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CO2 in Earth’s Ice Age Cycles  

Mathis P. Hain and Daniel M. Sigman

Earth's history is marked by episodes of large-scale continental glaciation. Most recently, beginning 3 million years ago, northern hemispheric glaciation expanded and developed cyclic variations known as the ice age cycles. With the 19th-century discovery of these cycles in ice extent and climate, changes in atmospheric carbon dioxide (CO2) concentration were proposed as a possible cause. Since the 1980s, scientists have produced detailed reconstructions revealing that, during ice ages, atmospheric CO2 was as much as a third lower than its preindustrial concentration—enough to explain almost half of the approximately 5 °C ice age cooling by weakening the Earth’s natural greenhouse effect. The consensus is that the ice age climate cycles result from cyclic changes in Earth’s orbit, which redistribute sunlight between regions and seasons but do not in themselves significantly heat or cool the globe on an annual-average basis. If so, the regional and seasonal effects of orbital change must cause changes in aspects of the Earth system that then induce changes in global annual-average climate. Changes in the reflection of sunlight by the ice sheets are widely believed to have played such a role. Atmospheric CO2 appears to be a second key Earth system property, and one that caused the ice age cycles to be global rather than simply regional phenomena. The ocean was likely the dominant driver of atmospheric CO2 change between warm “interglacial” and cold “glacial” periods, through multiple aspects of its behavior. First, ice age cooling and other changes allowed bulk global seawater to absorb additional CO2 from the atmosphere. Second, during ice ages, the ocean’s “biological carbon pump” was stronger: Ocean plankton and their sinking debris more effectively removed CO2 from surface waters and the atmosphere, sequestering it in the ocean interior. Polar ocean changes were key to this stronger biological pump, involving some combination of changes in biological productivity, ocean circulation, and air–sea gas exchange. Third, the net effect of these ocean changes was to enhance deep ocean CO2 storage and thus to dissolve calcium carbonate sediment off the seafloor, changing the ocean’s acid/base chemistry so that it absorbed additional CO2 from the atmosphere. The specific polar ocean changes that drove the strengthening of the biological carbon pump and the ensuing seafloor calcium carbonate response are a topic of ongoing debate.

Article

Energetics of the Climate System  

Jin-Song von Storch

The energetics considerations based on Lorenz’s available potential energy A focus on identification and quantification of processes capable of converting external energy sources into the kinetic energy of atmospheric and oceanic general circulations. Generally, these considerations consist of: (a) identifying the relevant energy compartments from which energy can be converted against friction to kinetic energy of motions of interests; (b) formulating for these energy compartments budget equations that describe all possible energy pathways; and (c) identifying the dominant energy pathways using realistic data. In order to obtain a more detailed description of energy pathways, a partitioning of motions, for example, into a “mean” and an “eddy” component, or into a diabatic and an adiabatic component, is used. Since the budget equations do not always suggest the relative importance of all possible pathways, often not even the directions, data that describe the atmospheric and the oceanic state in a sufficiently accurate manner are needed for evaluating the energy pathways. Apart from the complication due to different expressions of A , ranging from the original definition by Lorenz in 1955 to its approximations and to more generally defined forms, one has to balance the complexity of the respective budget equations that allows the evaluation of more possible energy pathways, with the quality of data available that allows sufficiently accurate estimates of energy pathways. With regard to the atmosphere, our knowledge, as inferred from the four-box Lorenz energy cycle, has consolidated in the last two decades, by, among other means, using data assimilation products obtained by combining observations with realistic atmospheric general circulation models (AGCMs). The eddy kinetic energy, amounting to slightly less than 50% of the total kinetic energy, is supported against friction through a baroclinic pathway “fueled” by the latitudinally dependent diabatic heating. The mean kinetic energy is supported against friction by converting eddy kinetic energy via inverse cascades. For the ocean, our knowledge is still emerging. The description through the four-box Lorenz energy cycle is approximative and was only estimated from a simulation of a 0 . 1 ° oceanic general circulation models (OGCM) realistically forced at the sea surface, rather than from a data assimilation product. The estimates obtained so far suggest that the oceanic eddy kinetic energy, amounting almost 75% of the total oceanic kinetic energy, is supported against friction through a baroclinic pathway similar to that in the atmosphere. However, the oceanic baroclinic pathway is “fueled” to a considerable extent by converting mean kinetic energy supported by winds into mean available potential energy. Winds are also the direct source of the kinetic energy of the mean circulation, without involving noticeable inverse cascades from transients, at least not for the ocean as a whole. The energetics of oceanic general circulation can also be examined by separating diabatic from adiabatic processes. Such a consideration is thought to be more appropriate for understanding the energetics of the oceanic meridional overturning circulation (MOC), since this circulation is sensitive to density changes induced by diabatic mixing. Further work is needed to quantify the respective energy pathways using realistic data.

Article

Multiple Equilibria in the Climate System  

Henk A. Dijkstra

The idea that under the same external forcing conditions, the climate system is able to have several (statistical) equilibrium states is both fascinating and worrying: fascinating because the interaction of different positive and negative feedbacks can then lead to different large-scale reorganizations of the transport of heat (and other properties) over the globe; worrying because perturbations on the current equilibrium state can then unexpectedly cause transitions in large-scale transport properties, with potential disastrous changes in regional weather conditions. In this article, the development of the idea to explain peculiar climate changes using multiple equilibrium states is presented.

Article

Ocean Mixing  

Carl Wunsch

Oceanic mixing is one of the major determinants of the ocean circulation and its climatological influences. Existing distributions of mixing properties determine the rates of storage and redistribution within the climate system of fundamental scalar tracers including heat, fresh water, oxygen, carbon, and others. Observations have overturned earlier concepts that mixing rates might be approximately uniform throughout the ocean volume, with profound implications for determining the circulation and its properties. Inferences about past and potential future oceanic circulations and the resulting climate influence require determination of changed energy inputs and the expected consequent adjustment of mixing processes and their influence.