Rising air forces winds poleward. The result is prevailing easterly winds along the equator, westerlies in the temperate latitudes, and easterlies towards the poles. It also affects precipitation. As the warm air rises over the equator it cools and drops rain. Hence the rain forests over the equator.
Note the location of the Sahara, the Sonora, the Alticama, and other major deserts. Therefore geological events such as the closing of the Isthmus of Panama can have a major effect on climate. These major forces have a significant impact on local weather. Air is brought to San Francisco by prevailing westerlies is coming off the ocean, whereas air in Baltimore has traveled over an entire continent.
The density of ocean water depends on salinity and temperature. Heavy water sinks, most notably in the North Atlantic. Therefore, contends the book, the deep oceans all over the world tend to contain water that sank in the North Atlantic. The geology of the Pacific Ocean doesn't force the same effect. A major theme of the book concerns the chemistry of carbon and carbon dioxide.
Mobile carbon dioxide continually taken out of the system by chemical processes, but continually restored by other processes such as respiration and of course burning fossil fuels. It is worth looking at where the carbon on earth is located to understand what a small fraction is really involved in this question of global warming. If CO2 in the atmosphere is 1, the relative abundance of CO2 elsewhere is: As CO2 in atmosphere Carbon dioxide continues to be released from the Earth's interior by volcanic activity.
The chapter closes with the mention of weathering. Carbon dioxide combines with water in the air to form carbonic acid H2CO3. It enters into chemical reactions with rocks on the earth. The carbon is locked into molecules such as calcium carbonate and swept into the oceans where it gets buried in deep sediments. Weathering, like most chemical processes, speeds up with increased heat.
This is a beneficial feedback loop. The warmer the earth, the more carbon dioxide gets removed from the air. The questions about what the Earth's climate history has been are just as fascinating, and delightfully apolitical.
Princeton Primers in Climate
The techniques by which scientists are able to estimate past climates, atmospheres and temperatures are truly ingenious. One major technique involves isotopes. Carbon has an atomic weight of However, there is a stable isotope, carbon 13, with seven neutrons and an unstable one, carbon, with eight. We know about carbon dating. Carbon decays, with a half-life of years. Carbon remaining in a specimen gives an approximation of its age.
But these are only good back through the last ice age. The primary technique involves stable isootopes. Stable isotopes of carbon and oxygen are available in the atmosphere in known ratios. Molecules incorporating these different isotopes have different weights. Since lighter molecules are a bit more chemically active than heavier ones, the fraction of heavier isotopes tends to be less in some chemical compounds than in the surrounding environment.
How much less is a function of the chemical environment. When carbon is readily available, the preference for lighter carbon 12 will be more strongly expressed. If carbon is not so abundant, the chemical reactions take what they can. The ratio of carbon 12 to carbon 13 will be closer to that of the pool from which the carbon is taken. Scientists are clever at using the ratios of isotopes to deduce the chemical makeup and the ambient temperatures of ancient atmospheres.
Read the book for details. Scientists use these techniques to answer a baffling question. Yet it was not. There is evidence of glacial activity and liquid water. They use a variety of devices to infer that there were other major gases in the Earth's atmosphere, among them methane CH4 and carbonyl sulfate OCS, or COS which served as greenhouse gases. Precambrian glaciations This chapter discusses the "Snowball Earth" theory. The geological record of glaciations, metabolic activities by single celled plants, and chemical reactions involving carbon, silicon and other elements indicate four or so glaciations, some of which may have covered the entire earth, in the Precambrian era, prior to million years ago, when oceanic multicellular life was just getting started.
This about feedback mechanisms. If the earth was totally frozen over, albedo would be so high that it would tend to stay frozen. However, carbon dioxide would continue to enter the atmosphere from volcanic activity. At such low temperatures there would be little chemical activity to remove it. This chapter discusses the theories as to the various glaciations and their reversals. Obviously, there would be no animals without plants to eat. CO2 levels were higher over much of this period; to ppm. Some put the figure as high as ppm.
The most authoritative figure, Robert Berner, incorporated a vast database of geological observations and chemical formulas into a model called GEOCARB, which would support a lower figure. Ceteris paribus, temperature goes up 2. It shows CO2 levels varying significantly, from about 20 times today's level million years ago clear down to just about today's level million years ago, then back up to five times as much before returning to the current low level.
Fossil evidence includes the stomata of leaves. When carbon dioxide is scarce leaves have many shallow stomata; when it is abundant they have fewer and deeper ones. This correlates with the other evidence. Previous chapters dealt extensively with the action in shallow seas, in which a great deal of photosynthesis takes place in CO2 exchange with the atmosphere.
Patterns of waves and sediments and in seabeds tell that story. More plate movement means more outgassing volcanic emission of CO2 from inside the earth. The evidence of outgassing is in the form of warmer seas. Therefore, sea level and ocean temperature are proxies for tectonic movements. The formation of sedimentary rocks supports this analysis: The isotope ratios discussed above are useful in analyzing the remains of plants. Bender compares the output of the model with other analysis. They don't agree entirely, but enough to give a general picture.
Interestingly, he cites a sharp CO2 minimum about 65 million years ago in the fossil record but not in the model. This is the time of the great extinction. Is it possible that without animals to eat the vegetation, it depleted most of the CO2 in the atmosphere? Lastly, CO2 in the atmosphere decreased over the past 55 million years.
There is a fair amount of discrepancy in the analysis of how much and when. Some analysts put the peak level at about ppm compared to the base rate of just prior to the industrial age. It embraces the Cambrian, Ordovician, Silurian, Devonian, Mississippian Pennsylvanian in Permian ages, stretching from to million years ago. The earth was glaciated from about to million years ago.
The evidence is that CO2 concentrations were low. The evidence for glaciation is direct: Glacial debris embedded in sedimentary rock called drop stones.
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Some deposits show seasonal variations, with fine deposits throughout the year book and coarse stones being dropped in the spring thaws. This was a time in geological history when all of the large continents were united in Gondwanaland, centered around the South Pole for the most part. This Continental configuration lent itself to the accumulation of ice and a general cold climate.
Bender concludes that there is still a lot to be known. There are conflicting theories as to how warm the tropics were and how far into the temperate latitudes the warmth might have extended. Chapter 6 Equable Climates of the Mesozoic and Paleogene The earth was substantially unglaciated from the upper Permian through the Triassic, Jurassic and Cretaceous periods leading to the great extinction 65 million years ago. This warmth continued through the Paleocene and Eocene periods of the Cenozoic era, up to about 34 million years ago.
Not only was it warmer, but the distribution of warmth was different. It was warmer at high latitudes. There is also discussion of the warmth of the deep oceans, inferred from the isotope ratios of plankton skeletons. The Paleocene Eocene Thermal Maximum The duration was about , years, at the beginning of the Eocene era 55 million years ago. The observation is supported by isotope readings. The event was sudden: It came on suddenly, and then died back at a rate consistent with the rate at which outgassed CO2 from volcanic activity dissipates via weathering, discussed above.
There are several theories as to where the carbon came from. It could have been fires, uplift of shallow oceans exposing organic material, the dying of forests, or have been related to volcanic action and outgassing. Two lesser events following at about 2 million year intervals are equally unexplained. Long Cooling of the Cenozoic The last 50 million years of Earth's history have shown a gradual cooling, including some dramatic ice ages. There may have been ice in the paleogene, but certainly not as extensive as today. There may have been some transient ice sheets on Antarctica proceeding the permanent glaciation that began 34 million years ago.
Temperate forests were replaced by grassland and steppe. Ugulate animals were replaced by ruminants. Smaller species of animals became more prevalent. Not all of the evidence goes the same direction, but this is the predominant trend. The deep oceans show the same pattern. The depth at which the calcium shells of plankton ceased to be dissolved in seawater continues to grow deeper, indicating that the water got colder. Some changes attributable to changes in the Earth's orbit and thousand year cycles , the tilt of the earth 41, year cycle and precession 21, years.
The interesting fact is that the biological record is sensitive enough to accurately record these changes. The latter marked a point of no return, leading to the intense glaciations over the period of man's evolution. The most obvious reason for cooling in the Oligocene and Miocene periods was falling carbon dioxide. CO2 was significantly more abundant in the Eocene than the more recent times. There was a large decrease about the time of the Oligocene boundary, and CO2 probably didn't vary much during the Miocene Vegetation on earth changed near the end of the Cretaceous 65 million years ago.
Back then high latitude areas that are now covered by tundra and ice sheets were forested. Second, the planet was wetter, with more forest and less desert. Lastly, the chemistry of photosynthesis changed. This resulted in profound changes in animals. Grasses are seasonally arid and prone to fires, destroying trees, favoring grasses. Animals evolved to eat grass. Grasses got tougher; teeth got stronger. Plants that metabolize carbon dioxide via the C4 path are more efficient with lower concentrations of CO2. Going back to isotopes again, C3 and C4 plants take up different ratios of isotopes.
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These isotopes are preserved in the teeth of the animals that eat them, providing a proxy for the replacement of forest by grass. The Asian monsoons appeared about this time. The air gets hotter over land, rising and squeezing out precipitation, and drawing more precipitation and off the oceans. The Himalaya Mountains rose, with rain on the south and east and a rain shadow to the west. Grazing animals replace browsing animals. In aggregate, these changes set the stage for increasingly intense cyclical glaciations in the northern hemisphere.
Two factors driving the ice ages of the last 2. There were feedback loops: We are currently living in the warm phase of a , year glacial cycle. Bender now tightens the timeframe of his reportage. Whereas it has been in millions of years, he focuses on the height of the last Ice Age 20, years ago, when our ancestors inhabited most of Eurasia. Nine ice sheets covered the earth: The snow line descended on the mountains in temperate and tropical climes. CO2 can be measured directly for such recent geological periods through gases trapped in ice cores. Bender cites a rise of 80 ppm, from to x 10, years ago.
It continued to rise to preindustrial. Methane doubled, from to ppb. There is a strong connection between the change in insolation due to Milankovitch orbital forcing and deglaciation. As changes in the Earth's orbit and tilt bring the northern hemisphere closer to the sun in summer, the glaciers melt. Smaller glaciers mean greater albedo, creating a positive feedback loop that continues until it reaches a cyclical maximum.
Whether the earth is heading toward warmer or cooler extremes, the feedbacks run out of steam and limit the extremes to something like today's world that the warm and, or at the other end, the world of the last glacial maximum. They involve CO2 exchange in the southern oceans; the salinity of the North Atlantic and hence the formation of new deep ocean waters; geothermal heating under glaciers, in which the glacier itself ironically acts as a blanket, protecting the rock from cold air masses.
The take-home point from this chapter are that the earth is close to the temperature maximum in the , year cycle. Rapid Climate Change During the Last Glacial Period The chapter includes a diagram of five different indicators of historical climate, ice core studies and isotope analyses.
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All of them show rapid warming starting about 20, years ago after a period of gradual cooling starting 60, years ago. Antarctica show similar cycles, not necessarily synchronized with Greenland. Continental areas were affected as indicated by the abundance of methane. Wetlands and rain were more extensive when Greenland was warm. Counterintuitively, when Greenland was at its coldest, sea ice and sea levels rose. The best studied warming occurred 11, years ago, ending the Younger Dryas cold episode. The warming reached its full magnitude in two decades or less.
The evidence, once again, consists of isotope studies and sediment dropped from glaciers. A major source of glaciers was the Hudson Bay flowing east through the Hudson Strait ; they dropped sediments they had scoured into the Atlantic. Polar regions appear throughout this book to be the most intensely studied for two reasons: Changes in the tropics largely affect the biomass, which does not leave as much of a geological record.
The deep oceans, like the polar regions, are a good source because of their stability. Calcium carbonate remains of plankton on the surface and formifera on the ocean bottom accumulate continually. Lake levels in the great basin region of the United States, centered in Nevada, correlate to the Greenland record. There is a year cycle of interstadial events. Bender summarizes that rapid climate change events were permanent features of Ice Age earth.
There were major shifts in temperature in the upper latitudes, and changes in precipitation in the lower latitudes. The hemispheres were somewhat out of phase. And ultimately, there is no complete explanation of rapid climate change. The Holocene The Holocene era begins with the end of the Younger Dyass cold episode 11, years ago. The angle of the tilt changes on a 40, year cycle and the orientation of the axis of the spin changes on a 26, year cycle. Paleoclimate by Michael L. At one extreme, Earth has been glaciated from the poles to the equator for periods that may have lasted millions of years.
At another, temperatures were once so warm that the Canadian Arctic was heavily forested and large dinosaurs lived on Antarctica. Paleoclimatology is the study of such changes and their causes.
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Studying Earth's long-term climate history gives scientists vital clues about anthropogenic global warming and how climate is affected by human endeavor. In this book, Michael Bender, an internationally recognized authority on paleoclimate, provides a concise, comprehensive, and sophisticated introduction to the subject. After briefly describing the major periods in Earth history to provide geologic context, he discusses controls on climate and how the record of past climate is determined. The heart of the book then proceeds chronologically, introducing the history of climate changes over millions of years--its patterns and major transitions, and why average global temperature has varied so much.
The book ends with a discussion of the Holocene the past 10, years and by putting manmade climate change in the context of paleoclimate. The most up-to-date overview on the subject, "Paleoclimate" provides an ideal introduction to undergraduates, nonspecialist scientists, and general readers with a scientific background. Reviews " Paleoclimate gives the reader a concise, clear view of how Earth's climate has changed over geologic time and the major drivers for this change. I heartily recommend the book for those interested in understanding Earth's rich climate complexity.