The History of Earth's Climate GEOS 342 Third Exam Study Guide

Orbital Cycles (Milankovitch Cycles)

(regular in period) and so we can represent them as sine waves.

Little Ice Age (LIA)

A period from about 1250 to 1900 when temperatures cooled in many regions of the world. This seems to have been caused by widespread volcanic eruptions and lower levels of carbon dioxide and methane in the atmosphere.

Heinrich events are

A) periods of unusually rapid deposition of ice rafted debris in the N. Atlantic Ocean, separated by 7-12 kyrs. B) initiated by unusual warmth C) periods of minimal ice-rafted debris in the N. Atlantic Ocean, separated by 1-2 kyrs.

boundary conditions for the Last glacial Maximum

was the most recent time during the Last Glacial Period that ice sheets were at their greatest extent. Vast ice sheets covered much of North America, Northern Europe, and Asia and profoundly affected Earth's climate by causing drought, desertification, and a large drop in sea levels.[1] According to Clark et al., growth of ice sheets commenced 33,000 years ago and maximum coverage was between 26,500 years and 19-20,000 years ago, when deglaciation commenced in the Northern Hemisphere, causing an abrupt rise in sea level. Decline of the West Antarctica ice sheet occurred between 14,000 and 15,000 years ago, consistent with evidence for another abrupt rise in the sea level about 14,500 years ago.

ocean acidification

when CO2 dissolves in seawater, it reacts with water to form carbonic acid, which lowers ocean pH

δ18O in ice cores

δ18O can be used with ice cores to determine the temperature from when the ice was formed. Lisiecki and Raymo (2005) used measurements of δ18O in benthic foraminifera from 57 globally distributed deep sea sediment cores, taken as a proxy for the total global mass of glacial ice sheets, to reconstruct the climate for the past five million years.[3] The stacked record of the 57 cores was orbitally tuned to an orbitally driven ice model, the Milankovitch cycles of 41 ky (obliquity), 26 ky (precession) and 100 ky (eccentricity), which are all assumed to cause orbital forcing of global ice volume. Over the past million years, there have been a number of very strong glacial maxima and minima, spaced by roughly 100 ky. As the observed isotope variations are similar in shape to the temperature variations recorded for the past 420 ky at Vostok Station, the figure shown on the right aligns the values of δ18O (right scale) with the reported temperature variations from the Vostok ice core (left scale), following Petit et al. (1999)

δ18-O lags insolation by____years

6000

LGM climate

According to Blue Marble 3000 (a video by the Zurich University of Applied Sciences), the average global temperature around 19,000 BC (about 21,000 years ago) was 9 °C (48 °F).[7] This is about 6°C (11°F) colder than the 2013-2017 average. According to the United States Geological Survey (USGS), permanent summer ice covered about 8% of Earth's surface and 25% of the land area during the last glacial maximum.[8] The USGS also states that sea level was about 125 meters (410 feet) lower than in present times (2012).[8] When comparing to the present, the average global temperature was 15 °C (59 °F) for the 2013-2017 period.[9] Currently (as of 2012), about 3.1% of Earth's surface and 10.7% of the land area is covered in year-round ice.[8] The formation of an ice sheet or ice cap requires both prolonged cold and precipitation (snow). Hence, despite having temperatures similar to those of glaciated areas in North America and Europe, East Asia remained unglaciated except at higher elevations. This difference was because the ice sheets in Europe produced extensive anticyclones above them. These anticyclones generated air masses that were so dry on reaching Siberia and Manchuria that precipitation sufficient for the formation of glaciers could never occur (except in Kamchatka where these westerly winds lifted moisture from the Sea of Japan). The relative warmth of the Pacific Ocean due to the shutting down of the Oyashio Current and the presence of large 'east-west' mountain ranges were secondary factors preventing continental glaciation in Asia. All over the world, climates at the Last Glacial Maximum were cooler and almost everywhere drier. In extreme cases, such as South Australia and the Sahel, rainfall could be diminished by up to 90% from present, with florae diminished to almost the same degree as in glaciated areas of Europe and North America. Even in less affected regions, rainforest cover was greatly diminished, especially in West Africa where a few refugia were surrounded by tropical grasslands. The Amazon rainforest was split into two large blocks by extensive savanna, and the tropical rainforests of Southeast Asia probably were similarly affected, with deciduous forests expanding in their place except on the east and west extremities of the Sundaland shelf. Only in Central America and the Chocó region of Colombia did tropical rainforests remain substantially intact - probably due to the extraordinarily heavy rainfall of these regions. A map of vegetation patterns during the last glacial maximum. Most of the world's deserts expanded. Exceptions were in what is now the western United States, where changes in the jet stream brought heavy rain to areas that are now desert and large pluvial lakes formed, the best known being Lake Bonneville in Utah. This also occurred in Afghanistan and Iran, where a major lake formed in the Dasht-e Kavir. In Australia, shifting sand dunes covered half the continent, whilst the Chaco and Pampas in South America became similarly dry. Present-day subtropical regions also lost most of their forest cover, notably in eastern Australia, the Atlantic Forest of Brazil, and southern China, where open woodland became dominant due to drier conditions. In northern China - unglaciated despite its cold climate - a mixture of grassland and tundra prevailed, and even here, the northern limit of tree growth was at least 20° farther south than today. In the period before the Last Glacial Maximum, many areas that became completely barren desert were wetter than they are today, notably in southern Australia, where Aboriginal occupation is believed to coincide with a wet period between 40,000 and 60,000 years Before Present (BP, a formal measurement of uncalibrated radiocarbon years, counted from 1950 CE).

Green Sahara

"Holocene wet phase" • Ca. 9300 BC to ca. 3500 BC • Warmer and wetter • Formation of playas • Steppe/dry savannah vegetation

La Nina

"Normal" year, easterly trade winds and ocean currents pool warm water in the western Pacific, allowing upwelling of nutrient rich water off the West coast of South America.

Delta O-18 in Ice Sheets

By the time water vapor reaches the polar ice caps very little O-18 is left and δO-18 is very negative. More positive δ18O associated with NOT JUST temperature BUT ALSO size of ice sheets.

How did early humans affect Carbon Dioxide levels

Deforestation is the primary preindustrial source of anthropogenic CO2, with much smaller contributions from early burning of coal and peat [Ruddiman, 2003, 2007]. The early spread of agriculture in Figure 6 requires substantial late Holocene deforestation of Eurasia, Africa, and the Americas, consistent with many studies [Ellis et al., 2013]. Two regions—Europe and China—have sufficient paleoecological and/or archaeological evidence to assess the extent of early forest clearance. Archaeological data document the spread of crops and livestock across Europe, starting 9000 years ago in the southeast and spanning all arable regions by 5500 years ago [Zohary et al., 2012; Colledge et al., 2013]. The arrival and subsequent intensification of agriculture across this naturally wooded continent necessitated forest clearance, and radiocarbon-dated fossil pollen records from sediment cores provide detailed information about changes in vegetation. Pollen data sets for northern and central Europe [Fyfe et al., 2015; Woodbrige et al., 2014] record a shift from forest cover to more open vegetation that began 6000 to 5000 years ago and was complete by the start of the industrial era (Figures 9a and 9b). The Mediterranean region was not included in either analysis, but comparably, early deforestation seems likely because of extensive land use by Greek and Roman civilizations. Consistent with this evidence, compilations of archaeological site density as a population proxy show abrupt increases after 6000 years ago [Timpson et al., 2014]. These trends lag slightly behind the start of the anomalous CO2 rise (Figures 2b and 3b). Further support for a large early human footprint in Europe comes from mitochondrial DNA analyses [Gignoux et al., 2011] and population modeling studies [Wirtz and Lemmen, 2003; Lemmen, 2010]. In Britain and France, forests had already been reduced to near-modern levels by 2500 to 2200 years ago [Fyfe et al., 2015; Woodbrige et al., 2014]. Compilations of charcoal abundance in European sediments generally show higher burning levels 3000 years ago than in later times [Marlon et al., 2013]. In addition, analysis of the fire indicator levoglucosan in the NEEM ice core record from Greenland points to increasing amounts of burning until 2500 years ago, followed by a leveling off and then a decrease [Zennaro et al., 2015]. These regional indications of reduced burning could have contributed to the slowing of the CO2 rise after 2500 years ago (Figures 2b and 3b). Although forest clearance trends in Europe provide support for the anthropogenic explanation of the late Holocene CO2 increase at Dome C, other regions need to be examined to reach a global view. A summary of pollen evidence from east central China [Ren, 2007] indicates widespread forest cover until 8000 years ago, followed by a persistent decrease attributed mostly to anthropogenic clearance, especially after 6000 years ago. These pollen records, however, are not adjusted for differential productivity of trees and open vegetation, and making these adjustments would likely reduce estimated forest cover. Also, in Southeast Asia, gradual weakening of the summer monsoon, and the resulting drying effect on vegetation, complicates attempts to isolate the unique effect of clearance by humans.

possible causes of climate change in the last millennium

Forcing here refers to any disturbance that alters the Earth's total energy balance. These forcings can either be of natural or anthropogenic origin. The global warming observed over the past century has been unequivocally attributed to the impact of human activities, specifically the increase in the concentration of greenhouse gases in the atmosphere (see The Climate Machine and A Carbon Cycle Disturbed by Human Activities). The man-made changes in climate were not negligible until 1850. For example, land use change through deforestation has significantly affected regional temperatures. However, they have had a weaker influence at the global level. At the scale of the last millennium, the two natural forcings that can potentially play a major role are variations in solar activity (see Solar Activity Variability and Climate Impacts: the case of recent centuries) and major volcanic eruptions. Astronomical forcing, which is dominant at longer time scales, and in particular one of the drivers of glacial-interglacial cycles, has a very limited effect on this period. 3.2. Response to solar and volcanic forcing Many studies have been devoted to the impact of changes in solar activity on temperatures over the past few millennia. It is expected that a higher solar irradiance (see Solar Activity Variability and Climate Impacts: the case of recent centuries) will lead to higher temperatures and this is what is simulated by climate models (see section 4). However, the effect is probably relatively small at the millennium scale and it has not yet been possible to formally detect an impact of solar forcing in global temperature variations reconstructed from paleoclimatic records. Figure 3. Global temperatures simulated by different climate models (PAGES 2K-PMIP3, 2015, [6]) forced by reconstructions of natural and anthropogenic forcings. The curves were smoothed using a 25-year window. [Source: https://www.clim-past.net/11/1673/2015/cp-11-1673-2015.pdf, article CC Attribution 3.0 License; reference [6]] The influence of major volcanic eruptions is clearer. Volcanic eruptions emit aerosols (mainly sulphates) into the atmosphere that reflect back into space and absorb some of the incident solar radiation, reducing the amount reaching the ground and thus causing cooling in the few years following the eruption [5]. For this cooling to be perceptible on a large scale, the volcanic eruption must be powerful enough to send aerosols into the stratosphere at an altitude of over 10 km. The dust or elements that remain in the lower layers of the atmosphere settle too quickly or are washed away by rainfall. Although the impact of an individual eruption usually lasts only a few years, several nearby eruptions can have a longer-term influence. In particular, the large number of eruptions at the beginning of the 19th century largely explains why this period was particularly cold compared to the rest of the millennium, as simulated by climate models (Figure 3) [6]. 3.3. Role of natural variability However, a significant part of the observed changes cannot be linked to these external causes, and is directly related to internal climate variations (see Climate Variability: The North Atlantic Oscillation Example). Internal variability results from interactions between the different components of the climate system. It can, for example, be induced by a temporary change in winds or ocean currents influencing the transport of thermal energy from one point of the Earth to another. This implies that internal variability is often characterized by warming in some regions and cooling in others, which leads to offsets when estimating an overall average. The magnitude of changes associated with internal variability is therefore smaller at the global scale than at the regional scale. But it is far from negligible, even globally, as evidenced by the several tenths of a degree increase in global temperature due to an El Niño event. 4. Simulations of the climate of the last millennium 4.1. Agreement between simulated temperatures and reconstructions Climate models (see Biosphere, hydrosphere and Cryosphere models) are used to simulate past or future climate evolution. For the last millennium, they are forced by realistic estimates of natural (volcanic and solar) and anthropogenic forcings. Their results are then a source of information on past climates that complements climate reconstructions based on natural archives. Although a disparity exists in their results (Figure 3), a comparison with Figure 2 shows that the models are able to simulate a large-scale temperature evolution in good agreement with the reconstructions, also showing a general cooling trend between the 11th and 19th centuries before recent warming. This is an excellent test of their validity and enhances their credibility to understand past variations and predict future changes.

when did humans migrate out-of-Africa? What was the climate like during this migration?

Humans migrated out of Africa as the climate shifted from wet to dry about 60,000 years ago, according to new paleoclimate research. What the northeast Africa climate was like when people migrated from Africa into Eurasia between 70,000 and 55,000 years ago is still uncertain. The new research shows around 70,000 years ago, the Horn of Africa climate shifted from a wet phase called 'Green Sahara' to even drier than the region is now.

Orbital configuration favoring ice sheet growth

June aphelion and little tilt gives colder poles

How does climate affect civilizations

In northern Peru and central Chile climate change is cited as the driving force in a series of migration patterns from about 15,000 B.C. to approximately 4,500 B.C. Between 11,800 B.C. and 10,500 B.C. evidence suggests seasonal migration from high to low elevation by the natives while conditions permitted a humid environment to persist in both areas. Around 9,000 B.C. the lakes that periodically served as a home to the natives dried up and were abandoned until 4,500 B.C.[9] This period of abandonment is a blank segment of the archeological record known in Spanish as the silencio arqueológico. During this break, there exists no evidence of activity by the natives in the lakes area. The correlation between climate and migratory patterns leads historians to believe the Central Chilean natives favored humid, low-elevation areas especially during periods of increased aridity.[9] The different inhabitants of Greenland, specifically in the west, migrated primarily in response to temperature change. The Saqqaq people arrived in Greenland around 4,500 B.P. and experienced moderate temperature variation for the first 1,100 years of occupation; near 3,400 B.P. a cooling period began that pushed the Saqqaq toward the west. A similar temperature fluctuation occurred around 2,800 B.P. that led to the abandonment of the inhabited Saqqaq region; this temperature shift was a decrease in temperature of about 4 °C over 200 years.[10] Following the Saqqaq dominance, other groups such as the Dorset people inhabited west Greenland; the Dorset were sea-ice hunters that had tools adapted to the colder environment. The Dorset appeared to leave the region around 2,200 B.P. without clear connection to the changing environment. Following the Dorset occupation, the Norse began to appear around 1,100 B.P. in west Greenland during a significant warming period.[12] However, a sharp decrease in temperature beginning in 850 B.P. of about 4 °C in 80 years is thought to contribute to the demise of initial Norse occupation in western Greenland.[10] In Historical China over the past 2,000 years, migration patterns have centered around precipitation change and temperature fluctuation. Pastoralists moved in order to feed the livestock that they cared for and to forage for themselves in more plentiful areas.[11] During dry periods or cooling periods the nomadic lifestyle became more prevalent because pastoralists were seeking more fertile ground. The precipitation was a more defining factor than temperature in terms of its effects on migration. The trend of the migrating Chinese showed that the northern pastoralists were more affected by the fluctuation in precipitation than the southern nomads. In a majority of cases, pastoralists migrated further southward during changes in precipitation.[11] These movements were not classified by one large event or a specific era of movement; rather, the relationship between climate and nomadic migration is relevant from "a long term perspective and on a large spatial scale."[11] The Natufian population in the Levant was subject to two major climactic changes that influenced the development and separation of their culture. As a consequence of increased temperature, the expansion of the Mediterranean woodlands occurred approximately 13,000 years ago; with that expansion came a shift to sedentary foraging adopted by the surrounding population.[7] Thus, a migration toward the higher-elevation woodlands took place and remained constant for nearly 2,000 years. This era ended when the climate became more arid and the Mediterranean forest shrank 11,000 years ago. Upon this change, some of the Natufian populations nearest sustainable land transitioned into an agricultural way of life; sustainable land was primarily near water sources. Those groups that did not reside near a stable resource returned to the nomadic foraging that was prevalent prior to sedentary life.[7]

Orbital configuration favoring ice sheet melting

June perihelion and large tilt gives warmer poles

Geoengineering

Manipulation of earths climate system to counteract the effects of climate change caused by greenhouse gas emissions.

ocean acidification

Ocean acidification is sometimes called "climate change's equally evil twin," and for good reason: it's a significant and harmful consequence of excess carbon dioxide in the atmosphere that we don't see or feel because its effects are happening underwater. At least one-quarter of the carbon dioxide (CO2) released by burning coal, oil and gas doesn't stay in the air, but instead dissolves into the ocean. Since the beginning of the industrial era, the ocean has absorbed some 525 billion tons of CO2 from the atmosphere, presently around 22 million tons per day.

Orbital influence on monsoons

Perhelion has the greatest effect on the strength

Glacial Effect by obliquity creates High obliquity = more sun in summer = more melting of ice sheets.

Regularly spaced (41,000 yr) glacial periods

projections for sea level rise

Since at least the start of the 20th century, the average global sea level has been rising. Between 1900 and 2016, the sea level rose by 16-21 cm (6.3-8.3 in).[2] More precise data gathered from satellite radar measurements reveal an accelerating rise of 7.5 cm (3.0 in) from 1993 to 2017,[3]:1554 which is a trend of roughly 30 cm (12 in) per century. This acceleration is due mostly to human-caused global warming, which is driving thermal expansion of seawater and the melting of land-based ice sheets and glaciers.[4] Between 1993 and 2018, thermal expansion of the oceans contributed 42% to sea level rise; the melting of temperate glaciers, 21%; Greenland, 15%; and Antarctica, 8%.[3]:1576 Climate scientists expect the rate to further accelerate during the 21st century.[5]:62 Projecting future sea level is challenging, due to the complexity of many aspects of the climate system. As climate research into past and present sea levels leads to improved computer models, projections have consistently increased. For example, in 2007 the Intergovernmental Panel on Climate Change (IPCC) projected a high end estimate of 60 cm (2 ft) through 2099,[6] but their 2014 report raised the high-end estimate to about 90 cm (3 ft).[7] A number of later studies have concluded that a global sea level rise of 200 to 270 cm (6.6 to 8.9 ft) this century is "physically plausible".[8][3][9] A conservative estimate of the long-term projections is that each Celsius degree of temperature rise triggers a sea level rise of approximately 2.3 meters (4.2 ft/degree Fahrenheit) over a period of two millennia (2,000 years): an example of climate inertia.[2] The sea level will not rise uniformly everywhere on Earth, and it will even drop slightly in some locations, such as the Arctic.[10] Local factors include tectonic effects and subsidence of the land, tides, currents and storms. Sea level rises can influence human populations considerably in coastal and island regions.[11] Widespread coastal flooding is expected with several degrees of warming sustained for millennia.[12] Further effects are higher storm-surges and more dangerous tsunamis, displacement of populations, loss and degradation of agricultural land and damage in cities.[13][14][15] Natural environments like marine ecosystems are also affected, with fish, birds and plants losing parts of their habitat.[16] Societies can respond to sea level rise in three different ways: to retreat, to accommodate and to protect. Sometimes these adaptation strategies go hand in hand, but at other times choices have to be made among different strategies.[17] Ecosystems that adapt to rising sea levels by moving inland might not always be able to do so, due to natural or artificial barriers.[18]

Measuring past sea level with Acropora palmata Corals, always grows in shallow water (<5m).

Take cores through old reefs, date them (with U/Th radioactive decay method) and then you can tell where sea level once was!

biological pump

The biological pump can be divided into three distinct phases,[3] the first of which is the production of fixed carbon by planktonic phototrophs in the euphotic (sunlit) surface region of the ocean. In these surface waters, phytoplankton use carbon dioxide (CO2), nitrogen (N), phosphorus (P), and other trace elements (barium, iron, zinc, etc.) during photosynthesis to make carbohydrates, lipids, and proteins. Some plankton, (e.g. coccolithophores and foraminifera) combine calcium (Ca) and dissolved carbonates (carbonic acid and bicarbonate) to form a calcium carbonate (CaCO3) protective coating. Once this carbon is fixed into soft or hard tissue, the organisms either stay in the euphotic zone to be recycled as part of the regenerative nutrient cycle or once they die, continue to the second phase of the biological pump and begin to sink to the ocean floor. The sinking particles will often form aggregates as they sink, greatly increasing the sinking rate. It is this aggregation that gives particles a better chance of escaping predation and decomposition in the water column and eventually make it to the sea floor. The fixed carbon that is either decomposed by bacteria on the way down or once on the sea floor then enters the final phase of the pump and is remineralized to be used again in primary production. The particles that escape these processes entirely are sequestered in the sediment and may remain there for millions of years. It is this sequestered carbon that is responsible for ultimately lowering atmospheric CO2.

1000 Years Ice Age Cycles, Eccentricity controlled?

Thus the ice ages can't be controlled by eccentricity. Instead they must be paced by obliquity and precession. Eccentricity could have an indirect effect by modulating precession, or the 100 ka cycles could just be three obliquity cycles.

Carbon dioxide Biological Pump In the Glacial Period

We know there is less in the atmosphere and less in veg/soil (glacial climates had less vegetation) so the deep ocean is the only option on these timescales.

Ice Sheet equilibrium line

Where Zone of accumulation/zone of ablation are demarcated and controls ice formation or melting

Zone of accumulation/zone of ablation

Zone of Accumulation: Glaciers form in the zone of accumulation. This is the portion of the glacier in which the growth of ice is greater than its depletion. The zone of accumulation for the large continental ice sheets resides at high altitudes. For alpine glaciers, the zone of accumulation is at high altitude where cold temperatures prevent melting in summertime. The accumulation zone comprises 60-70% of the total surface area in a healthy glacier. Zone of Ablation: Ablation is the depletion of ice or snow from the glacier. Ablation may result from wind erosion, melting, and evaporation. The zone of ablation is the part of the glacier where ice melts faster than it can be replaced by snowfall. The base of the glacier is called the terminus, snout or toe. This is where the glacier deposits all manner of debris. The smaller rocks and soil are called moraine. The extremity of the ablation zone, where the glacier thins away to nothing, is called the ice front.

El Nino

an irregularly occurring and complex series of climatic changes affecting the equatorial Pacific region and beyond every few years, characterized by the appearance of unusually warm, nutrient-poor water off northern Peru and Ecuador, typically in late December.

sapropels

black organic-rich muds deposited on the Mediterranean seafloor as a result of strong inflow from the Nile River, which stifles delivery of oxygen to the deep parts of the basin

precession on radiation

causes opposite changes b/t hemispheres • Example: June perihelion = stronger NH summers and winters, weaker SH summers and winters

Precession has a 23 ka cycle and is modulated by

eccentricity.

CO2 During Glacial Cycles

follows the ice ages closely and could act as a feedback

industrial aerosols

from ground level pollution cancel some warming from greenhouse gases sulfates, nitrates, dust, industrial soot, forest fires

Millennial-scale oscillations:

indicate the earth flips back and forth between warmer states and colder states

anoxia

lack of oxygen

Big ice sheets means

lower sea level

Isostatic adjustments are the result of the buoyancy of Earth's lithosphere as it floats on the ________ below which is denser and plastic like.

mantle

High obliquity

more sun in summer, less in winter • warmer summers, colder winters for both hemispheres

Perihelion

orbital point nearest the sun

The biological pump is driven by:

primary producers

glacial termination is

rapid

Last Glacial Maximum (LGM)

refers to a period in the Earth's climate history when ice sheets were at their most recent maximum extension, between 26,500 and 19,000-20,000 years ago, marking the peak of the last glacial period. During this time, vast ice sheets covered much of North America, northern Europe and Asia.

eccentricity

refers to how elliptical the Earth's orbit is. 100,000 and 400,00 year cycles.

Modulation

refers to repetitive changes in the amplitude of the cycle. It is not a cycle in of itself.

precession of the equinoxes

refers to the change in the timing of perihelion in the seasonal cycle. This changes due to the wobble in the Earth's rotation.

Glacial inception is

slow

Description of vegetation patterns During LGM

s 4.1 Index to the Regions This text is divided up on a regional basis, roughly corresponding to traditional notions of "the continents", although the detailed choice of boundaries for each region is fairly arbitrary. The legend for the maps is found in Figure 3. The order of the regional treatment is as follows: North and central America: includes the U.S.A. along with Mexico and the Caribbean, Canada, Greenland, "Beringia" and central America; South America: includes the South American area and the Falkland Islands; Africa: includes Madagascar, Arabia, and the Levant; Europe: includes the area eastwards to the Urals, and also Asia Minor; Eurasia: includes northern Eurasia, mostly Russia east of the Urals, southern and eastern Asia, the Middle East, and from the central Asian desert southwards and eastwards to Malaysia/Indonesia; Australasia: includes Australia, New Guinea, and New Zealand. Ray, N., and J. M. Adams. 2001. Internet Archaeology 11 (http://intarch.ac.uk/journal/issue11/rayadams_toc.html) - 11 - 4.2 North and Central America In North America, the dominant feature was the presence of a vast ice sheet covering Canada. Forest dominated the eastern USA, but it was more open and contained trees adapted to the cooler climates. Regionally specific altitudinal zones: in the Rockies in the west, altitudinal zones were lowered: For the LGM western USA montane mosaic region, between around 26 and 42 Deg.N the following vegetation zones seem to have predominated: alpine tundra, above 2500m, subalpine parkland of open stands of spruce, pines, and fir between 1500-2500m, and scrub/woodland below about 1500m (Tallis 1991). In the Cordillera region of the western USA, the areas below 500m altitude were "semi-desert". Everything below 500m was semi-desert, and scrub in the 500-1500m range. Same for the whole "desert" area to the south, covering Texas and northern Mexico; if below 500m the vegetation was ascribed to the semi-desert category, above 500m, it was reconstructed as a scrub. In the Sierra Nevada of California, the areas above 2000m were labeled as ice (Tallis 1991). 4.3 South America was slightly cooler and generally drier than at present. It appears that the Amazonian rainforest was substantially reduced in the area (though large uncertainties remain). The Atlantic forest of Brazil was also much diminished. Some desert and semi-desert areas formed in what are presently grassland and scrub zones. Regionally specific altitudinal zones: "permanent ice" above 4100m throughout the Andes at the LGM, based on Hooghiemstra (1989). Ray, N., and J. M. Adams. 2001. Internet Archaeology 11 4.4 Africa Africa was slightly cooler but much drier than at present. The Sahara Desert and the Namib Desert were both expanded, and in equatorial Africa, there was relatively little forest cover. Regionally specific altitudinal zones: In the eastern part of South Africa: areas above 1000m were labeled "temperate steppe grassland" based on Coetzee and van Zinderen Bakker (1988). In the central part of the Sahara desert, areas above 1500m were reconstructed as "semi-desert", based on the fact that some winter rains occurred in altitude and maintained scattered vegetation ( - 13 - 4.5 Europe Ice sheets covered northern Europe and Scandinavia. Most of the rest of northern Europe resembled semi-desert, with a mixture of tundra and grassland elements (steppe-tundra). In southern Europe, vegetation resembled a semi-desert steppe, with scattered pockets of trees in moist areas.

obliquity cycle

the change in the tilt of the Earth's axis relative to the plane of the ecliptic over a 41,000-year period.

pH scale measures

the concentration of hydrogen ions in a solution

In the tropics, oxygen isotopes

trace both temperature and the amount of rain.

Medieval Warm Period

was a time of warm climate in the North Atlantic region lasting from c. 950 to c. 1250.[2] It was likely[3] related to warming elsewhere[4][5][6] while some other regions were colder, such as the tropical Pacific. Average global mean temperatures have been calculated to be similar to early-mid-20th-century warming. Possible causes of the Medieval Warm Period include increased solar activity, decreased volcanic activity, and changes to ocean circulation.[7] The period was followed by a cooler period in the North Atlantic and elsewhere termed the Little Ice Age. Some refer to the event as the Medieval Climatic Anomaly as this term emphasizes that climatic effects other than temperature were important.[8][9] It is thought that between c. 950 and c. 1100 was the Northern Hemisphere's warmest period since the Roman Warm Period. It was only in the 20th and 21st centuries that the Northern Hemisphere experienced higher temperatures. Climate proxy records show peak warmth occurred at different times for different regions, indicating that the Medieval Warm Period was not a globally uniform event.[10

the pleistocene ice sheets greatly affected ______ patterns over large regions of north america

weather

How did early humans affect Methan levels

Anthropogenic methane emissions during the late Holocene came from several sources, one of which was irrigated rice paddies. Fuller et al. [2011] used archaeological and archaeobotanical data to map the spread of irrigated rice from origins in the Yangtze River Valley more than 5000 years ago across the rest of southeastern Asia by 1000 years ago (Figure 7a). They used this mapped spread of irrigated rice to estimate changes in atmospheric CH4 concentrations based on two assumptions: (1) the spatial density of wet-rice farming grew with the square root of the estimated population size after the first arrival of irrigated rice in each area and (2) modern relationships can be used to convert estimates of total irrigated rice area to CH4 emissions and enhanced atmospheric CH4 concentrations between 5000 and 1000 years ago (Figure 7b). With this method, they found that the spread of irrigated rice caused an estimated 70 ppb increase in atmospheric CH4 by 1000 years ago, a substantial fraction of the 100 ppb increase measured at Dome C (Figure 7c). The CH4 rise measured at Dome C is only part of the proposed methane anomaly, because CH4 values fell during previous interglaciations but did not do so during the late Holocene (Figures 2a and 3a). As a result, the full anthropogenic anomaly proposed for 5000 to 1000 years ago needs to be at least twice as large as the observed increase to confirm the full hypothesis. The spread of CH4-emitting livestock across Asia and Africa after 5000 years ago (Figure 8) would also have contributed significantly to the growing CH4 anomaly [Fuller et al., 2011], along with livestock expansion in Europe after initial entry of agriculture from southwest Asia [Zohary et al., 2012; Colledge et al., 2013]. Increased biomass burning of weeds and crop residues would also have added to the anthropogenic CH4 total as farming spread. Using a model-based reconstruction of potential natural sources of methane, Singarayer et al. [2011] concluded that a significant part of the late Holocene CH4 rise could have been caused by increased emissions from South America due to greater Southern Hemisphere summer insolation that enhanced monsoon strength in the western Amazon [Selzer et al., 2000]. But the early-interglacial CH4 responses shown in Figures 2a and 3a raise questions about this conclusion. All previous CH4 trends fell despite similar or larger increases in Southern Hemisphere insolation forcing [Berger, 1978], which should have driven stronger South American monsoons and caused greater CH4 emissions. It appears that those larger Southern Hemisphere methane contributions in the past were overwhelmed by even larger decreases from shrinking wetlands in the Northern Hemisphere. Given this Northern Hemisphere dominance through previous interglaciations that span 800,000 y

Ice feedbacks • Ice-albedo feedback • Ice-elevation feedback: as ice sheets become higher (2-3 km) more ice is above eq line (more accumulation). Two feedbacks are important when talking about waxing/ waning of ice sheet

Ice-albedo feedback Ice-elevation feedback: as ice sheets become higher (2-3 km) more ice is above eq line (more accumulation). Two feedbacks are important when talking about waxing/ waning of ice sheet

Seasonal Radiation

orbital cycles affect the amount of shortwave radiation (sunlight) reaching different parts of Earth.

Heinrich Events And Deep ocean Currents

Atlantic Ocean Circulation During the Last Ice Age There is strong evidence that the circulation of the deep Atlantic during the peak of the last Ice Age, or the Last Glacial Maximum (LGM; ~22,000 to 19,000 years ago) was different from the modern circulation (Boyle & Keigwin 1987, Duplessy et al. 1988, Marchal & Curry 2008). Compilations of deepwater δ13C and CdW for the LGM (Figure 5) show several features that contrast with their modern distributions. Whereas much of the modern deep western Atlantic has similar δ13C values because it is filled with NADW, during the LGM, the range of δ13C values was larger than today, with higher values in NADW and lower values in AABW. The main core of high-δ13C, low-CdW NADW was at least 1000 meters shallower than today, probably because the density difference between surface waters and deep water was reduced — surface salinity may have decreased as a result of less evaporation due to colder glacial temperatures, and as a result of input of freshwater from glaciers surrounding the North Atlantic (Boyle & Keigwin 1987). In the western Atlantic, depths below ~2 km were filled with AABW. Radiocarbon data suggest that deepwater was older (Keigwin & Schlegel 2002), consistent with less NADW and more AABW as indicated by the δ13C and CdW of benthic foraminifera. Glacial δ13C data from the eastern Atlantic suggest that the boundary between glacial AABW and glacial NADW may have been shallower than in the western Atlantic (Sarnthein et al. 1994), although the difference may be the result of local effects caused by increased glacial productivity and higher rates of remineralization of low-δ13C organic carbon in the eastern basin. Inferences using other kinds of proxy data of deep Atlantic circulation are consistent with the changes inferred from δ13C, Cd/Ca and 14C of benthic foraminifera (Lynch-Steiglitz et al. 2007).

RCP projected Temperature change

In RCP 8.5 emissions continue to rise throughout the 21st century.[9] Since AR5 this has been thought to be very unlikely, but still possible as feedbacks are not well understood.[10] The high concentration pathways depend on assumptions of abundant fossil fuel for future production. Researchers have questioned whether remaining world supply can meet such demand. Wang et al conducted a study considering 116 different projections for 21st century fossil fuel production forecasts published in scientific literature and by mainstream energy institutes comprising a wide range of scenarios. The study found that high-concentration pathways may be overestimating future supply of fossil fuels, and in particular RCP8.5 appeared to be an extremely high overestimation.[11] The results indicated that "most climate projections made with current knowledge ... overestimate future climate change due to making what appear to be unrealistic assumptions on the increase in usage of fossil fuels." This study found that a likely upper bound for 21st century CO2 concentration would be about 610ppm, associated with about 2.6 degrees of warming above pre-industrial levels. The study also indicated, however, that even under these supply constraints, CO2 concentration and warming were still likely to exceed the accepted "dangerous" level of 410 ppm and 2 degrees, respectively, by 2100.

PETM (Paleocene-Eocene Thermal Maximum)

Likely, the cause of these warm deep waters was that they came from different surface ocean locations than they do today (see Module 6 for more detail on the sources of modern deep waters), combined with the warming that took place in the surface ocean. As warmer waters hold less oxygen than cold waters, PETM deep waters in many locations likely were possibly close to a condition that is known as hypoxia (we will learn more about this in Module 6). The word hypoxia does not sound too distressing, but imagine for a minute that you are a fish and you needed oxygen for respiration. Hypoxia would have been truly awful for such a creature! Finally, the input of so much CO2 into the ocean caused ocean waters to become more acidic and led to a condition known as ocean acidification (sorry to keep jumping ahead but we will learn a lot more about this in Module 7!). Acidification of the deep ocean during the PETM is well accepted and is observed by complete dissolution of all CaCO3 shells that rained down on the seafloor. For creatures that require a shell for protection against predators and to protect the soft cellular parts from the harsh ocean, acidification would have been disastrous. By comparison, the shallow ocean experienced a much more minor decrease in its acidity and shelly creatures continued to thrive there.

Climate change and early human evolution

Over time (from left to right), new adaptations may evolve during periods of (A) relatively stable environment; (B) directional or progressive change, such as from wet to dry; or (C) highly variable habitat, as predicted by the variability selection hypothesis. One way organisms can cope with environmental fluctuation is through genetic adaptation, where several alleles, or different versions of genes, are present in the population at different frequencies. As conditions change, natural selection favors one allele or genetic variant over another. Genes that can facilitate a range of different forms under different environments (phenotypic plasticity) can also help an organism adapt to changing conditions. Another response to environmental change is to evolve structures and behaviors that can be used to cope with different environments. The selection of these structures and behaviors as a result of environmental instability is known as variability selection. This hypothesis differs from those based on consistent environmental trends. Environmental change in an overall direction leads to specializations for those specific conditions. But if the environment becomes highly variable, specializations for particular environments would be less advantageous than structures and behaviors that enable coping with changing and unpredictable conditions. Variability selection refers to the benefits conferred by variations in behavior that help organisms survive change. To test the variability selection hypothesis, and to compare it with habitat-specific hypotheses, Potts examined the hominin fossil record and the records of environmental change during the time of human evolution. If environmental instability was the key factor favoring human adaptations, new adaptations would be expected to occur during periods of increased environmental variability, and these adaptations would have improved the ability of early human ancestors to deal with habitat change and environmental diversity. Overall, the hominin fossil record and the environmental record show that hominins evolved during an environmentally variable time. Higher variability occurred as changes in seasonality produced large-scale environmental fluctuations over periods that often lasted tens of thousands of years. The variability selection hypothesis implies that human traits evolved over time because they enabled human ancestors to adjust to environmental uncertainty and change. The hypothesis addresses the matter of how, exactly, adaptability can evolve over time.

Milankovic theory of climate

Postulates that ice sheet growth is paced by orbital cycles; specifically, that ice sheets grow when summer radiation is low. • Predicts that 65˚N is the most sensitive latitude b/c that is where NH ice sheets first grow and last melt

Representative Concentration Pathway (RCP)

is a greenhouse gas concentration (not emissions) trajectory adopted by the IPCC. Four pathways were used for climate modeling and research for the IPCC fifth Assessment Report (AR5) in 2014. The pathways describe different climate futures, all of which are considered possible depending on the volume of greenhouse gases (GHG) emitted in the years to come. The RCPs; originally RCP2.6, RCP4.5, RCP6, and RCP8.5; are labelled after a possible range of radiative forcing values in the year 2100 (2.6, 4.5, 6, and 8.5 W/m2, respectively).[1][2][3] Since AR5 the original pathways are being considered together with Shared Socioeconomic Pathways: as are new RCPs such as RCP1.9, RCP3.4 and RCP7.[

Delta Oxygen-18

δ18O or delta-O-18 is a measure of the ratio of stable isotopes oxygen-18 (18O) and oxygen-16 (16O). It is commonly used as a measure of the temperature of precipitation, as a measure of groundwater/mineral interactions, and as an indicator of processes that show isotopic fractionation, like methanogenesis. In paleosciences, 18O:16O data from corals, foraminifera and ice cores are used as a proxy for temperature.