Causal or casual link between the rise of nannoplankton calcification and a tectonically-driven massive decrease in Late Triassic atmospheric CO2?


Y Godderis, Y Donnadieu, C de Vargas, RT Pierrehumbert, G Dromart, B van de Schootbrugge

Data discrepancies in solar-climate link.

Science (New York, N.Y.) 320 (2008) 746-

RT Pierrehumbert

Causal or casual link between the rise of nannoplankton calcification and an abrupt tectonically-driven atmospheric CO2 decline in the Late Triassic?


Y Godderis, Y Donnadieu, C De Vargas, RT Pierrehumbert, G Dromart, BV De Schootbrugge

Optimal surface excitation of the thermohaline circulation

Journal of Physical Oceanography 38 (2008) 1820-1830

L Zanna, E Tziperman

The amplification of thermohaline circulation (THC) anomalies resulting from heat and freshwater forcing at the ocean surface is investigated in a zonally averaged coupled ocean-atmosphere model. Optimal initial conditions of surface temperature and salinity leading to the largest THC growth are computed, and so are the structures of stochastic surface temperature and salinity forcing that excite maximum THC variance (stochastic optimals). When the THC amplitude is defined as its sum of squares (equivalent to using the standard L2 norm), the nonnormal linearized dynamics lead to an amplification with a time scale on the order of 100 yr. The optimal initial conditions have a vanishing THC anomaly, and the complex amplification mechanism involves the advection of both temperature and salinity anomalies by the mean flow and of the mean temperature and salinity by the anomaly flow. The L2 characterization of THC anomalies leads to physically interesting results, yet to a mathematically singular problem. A novel alternative characterizing the THC amplitude by its maximum value, as often done in general circulation model studies, is therefore introduced. This complementary method is shown to be equivalent to using the L-infinity norm, and the needed mathematical approach is developed and applied to the THC problem. Under this norm, an amplification occurs within 10 yr explained by the classic salinity advective feedback mechanism. The analysis of the stochastic optimals shows that the character of the THC variability may be very sensitive to the spatial pattern of the surface forcing. In particular, a maximum THC variance and long-time-scale variability are excited by a basin-scale surface forcing pattern, while a significantly higher frequency and to some extent a weaker variability are induced by a smooth and large-scale, yet mostly concentrated in polar areas, surface forcing pattern. Overall, the results suggest that a large THC variability can be efficiently excited by atmospheric surface forcing, and the simple model used here makes several predictions that would be interesting to test using more complex models. © 2008 American Meteorological Society.

On the relative humidity of the atmosphere

in The Global Circulation of the Atmosphere, Princeton University Press (2007) 6

RT Pierrehumbert

Intercomparison of tropical tropospheric humidity in GCMs with AMSU-B water vapor data


H Brogniez, RT Pierrehumbert

Investigating plausible mechanisms to trigger a deglaciation from a hard snowball Earth


G Le Hir, G Ramstein, Y Donnadieu, RT Pierrehumbert

An ocean of air: A natural history of the atmosphere

NATURE 447 (2007) 911-911

RT Pierrehumbert

The dynamics behind Titan's methane clouds.

Proceedings of the National Academy of Sciences of the United States of America 103 (2006) 18421-18426

JL Mitchell, RT Pierrehumbert, DMW Frierson, R Caballero

We present results of an axisymmetric global circulation model of Titan with a simplified suite of atmospheric physics forced by seasonally varying insolation. The recent discovery of midlatitude tropospheric clouds on Titan has caused much excitement about the roles of surface sources of methane and the global circulation in forming clouds. Although localized surface sources, such as methane geysers or "cryovolcanoes," have been invoked to explain these clouds, we find in this work that clouds appear in regions of convergence by the mean meridional circulation and over the poles during solstices, where the solar forcing reaches its seasonal maximum. Other regions are inhibited from forming clouds because of dynamical transports of methane and strong subsidence. We find that for a variety of moist regimes, i.e., with the effect of methane thermodynamics included, the observed cloud features can be explained by the large-scale dynamics of the atmosphere. Clouds at the solsticial pole are found to be a robust feature of Titan's dynamics, whereas isolated midlatitude clouds are present exclusively in a variety of moist dynamical regimes. In all cases, even without including methane thermodynamics, our model ceases to produce polar clouds approximately 4-6 terrestrial years after solstices.

Modelling the primary control of paleogeography on Cretaceous climate


Y Donnadieu, R Pierrehumbert, R Jacob, F Fluteau

A GEOCLIM simulation of climatic and biogeochemical consequences of Pangea breakup


Y Donnadieu, Y Godderis, R Pierrehumbert, G Dromart, F Fluteau, R Jacob

Using microwave observations to assess large-scale control of free tropospheric water vapor in the mid-latitudes


H Brogniez, RT Pierrehumbert

Climate Change: A Catastrophe in Slow Motion

Chicago Journal of International Law 6 (2006) 6

RT Pierrehumbert

Climate dynamics of a hard snowball Earth


RT Pierrehumbert

Nonnormal amplification of the thermohaline circulation

Journal of Physical Oceanography 35 (2005) 1593-1605

L Zanna, E Tziperman

A simple zonally averaged coupled ocean-atmosphere model, with a relatively high resolution in the meridional direction, is used to examine physical mechanisms leading to transient amplification of thermohaline circulation (THC) anomalies. It is found that in a stable regime, in which small perturbations eventually decay, there are optimal initial conditions leading to a dramatic amplification of initial temperature and salinity anomalies in addition to the THC amplification. The maximum amplification occurs after about 40 years, and the eventual decay is on a centennial time scale. The initial temperature and salinity anomalies are considerably amplified by factors of a few hundreds and 20, respectively. The initial conditions leading to this amplification are characterized by mutually canceling initial temperature and salinity anomalies contributions to the THC anomaly, such that the initial THC anomaly vanishes. The mechanism of amplification is analyzed and found to be the result of an interaction between a few damped (oscillatory and nonoscillatory) modes with decay time scales lying in a range of 20-800 years. The amplification mechanism is also found to be distinct from the advertive feedback leading to THC instabilities for large freshwater forcing. © 2005 American Meteorological Society.

High levels of atmospheric carbon dioxide necessary for the termination of global glaciation.

Nature 429 (2004) 646-649

RT Pierrehumbert

The possibility that the Earth suffered episodes of global glaciation as recently as the Neoproterozoic period, between about 900 and 543 million years ago, has been widely discussed. Termination of such 'hard snowball Earth' climate states has been proposed to proceed from accumulation of carbon dioxide in the atmosphere. Many salient aspects of the snowball scenario depend critically on the threshold of atmospheric carbon dioxide concentrations needed to trigger deglaciation. Here I present simulations with a general circulation model, using elevated carbon dioxide levels to estimate this deglaciation threshold. The model simulates several phenomena that are expected to be significant in a 'snowball Earth' scenario, but which have not been considered in previous studies with less sophisticated models, such as a reduction of vertical temperature gradients in winter, a reduction in summer tropopause height, the effect of snow cover and a reduction in cloud greenhouse effects. In my simulations, the system remains far short of deglaciation even at atmospheric carbon dioxide concentrations of 550 times the present levels (0.2 bar of CO2). I find that at much higher carbon dioxide levels, deglaciation is unlikely unless unknown feedback cycles that are not captured in the model come into effect.

Warming the world.

Nature 432 (2004) 677-

RT Pierrehumbert

Hydrothermal plume dynamics on Europa: Implications for chaos formation


JC Goodman, GC Collins, J Marshall, RT Pierrehumbert

Hydrothermal plume dynamics on Europa: Implications for chaos formation

Journal of Geophysical Research E: Planets 109 (2004)

JC Goodman, GC Collins, J Marshall, RT Pierrehumbert

Hydrothermal plumes may be responsible for transmitting radiogenic or tidally generated heat from Europa's rocky interior through a liquid ocean to the base of its ice shell. This process has been implicated in the formation of chaos regions and lenticulae by melting or exciting convection in the ice layer. In contrast to earlier work, we argue that Europa's ocean should be treated as an unstratified fluid. We have adapted and expanded upon existing work describing buoyant plumes in a rotating, unstratified environment. We discuss the scaling laws governing the flow and geometry of plumes on Europa and perform a laboratory experiment to obtain scaling constants and to visualize plume behavior in a Europa-like parameter regime. We predict that hydrothermal plumes on Europa are of a lateral scale (at least 25-50 km) comparable to large chaos regions; they are too broad to be responsible for the formation of individual lenticulae. Plume heat fluxes (0.1-10 W/m2) are too weak to allow complete melt-through of the ice layer. Current speeds in the plume (3-8 mm/s) are much slower than indicated by previous studies. The observed movement of ice blocks in the Conamara Chaos region is unlikely to be driven by such weak flow. Copyright 2004 by the American Geophysical Union.

Decay of passive scalars under the action of single scale smooth velocity fields in bounded two-dimensional domains: From non-self-similar probability distribution functions to self-similar eigenmodes (vol E 66, art no 056302, 2002)

PHYSICAL REVIEW E 68 (2003) ARTN 019903

J Sukhatme, RT Pierrehumbert