Climate change additionally the emergence of agriculture
The first known examples of animal domestication occurred in western Asia between 11,000 and 9,500 years ago when goats and sheep were first herded, whereas examples of plant domestication date to 9,000 years ago when wheat, lentils, rye, and barley were first cultivated. This phase of technological increase occurred during a time of climatic transition that accompanied the last glacial period. A number of scientists have suggested that, although climate change imposed stresses on hunter-gatherer-forager societies by causing rapid shifts in resources, it also provided opportunities as new plant and animal resources appeared.
Glacial and interglacial cycles of the Pleistocene
The glacial period that peaked 21,500 years ago was only the most recent of five glacial periods in the last 450,000 years. In fact, the Earth system has alternated between glacial and interglacial regimes for more than two million years, a period of time known as the Pleistocene. The duration and severity of the glacial periods increased during this period, through a particularly sharp change occurring between 900,000 and 600,000 years ago. Earth is currently within the most recent interglacial period, which started 11,700 years ago and is commonly known as the Holocene Epoch.
The continental glaciations regarding the Pleistocene left signatures on the landscape in the form of glacial deposits and landforms; however, the best knowledge of the magnitude and timing of the various glacial and interglacial periods comes from oxygen isotope records in ocean sediments. These records provide both a direct measure of sea level and an indirect measure of global ice volume. Water molecules composed of a lighter isotope of oxygen, 16O, are evaporated more readily than molecules bearing a heavier isotope, 18O. Glacial periods are characterized by high 18O concentrations and represent a net transfer of water, especially with 16O, from the oceans to the ice sheets. Oxygen isotope records indicate that interglacial periods have typically lasted 10,000–15,000 years, and maximum glacial periods were of similar length. Most of the past 500,000 years—approximately 80 percent—have been spent within various intermediate glacial states that were warmer than glacial maxima but cooler than interglacials. During these intermediate times, substantial glaciers occurred over much of Canada and probably covered Scandinavia as well. These intermediate states were not constant; they were characterized by continual, millennial-scale climate variation. There has been no average or typical state for global climate during Pleistocene and Holocene times; the Earth system has been in continual flux between interglacial and glacial patterns.
The cycling regarding the Earth system between glacial and interglacial modes has been ultimately driven by orbital variations. However, orbital forcing is by itself insufficient to explain all of this variation, and Earth system scientists are focusing their attention on the interactions and feedbacks between the array components of the Earth system. For example, the initial development of a continental ice sheet increases albedo over a portion of Earth, reducing surface absorption of sunlight and leading to further cooling. Similarly, changes in terrestrial vegetation, such as the replacement of forests by tundra, feed back into the atmosphere via changes in both albedo and latent heat flux from evapotranspiration. Forests—particularly those of tropical and temperate areas, with their large leaf area—release great amounts of water vapour and latent heat through transpiration. Tundra plants, which are much smaller, possess tiny leaves designed to slow water loss; they release only a small fraction of the water vapour that forests do.
The blue areas are those that were covered by ice sheets in the past. The Kansan and Nebraskan sheets overlapped practically equivalent areas, additionally the Wisconsin and Illinoisan sheets covered approximately the same territory. In the high altitudes of the West are the Cordilleran ice sheets. An area at the junction of Wisconsin, Minnesota, Iowa, and Illinois was never entirely covered with ice.Encyclopædia Britannica, Inc.
Europe, like North America, had four periods of glaciation. Successive ice caps reached limits that differed only slightly. The area covered by ice at any time is shown in white.Encyclopædia Britannica, Inc.
The advancement in ice core records that atmospheric concentrations of two potent greenhouse gases, carbon dioxide and methane, have decreased during past glacial periods and peaked during interglacials indicates important feedback processes in the Earth system. Reduction of greenhouse gas concentrations during the transition to a glacial phase would reinforce and amplify cooling already under way. The reverse is true for transition to interglacial periods. The glacial carbon sink remains a topic of considerable research activity. A full understanding of glacial-interglacial carbon dynamics requires knowledge of the complex interplay among ocean chemistry and circulation, ecology of marine and terrestrial organisms, ice sheet dynamics, and atmospheric chemistry and circulation.
The last great cooling
The Earth system has undergone a general cooling trend for the past 50 million years, culminating in the development of permanent ice sheets in the Northern Hemisphere about 2.75 million years ago. These ice sheets expanded and contracted in a regular rhythm, with each glacial maximum separated from adjacent ones by 41,000 years ( based on the cycle of axial tilt). As the ice sheets waxed and waned, global climate drifted steadily toward cooler conditions characterized by increasingly severe glaciations and increasingly cool interglacial phases. Beginning around 900,000 years ago, the glacial-interglacial cycles shifted frequency. Ever since, the glacial peaks have been 100,000 years apart, additionally the Earth system has spent more time in cool phases than before. The 41,000-year periodicity has continued, with smaller fluctuations superimposed on the 100,000-year cycle. In addition, a smaller, 23,000-year cycle has occurred through both the 41,000-year and 100,000-year cycles.
The 23,000-year and 41,000-year cycles are driven ultimately by two components of Earth’s orbital geometry: the equinoctial precession cycle (23,000 years) and the axial-tilt cycle (41,000 years). Although the third parameter of Earth’s orbit, eccentricity, varies on a 100,000-year cycle, its magnitude is insufficient to explain the 100,000-year cycles of glacial and interglacial periods of the past 900,000 years. The origin regarding the periodicity present in Earth’s eccentricity is an important question in current paleoclimate research.
Climate Change Through Geologic Time
The Earth system has undergone dramatic changes throughout its 4.5-billion-year history. These have included climatic changes diverse in mechanisms, magnitudes, rates, and consequences. Many of these past changes are obscure and controversial, and some have now been discovered only recently. Nevertheless, the history of life has been strongly influenced by these changes, some of which radically altered the course of evolution. Life itself is implicated as a causative agent of some of these changes, as the processes of photosynthesis and respiration have largely shaped the chemistry of Earth’s atmosphere, oceans, and sediments.
The Cenozoic Era—encompassing the past 65.5 million years, the time that has elapsed since the mass extinction event marking the Cretaceous Period—has a broad range of climatic variation characterized by alternating intervals of global warming and cooling. Earth has experienced both extreme warmth and extreme cold during this period. These changes have been driven by tectonic forces, which have altered the positions and elevations of the continents as well as ocean passages and bathymetry. Feedbacks between different components of the Earth system (atmosphere, biosphere, lithosphere, cryosphere, and oceans in the hydrosphere) are being increasingly recognized as influences of global and regional climate. In particular, atmospheric concentrations of carbon dioxide have varied substantially during the Cenozoic for reasons that are poorly understood, though its fluctuation must have involved example of topic outline about global warming feedbacks between Earth’s spheres.
Orbital forcing is also evident in the Cenozoic, although, when compared on such a vast era-level timescale, orbital variations can be seen as oscillations against a slowly changing backdrop of lower-frequency climatic trends. Descriptions of the orbital variations have evolved according to the growing understanding of tectonic and biogeochemical changes. a pattern promising from recent paleoclimatologic studies suggests that the climatic outcomes of eccentricity, precession, and axial tilt have been amplified during cool phases of the Cenozoic, whereas they have been dampened during warm phases.
The meteor sex education as you like it summary impact that occurred at or very close to the end of the Cretaceous came at a time of global warming, which continued into the early Cenozoic. Tropical and flora that are subtropical fauna occurred at high latitudes until at least 40 million years ago, and geochemical records of marine sediments have indicated the presence of warm oceans. The interval of maximum temperature occurred during the late Paleocene and early Eocene epochs (58.7 million to 40.4 million years ago). The highest global temperatures of the Cenozoic occurred during the Paleocene-Eocene Thermal Maximum (PETM), a short interval lasting more or less 100,000 years. Although the underlying causes are ambiguous, the onset of the PETM about 56 million years ago was rapid, occurring within a few thousand years, and ecological consequences were large, with widespread extinctions in both marine and terrestrial ecosystems. Sea surface and continental air temperatures increased by more than 5 °C (9 °F) during the transition into the PETM. Sea surface temperatures in the high-latitude Arctic may have been as warm as 23 °C (73 °F), comparable to modern subtropical and warm-temperate seas. Following the PETM, global temperatures declined to pre-PETM levels, but they gradually increased to near-PETM levels over the next few million years during a period known as the Eocene Optimum. This temperature maximum was followed by a steady decline in global temperatures toward the Eocene-Oligocene boundary, which occurred about 33.9 million years ago. These changes are well-represented in marine sediments and in paleontological records from the continents, where vegetation zones moved Equator-ward. Mechanisms underlying the cooling trend are under study, but it is most likely that tectonic movements played a important role. This period saw the gradual opening of the sea passage between Tasmania and Antarctica, followed by the opening regarding the Drake Passage between South America and Antarctica. The latter, which isolated Antarctica within a cold polar sea, produced global effects on atmospheric and oceanic circulation. Recent evidence suggests that decreasing atmospheric concentrations of carbon dioxide during this period may have initiated a steady and irreversible cooling trend over the next few million years.
A continental ice sheet developed in Antarctica during the Oligocene Epoch, persisting until a rapid warming event took place 27 million years ago. The late Oligocene and early to mid-Miocene epochs (28.4 million to 13.8 million years ago) were relatively warm, though not nearly as warm as the Eocene. Cooling resumed 15 million years ago, additionally the Antarctic Ice Sheet expanded again to cover much of the continent. The cooling trend continued through the late Miocene and accelerated into the early Pliocene Epoch, 5.3 million years ago. During this period the Northern Hemisphere remained ice-free, and paleobotanical studies show cool-temperate Pliocene floras at high latitudes on Greenland additionally the Arctic Archipelago. The Northern Hemisphere glaciation, which began 3.2 million years ago, was driven by tectonic events, such as the closing of the Panama seaway and the uplift of the Andes, the Tibetan Plateau, and western parts of North America. These tectonic events led to changes in the circulation of the oceans additionally the atmosphere, which in turn fostered the development of persistent ice at high northern latitudes. Small-magnitude variations in carbon dioxide concentrations, which was indeed relatively low since at least the mid-Oligocene (28.4 million years ago), are also thought to have contributed to this glaciation.
The Phanerozoic Eon (542 million years ago to the present), which includes the entire span of complex, multicellular life on Earth, has witnessed an extraordinary array of climatic states and transitions. The sheer antiquity of many of these regimes and events renders them difficult to understand in detail. However, a number of periods and transitions are well known, owing to good geological records and intense study by scientists. Furthermore, a coherent pattern of low-frequency climatic variation is promising, in which the Earth system alternates between warm (‘greenhouse’) phases and cool (‘icehouse’) phases. The warm phases are characterized by high temperatures, high sea levels, and an absence of continental glaciers. Cool phases in turn are marked by low temperatures, low sea levels, additionally the presence of continental ice sheets, at high latitudes. Superimposed on these alternations are higher-frequency variations, where cool periods are embedded within greenhouse phases and warm periods are embedded within icehouse phases. For example, glaciers developed for a brief period (between 1 million and 10 million years) during the late Ordovician and early Silurian, in the middle of the early Paleozoic greenhouse phase (542 million to 350 million years ago). Similarly, warm periods with glacial retreat occurred within the late Cenozoic cool period during the late Oligocene and early Miocene epochs.
The Earth system has been in an icehouse phase for the past 30 million to 35 million years, ever since the development of ice sheets on Antarctica. The previous major icehouse phase occurred between about 350 million and 250 million years ago, during the Carboniferous and Permian periods of the late Paleozoic Era. Glacial sediments dating to this period have now been identified in much of Africa as well as in the Arabian Peninsula, South America, Australia, India, and Antarctica. At the time, all those regions were part of Gondwana, a high-latitude supercontinent in the Southern Hemisphere. The glaciers atop Gondwana extended to at least 45° S latitude, similar to the latitude reached by Northern Hemisphere ice sheets during the Pleistocene. Some late Paleozoic glaciers extended even further Equator-ward—to 35° S. perhaps one of the most striking attributes of this time period are cyclothems, repeating sedimentary beds of alternating sandstone, shale, coal, and limestone. The great coal deposits of North America’s Appalachian region, the American Midwest, and northern Europe are interbedded in these cyclothems, that may represent repeated transgressions (producing limestone) and retreats (producing shales and coals) of ocean shorelines in response to orbital variations.
The two most prominent warm phases in Earth history occurred during the Mesozoic and early Cenozoic eras (approximately 250 million to 35 million years ago) and the early and mid-Paleozoic ( more or less 500 million to 350 million years ago). Climates of each of these greenhouse periods were distinct; continental positions and ocean bathymetry were very different, and terrestrial vegetation was absent from the continents until relatively late in the Paleozoic warm period. Both of these periods experienced substantial long-term climate variation and change; increasing evidence indicates brief glacial episodes during the mid-Mesozoic.
Understanding the mechanisms underlying icehouse-greenhouse dynamics is an important area of research, involving an interchange between geologic records additionally the modeling regarding the Earth system and its components. Two processes have been implicated as drivers of Phanerozoic climate change. First, tectonic forces caused changes in the positions and elevations of continents additionally the bathymetry of oceans and seas. Second, variations in greenhouse gases were also important drivers of climate, though at these long timescales they were largely controlled by tectonic processes, in which sinks and sources of greenhouse gases varied.
Climates of early Earth
The pre-Phanerozoic interval, also known as Precambrian time, comprises some 88 percent of the time elapsed since the origin of Earth. The pre-Phanerozoic is a poorly understood phase of Earth system history. Much of the sedimentary record of the atmosphere, oceans, biota, and crust of the early Earth has been obliterated by erosion, metamorphosis, and subduction. However, range pre-Phanerozoic records have been found in various parts of the world, mainly from the later portions of the period. Pre-Phanerozoic Earth system history is an acutely active area of research, in part because of its importance in understanding the origin and early evolution of life on Earth. Furthermore, the chemical composition of Earth’s atmosphere and oceans largely developed during this period, with living organisms playing a active role. Geologists, paleontologists, microbiologists, planetary geologists, atmospheric scientists, and geochemists are focusing intense efforts on understanding this period. Three areas of particular interest and debate are the ‘faint young Sun paradox,’ the role of organisms in shaping Earth’s atmosphere, additionally the possibility that Earth went through one or more ‘snowball’ phases of global glaciation.
Faint young Sun paradox
Astrophysical studies indicate that the luminosity of the Sun was much lower during Earth’s early history than it has been in the Phanerozoic. In fact, radiative output was low enough to suggest that all surface water on Earth should have been frozen solid during its early history, but evidence reveals that it was not. The solution to this ‘faint young Sun paradox’ appears to lie in the presence of unusually high concentrations of greenhouse gases at the time, particularly methane and carbon dioxide. As solar luminosity gradually increased through time, concentrations of greenhouse gases would have to have been much higher than today. This circumstance would have caused Earth to heat up beyond life-sustaining levels. Therefore, greenhouse gas concentrations must have decreased proportionally with increasing solar radiation, implying a feedback method to regulate greenhouse gases. One of these mechanisms might have been rock weathering, that will be temperature-dependent and serves as a important sink for, in the place of source of, carbon dioxide by removing sizable amounts of this gas from the atmosphere. Scientists are also looking to biological processes ( many of which also serve as carbon dioxide sinks) as complementary or alternative regulating mechanisms of greenhouse gases on the young Earth.
Photosynthesis and atmospheric chemistry
The evolution by photosynthetic bacteria of a new photosynthetic pathway, substituting water (H2O) for hydrogen sulfide (H2S) as a reducing agent for carbon dioxide, had dramatic consequences for Earth system geochemistry. Molecular oxygen (O2) is given off as a by-product of photosynthesis making use of the H2O pathway, that will be energetically more efficient than the more primitive H2S pathway. Using H2O as a reducing agent in this process led to the large-scale deposition of banded-iron formations, or BIFs, a source of 90 percent of present-day iron ores. Oxygen present in ancient oceans oxidized dissolved iron, which precipitated out of solution onto the ocean floors. This deposition process, in which oxygen was used up as fast as it was produced, continued for millions of years until most of the iron dissolved in the oceans was precipitated. By more or less 2 billion years ago, oxygen was able to accumulate in dissolved form in seawater and to outgas to the atmosphere. Although oxygen does not have greenhouse gas properties, it plays important indirect roles in Earth’s climate, particularly in phases of the carbon cycle. Scientists are studying the role of oxygen and other contributions of early life to the development of the Earth system.
Snowball Earth hypothesis
Geochemical and sedimentary evidence indicates that Earth experienced as many as four extreme cooling events between 750 million and 580 million years ago. Geologists have proposed that Earth’s oceans and land surfaces were covered by ice from the poles to the Equator during these events. This ‘Snowball Earth’ hypothesis is a subject of intense study and discussion. Two important questions arise from this hypothesis. First, how, once frozen, could Earth thaw? Second, how could life survive periods of global freezing? a proposed solution to the first question involves the outgassing of massive amounts of carbon dioxide by volcanoes, which could have warmed the planetary surface rapidly, especially given that major carbon dioxide sinks (rock weathering and photosynthesis) would have been dampened by a frozen Earth. a possible answer to the second question may lie in the existence of present-day life-forms within hot springs and deep-sea vents, which would have persisted long ago despite the frozen state of Earth’s surface.
A counter-premise known as the ‘Slushball Earth’ hypothesis contends that Earth was not completely frozen over. Rather, in addition to massive ice sheets covering the continents, parts of the planet (especially ocean areas near the Equator) could have been draped only by a thin, watery layer of ice amid areas of open sea. Under this scenario, photosynthetic organisms in low-ice or ice-free regions could continue to capture sunlight efficiently and survive these periods of extreme cold.
Abrupt Climate Changes In Earth History
An important new area of research, abrupt climate change, has developed since the 1980s. This research has been empowered by the advancement, in the ice core records of Greenland and Antarctica, of evidence for abrupt shifts in regional and global climates of the past. These events, which have also been documented in ocean and continental records, involve sudden shifts of Earth’s climate system from 1 equilibrium state to another. Such shifts are of considerable scientific concern because they can reveal something about the controls and sensitivity of the climate system. In particular, they point out nonlinearities, the so-called ‘tipping points,’ where small, gradual changes in one component of the system can lead to a large change in the entire system. Such nonlinearities arise from the complex feedbacks between components of the Earth system. For example, during the Younger Dryas event (see below) a gradual rise in the release of fresh water to the North Atlantic Ocean led to an abrupt shutdown regarding the thermohaline circulation in the Atlantic basin. Abrupt climate shifts are of great societal concern, for any such shifts in tomorrow might be so rapid and radical as to outstrip the capacity of agricultural, ecological, industrial, and economic systems to respond and adapt. Climate scientists are using the services of social scientists, ecologists, and economists to assess society’s vulnerability to such ‘climate surprises.’
The Younger Dryas event (12,800 to 11,600 years ago) is the most intensely studied and best-understood example of abrupt climate change. The event took place during the last deglaciation, a period of global warming as soon as the Earth system was in transition from a glacial mode to an interglacial one. The Younger Dryas was marked by a sharp drop in temperatures in the North Atlantic region; cooling in northern Europe and eastern North America is estimated at 4 to 8 °C (7.2 to 14.4 °F). Terrestrial and marine records indicate that the Younger Dryas had detectable outcomes of lesser magnitude over most other regions of Earth. The termination of the Younger Dryas was very rapid, occurring within a decade. The Younger Dryas resulted from an abrupt shutdown regarding the thermohaline circulation in the North Atlantic, that will be critical for the transport of heat from equatorial regions northward (today the Gulf Stream is a part of that circulation). the shutdown regarding the thermohaline circulation is under study; an influx of large volumes of freshwater from melting glaciers into the North Atlantic has been implicated, although other factors probably played a role.
The Younger Dryas event was characterized by a substantial and relatively sudden drop in temperature between 12,800 and 11,600 years ago. In addition to cold regions, the evidence of this temperature change has been discovered in tropical and subtropical regions.
Paleoclimatologists are devoting increasing attention to identifying and studying other abrupt changes. The Dansgaard-Oeschger cycles of the last glacial period are now recognized as representing alternation between two climate states, with rapid transitions from 1 state to the other. A 200-year-long cooling event in the Northern Hemisphere approximately 8,200 years ago resulted from the rapid draining of glacial Lake Agassiz into the North Atlantic via the Great Lakes and St. Lawrence drainage. This event, characterized as a miniature version of the Younger Dryas, had ecological impacts in Europe and North America that included a rapid decline of hemlock populations in New England forests. In addition, evidence of another such transition, marked by a rapid drop in the water levels of lakes and bogs in eastern North America, occurred 5,200 years ago. It is recorded in ice cores from glaciers at high altitudes in tropical regions as well as tree-ring, lake-level, and peatland samples from temperate regions.
Abrupt climatic changes occurring before the Pleistocene have also been documented. A transient thermal maximum has been documented near the Paleocene-Eocene boundary (55.8 million years ago), and evidence of rapid cooling events are observed near the boundaries between both the Eocene and Oligocene epochs (33.9 million years ago) together with Oligocene and Miocene epochs (23 million years ago). All three of these events had global ecological, climatic, and biogeochemical consequences. Geochemical evidence indicates that the warm event occurring at the Paleocene-Eocene boundary was associated with a rapid rise in atmospheric carbon dioxide concentrations, possibly resulting from the massive outgassing and oxidation of methane hydrates (a compound whose chemical structure traps methane within a lattice of ice) from the ocean floor. The two cooling events appear to have resulted from a transient series of positive feedbacks among the atmosphere, oceans, ice sheets, and biosphere, similar to those observed in the Pleistocene. Other abrupt changes, such as the Paleocene-Eocene Thermal Maximum, are recorded at various points in the Phanerozoic.
Abrupt climate changes can evidently be caused by a variety of processes. Rapid changes in an external factor can push the climate system into a new mode. Outgassing of methane hydrates and the sudden influx of glacial meltwater into the ocean are examples of such external forcing. Alternatively, gradual changes in external factors can lead to the crossing of a threshold; the climate system is unable to return to the former equilibrium and passes rapidly to a new one. Such nonlinear system behaviour is a potential concern as human activities, such as fossil-fuel combustion and land-use change, alter important components of Earth’s climate system.
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Humans and other species have survived countless climatic changes in the past, and humans are a definite notably adaptable species. Adjustment to climatic changes, whether it is biological (as in the actual situation of other species) or cultural (for humans), is easiest and least catastrophic as soon as the changes are gradual and certainly will be anticipated to large extent. Rapid changes are more difficult to adapt to and incur more disruption and risk. Abrupt changes, especially unanticipated climate surprises, put human cultures and societies, as well as both the populations of other species additionally the ecosystems they inhabit, at considerable risk of severe disruption. Such changes may well be within humanity’s capacity to adapt, not without paying severe penalties in the form of economic, ecological, agricultural, human health, and other disruptions. Knowledge of past climate variability provides directions on the natural variability and sensitivity regarding the Earth system. This knowledge also helps identify the risks associated with altering the Earth system with greenhouse gas emissions and regional to global-scale changes in land cover.