Climate change plus the emergence of agriculture

Climate change plus 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 final glacial period. A number of researchers 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, by having a specially 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 associated with the Pleistocene left signatures regarding the landscape in the form of glacial deposits and landforms; nonetheless, 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 more substantial isotope, 18O. Glacial periods are characterized by high 18O concentrations and represent a net transfer of water, specially with 16O, from the oceans towards 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; these people were seen as an 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 associated with the Earth system between glacial and interglacial modes has been eventually driven by orbital variations. However, orbital forcing is by itself insufficient to explain all of this variation, and Earth system researchers are focusing their attention regarding the interactions and feedbacks involving the variety 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 resulting in further cooling. Similarly, changes in terrestrial vegetation, including 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 just 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 virtually similar areas, plus the Wisconsin and Illinoisan sheets covered around the same territory. Into 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 finding 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 into the Earth system. Reduced total 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 blood flow, ecology of marine and terrestrial organisms, ice sheet dynamics, and atmospheric chemistry and blood flow.

The final great cooling

The Earth system has undergone a general cooling trend for the past 50 million years, culminating into 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). While the ice sheets waxed and waned, global climate drifted steadily toward cooler conditions seen as an 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, plus 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 eventually 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 of this 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. A number of these past changes are obscure and controversial, and some have now been discovered only recently. Nevertheless, the history of life was strongly influenced by these changes, a number of which radically altered the course of evolution. Life itself is implicated as being a causative agent of some of these changes, while the processes of photosynthesis and respiration have largely shaped the chemistry of Earth’s atmosphere, oceans, and sediments.

Cenozoic climates

The Cenozoic Era—encompassing the past 65.5 million years, the time which includes 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 were driven by tectonic forces, which have altered the jobs 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 into the hydrosphere) are being increasingly seen as influences of global and regional climate. In particular, atmospheric concentrations of skin tightening and have varied substantially during the Cenozoic for reasons which can be poorly comprehended, though its fluctuation must have involved feedbacks between Earth’s spheres.

Orbital forcing is also evident into 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 in line with the growing understanding of tectonic and biogeochemical changes. a pattern growing from recent paleoclimatologic studies suggests that the climatic aftereffects 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 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 clear 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 around 100,000 years. Although the underlying causes are confusing, 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, nevertheless they gradually increased to near-PETM levels within 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 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 associated with 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 blood flow. Recent evidence suggests that decreasing atmospheric concentrations of skin tightening and 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 destination 27 climate change thesis paper 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, plus 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 plus the Arctic Archipelago. The Northern Hemisphere glaciation, which began 3.2 million years ago, was driven by tectonic events, including the closing of the Panama seaway as well as the uplift of the Andes, the Tibetan Plateau, and western elements of North America. These tectonic events led to changes in the blood flow of the oceans plus the atmosphere, which in turn fostered the development of persistent ice at high northern latitudes. Small-magnitude variations in carbon dioxide concentrations, which have been relatively low since at least the mid-Oligocene (28.4 million years ago), may also be thought to have contributed to this glaciation.

Phanerozoic climates

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 selection of climatic states and transitions. The sheer antiquity of many of these regimes and events renders them difficult to understand in detail. Nonetheless, a number of periods and transitions are well known, owing to good geological records and intense study by researchers. Also, a coherent pattern of low-frequency climatic variation is growing, in which the Earth system alternates between warm (‘greenhouse’) phases and cool (‘icehouse’) phases. The warm phases are seen as an high temperatures, high sea levels, and an absence of continental glaciers. Cool phases in turn are marked by low temperatures, low sea levels, plus 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 last 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, every one of these regions were part of Gondwana, a high-latitude supercontinent into the Southern Hemisphere. The glaciers atop Gondwana stretched to at least 45° S latitude, just like 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 top features 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, which might 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) plus the early and mid-Paleozoic ( around 500 million to 350 million years ago). Climates of each of those greenhouse periods were distinct; continental jobs 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 plus the modeling associated with the Earth system as well as its components. Two processes were implicated as drivers of Phanerozoic climate change. First, tectonic forces caused changes in the jobs and elevations of continents plus 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 resources 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 comprehended phase of Earth system history. Much of the sedimentary record of the atmosphere, oceans, biota, and crust of the early Earth was obliterated by erosion, metamorphosis, and subduction. Nonetheless, quantity of pre-Phanerozoic records were found in various parts of the world, mainly from the later portions of the period. Pre-Phanerozoic Earth system history is an incredibly active area of research, in part because of its significance in understanding the origin and early evolution of life on Earth. Also, 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 researchers, 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, plus 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 is often into 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 perfect solution is to this ‘faint young Sun paradox’ appears to lie into the presence of unusually high concentrations of greenhouse gases at the time, specially methane and carbon dioxide. As solar luminosity gradually increased through time, concentrations of greenhouse gases would have to were greater than today. This circumstance could have caused Earth to heat up beyond life-sustaining levels. Therefore, greenhouse gas concentrations should have decreased proportionally with increasing solar radiation, implying a feedback method to regulate greenhouse gases. One of these mechanisms might have been rock weathering, which can be temperature-dependent and serves as a important sink for, as opposed to source of, carbon dioxide by removing considerable amounts of this gas from the atmosphere. Researchers may also be looking to biological processes ( many of which also serve as carbon dioxide sinks) as complementary or alternative regulating mechanisms of greenhouse gases regarding 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 being a reducing agent for skin tightening and, had dramatic consequences for Earth system geochemistry. Molecular oxygen (O2) is given off as being a by-product of photosynthesis utilising the H2O pathway, which can be energetically more effective compared to the more primitive H2S pathway. Using H2O as being a reducing agent in this technique 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 into the oceans was precipitated. By around 2 billion years ago, oxygen was able to accumulate in dissolved form in seawater and to outgas towards 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. Researchers are studying the role of oxygen and other contributions of early life towards 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 towards the Equator during these events. This ‘Snowball Earth’ hypothesis is a subject of intense study and discussion. Two important questions arise using this hypothesis. First, how, once frozen, could Earth thaw? Second, how could life survive periods of global freezing? a proposed solution to the initial question involves the outgassing of massive amounts of skin tightening and by volcanoes, which could have warmed the planetary surface rapidly, specially given that major carbon dioxide sinks (rock weathering and photosynthesis) could have been dampened by way of a frozen Earth. a possible answer to the second question may lie into 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, elements of the planet (especially ocean areas near the Equator) could have been draped only by way of 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, is rolling out since the 1980s. This research has been empowered by the finding, into 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 a single equilibrium state to another. Such shifts are of considerable clinical concern because they can reveal anything about the controls and sensitivity of the climate system. In particular, they highlight nonlinearities, the so-called ‘tipping points,’ where small, gradual changes in one component of the system can cause a large change in the entire system. Such nonlinearities arise from the complex feedbacks between components of the Earth system. As an example, during the Younger Dryas event (see below) a gradual escalation in the release of fresh water towards the North Atlantic Ocean led to an abrupt shutdown of this thermohaline blood flow into the Atlantic basin. Abrupt climate shifts are of great societal concern, for any such shifts in the long run might be so rapid and radical as to outstrip the capacity of agricultural, ecological, manufacturing, and economic systems to respond and adapt. Climate scientists are dealing with social researchers, ecologists, and economists to assess society’s vulnerability to such ‘climate surprises.’

The Younger Dryas event (12,800 to 11,600 as you like it quick summary by scene years ago) is the most intensely studied and best-understood example of abrupt climate change. The event occurred during the last deglaciation, a period of global warming if the Earth system was in transition from a glacial mode to an interglacial one. The Younger Dryas was marked by way of a sharp drop in temperatures into the North Atlantic region; cooling in northern Europe and eastern North America is believed at 4 to 8 °C (7.2 to 14.4 °F). Terrestrial and marine records indicate that the Younger Dryas had detectable aftereffects of lesser magnitude over most other elements of Earth. The termination of the Younger Dryas was very rapid, occurring within a decade. The Younger Dryas resulted from an abrupt shutdown of this thermohaline blood flow into the North Atlantic, which can be critical for the transport of heat from equatorial regions northward (today the Gulf Stream is a part of that blood flow). the shutdown of this thermohaline blood flow is under study; an influx of large volumes of freshwater from melting glaciers into the North Atlantic has been implicated, although other elements probably played a role.

The Younger Dryas event was seen as an a substantial and relatively sudden drop in temperature between 12,800 and 11,600 years ago. In addition to cold regions, the evidence with this temperature change was 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 seen as representing alternation between two climate states, with rapid transitions from a single 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 being 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 way of a rapid drop into the water quantities of lakes and bogs in eastern North America, occurred 5,200 years ago. It really 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 those 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 escalation in atmospheric skin tightening and 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 group of positive feedbacks on the list of atmosphere, oceans, ice sheets, and biosphere, just like those observed in the Pleistocene. Other abrupt changes, including the Paleocene-Eocene Thermal Maximum, are recorded at various points into the Phanerozoic.

Abrupt climate changes can evidently be caused by a variety of processes. Rapid changes in an exterior factor can push the climate system in to a new mode. Outgassing of methane hydrates as well as the sudden influx of glacial meltwater into the ocean are examples of such exterior forcing. Alternatively, gradual changes in exterior elements can cause 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 being activities, such as fossil-fuel combustion and land-use change, alter important components of Earth’s climate system.

climate change: marine ecosystemThe effects of climate change on marine ecosystems.Contunico © ZDF Enterprises GmbH, MainzSee all videos for this article

Humans and other species have survived countless climatic changes in yesteryear, and humans certainly are a notably adaptable species. Adjustment to climatic changes, whether it is biological (as with the actual situation of other species) or cultural (for humans), is easiest and least catastrophic if the changes are gradual and may be anticipated to large extent. Rapid changes are more difficult to adapt to and incur more disruption and risk. Abrupt changes, specially unanticipated climate surprises, put human cultures and societies, as well as both the populations of other species plus the ecosystems they inhabit, at considerable threat of severe disruption. Such changes may well be within humanity’s capacity to adapt, yet not without paying severe penalties in the form of economic, ecological, agricultural, human being health, and other disruptions. Knowledge of past climate variability provides instructions regarding the natural variability and sensitivity associated with the Earth system. This knowledge also helps identify the risks connected with altering the Earth system with greenhouse gas emissions and regional to global-scale changes in land cover.

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