Ekman transport, part of Ekman motion theory first investigated in 1902 by Vagn Walfrid Ekman, refers to the wind-driven net transport of the surface layer of a fluid that, due to the Coriolis effect, occurs at 90° to the direction of the surface wind. This phenomenon was first noted by Fridtjof Nansen, who recorded that ice transport appeared to occur at an angle to the wind direction during his Arctic expedition during the 1890s; the direction of transport is dependent on the hemisphere: in the northern hemisphere, transport occurs at 90° clockwise from wind direction, while in the southern hemisphere it occurs at a 90° counterclockwise. Ekman theory explains the theoretical state of circulation if water currents were driven only by the transfer of momentum from the wind. In the physical world, this is difficult to observe because of the influences of many simultaneous current driving forces. Though the following theory technically applies to the idealized situation involving only wind forces, Ekman motion describes the wind-driven portion of circulation seen in the surface layer.
Surface currents flow at a 45° angle to the wind due to a balance between the Coriolis force and the drags generated by the wind and the water. If the ocean is divided vertically into thin layers, the magnitude of the velocity decreases from a maximum at the surface until it dissipates; the direction shifts across each subsequent layer. This is called the Ekman spiral; the layer of water from the surface to the point of dissipation of this spiral is known as the Ekman layer. If all flow over the Ekman layer is integrated, the net transportation is at 90° to the right of the surface wind in the northern hemisphere; some assumptions of the fluid dynamics involved in the process must be made in order to simplify the process to a point where it is solvable. The assumptions made by Ekman were: no boundaries; the simplified equations for the Coriolis force in the x and y directions follow from these assumptions: 1 ρ ∂ τ x ∂ z = − f v, 1 ρ ∂ τ y ∂ z = f u, where τ is the wind stress, ρ is the density, u is the East-West velocity, v is the north-south velocity.
Integrating each equation over the entire Ekman layer: τ x = − M y f, τ y = M x f, where M x = ∫ 0 z ρ u d z, M y = ∫ 0 z ρ v d z. Here M x and M y represent the zonal and meridional mass transport terms with units of mass per unit time per unit length. Contrarily to common logic, north-south winds cause mass transport in the East-West direction. In order to understand the vertical velocity structure of the water column, equations 1 and 2 can be rewritten in terms of the vertical eddy viscosity term. ∂ τ x ∂ z = ρ A z ∂ 2 u ∂ z 2, ∂ τ y ∂ z = ρ A z ∂ 2 v ∂ z 2, where A z is the vertical eddy viscosity coefficient. This gives a set of differential equations of the form A z ∂ 2 u ∂ z 2 = − f v, A z ∂ 2 v ∂ z 2 = f u. In order to solve this system of two differ
A Heinrich event is a natural phenomenon in which large armadas of icebergs break off from glaciers and traverse the North Atlantic. First described by marine geologist Hartmut Heinrich, they occurred during five of the last seven glacial periods or "ice ages" over the past 640,000 years. Heinrich events are well documented for the last glacial period but notably absent from the penultimate glaciation; the icebergs contained rock mass, eroded by the glaciers, as they melted, this material was dropped to the sea floor as ice rafted debris. The icebergs' melting caused extensive amounts of fresh water to be added to the North Atlantic; such inputs of cold and fresh water may well have altered the density-driven, thermohaline circulation patterns of the ocean, coincide with indications of global climate fluctuations. Various mechanisms have been proposed to explain the cause of Heinrich events, most of which imply instability of the massive Laurentide ice sheet, a continental glacier covering north eastern North America during the last glacial period.
Other northern hemisphere ice sheets were involved as well. However, the initial cause of this instability is still debated; the strict definition of Heinrich events is the climatic event causing the IRD layer observed in marine sediment cores from the North Atlantic: a massive collapse of northern hemisphere ice shelves and the consequent release of a prodigious volume of icebergs. By extension, the name "Heinrich event" can refer to the associated climatic anomalies registered at other places around the globe, at the same time periods; the events are rapid: they last less than a millennium, a duration varying from one event to the next, their abrupt onset may occur in mere years. Heinrich events are observed in many North Atlantic marine sediment cores covering the last glacial period; some identify the Younger Dryas event as a Heinrich event, which would make it event H0. Heinrich events appear related to some, but not all, of the cold periods preceding the rapid warming events known as Dansgaard-Oeschger events, which are best recorded in the NGRIP Greenland ice core.
However, difficulties in synchronising marine sediment cores and Greenland ice cores to the same time scale cast aspersions on the accuracy of that statement. Heinrich's original observations were of six layers in ocean sediment cores with high proportions of rocks of continental origin, "lithic fragments", in the 180 μm to 3 mm size range; the larger size fractions cannot be transported by ocean currents, are thus interpreted as having been carried by icebergs or sea ice which broke off glaciers or ice shelves, dumped debris onto the sea floor as the icebergs melted. Geochemical analyses of the IRD can provide information about the origin of these debris: the large Laurentide ice sheet covering North America for Heinrich events 1, 2, 4 and 5, on the contrary, European ice sheets for the minor events 3 and 6; the signature of the events in sediment cores varies with distance from the source region. For events of Laurentide origin, there is a belt of IRD at around 50° N, known as the Ruddiman belt, expanding some 3,000 km from its North American source towards Europe, thinning by an order of magnitude from the Labrador Sea to the European end of the present iceberg route.
During Heinrich events, huge volumes of fresh water flow into the ocean. For Heinrich event 4, based on a model study reproducing the isotopic anomaly of oceanic oxygen 18, the fresh water flux has been estimated to 0.29±0.05 Sverdrup with a duration of 250±150 years, equivalent to a fresh water volume of about 2.3 million cubic kilometres or a 2 ± 1 m sea-level rise. Several geological indicators fluctuate in time with these Heinrich events, but difficulties in precise dating and correlation make it difficult to tell whether the indicators precede or lag Heinrich events, or in some cases whether they are related at all. Heinrich events are marked by the following changes: Increased δ18O of the northern seas and East Asian stalactites, which by proxy suggests falling global temperature Decreased oceanic salinity, due to the influx of fresh water Decreased sea surface temperature estimates off the West African coast through biochemical indicators known as alkenones Changes in sedimentary disturbance caused by burrowing animals Flux in planktonic isotopic make-up Pollen indications of cold-loving pines replacing oaks on the North American mainland Decreased foramaniferal abundance – which due to the pristine nature of many samples cannot be attributed to preservational bias and has been related to reduced salinity Increased terrigenous runoff from the continents, measured near the mouth of the Amazon River Increased grain size in wind-blown loess in China, suggesting stronger winds Changes in relative Thorium-230 abundance, reflecting variations in ocean current velocity Increased deposition rates in the northern Atlantic, reflected by an increase in continentally derived sediments relative to background sedimentation Expansion o
Thermohaline circulation is a part of the large-scale ocean circulation, driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents travel polewards from the equatorial Atlantic Ocean, cooling en route, sinking at high latitudes; this dense water flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters upwell in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. On their journey, the water masses transport both mass of substances around the globe; as such, the state of the circulation has a large impact on the climate of the Earth. The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt.
On occasion, it is used to refer to the meridional overturning circulation. The term MOC is more accurate and well defined, as it is difficult to separate the part of the circulation, driven by temperature and salinity alone as opposed to other factors such as the wind and tidal forces. Moreover and salinity gradients can lead to circulation effects that are not included in the MOC itself; the movement of surface currents pushed by the wind is intuitive. For example, the wind produces ripples on the surface of a pond, thus the deep ocean—devoid of wind—was assumed to be static by early oceanographers. However, modern instrumentation shows that current velocities in deep water masses can be significant. In general, ocean water velocities range from fractions of centimeters per second to sometimes more than 1 m/s in surface currents like the Gulf Stream and Kuroshio. In the deep ocean, the predominant driving force is differences in density, caused by salinity and temperature variations. There is confusion over the components of the circulation that are wind and density driven.
Note that ocean currents due to tides are significant in many places. There they are thought to facilitate mixing processes diapycnal mixing; the density of ocean water is not globally homogeneous, but varies and discretely. Defined boundaries exist between water masses which form at the surface, subsequently maintain their own identity within the ocean, but these sharp boundaries are not to be imagined spatially but rather in a T-S-diagram where water masses are distinguished. They position themselves above or below each other according to their density, which depends on both temperature and salinity. Warm seawater is thus less dense than cooler seawater. Saltier water is denser than fresher water because the dissolved salts fill interstices between water molecules, resulting in more mass per unit volume. Lighter water masses float over denser ones; this is known as "stable stratification" as opposed to unstable stratification where denser waters are located over less dense waters. When dense water masses are first formed, they are not stably stratified, so they seek to locate themselves in the correct vertical position according to their density.
This motion is called it orders the stratification by gravitation. Driven by the density gradients this sets up the main driving force behind deep ocean currents like the deep western boundary current; the thermohaline circulation is driven by the formation of deep water masses in the North Atlantic and the Southern Ocean caused by differences in temperature and salinity of the water. The great quantities of dense water sinking at high latitudes must be offset by equal quantities of water rising elsewhere. Note that cold water in polar zones sink rapidly over a small area, while warm water in temperate and tropical zones rise more across a much larger area, it slowly returns poleward near the surface to repeat the cycle. The continual diffuse upwelling of deep water maintains the existence of the permanent thermocline found everywhere at low and mid-latitudes; this model was described by Henry Stommel and Arnold B. Arons in 1960 and is known as the Stommel-Arons box model for the MOC; this slow upward movement is approximated to be about 1 centimeter per day over most of the ocean.
If this rise were to stop, downward movement of heat would cause the thermocline to descend and would reduce its steepness. The dense water masses that sink into the deep basins are formed in quite specific areas of the North Atlantic and the Southern Ocean. In the North Atlantic, seawater at the surface of the ocean is intensely cooled by the wind and low ambient air temperatures. Wind moving over the water produces a great deal of evaporation, leading to a decrease in temperature, called evaporative cooling related to latent heat. Evaporation removes only water molecules, resulting in an increase in the salinity of the seawater left behind, thus an increase in the density of the water mass along with the decrease in tem
The Pacific Ocean is the largest and deepest of Earth's oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean in the south and is bounded by Asia and Australia in the west and the Americas in the east. At 165,250,000 square kilometers in area, this largest division of the World Ocean—and, in turn, the hydrosphere—covers about 46% of Earth's water surface and about one-third of its total surface area, making it larger than all of Earth's land area combined; the centers of both the Water Hemisphere and the Western Hemisphere are in the Pacific Ocean. The equator subdivides it into the North Pacific Ocean and South Pacific Ocean, with two exceptions: the Galápagos and Gilbert Islands, while straddling the equator, are deemed wholly within the South Pacific, its mean depth is 4,000 meters. The Mariana Trench in the western North Pacific is the deepest point in the world, reaching a depth of 10,911 meters; the western Pacific has many peripheral seas. Though the peoples of Asia and Oceania have traveled the Pacific Ocean since prehistoric times, the eastern Pacific was first sighted by Europeans in the early 16th century when Spanish explorer Vasco Núñez de Balboa crossed the Isthmus of Panama in 1513 and discovered the great "southern sea" which he named Mar del Sur.
The ocean's current name was coined by Portuguese explorer Ferdinand Magellan during the Spanish circumnavigation of the world in 1521, as he encountered favorable winds on reaching the ocean. He called it Mar Pacífico, which in both Portuguese and Spanish means "peaceful sea". Important human migrations occurred in the Pacific in prehistoric times. About 3000 BC, the Austronesian peoples on the island of Taiwan mastered the art of long-distance canoe travel and spread themselves and their languages south to the Philippines and maritime Southeast Asia. Long-distance trade developed all along the coast from Mozambique to Japan. Trade, therefore knowledge, extended to the Indonesian islands but not Australia. By at least 878 when there was a significant Islamic settlement in Canton much of this trade was controlled by Arabs or Muslims. In 219 BC Xu Fu sailed out into the Pacific searching for the elixir of immortality. From 1404 to 1433 Zheng He led expeditions into the Indian Ocean; the first contact of European navigators with the western edge of the Pacific Ocean was made by the Portuguese expeditions of António de Abreu and Francisco Serrão, via the Lesser Sunda Islands, to the Maluku Islands, in 1512, with Jorge Álvares's expedition to southern China in 1513, both ordered by Afonso de Albuquerque from Malacca.
The east side of the ocean was discovered by Spanish explorer Vasco Núñez de Balboa in 1513 after his expedition crossed the Isthmus of Panama and reached a new ocean. He named it Mar del Sur because the ocean was to the south of the coast of the isthmus where he first observed the Pacific. In 1519, Portuguese explorer Ferdinand Magellan sailed the Pacific East to West on a Spanish expedition to the Spice Islands that would result in the first world circumnavigation. Magellan called the ocean Pacífico because, after sailing through the stormy seas off Cape Horn, the expedition found calm waters; the ocean was called the Sea of Magellan in his honor until the eighteenth century. Although Magellan himself died in the Philippines in 1521, Spanish Basque navigator Juan Sebastián Elcano led the remains of the expedition back to Spain across the Indian Ocean and round the Cape of Good Hope, completing the first world circumnavigation in a single expedition in 1522. Sailing around and east of the Moluccas, between 1525 and 1527, Portuguese expeditions discovered the Caroline Islands, the Aru Islands, Papua New Guinea.
In 1542–43 the Portuguese reached Japan. In 1564, five Spanish ships carrying 379 explorers crossed the ocean from Mexico led by Miguel López de Legazpi, sailed to the Philippines and Mariana Islands. For the remainder of the 16th century, Spanish influence was paramount, with ships sailing from Mexico and Peru across the Pacific Ocean to the Philippines via Guam, establishing the Spanish East Indies; the Manila galleons operated for two and a half centuries, linking Manila and Acapulco, in one of the longest trade routes in history. Spanish expeditions discovered Tuvalu, the Marquesas, the Cook Islands, the Solomon Islands, the Admiralty Islands in the South Pacific. In the quest for Terra Australis, Spanish explorations in the 17th century, such as the expedition led by the Portuguese navigator Pedro Fernandes de Queirós, discovered the Pitcairn and Vanuatu archipelagos, sailed the Torres Strait between Australia and New Guinea, named after navigator Luís Vaz de Torres. Dutch explorers, sailing around southern Africa engaged in discovery and trade.
In the 16th and 17th centuries Spain considered the Pacific Ocean a mare clausum—a sea closed to other naval powers. As the only known entrance from the Atlantic, the Strait of Magellan was at times patrolled by fleets sent to prevent entrance of non-Spanish ships. On the western side of the Pacific Ocean the Dutch threatened the Spanish Philippines; the 18th cen
Journal of Geophysical Research
The Journal of Geophysical Research is a peer-reviewed scientific journal. It is the flagship journal of the American Geophysical Union, it contains original research on the physical and biological processes that contribute to the understanding of the Earth and solar system. It has seven sections: A, B, C, D, E, F, G. All current and back issues are available online for subscribers; the journal was founded under the name Terrestrial Magnetism by the American Geophysical Union's president Louis Agricola Bauer in 1896. It was entitled Terrestrial Magnetism and Atmospheric Electricity from 1899–1948. In 1980, three specialized sections were established: A: Space Physics, B: Solid Earth, C: Oceans. Subsequently, further sections have been added: D: Atmospheres in 1984, E: Planets in 1991, F: Earth Surface in 2003, G: Biogeosciences in 2005; the scopes of the current seven sections, published as separate issues, are: A: Space Physics covers aeronomy and magnetospheric physics, planetary atmospheres and magnetospheres and external solar physics, cosmic rays, heliospheric physics.
B: Solid Earth focuses on the physics and chemistry of the solid Earth and the liquid core of the Earth, paleomagnetism, marine geology/geophysics and physics of minerals, volcanology, geodesy and tectonophysics. C: Oceans covers physical and chemical oceanography. D: Atmospheres covers atmospheric properties and processes, including the interaction of the atmosphere with other components of the Earth system. E: Planets covers the geology, geochemistry, atmospheres and dynamics of the planets, asteroids, rings and meteorites. Studies of the Earth are included when they concern exogenic effects or the comparison of the Earth to other planets. F: Earth Surface focuses on the physical and biological processes that affect the form and function of the surface of the solid Earth over all temporal and spatial scales, including fluvial and coastal sediment transport. G: Biogeosciences focuses on the interface between biology and the geosciences and attempts to understand the functions of the Earth system across multiple spatial and temporal scales.
Each of the sections has one or more editors who are appointed by and serve at the pleasure of the President of the American Geophysical Union for terms of three to four years. Each editor can in turn appoint associate editors. According to the Editor-in-Chief of JGR-Space Physics, "With the switch to Wiley, the separate sections of JGR were given distinct ISSN numbers; this means that in a couple of years, each section of JGR will have its own Impact Factor." The journal is indexed by GEOBASE, GeoRef, PubMed, Web of Science, several CSA indexes. It published 2995 articles in 2010. According to the Journal Citation Reports, the journal has a 2010 impact factor of 3.303, ranking it 15th out of 165 journals in the category "Geosciences, Multidisciplinary". Journal of Geophysical Research—Atmospheres was the 6th most cited publication on climate change between 1999 and 2009. Among the most cited papers in the Journal of Geophysical Research are: Cande, D. V.. C.. "Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic".
Journal of Geophysical Research. 100. Pp. 6093–6095. Bibcode:1995JGR...100.6093C. doi:10.1029/94JB03098. Brune, J. N.. "Tectonic stress and the spectra of seismic shear Waves from earthquakes". Journal of Geophysical Research. 75. Pp. 4997–5009. Bibcode:1970JGR....75.4997B. Doi:10.1029/JB075i026p04997. Parsons, B.. "Analysis of variation of ocean-floor bathymetry and heat-flow with age". Journal of Geophysical Research. 82. Pp. 803–827. Bibcode:1977JGR....82..803P. Doi:10.1029/JB082i005p00803. Minster, J. B.. H.. "Present-day plate motions". Journal of Geophysical Research. 83. Pp. 5331–5354. Bibcode:1978JGR....83.5331M. Doi:10.1029/JB083iB11p05331. Alex Guenther, C. Nicholas Hewitt, David Erickson, Ray Fall, Chris Geron, Tom Graedel, Peter Harley, Lee Klinger, Manuel Lerdau, W. A. Mckay, Tom Pierce, Bob Scholes, Rainer Steinbrecher, Raja Tallamraju, John Taylor, Pat Zimmerman. "A global model of natural volatile organic compound emissions". Journal of Geophysical Research. 100. Pp. 8873–8892. Bibcode:1995JGR...100.8873G.
Doi:10.1029/94JD02950. CS1 maint: Uses authors parameter Kennel, C. F.. E.. "Limit on stably trapped particle fluxes". Journal of Geophysical Research. 71. Pp. 1–28. Bibcode:1966JGR....71....1K. Doi:10.1029/JZ071i001p00001. Birch, F.. "Elasticity and constitution of the Earth interior". Journal of Geophysical Research. 57. Pp. 227–286. Bibcode:1952JGR....57..227B. Doi:10.1029/JZ057i002p00227. List of scientific journals in earth and atmospheric sciences Official website
The Fram Strait is the passage between Greenland and Svalbard, located between 77°N and 81°N latitudes and centered on the prime meridian. The Greenland and Norwegian Seas lie south of Fram Strait, while the Nansen Basin of the Arctic Ocean lies to the north. Fram Strait is noted for being the only deep connection between the Arctic Ocean and the World Oceans; the dominant oceanographic features of the region are the West Spitsbergen Current on the east side of the strait and the East Greenland Current on the west. Fram Strait is the northernmost ocean area having ice-free conditions throughout the year; the width of the strait is about 450 km, but because of the wide continental shelves of Greenland and Spitsbergen, the deep portion of Fram Strait is only about 300 km wide. The ocean over the Greenland continental shelf is covered with ice. Within Fram Strait, the sill connecting the Arctic and Fram Strait is 2545 m deep; the Knipovich Ridge, the northernmost section of the Mid-Atlantic Ridge, extends northward through the strait to connect to the Nansen-Gakkel Ridge of the Arctic Ocean.
A rift valley, caused by sea-floor spreading, runs parallel to the Knipovich Ridge. The Molloy Deep within Fram Strait is the deepest point of the Arctic; this small basin at 79°8.5′N and 2°47′E has a maximum depth of 5607 m. The Yermak Plateau, with a mean depth of about 650 m, lies to the northwest of Spitsbergen. Fram Strait was home to a large population of Bowhead whales called the Greenland right whale. By mid-17th century, the Svalbard population of Bowhead whales was reduced to near extinction by excessive whaling. Western Fram Strait may be a wintering ground for this Critically Endangered population; the use of the name "Fram Strait" for the passage between Spitsbergen and Greenland appears to have come into common use in the oceanographic literature in the 1970s. Fram Strait is named after the Norwegian ship Fram. In an 1893 expedition led by Fridtjof Nansen, the Fram drifted for two years across the Arctic before exiting the Arctic through what is now known as Fram Strait. According to glaciologist and geographer Moira Dunbar, an early adopter of the name, the name "Fram Strait" originated in the Russian scientific literature.
While in common use in the oceanographic scientific literature, the name appears to be unofficial. Fram Strait is the only deep-water connection between the Arctic. Other gateways are the Barents Sea Opening, the Bering Strait and various small channels in the Canadian Arctic Archipelago, they are all shallower than Fram Strait, leaving Fram Strait the only route by which deep water can be exchanged between the Atlantic and Arctic Oceans. This exchange occurs in both directions, with specific water masses identified with specific regions flowing between the Oceans. Water with characteristics of the deep Canadian and Eurasian Basins of the Arctic are observed leaving the Arctic in the deep western side of Fram Strait, for example. On the eastern side, cold water from the Norwegian Sea is observed entering the Arctic below the West Spitsbergen Current. In recent years the nature and interactions of these water masses have been changing, symptoms of the changes occurring with the ocean's climate.
Warm, salty water is transported northward from the Atlantic by the West Spitsbergen Current in the east of the strait. The West Spitsbergen Current is the northernmost branch of the North Atlantic Current system; this water forms. The sub-surface flow has a strong seasonality with a minimal volume transport in winter; this current transports internal energy into the Arctic Ocean. The northward velocity is maximum in winter, so the heat transport is highest in winter. On the west side of the strait, the East Greenland Current flows southward on the Greenland Shelf; the current carries is cold and fresh water out of the Arctic that corresponds to a water mass called Polar water. The Fram Strait area is located downwind of the Transpolar Drift and therefore covered by multi-year ice in the west of the strait, next to the coast of Greenland. 90% of sea ice exported from the Arctic is transported by the East Greenland Current. Sea ice corresponds to fresh water, since its salt content of 4 per mil is much less than the 35 per mil for sea water.
The Alfred Wegener Institute for Polar and Marine Research and the Norwegian Polar Institute have maintained long term monitoring measurements in Fram Strait to obtain volume- and energy-budgets through this choke point. The observations serve to assess the development of the Arctic Ocean as a sink for terrestrial organic carbon; the AWI=NPI observing array consists of a line of up to 16 moorings across Fram Strait. The mooring line has been maintained since 1997 with a spacing of 25 km. At up to five different depths, the moored array measures the water velocity and salinity of the water column. Computer simulations suggest that 60 to 70% of the fluctuation of the sea ice flowing through the Fram Strait is correlated with a 6–7 year fluctuation in which the Icelandic Low Pressure system extends eastward into the Barents Sea; the amount of sea ice passing through the Fram Strait varies from year to year and affects the global climate through its influence on thermohaline circulation. The warming in the Fram Strait region has amplified Arctic shrinkage, serves as a positive feedback mechanism for transporting more internal energy to the Arctic Ocean.
In the past century, the sea surface temperature at Fram Strait has on average warmed 1.9 °C, is 1.4 °C warmer than during the Medieval Warm Period
Global warming is a long-term rise in the average temperature of the Earth's climate system, an aspect of climate change shown by temperature measurements and by multiple effects of the warming. Though earlier geological periods experienced episodes of warming, the term refers to the observed and continuing increase in average air and ocean temperatures since 1900 caused by emissions of greenhouse gasses in the modern industrial economy. In the modern context the terms global warming and climate change are used interchangeably, but climate change includes both global warming and its effects, such as changes to precipitation and impacts that differ by region. Many of the observed warming changes since the 1950s are unprecedented in the instrumental temperature record, in historical and paleoclimate proxy records of climate change over thousands to millions of years. In 2013, the Intergovernmental Panel on Climate Change Fifth Assessment Report concluded, "It is likely that human influence has been the dominant cause of the observed warming since the mid-20th century."
The largest human influence has been the emission of greenhouse gases such as carbon dioxide and nitrous oxide. Climate model projections summarized in the report indicated that during the 21st century, the global surface temperature is to rise a further 0.3 to 1.7 °C to 2.6 to 4.8 °C depending on the rate of greenhouse gas emissions and on climate feedback effects. These findings have been recognized by the national science academies of the major industrialized nations and are not disputed by any scientific body of national or international standing. Future climate change effects are expected to include rising sea levels, ocean acidification, regional changes in precipitation, expansion of deserts in the subtropics. Surface temperature increases are greatest in the Arctic, with the continuing retreat of glaciers and sea ice. Predicted regional precipitation effects include more frequent extreme weather events such as heat waves, wildfires, heavy rainfall with floods, heavy snowfall. Effects directly significant to humans are predicted to include the threat to food security from decreasing crop yields, the abandonment of populated areas due to rising sea levels.
Environmental impacts appear to include the extinction or relocation of ecosystems as they adapt to climate change, with coral reefs, mountain ecosystems, Arctic ecosystems most threatened. Because the climate system has a large "inertia" and greenhouse gases will remain in the atmosphere for a long time, climatic changes and their effects will continue to become more pronounced for many centuries if further increases to greenhouse gases stop. Possible societal responses to global warming include mitigation by emissions reduction, adaptation to its effects, possible future climate engineering. Most countries are parties to the United Nations Framework Convention on Climate Change, whose ultimate objective is to prevent dangerous anthropogenic climate change. Parties to the UNFCCC have agreed that deep cuts in emissions are required and that global warming should be limited to well below 2.0 °C compared to pre-industrial levels, with efforts made to limit warming to 1.5 °C. Some scientists call into question climate adaptation feasibility, with higher emissions scenarios, or the two degree temperature target.
Public reactions to global warming and concern about its effects are increasing. A 2015 global survey showed that a median of 54% of respondents consider it "a serious problem", with significant regional differences: Americans and Chinese are among the least concerned. Multiple independently produced datasets confirm that between 1880 and 2012, the global average surface temperature increased by 0.85 °C. Since 1979 the rate of warming has doubled. Climate proxies show the temperature to have been stable over the one or two thousand years before 1850, with regionally varying fluctuations such as the Medieval Warm Period and the Little Ice Age. Although the increase of the average near-surface atmospheric temperature is used to track global warming, over 90% of the additional energy stored in the climate system over the last 50 years has accumulated in the oceans; the rest warmed the continents and the atmosphere. The warming evident in the instrumental temperature record is consistent with a wide range of observations, as documented by many independent scientific groups.
Examples include sea level rise, widespread melting of snow and land ice, increased heat content of the oceans, increased humidity, the earlier timing of spring events, e.g. the flowering of plants. Global warming refers with the amount of warming varying by region. Since 1979, global average land temperatures have increased about twice as fast as global average ocean temperatures; this is due to the larger heat capacity of the oceans and because oceans lose more heat by evaporation. Where greenhouse gas emissions occur does not impact the location of warming because the major greenhouse gases persist long enough to diffuse across the planet, although localized black carbon deposits on snow and ice do contribute to Arctic warming; the Northern Hemisphere and North Pole have heated much faster than the South Pole and Southern Hemisphere. The Northern Hemisphere not only has much more land, its arrangement around the Arctic Ocean has resulted in the maximum surface area flipping from reflective snow and ice cover to ocean and land surfaces that absorb more sunlight.