Marie-France Loutre

INSOLATION VALUES FOR THE CLIMATE OF THE LAST 10 MILLION YEARS

New values for the astronomical parameters of the Earths orbit and rotation (eccentricity, obliquity and precession) are proposed for paleoclimatic research related to the Late Miocene, the Pliocene and the Quaternary. They have been obtained from a numerical solution of the Lagrangian system of the planetary point masses and from an analytical solution of the Poisson equations of the Earth-Moon system. The analytical expansion allows the direct determination of the main frequencies with their phase and amplitude. Numerical and analytical comparisons with the former astronomical solution BER78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated. The corresponding insolation values have also been computed. This analysis leads to the conclusion that the new values are expected to be reliable over the last 5 Ma in the time domain and at least over the last 10 Ma in the frequency domain.

Alternate interpretation of the Messinian salinity crisis: Controversy resolved?

A high-resolution chronology of the Mediterranean Messinian salinity crisis is proposed. Two types of evaporite deposition may be distinguished: those in marginal areas vs. those in basinal ones. Their diachroneity is deduced from the stratigraphic relationships linking these evaporites to a major Messinian erosional surface, A two-step model is proposed for the evolution of the salinity crisis through time. During the first phase (from 5.75 to 5.60 Ma), the deposition of marginal evaporites took place in response to a modest sea-level fall; in the second interval (from 5.60 to 5.32 Ma), the Mediterranean basin became isolated. During this later period, the deposition of basinal evaporites and the cutting of the Mediterranean canyons took place.

Stability of the astronomical frequencies over the earth's history for paleoclimate studies

The expected changes over the past 500 million years in the principal astronomical frequencies influencing the Earths climate may be strong enough to be detectable in the geological records, and such effects have been inferred in several cases. Calculations suggest that the shortening of the Earth-moon distance and of the length of the day back in time induced a shortening of the fundamental periods for the obliquity and climatic precession, from 54 to 35, 41 to 29, 23 to 19, and 19 to 16 thousand years over the last half-billion years. At the same time, the precessional constant increased from 50 to 61 arc seconds per year. The changes in the frequencies of the planetary system due to its chaotic motion are much smaller; their influence on the changes of the periods of climatic precession, obliquity, and eccentricity of the Earths orbit around the sun can be neglected. Eccentricity periods used for Quaternary climate studies may therefore be considered to have been more or less constant for pre-Quaternary times.

The contribution of orbital forcing to the progressive intensification of Northern Hemisphere glaciation

In this study, we reconstruct the timing of the onset of Northern Hemisphere glaciation. This began in the late Miocene with a significant build-up of ice on Southern Greenland. However, progressive intensification of glaciation did not begin until 3.5-3 Ma, when the Greenland ice sheet expanded to include Northern Greenland. Following this stage we suggest that the Eurasian Arctic and Northeast Asia were glaciated at approximately 2.74 Ma, 40 ka before the glaciation of Alaska (2.70 Ma) and about 200 ka before significant glaciation of the North East American continent (2.54 Ma). We also review the suggested causes of Northern Hemisphere glaciation. Tectonic changes, such as the uplift of the Himalayan and Tibetan Plateau, the deepening of the Bering Strait and the emergence of the Panama Isthmus, are too gradual to account entirely for the speed of Northern Hemisphere glaciation. We, therefore, postulate that tectonic changes may have brought global climate to a critical threshold, but the relatively rapid variations in the Earths orbital parameters and thus insolation, triggered the intensification of Northern Hemisphere glaciation. This theory is supported by computer simulations, which despite the relative simplicity of the model and the approximation of some factors (e.g. using a linear carbon dioxide scenario, neglecting the geographical difference between the Pliocene and the present) suggest that it is possible to build-up Northern Hemisphere ice sheets, between 2.75 and 2.55 Ma, by varying only the insolation controlled by the orbital parameters

Marine Isotope Stage 11 as an analogue for the present interglacial

Past analogues for our present interglacial or even warmer periods have been sought in order to better understand our present and future climate. Marine Isotope Stage (MIS) 5, more precisely substage 5e, has long been considered to be a good candidate. However, there were some elements against this analogy in the data themselves [Kukla et al. Quat. Sci. Rev. 16 (6) (1997) 605], as well as in the mechanisms [Berger, 1989 Response of the climate system to CO2 and astronomical forcings. In: Paleo-Analogs, IPCC Working Group I, Bath, 20-21 November 1989] and forcing related to both periods. Here we suggest that the period from 405 to 340 ka before present (BP), including a large part of Marine Isotope Stage 11, could be a good analogue for future climate. The insolation over this interval shows a strong linear correlation with the insolation signal over the recent past and the future. In addition, simulations using the climate model developed in Louvain-la-Neuve (LLN 2-D NH) show that both MIS 11 and the future are characterized by small amount (if any) of continental ice, with almost no variation during the whole interval. In contrast, MIS 5 is exhibiting larger variability in simulated ice volume. This confirms that the interval [405-340 ka BP] may lead to a better understanding of our present and future warm climate

Long-term climate commitments projected with climate-carbon cycle models

Eight earth system models of intermediate complexity (EMICs) are used to project climate change commitments for the recent Intergovernmental Panel on Climate Change’s (IPCC’s) Fourth Assessment Report (AR4). Simulations are run until the year 3000 A.D. and extend substantially farther into the future than conceptually similar simulations with atmosphere–ocean general circulation models (AOGCMs) coupled to carbon cycle models. In this paper the following are investigated: 1) the climate change commitment in response to stabilized greenhouse gases and stabilized total radiative forcing, 2) the climate change commitment in response to earlier CO2 emissions, and 3) emission trajectories for profiles leading to the stabilization of atmospheric CO2 and their uncertainties due to carbon cycle processes. Results over the twenty-first century compare reasonably well with results from AOGCMs, and the suite of EMICs proves well suited to complement more complex models. Substantial climate change commitments for sea level rise and global mean surface temperature increase after a stabilization of atmospheric greenhouse gases and radiative forcing in the year 2100 are identified. The additional warming by the year 3000 is 0.6–1.6 K for the low-CO2 IPCC Special Report on Emissions Scenarios (SRES) B1 scenario and 1.3–2.2 K for the high-CO2 SRES A2 scenario. Correspondingly, the post-2100 thermal expansion commitment is 0.3–1.1 m for SRES B1 and 0.5–2.2 m for SRES A2. Sea level continues to rise due to thermal expansion for several centuries after CO2 stabilization. In contrast, surface temperature changes slow down after a century. The meridional overturning circulation is weakened in all EMICs, but recovers to nearly initial values in all but one of the models after centuries for the scenarios considered. Emissions during the twenty-first century continue to impact atmospheric CO2 and climate even at year 3000. All models find that most of the anthropogenic carbon emissions are eventually taken up by the ocean (49%–62%) in year 3000, and that a substantial fraction (15%–28%) is still airborne even 900 yr after carbon emissions have ceased. Future stabilization of atmospheric CO2 and climate change requires a substantial reduction of CO2 emissions below present levels in all EMICs. This reduction needs to be substantially larger if carbon cycle–climate feedbacks are accounted for or if terrestrial CO2 fertilization is not operating. Large differences among EMICs are identified in both the response to increasing atmospheric CO2 and the response to climate change. This highlights the need for improved representations of carbon cycle processes in these models apart from the sensitivity to climate change. Sensitivity simulations with one single EMIC indicate that both carbon cycle and climate sensitivity related uncertainties on projected allowable emissions are substantial.

Modelling northern hemisphere ice volume over the last 3 Ma

The Northern Hemisphere ice-sheet volumes over the last 3 Ma were simulated using the LLN 2-D model. The forcings were both insolation and CO2 concentration. Different atmospheric CO2 scenarios were used because of a lack of CO2 reconstruction over this remote past. With constant CO2 concentrations, the simulated ice volume does not show any gradual increase as recorded in the marine sediments. Moreover, its spectrum changes according to the chosen value of CO2: when CO2 is fixed at 220 ppmv, the simulated ice volume is dominated by the similar to 100 ka period; if a pre-industrial CO2 concentration of 280 ppmv is used, the simulation is dominated by the similar to 41 ka period. By using a linearly decreasing CO2 concentration going from 320 ppmv at 3 Ma BP to 200 ppmv at present, the simulated changes in the power spectrum are in agreement with those obtained from the sedimentary records: roughly before 1 Ma BP the dominating period is similar to 41 ka, afterwards the similar to 100 ka period becomes dominant. This transition is accompanied by a gradual increase of the ice volume. Potential mechanisms for this transition are discussed. Using cyclic CO2 fluctuations over the last 0.6 Ma does not change significantly the spectral characteristics of the simulated ice volume, but amplifies the amplitude of its variations

Millennial total sea-level commitments projected with the Earth system model of intermediate complexity LOVECLIM

Sea-level is expected to rise for a long time to come, even after stabilization of human-induced climatic warming. Here we use simulations with the Earth system model of intermediate complexity LOVECLIM to project sea-level changes over the third millennium forced with atmospheric greenhouse gas concentrations that stabilize by either 2000 or 2100 AD. The model includes 3D thermomechanical models of the Greenland and Antarctic ice sheets coupled to an atmosphere and an ocean model, a global glacier melt algorithm to account for the response of mountain glaciers and ice caps, and a procedure for assessing oceanic thermal expansion from oceanic heat uptake. Four climate change scenarios are considered to determine sea-level commitments. These assume a 21st century increase in greenhouse gases according to SRES scenarios B1, A1B and A2 with a stabilization of the atmospheric composition after the year 2100. One additional scenario assumes 1000 years of constant atmospheric composition from the year 2000 onwards. For our preferred model version, we find an already committed total sea-level rise of 1.1 m by 3000 AD. In experiments with greenhouse gas concentration stabilization at 2100 AD, the total sea-level rise ranges between 2.1 m (B1), 4.1 m (A1B) and 6.8 m (A2). In all scenarios, more than half of this amount arises from the Greenland ice sheet, thermal expansion is the second largest contributor, and the contribution of glaciers and ice caps is small as it is limited by the available ice volume of maximally 25 cm of sea-level equivalent. Additionally, we analysed the sensitivity of the sea-level contributions from an ensemble of nine different model versions that cover a large range of climate sensitivity realized by model parameter variations of the atmosphere–ocean model. Selected temperature indices are found to be good predictors for sea-level contributions from the different components of land ice and oceanic thermal expansion after 1000 years.

Insolation and Earth's orbital periods

Solar irradiance received on a horizontal surface depends on the solar output, the semimajor axis of the elliptical orbit of the Earth around the sun (a), the distance from the Earth to the sun (r), and the zenith distance (z). The spectrum of the distance, r, for a given value of the true longitude, lambda, displays mainly the precessional periods and, with much less power, half precession periods, eccentricity periods, and some combination tones. The zenith distance or its equivalent, the elevation angle (E), is only a function of obliquity (epsilon) for a given latitude, phi, true longitude, and hour angle, H. Therefore the insolation at a given constant value of z is only a function of precession and eccentricity. On the other hand, the value of the hour angle, H, corresponding to this fixed value of z varies with epsilon, except for the equinoxes, where H corresponding to a constant z also remains constant through time. Three kinds of insolation have been computed both analytically and numerically: the instantaneous insolation (irradiance) at noon, the daily irradiation, and the irradiations received during particular time intervals of the day defined by two constant values of the zenith distance (diurnal irradiations). Mean irradiances (irradiations divided by the length of the time interval over which they are calculated) are also computed for different time intervals, like the interval between sunrise and sunset, in particular. Examples of these insolations are given in this paper for the equinoxes and the solstices. At the equinoxes, for each latitude, all insolations are only a function of precession (this invalidates the results obtained by Cerveny [1991)). At the solstices, both precession and obliquity are present, although precession dominates for most of the latitudes. Because the lengths of the astronomical seasons are secularly variable (in tenus of precession only), a particular calendar day does not always correspond to the same position relative to the sun through geological time. Similarly, a given longitude of the Sun on its orbit does not correspond to the same calendar day. For example, 103 kyr ago, assuming arbitrarily that the spring equinox is always on March 21, autumn began on September 13, and 114 kyr ago, it began on September 27, the length of the summer season being 85 and 98 calendar days, respectively, at these remote times in the past.

Future climatic changes : Are we entering an exceptionally long interglacial?

Various experiments have been conducted using theLouvain-la-Neuve two-dimensional Northern Hemisphereclimate model (LLN 2-D NH) to simulate climate for thenext 130 kyr into the future. Simulations start withvalues representing the present-day NorthernHemisphere ice sheet, using different scenarios forfuture CO2 concentrations. The sensitivity of themodel to the initial size of the Greenland ice sheet,and to possible impacts of human activities, has alsobeen tested. Most of the natural scenarios indicatethat: (i) the climate is likely to experience a longlasting (∼50 kyr) interglacial; (ii) the next glacialmaximum is expected to be most intense at around 100kyr after present (AP), with a likely interstadial at∼60 kyr AP; and (iii) after 100 kyr AP continentalice rapidly melts, leading to an ice volume minimum 20kyr later. However, the amplitude and, to a lesserextent, the timing of future climatic changes dependon the CO2 scenario and on the initial conditionsrelated to the assumed present-day ice volume.According to our modelling experiments, mansactivities over the next centuries may significantlyaffect the ice-sheets behaviour for approximately thenext 50 kyr. Finally, the existence of thresholds inCO2 and insolation, earlier shown to besignificant for the past, is confirmed to be alsoimportant for the future.