolution of multicellular animals, posing the question of how early life survived under such environmental stress. Here we present computer simulations of this unusual climate stage with a coupled climate/ice-sheet model. To simulate a snowball Earth, we use only a reduction in the solar constant compared to present-day conditions and we keep atmospheric CO2 concentrations near present levels. We find rapid transitions into and out of full glaciation that are consistent with the geological evidence. When we combine these results with a general circulation model, some of the simulations result in an equatorial belt of open water that may have provided a refugium for multicellular animals.

Effect of vegetation on an ice-age climate model simulation

A growing number of studies suggest that vegetation changes can significantly influence regional climate variations. Herein we utilize a climate model (GENESIS) with a land surface vegetation package to evaluate the potential role of the very large vegetation changes that occurred during the last glacial maximum (LGM). In particular, we focus on the potential response to a significant reduction in the area of tropical rainforest. Simulations employed a global vegetation reconstruction for the LGM and Climate/Long-Range Investigation, Mapping and Prediction (CLIMAP) sea surface temperature (SST) estimates. Results indicate that expansion of dryland vegetation causes a 15–30% additional LGM cooling for Australia (0.4°C) and Africa (0.9°C), respectively. Turnover from conifer to tundra also causes cooling of 2°–4°C or more in western Europe and Siberia. However, for the largest rainforest area (Amazon Basin), inclusion of realistic vegetation increased modeled temperatures 2°–4°C and decreased precipitation by 10–35%. These latter results are similar to those obtained with sensitivity experiments of the effects of future Amazon deforestation. Initial assessment of the potential effect of decreased stomatal resistance due to lower ice age CO2 levels indicates little significant response to this effect. Comparison of model-predicted low-elevation LGM temperature changes with estimates from proxy data indicate that inclusion of realistic vegetation estimates for the LGM results in slightly more than 50% agreement between models and data for low-elevation sites in low-mid latitudes. Data at variance with model predictions would appear to be explainable by considering additional changes in vegetation, ice age dust, or a 1°–2°C cooling below CLIMAP values. This conclusion is at variance with a 3°–4°C tropical cooling suggested by some studies for explaining estimated land temperature changes during the LGM. In some western European sites model temperatures are colder than proxy data by 2°–8°C. This model-data discrepancy may be explained by less sea ice in the subpolar North Atlantic than stipulated by CLIMAP, a conclusion consistent with new marine data from that region.

Modeling ocean heat content changes during the last millennium

[1] Observational studies show a significant increase in ocean heat content over the last half century. Herein we estimate heat content changes during the last millennium with a climate model whose forcing terms have been best-fit to surface proxy data. The model simulates the observed late 20th century ocean heat content increase and a comparable Little Ice Age minimum. When glacial advances are factored in, these results imply a sea level fall after the Middle Ages that is consistent with some geologic data. The present ocean heat content increase can be traced back to the mid-19th century, with a near-linear rate of change during the 20th century.

Estimating Carboniferous sea-level fluctuations from Gondwanan ice extent

The magnitude of Carboniferous sea-level fluctuations is not well known, but the estimates are important for constraining cyclothem models. In this paper we apply area-volume relations developed for Quaternary ice sheets to estimate the glacio-eustatic component of sea-level fluctuations in the Carboniferous. Utilizing the best estimate of Gondwanan ice area at the time of its greatest extent, we arrive at a figure of 60 ±15 m for the isostasy-corrected glacio-eustatic component. This value is nearer the lower limit of sea-level estimates for cyclothem fluctuations from other sources.

Modeling late Paleozoic glaciation

Late Paleozoic glaciation on Gondwana is associated with changes in geography, solar luminosity, and estimated CO{sub 2} levels. To assess the relative importance of these boundary conditions, the authors conducted a suite of climate model simulations for the periods before, during, and after peak mid-Carboniferous ({approximately}300 Ma) glaciation (340, 300, and 255 and 225 Ma, respectively). Orbital insolation values favorable for glaciation and interglaciation were used for each time interval. Results indicate that changes in geography cause significant changes in snow area, but the temporal trend is not consistent with the geologic record for glaciation. Combined CO{sub 2}-plus-geography changes yield the best agreement with observations. In addition, interglacial orbital configurations result in almost ice-free conditions for the glacial interval at 300 Ma, at a time of low CO{sub 2}. The large simulated glacial-interglacial snowline fluctuations for Permian-Carboniferous time may explain cyclothem fluctuations at these times. Overall, results support the importance of the CO{sub 2} paradigm, but also indicate that a fuller understanding of past climate change requires consideration of paleogeographic, luminosity, and orbital insolation changes.

Effect of decreased solar luminosity on late Precambrian ice extent

The latest Precambrian (∼0.57 Ga) was marked by extensive glaciation on a supercontinent. Ice cover may have been in lower latitudes than during the Pleistocene. Deglaciation and breakup of the supercontinent were followed by the first appearance/expansion of metazoans. Herein we report results from a seasonal climate model that clarify some of the processes operating during this important time interval. We demonstrate that, because solar luminosity was about 6% less than present, the modeled snowline was approximately 15° equatorward of its modeled Pleistocene limit. The significance of this response depends on choice of paleogeographic reconstruction. If the supercontinent was located entirely in low latitudes, the freezing line changes would not be enough to trigger glaciation on land. However, the luminosity changes are much more important if the supercontinent extended into midlatitudes (∼50° paleolatitude). Such a configuration has extensive summer snowcover and would provide a “seed” area for ice growth into lower latitudes. We postulate that if the large snowline changes we simulate were coupled to an ice sheet model, the ice margin could have reached to within 25° of the equator. Such a response could reconcile models and geologic data, but the reconciliation would critically depend on a more precise definition of low latitude glaciation, that is, whether the ice was at 25–30° or 0° latitude. Additional simulations for one Precambrian/Cambrian boundary reconstruction (∼0.54 Ga) suggest that reduction in late Precambrian snow cover might simply reflect movement of a midlatitude supercontinent into lower latitudes. The deglaciation could have been associated with a sea level rise of as much as 250–300 m, creating a much larger area for habitat occupation by benthic biota. Although more work is required on this topic, our results could explain both glaciation and deglaciation, with the explanation critically dependent on choice of paleogeographic reconstruction and more precise descriptions of late Precambrian ice sheet locations.

Reconciling Late Ordovician (440 Ma) glaciation with very high (14X) CO2 levels

Geochemical data and models suggest a positive correlation between carbon dioxide changes and climate during the last 540 m.y. The most dramatic exception to this correlation involves the Late Ordovician (440 Ma) glaciation, which occurred at a time when CO2 levels may have been much greater than present (14–16X?). Since decreased solar luminosity at that time only partially offset increased radiative forcing from CO2, some other factor needs to be considered to explain the glaciation. Prior work with energy balance models (EBMs) suggested that the unique geographic configuration of Gondwanaland at that time may have resulted in a small area of parameter space permitting permanent snow cover and higher CO2 levels. However, the crude snow and sea ice parameterizations in the EBM left these conclusions open to further scrutiny. Herein we present results from four experiments with the GENESIS general circulation model with CO2 levels 14X greater than present, solar luminosity reduced 4.5%, and an orbital configuration set for minimum summer insolation receipt. We examined the effects of different combinations of ocean heat transport and topography on high-latitude snow cover on Gondwanaland. For the no-elevation simulations we failed to simulate permanent summer snow cover. However, for the slightly elevated topography cases (300–500 m), permanent summer snow cover occurs where geological data indicate the Ordovician ice sheet was present. These results support the hypothesis based on EBM studies. Further results indicate that although average runoff per grid point increases substantially for the Ordovician runs, the decreased land area results in global runoff 10–30% less than present, with largest runoff reductions for flat topography. This response has implications for CO2-runoff/weathering parameterizations in geochemical models. Finally, simulated tropical sea surface temperatures (SSTs) are the same or only marginally warmer than present. This result is consistent with evidence from other warm time intervals indicating small changes in tropical SSTs during time of high CO2.

GCM response to Late Precambrian (∼590 Ma) ice—covered continents

Recent coupled energy balance/ice sheet modeling studies indicate that ice-covered continents can be simulated for the Late Precambrian with 6% solar constant reduction. We examine the ocean mixed layer response to such an ice sheet with the GENESIS 2 general circulation model and CO2 levels varying from 0.5–2.5 times present. The ocean ices over completely at 0.5–1.0X present levels, with the final phase of sea ice growth occurring within a single model year. A qualitatively different 2.5X CO2 solution is close to equilibrium and yields open water between ∼25 °N and ∼25 °S paleolatitude. The prevailing wind patterns suggest that an equatorial Pacific-type circulation may have developed in part of the Neoproterozoic ocean.

Toward reconciliation of Late Ordovician (∼440 Ma) glaciation with very high CO2 levels

Although Phanerozoic glaciations usually coincided with times of estimated low atmospheric CO2, the Late Ordovician (440 Ma) glaciation is a significant exception. CO2 levels during that time may have been as much as 10 times greater than present. In an earlier paper we suggested that the unique geographic configuration of Gondwanaland may explain such a response, as the edge of the supercontinent was essentially tangent to the south pole, and the moderating effect of the nearby ocean may have suppressed the magnitude of summer warming on the landmass, thereby allowing glacial inception. One limitation to the earlier study was that it used a linear energy balance model (EBM). In this paper we further test the above hypothesis in a suite of experiments with a nonlinear EBM that allows for snow-albedo feedback. We also utilize updated estimates for CO2 levels, decreases in solar luminosity, and variations in orbital forcing. Baseline experiments with no changes in luminosity or CO2 resulted in an ice-covered area of 6.3 ×106 km2, 53% of the estimated area covered by the Ordovician ice sheet. Additional experiments for different combinations of orbital forcing, 7X/13X CO2, and −3.5%/−5.0% luminosity yielded 0–35% of the estimated ice area in the Late Ordovician. A crude estimate of possible topographic influences increased these numbers to7–47% of total estimated ice area. Additional factors related to ice sheet growth should increase these values somewhat. These results provide additional support for the high CO2/glaciation explanation, with the caveat that even the partial success occurs only when parameters are at the extreme end of their permissible range. The estimated duration of Ordovician glaciation is also consistent with the migration of Gondwanaland across the south pole, with a centrally located pole yielding ice-free conditions in the summer. Thus identical levels of external forcing yield either glaciated or ice-free conditions, with the solution dependent on location of the landmass. Although more work is required on this topic, our experiments suggest that there may be a relatively parsimonious explanation for this perplexing paleoclimate paradox.The results lend further support to the proposition that paleogeography significantly modifies the role of CO2 in the long-term evolution of climate.

Climate model comparison of Gondwanan and Laurentide glaciations

Geologic studies indicate that the Carboniferous glaciation on Gondwanaland was approximately as extensive as the ice sheets during the Pleistocene. However, there is one major difference between the climate boundary conditions for the two ice sheets: the Gondwanan ice sheet was located on a supercontinent. Three different levels of sensitivity experiments were conducted to examine the effect of the large landmass on the magnitude of summer warming over the ice sheets: (1) simulations with present solar luminosity and present orbital forcing resulted in summer temperatures over the Gondwanan Ice Sheet 12°–17°C greater than over the Pleistocene Laurentide Ice Sheet; (2) lowering the solar constant 3% or modifying the seasonal insolation cycle to a “cold summer orbit” reduced the warming but still led to significant differences between Gondwanan and Pleistocene simulations; and (3) the combined effect of lowering the solar constant and modifying the seasonal insolation cycle to a cold summer orbit resulted in temperature patterns over the Gondwanan Ice Sheet similar to the Pleistocene. Model simulations also predict tropical sea surface temperatures about 4°C less than at present as a result of reduced solar luminosity. These results suggest that conditions necessary to explain Gondwanan ice sheet stability may be known, but required boundary conditions would be more extreme than in the Pleistocene. Although a number of uncertainties remain in these calculations, they help to better define critical conditions for glaciation for one of the most prolonged periods of continuous glaciation in Earth history.