From the Inside Flap
"Michael Bender, a giant in the field, fits the excitement, rigor, and deep insights of paleoclimatology into a succinct text suitable for a semester-long course introducing this indispensable branch of environmental science."--Richard B. Alley, Pennsylvania State University
"The history of Earth's climate is an essential context for understanding anthropogenic climate change in the future. Michael Bender pulls together this vast area of science and distills it to the essentials, delivering a comprehensive view of the evolution of Earth's climate at a level useful to scientists and the general reader."--Daniel Schrag, Harvard University
"Paleoclimatology has been missing a concise, modern, overview textbook, and I think this book will fill that niche. There is much ground to cover and the text does a good job of bringing the reader up to speed on most of the important patterns and processes in Earth's climatic history. The book combines excellent coverage of basic physical and chemical aspects of the climate system with a long-term historical overview of the climate system in action."--Matthew Huber, Purdue University
"This concise but comprehensive history of Earth's climate hits all the right points and will serve equally well as an introductory textbook or as an entry into the field of paleoclimate."--David W. Lea, University of California, Santa Barbara
From the Back Cover
"Michael Bender, a giant in the field, fits the excitement, rigor, and deep insights of paleoclimatology into a succinct text suitable for a semester-long course introducing this indispensable branch of environmental science."--Richard B. Alley, Pennsylvania State University
"The history of Earth's climate is an essential context for understanding anthropogenic climate change in the future. Michael Bender pulls together this vast area of science and distills it to the essentials, delivering a comprehensive view of the evolution of Earth's climate at a level useful to scientists and the general reader."--Daniel Schrag, Harvard University
"Paleoclimatology has been missing a concise, modern, overview textbook, and I think this book will fill that niche. There is much ground to cover and the text does a good job of bringing the reader up to speed on most of the important patterns and processes in Earth's climatic history. The book combines excellent coverage of basic physical and chemical aspects of the climate system with a long-term historical overview of the climate system in action."--Matthew Huber, Purdue University
"This concise but comprehensive history of Earth's climate hits all the right points and will serve equally well as an introductory textbook or as an entry into the field of paleoclimate."--David W. Lea, University of California, Santa Barbara
About the Author
Michael L. Bender is professor of geosciences and atmospheric/ocean sciences at Princeton University, a fellow of the American Geophysical Union, and a member of the National Academy of Sciences. He has taught paleoclimate at the graduate and undergraduate levels, and published widely on the topic in Science, Nature, and other journals.
Excerpt. © Reprinted by permission. All rights reserved.
PALEOCLIMATE
By Michael L. BenderPRINCETON UNIVERSITY PRESS
Copyright © 2013 Princeton University Press
All rights reserved.
ISBN: 978-0-691-14555-6
Contents
List of Boxes..............................................................viPreface....................................................................viiAcknowledgments............................................................xiii1 Earth's Climate System...................................................12 The Faint Young Sun......................................................243 Precambrian Glaciations..................................................384 Regulation of the Earth System and Global Temperature....................545 The Late Paleozoic Ice Ages..............................................736 Equable Climates of the Mesozoic and Paleogene...........................977 The Paleocene-Eocene Thermal Maximum.....................................1258 The Long Cooling of the Cenozoic.........................................1449 The Origin of Northern Hemisphere Glaciation and the Pleistocene Ice
Ages.......................................................................17210 Rapid Climate Change during the Last Glacial Period.....................23511 The Holocene............................................................26412 Anthropogenic Global Warming in the Context of Paleoclimate.............287Glossary...................................................................295Index......................................................................303
<h2>CHAPTER 1</h2><p>EARTH'S CLIMATE SYSTEM</p><br><p>Earth's climate system includes all the realmsof the planet that interact to produce the seasonal marchof temperature, wind, and precipitation. Most importantare the atmosphere; the oceans, including their linkedchemical and biological processes; and the solid Earthinsofar as it influences CO<sub>2</sub> concentration in air. Atmosphericprocesses govern climate over time scales of afew years or less. The oceans influence climate changeover periods of decades to tens of millennia. Over periodsof a hundred thousand years or more, interactionsbetween the solid Earth and the surface environmentfix the CO<sub>2</sub> concentration of air and Earth's averagetemperature.</p><p>In this chapter, we discuss the physical and chemicalcontrols that determine the most important characteristicsof Earth's climate. We start by discussing the decreaseof pressure and temperature with elevation. We proceedto Earth's heat budget and the controls on global averagetemperature. We then examine the large scale circulationof Earth's atmosphere, and discuss how this circulationdictates prevailing wind directions at the surface andhow it determines what regions of the globe get a lot ofprecipitation and what regions are dry. We discuss oceancirculation, biological processes in the ocean, and howthese processes combine to change the partial pressure ofCO<sub>2</sub> in the atmosphere over periods of decades to millennia.We end with a brief description of the geological processesthat fix the average background CO<sub>2</sub> concentrationof the atmosphere over hundreds of thousands of years.</p><br><p><b>ATMOSPHERIC PROPERTIES AND CLIMATE</p><p>Pressure and temperatureas a function of altitude</b></p><p>Earth is heated by sunlight predominantly at groundlevel, which in turn warms the local lower atmosphere.Warm air expands and becomes buoyant, leading to verticalmixing. As air rises, it encounters lower pressuresand expands into the void. Atmospheric pressure decreaseswith elevation in a way that reflects hydrostaticequilibrium in the atmosphere. In this condition, airis stabilized at a given altitude by the balance betweengravity, which pulls the air mass down toward the surface,and the upward push exerted by the natural tendencyof a gas to expand.</p><p>At a given elevation, pressure is simply the weight perunit area of the overlying column of air. This condition isexpressed by the equation:</p><p>dρ/dz = -gρ = M<sub>air</sub>/RT, (1)</p><br><p>where ρ is density, z is height above the surface, g isgravitational acceleration, M<sub>air</sub> is the molar mass of air(29 gm/mole), R is the ideal gas constant, and T is Kelvintemperature. From this equation, one can show thatpressure decreases by a factor of 1/e (0.37) for every ~7–8km increase in elevation for typical air temperatures.</p><p>Temperatures are cold at higher altitudes because ofthe decrease of pressure with elevation. Consider a parcelof dry air large enough that it is not gaining or losingheat to the surrounding atmosphere. As this parcel rises,it encounters lower atmospheric pressure and "pushesout" into the surrounding air. In so doing, it uses energy,which leads it to cool. The cooling rate, or "lapse rate," isabout 10°/km. If the air is wet, water condenses as it risesand reaches the dew point. Latent heat is released, andthe lapse rate is smaller, typically 4–7°/km. Lower valuescorrespond to warmer saturated air, with more watervapor and greater potential to release latent heat.</p><p>This decrease with temperature reverses at an altitudeof about 11 km. The reversal is caused by the absorptionof high energy (ultraviolet or UV) light from the sun dueto reactions of the ozone cycle. These reactions are:</p><p>O<sub>2</sub> -> 2 O (2)</p><p>O<sub>2</sub> + O -> O<sub>3</sub> (3)</p><p>O<sub>3</sub> -> O<sub>2</sub> + O (4)</p><p>O + O<sub>3</sub> -> 2 O<sub>2</sub> (5)</p><br><p>Absorption of ultraviolet light by O<sub>2</sub> (oxygen) and O<sub>3</sub>(ozone) has the net effect of warming the surroundingair. It is this warming that causes the temperatureto increase with altitude above about 11 km elevation.This increase in temperature continues to an elevationof ~50 km, where pressure is about 1% of the sea levelvalue. Above 50 km, the temperature again begins to fallbecause O<sub>2</sub> is not abundant enough to allow significantO<sub>3</sub> produuction. There is an additional reversal at about85 km elevation.</p><p>The <i>troposphere</i> is the atmospheric layer from thesurface to the first temperature minimum, and the surfaceof minimum temperature is the &llt;i>tropopause</i>. The<i>stratosphere</i> is the overlying layer of air from about 11 to50 km elevation. These are the two lowest layers of theatmosphere.</p><br><p><b>Solar heating and radiative equilibrium</b></p><p>At the toooop of the atmosphere, the cross sectional area ofEarth receives heat from the sun at the rate of 1368 wattsm<sup>-2</sup>. Spread over Earth's entire daytime and nighttimespherical surface, the average heating is 4 times lower, at342 W m<sup>-2</sup>. Some of this heat is reflected back to space.The remainder is redistributed in various ways betweenthe land surface, ocean, and atmosphere. However, itis lost only by radiation of photons or electromagneticradiation to space. The loss is described by the Stefan-Boltzmannequation:</p><p>Rate of energy loss = σ T<sup>4</sup>, (6)</p><br><p>where T is Kelvin temperature and σ is the Stefan-Boltzmannconstant, 5.67 × 10<sup>-8</sup> w m<sup>-2</sup> K<sup>-4</sup>.</p><p>Earth's average surface temperature changes only veryslowly with time, so that heat must be lost at nearly thesame rate at which it is received. Equation (6) can thenbe rearranged to solve for temperature. The temperaturecalculated for a heat flux of 342 W m<sup>-2</sup> is 6°C, nottoo different from Earth's preindustrial average surfacetemperature of about 15°C. Unfortunately, there are twoserious omissions in this calculation. The first becomesapparent by examining figure 1.1: much of the light reachingthe Earth is simply reflected back to space, withoutever warming the surface and contributing to Earth'sheat budget. Sunlight is reflected by all surfaces, but thebrightest (most reflective) are clouds, snow and ice, anddeserts. Earth's global reflectance, or <i>albedo</i>, is 0.31. Correctingfor albedo, a value of -19°C is calculated, whichis ~36°C too low for Earth's surface temperature. What'swrong?</p><p>Actually, nothing. A value of -19°C is Earth's radiativeequilibrium temperature, but it is not achieved atthe surface. The reason for this is the greenhouse effect,which is illustrated in figure 1.2. The top panel (a) showsthe energy density of solar and Earth radiation as a functionof wavelength. Wavelength increases to the right;frequency, and energy of electromagnetic radiation,increase to the left. Because the surface of the sun is sohot (~6000 K), most solar energy is radiated in the visibleregion of the electromagnetic spectrum. Radiationfrom the cool Earth, on the other hand, is in the longerwavelength, lower energy infrared region. Panels (b)and (c) show the fate of radiation as it passes through theatmosphere. In these panels, white means that radiationis transmitted, and gray indicates that it is absorbed byinteractions with molecules of the gases in air. Absorbedradiation is used to kick electrons into higher energy levels,and to increase vibrational and rotational frequenciesof molecules. Panel (b) shows the absorption of radiationas a function of wavelength between the surface andthe top of the atmosphere. Most solar radiation passesthrough the atmosphere "intact"; it is this property thatallows us to view the sun, Moon, and stars. Most Earthradiation, on the other hand, is absorbed as it passesthrough the atmosphere. Absorption warms the air andleads to reradiation of the absorbed energy. Some of thisreradiated energy is transmitted downward toward thesurface, where it delivers an extra serving of heat. Thus,the surface is warmer than it would be if absorbing (orgreenhouse) gases were absent from the atmosphere.</p><p>Panel (c) shows the fraction of radiation absorbed between11 km and the top of the atmosphere. In this interval,almost all outgoing Earth radiation is transmitted,and there is no longer much warming of the atmospheredue to absorption of outgoing infrared radiation.</p><p>Ideally, there would be some level in the atmospherebelow which infrared radiation is largely absorbed, andabove which it is mostly transmitted. It is at this hypotheticallevel that Earth attains its radiative equilibriumtemperature of -19°C. This level is at about 5 km elevation.The average lapse rate in the troposphere is about6.5°C/km, so that temperature rises by 33° from theradiative equilibrium level to sea level. The calculated averagesea level temperature is 14°C, close to the observedglobal average. The concept of a single level where the infraredradiation ceases is greatly simplified, but the ideais correct.</p><p>Panel (d) illustrates the absorption of radiation by differentgases between the surface and the top of the atmosphere.In the ultraviolet, all absorption is due to O<sub>2</sub>and O<sub>3</sub>, illuminating the role of ozone as the UV shield.In the infrared (IR), most absorption of radiation is dueto water. Of the other so-called greenhouse gases, CO<sub>2</sub>is by far the most important absorber, followed by CH<sub>4</sub>(methane) and N<sub>2</sub>O (nitrous oxide). Ozone also absorbsin the IR, and there is thus a small contribution to thegreenhouse effect both from tropospheric and stratosphericozone.</p><br><p><b>ATMOSPHERIC CIRCULATION</b></p><p>Sunlight warms Earth's surface, which in turn warms theatmosphere. Under these conditions, one might expect ameridional (latitudinal) circulation system, with warmair rising at the equator, and cool air sinking at the poles.There would be poleward flow aloft, and equatorwardflow at the surface. In fact there are convection cells inthe atmosphere, but they are not quite this grand. In the"Hadley cell," air rising at the equator flows to a latitudeof about 30°, sinks to ground level, and flows back towardthe equator. In the "Polar cell," air rises at about60°. latitude, flows toward higher latitudes, sinks at thepoles, and again closes the loop by equatorward flow atthe surface.</p><p>Because of an effect known as the Coriolis force, windsare westerly (from the west) in the upper, poleward flowingair of the Hadley cell. The air flowing poleward must,in the absence of friction, conserve its angular momentum(mass × velocity × radius). Air at the equator ismoving toward the east at about the same rate that thesurface is spinning on its axis. As this equatorial air risesand moves poleward, its eastward velocity increases inan absolute reference frame because the radius of theEarth is decreasing. With respect to the underlyingground, its velocity is increasing even faster, because theeastward velocity of the ground becomes smaller as theair moves to higher latitudes. From our perspective, witha reference frame rotating along with the Earth, an airmass traveling toward the pole will accelerate toward theeast. The Coriolis force is the virtual force producing thisacceleration.</p><p>Think about air at the equator in relation to theground. In an absolute frame of reference, it is moving tothe east at a velocity of 1671 km/hr, due to rotation of thesolid Earth. In the absence of friction and turbulence,this air would be flowing to the east 484 km/hr fasterthan the ground when it reaches a latitude of 30°. Inpractice, there is such a feature in the upper troposphere:the jet stream, which flows eastward but at a lower velocityof 150–200 km/hr. Wind speeds must be low near thesurface due to friction, and even above the surface, frictionand other influences make wind speeds lower thanthose calculated when only considering conservation ofangular momentum.</p><p>High velocities aloft, coupled with the decrease intemperatures between the Hadley cell and the Polar cell(at latitudes of roughly 30–60°), lead to a very differentatmospheric circulation in these middle latitudes. Here,the circulation is much more chaotic, dominated bycyclones—large air masses rotating counterclockwise inthe Northern Hemisphere and clockwise in the SouthernHemisphere. Cyclones travel to the east, leading totransitional regions known as fronts and the variableweather so characteristic of the midlatitudes. Along withthe movement of cyclones, there are flows of warm airmasses toward the poles and cool air masses toward theequator. These flows lead to the transport of heat fromthe tropics toward the poles, and attenuate the meridionaltemperature gradient. For dynamic reasons, theretends to be net sinking of air at the southern boundaryof the midlatitude zone (around 30°) and rising air at thenorthern end (60°).</p><p>With this background it becomes fairly straightforwardto understand the distribution of surface winds.In the region of the Hadley cell (0–30°), air rises at theequator. Aloft, it flows toward the pole and to the east.At ground level, there is a return flow. Winds turn towardthe right (west) as they pass from a region wherethe surface is turning more slowly to a region where itis turning faster (the equator). The tropics are thereforeregions of easterly winds (from the east) known as thetrade winds. Between 30 and 60° latitude, there is a netwesterly flow, upon which are superimposed the airflowsassociated with rotating cyclones. Hence, surface windsin this region are variable, despite the mean flow to theeast. North of 60°, circulation of the polar cell is similarto that of the Hadley cell: air rises at the low-latitudeboundary, flows toward the east aloft, sinks at the poles,and the surface return flow is again easterly.</p><p>This dynamic picture also explains the distribution ofprecipitation. Air rises at the equator and at 60. (on average),then sinks at around 30° and at the poles. Whenair rises, there are two changes that influence the degreeof saturation of water vapor, and hence the amount ofprecipitation. First, rising air expands, decreasing theconcentration of water vapor relative to the saturationconcentration at which liquid will form. Second, risingair cools, lowering the equilibrium water vapor concentration.It turns out that the effects of cooling exceedthose of expansion, and the degree of saturation of watervapor increases as air rises. Consequently, there is a beltof heavy precipitation along the equator. Sinking leadsto the opposite effect: air warms, and its ability to holdmoisture rises. This effect leads to the areas of low precipitationin the Northern Hemisphere around 30° N, accountingfor the deserts of North Africa and the westernUnited States, and dry climates elsewhere in the subtropics.In midlatitudes, fronts lead to rising air masses andabundant precipitation. Precipitation is low at very highlatitudes because cold air can hold very little moisture.</p><p>Superimposed on these global patterns are importantlocal features. Perhaps the most important are temperatureand precipitation gradients over land produced byinteractions between land and the nearby sea. Heat receivedby land is absorbed at the very surface and transmittedto depth by conduction, which operates veryslowly and induces seasonal temperature cycles to depthsof only a few meters. Heat received at the sea surface penetratesmore deeply and is mixed rapidly to depths of tensto hundreds of meters. In other words, the land surfaceshares its heat to only a meter or so depth, while the oceansurface shares its heat to 100 m or more. Consequently,seasonal heating and cooling of land is much greater thanthat of the oceans. This feature manifests itself dramaticallyin two ways. First, in temperate and subpolar continentalareas, seasonality is much stronger in the eastthan in the west. For example, Baltimore is only 2° furthernorth than San Francisco, but its annual temperaturecycle is four times larger (25°C range of monthly averagetemperatures compared with 6°). These zonal gradientsare a sign of the prevailing westerly airflow in the middlelatitudes. Air in San Francisco originates from the PacificOcean, and reflects its attenuated seasonal temperaturecycle. On the other hand, air in Baltimore has crossed thecontinent and has acquired the large seasonal temperaturefluctuations in the center of North America.
(Continues...)Excerpted from PALEOCLIMATE by Michael L. Bender. Copyright © 2013 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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