Linda E. Sohl

Climate forcings and the initiation of low‐latitude ice sheets during the Neoproterozoic Varanger glacial interval

The GISS GCM was used to determine if a diverse set of climate forcings, alone or in combination, could have initiated the low-latitude ice sheets of the Varanger (∼600 Ma) glacial interval. The simulations use a realistic reconstruction of the paleocontinental distribution and test the following forcings, alone and in combination: 6% solar luminosity decrease, four atmospheric CO2 scenarios (1260, 315, 140, and 40 ppm), a 50% increase and a 50% decrease in ocean heat transports, and a change in obliquity to 60°. None of the forcings, individually, produced year-round snow accumulation on low-latitude continents, although the solar insolation decrease and 40 ppm CO2 scenarios allowed snow and ice to accumulate at high and middle latitudes. Combining forcings further cools the climate: when solar luminosity, CO2, and ocean heat transports were all decreased, annual mean freezing and snow accumulation extended across tropical continents. No simulation would have initiated low-latitude glaciation without contemporaneous glaciation at higher latitudes, a finding that matches the distribution of glacial deposits but which argues against high obliquity as a cause of the Varanger ice age. Low-level clouds increased in most scenarios, as did surface albedo, while atmospheric water vapor amounts declined; all are positive feedbacks that drive temperatures lower. In the most severe scenario, global snow and ice cover increased to 68%, compared to 12% under modern conditions, and water vapor dropped by 90%. These results do not necessarily preclude a ‘snowball’ Earth climate scenario for the Varanger glacial interval. However, either more severe forcings existed or radical changes occurred in the ocean/atmosphere system which are unaccounted for by the GCM. Also, as sea ice extent increased in these experiments, snow accumulation began to decline, because of an increasingly dry atmosphere. Under snowball Earth conditions, glaciation would be impossible, since the hydrological cycle would all but cease if the atmospheres primary moisture source were cut off.

Sea Surface Temperature of the mid-Piacenzian Ocean: A Data-Model Comparison

The mid-Piacenzian climate represents the most geologically recent interval of long-term average warmth relative to the last million years, and shares similarities with the climate projected for the end of the 21st century. As such, it represents a natural experiment from which we can gain insight into potential climate change impacts, enabling more informed policy decisions for mitigation and adaptation. Here, we present the first systematic comparison of Pliocene sea surface temperature (SST) between an ensemble of eight climate model simulations produced as part of PlioMIP (Pliocene Model Intercomparison Project) with the PRISM (Pliocene Research, Interpretation and Synoptic Mapping) Project mean annual SST field. Our results highlight key regional and dynamic situations where there is discord between the palaeoenvironmental reconstruction and the climate model simulations. These differences have led to improved strategies for both experimental design and temporal refinement of the palaeoenvironmental reconstruction.

Was Venus the first habitable world of our solar system

Present-day Venus is an inhospitable place with surface temperatures approaching 750K and an atmosphere 90 times as thick as Earths. Billions of years ago the picture may have been very different. We have created a suite of 3-D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 Gya, present-day Venus orbital parameters, an ocean volume consistent with current theory, and an atmospheric composition estimated for early Venus. Using these parameters we find that such a world could have had moderate temperatures if Venus had a rotation period slower than ~16 Earth days, despite an incident solar flux 46-70% higher than Earth receives. At its current rotation period, Venuss climate could have remained habitable until at least 715 million years ago. These results demonstrate the role rotation and topography play in understanding the climatic history of Venus-like exoplanets discovered in the present epoch.

Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D) 1.0: A General Circulation Model for Simulating the Climates of Rocky Planets

Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics (ROCKE-3D) is a three-dimensional General Circulation Model (GCM) developed at the NASA Goddard Institute for Space Studies for the modeling of atmospheres of solar system and exoplanetary terrestrial planets. Its parent model, known as ModelE2, is used to simulate modern Earth and near-term paleo-Earth climates. ROCKE-3D is an ongoing effort to expand the capabilities of ModelE2 to handle a broader range of atmospheric conditions, including higher and lower atmospheric pressures, more diverse chemistries and compositions, larger and smaller planet radii and gravity, different rotation rates (from slower to more rapid than modern Earths, including synchronous rotation), diverse ocean and land distributions and topographies, and potential basic biosphere functions. The first aim of ROCKE-3D is to model planetary atmospheres on terrestrial worlds within the solar system such as paleo-Earth, modern and paleo-Mars, paleo-Venus, and Saturns moon Titan. By validating the model for a broad range of temperatures, pressures, and atmospheric constituents, we can then further expand its capabilities to those exoplanetary rocky worlds that have been discovered in the past, as well as those to be discovered in the future. We also discuss the current and near-future capabilities of ROCKE-3D as a community model for studying planetary and exoplanetary atmospheres.