Global warming

Global warming is the rise in the average temperature of Earth's atmosphere and oceans since the late 19th century and its projected continuation. Since the early 20th century, Earth's mean surface temperature has increased by about 0.8 °C (1.4 °F), with about two-thirds of the increase occurring since 1980. Warming of the climate system is unequivocal, and scientists are more than 90% certain that it is primarily caused by increasing concentrations of greenhouse gases produced by human activities such as the burning of fossil fuels and deforestation. These findings are recognized by the national science academies of all major industrialized nations.

Climate model projections were summarized in the 2007 Fourth Assessment Report (AR4) by the Intergovernmental Panel on Climate Change (IPCC). They indicated that during the 21st century the global surface temperature is likely to rise a further 1.1 to 2.9 °C (2 to 5.2 °F) for their lowest emissions scenario and 2.4 to 6.4 °C (4.3 to 11.5 °F) for their highest. The ranges of these estimates arise from the use of models with differing sensitivity to greenhouse gas concentrations.^[9][10]
According to AR4, warming and related changes will vary from region to region around the globe. The effects of an increase in global temperature include a rise in sea levels and a change in the amount and pattern of precipitation, as well a probable expansion of subtropical deserts. Warming is expected to be strongest in the Arctic and would be associated with the continuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include a more frequent occurrence of extreme-weather events including heat waves, droughts and heavy rainfall, ocean acidification and species extinctions due to shifting temperature regimes. Effects significant to humans include the threat to food security from decreasing crop yields and the loss of habitat from inundation.
Proposed policy responses to global warming include mitigation by emissions reduction, adaptation to its effects, and possible future geoengineering. Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC), whose ultimate objective is to prevent dangerous anthropogenic (i.e., human-induced) climate change. Parties to the UNFCCC have adopted a range of policies designed to reduce greenhouse gas emissions¹⁰⁹ and to assist in adaptation to global warming.^:13:10 Parties to the UNFCCC have agreed that deep cuts in emissions are required, and that future global warming should be limited to below 2.0 °C (3.6 °F) relative to the pre-industrial level. Reports published in 2011 by the United Nations Environment Programme and the International Energy Agency^[24] suggest that efforts as of the early 21st century to reduce emissions may be inadequate to meet the UNFCCC's 2 °C target.

Observed temperature changes

The increase in ocean heat content is much larger than any other store of energy in the Earth’s heat balance over the two periods 1961 to 2003 and 1993 to 2003, and accounts for more than 90% of the possible increase in heat content of the Earth system during these periods.

Two millennia of mean surface temperatures according to different reconstructions from climate proxies, each smoothed on a decadal scale, with the instrumental temperature record overlaid in black.

The Earth's average surface temperature rose by 0.74±0.18 °C over the period 1906–2005. The rate of warming over the last half of that period was almost double that for the period as a whole (0.13±0.03 °C per decade, versus 0.07±0.02 °C per decade). The urban heat island effect is very small, estimated to account for less than 0.002 °C of warming per decade since 1900. Temperatures in the lower troposphere have increased between 0.13 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Climate proxies show the temperature to have been relatively 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.
The warming that is 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 (water expands as it warms), widespread melting of snow and ice increased heat content of the oceans, increased humidity, and the earlier timing of spring events, e.g., the flowering of plants. The probability that these changes could have occurred by chance is virtually zero.
Recent estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic Data Center show that 2005 and 2010 tied for the planet's warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding 1998 by a few hundredths of a degree. Estimates by the Climatic Research Unit (CRU) show 2005 as the second warmest year, behind 1998 with 2003 and 2010 tied for third warmest year, however, "the error estimate for individual years ... is at least ten times larger than the differences between these three years." The World Meteorological Organization (WMO) statement on the status of the global climate in 2010 explains that, "The 2010 nominal value of +0.53 °C ranks just ahead of those of 2005 (+0.52 °C) and 1998 (+0.51 °C), although the differences between the three years are not statistically significant..."

NOAA graph of Global Annual Temperature Anomalies 1950–2011, showing the El Niño-Southern Oscillation

Temperatures in 1998 were unusually warm because global temperatures are affected by the El Niño-Southern Oscillation (ENSO), and the strongest El Niño in the past century occurred during that year. Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode. 2010 was also an El Niño year. On the low swing of the oscillation, 2011 as an La Niña year was cooler but it was still the 11th warmest year since records began in 1880. Of the 13 warmest years since 1880, 11 were the years from 2001 to 2011. Over the more recent record, 2011 was the warmest La Niña year in the period from 1950 to 2011, and was close to 1997 which was not at the lowest point of the cycle.
Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade). Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation. The northern hemisphere warms faster than the southern hemisphere because it has more land and because it has extensive areas of seasonal snow and sea-ice cover subject to ice-albedo feedback. Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.
The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.

Feedback

Main article: Climate change feedback

Sea ice, shown here in Nunavut, in northern Canada, reflects more sunshine, while open ocean absorbs more, accelerating melting.

Feedback is a process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive feedback increases the change in the first quantity while negative feedback reduces it. Feedback is important in the study of global warming because it may amplify or diminish the effect of a particular process.
The main positive feedback in the climate system is the water vapor feedback. The main negative feedback is radiative cooling through the Stefan–Boltzmann law, which increases as the fourth power of temperature. Positive and negative feedbacks are not imposed as assumptions in the models, but are instead emergent properties that result from the interactions of basic dynamical and thermodynamic processes.
A wide range of potential feedback processes exist, such as Arctic methane release and ice-albedo feedback. Consequentially, potential tipping points may exist, which may have the potential to cause abrupt climate change.
For example, the "emission scenarios" used by IPCC in its 2007 report primarily examined greenhouse gas emissions from human sources. In 2011, a joint study by the US National Snow and Ice Data Center and National Oceanic and Atmospheric Administration calculated the additional greenhouse gas emissions that would emanate from melted and decomposing permafrost, even if policymakers attempt to reduce human emissions from the A1FI scenario to the A1B scenario. The team found that even at the much lower level of human emissions, permafrost thawing and decomposition would still result in 190 Gt C of permafrost carbon being added to the atmosphere on top of the human sources. Importantly, the team made three extremely conservative assumptions: (1) that policymakers will embrace the A1B scenario instead of the A1FI scenario, (2) that all of the carbon would be released as carbon dioxide instead of methane, which is more likely and over a 20 year lifetime has 72x the greenhouse warming power of CO₂, and (3) their model did not project additional temperature rise caused by the release of these additional gases These very conservative permafrost carbon dioxide emissions are equivalent to about 1/2 of all carbon released from fossil fuel burning since the dawn of the Industrial Age, and is enough to raise atmospheric concentrations by an additional 87±29 ppm, beyond human emissions. Once initiated, permafrost carbon forcing (PCF) is irreversible, is strong compared to other global sources and sinks of atmospheric CO₂, and due to thermal inertia will continue for many years even if atmospheric warming stops. A great deal of this permafrost carbon is actually being released as methane instead of carbon dioxide.IPCC 2007's temperature projections did not take any of the permafrost carbon emissions into account and therefore underestimate the degree of expected climate change.
Other research published in 2011 found that increased emissions of methane could instigate significant feedbacks that amplify the warming attributable to the methane alone. The researchers found that a 2.5-fold increase in methane emissions would cause indirect effects that increase the warming 250% above that of the methane alone. For a 5.2-fold increase, the indirect effects would be 400% of the warming from the methane alone.

The Politicization of Global Warming

The question of global warming has both a political and a scientific side to it. The scientific side addresses the veracity of various claims such as its existence, causes, future projections and its consequences given various actions. The political side asks what policies we should put in place given the information of science and our various values.

There is a tendency to want to decouple these two things from each other. However, the reality is in the public consciousness there already exists a significant coupling between the science and the politics; moreover, one side is doing a much better job of this than the other.

Over the course of the last decade, public belief in the veracity of global warming as a scientific fact has been politicized in the sense that a majority of the left thinks it is true and a majority of the right thinks it is false. It is a self identifying characteristic of many partisans. Public belief in scientific claims is not just a function of the scientific certainty but also is highly dependent on the politics.

Unfortunately, the politicization of this issue is not even. The left has, to its detriment, largely treated global warming as an entirely separate issue from other political issues - stemming I think from the reasonable idea to treat science as decoupled from politics and that the veracity of global warming stands or falls on scientific grounds, not political ones.  Scientific facts are largely inaccessible for independent public scrutiny; for most people the best that can be done is to establish consensus among scientists. As such it is the political motivations that really grip people.

The right, on the other hand has very closely tied the two concepts together. Belief in global warming is seen as an attempt for big government takeovers, regulations, taxes, strong international agreements and the like all of which are ideologically opposed.  It is an inherently political debate going on and explains why belief in global warming is such a partisan thing with a large number of people wildly in defiance of the scientific consensus. While the left argues we should fight for global warming because of the effects of global warming, the right argues we shouldn't fight - and indeed that the phenomenon doesn't exist or is overstated - because of political reasons such as variations on the themes of big government and restricting freedom.

The right is actually entirely justified in their fears. All of these political factors mentioned quickly follow from accepting the need to act. Global warming represents an enormous externality on market transactions and it is very natural that some form of fiat policies to compensate for this externality ought to occur. Of course, the precise nature of the solutions requires extensive debate and consideration, and certainly government solutions are not everything but it seems very likely they ought to play at least some role. There is thus a certain cognitive dissonance that occurs when one ideologically rejects policies the involve government intervention but believe in a theory that seems to so clearly require government intervention to mitigate. This pressure at least partially explains the partisan disbelief in global warming.

I believe there should be a shift in the framing of the global debate that widens it from being merely a scientific question but instead to a wider political question that aims to combat sole of the larger and more general issues at the same time. Right or wrongly, the global warming debate has become politicized and one side is asymmetrically winning this framing. This isn't to say global warming is never a political issue for the left, it is. Dion's Liberals in Canada, for example, ran - and lost - on an election platform centered around global warming. However, it didn't try to tie in global warming to any other issue and this was a political debate about global warming in and of itself.

Take the issue of campaign finance reform. This is a much broader issue often supported by progressives that tries to limit the relative amount of corporate money which infiltrates the political scene. It affects issues from the military/industrial complex to the financial system to corporate pollution. In particular, it extensively affects the politics of global warming because of the pervasive existence of campaign dollars and lobbying money from fossil fuel lobbies and other corporations that are heavy on pollution. Along with the ideological cognitive dissonance mentioned above, the denial of global warming as requiring political action or even as a scientific fact is at least partly explained by the existence of this powerful lobby. The left can thus broaden the political debate about global warming to include these kind of issues, advocating for systems that reduce the relative monetary dependence on corporate influence so as to help not just global warming issues but all sorts of other issues. It makes it possible for there to be considerable alliances between different advocacy groups and to present a larger, more all encompassing political platform that self reinforces itself.

I have previously talked about how global warming should be reframed in tandem with issues of peak oil and cheap energy depletion. This is an example of expanding the debate to a slightly larger political context. One can advocate for global warming as part of an economic policy that acknowledges that green energy is going to be an area of future economic growth and that it makes good economic sense with a high return on investment to invest in green energy. Intergenerational justice and various equality movements are all typically left wing strengths that can be tapped into on this issue as well.

In short, there are a plethora of different way one can integrate global warming as part of a larger political platform and not as simple a single, isolated plank of a political platform. The right has been quite successful at doing this by maintaining - sometimes with complete strawmen - how belief in global warming ties into various ideological positions that are supported by the right. This kind of reframing can and should occur if one aims to win the debate for the public opinion of global warming.

Original Post From : http://progressiveproselytizing.blogspot.com

The Story Of Our Beloved Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets. It is sometimes referred to as the world, the Blue Planet,^[21] or by its Latin name, Terra.^{[note 6]}
Earth formed approximately 4.54 billion years ago, and life appeared on its surface within one billion years.^[22] Earth's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer, which together with Earth's magnetic field blocked harmful solar radiation, and permitted formerly ocean-confined life to move safely to land.^[23] The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist. Estimates on how much longer the planet will to be able to continue to support life range from 500 million years (myr), to as long as 2.3 billion years (byr).

Earth's crust is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that is the solid ice of the Antarctic ice sheet and the sea ice that is the Polar ice packs. The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.
Earth interacts with other objects in space, especially the Sun and the Moon. During one orbit around the sun, the Earth rotates about its own axis 366.26 times, creating 365.26 solar days, or one sidereal year.^{[note 7]} The Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).^[27] The Moon is Earth's only natural satellite. It began orbiting the Earth about 4.53 billion years ago (bya). The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation.
The planet is home to millions of species, including humans.^[28] Both the mineral resources of the planet and the products of the biosphere contribute resources that are used to support a global human population.^[29] These inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including its personification as a planetary deity, its shape as flat, its position as the center of the universe, and in the modern Gaia Principle, as a single, self-regulating organism in its own right.

Name and etymology

The modern English noun earth developed from Middle English erthe (recorded in 1137), itself from Old English eorthe (dating from before 725), ultimately deriving from Proto-Germanic *erthō. Earth has cognates in all other Germanic languages, including Dutch aarde, German Erde, and Swedish, Norwegian, and Danish jord.^[30] The Earth is personified as a goddess in Germanic paganism (appearing as Jörð in Norse mythology, mother of the god Thor).^[31]
In general English usage, the name earth can be capitalized or spelled in lowercase interchangeably, either when used absolutely or prefixed with "the" (i.e. "Earth", "the Earth", "earth", or "the earth"). Many deliberately spell the name of the planet with a capital, both as "Earth" or "the Earth". This is to distinguish it as a proper noun, distinct from the senses of the term as a count noun or verb (e.g. referring to soil, the ground, earthing in the electrical sense, etc.). Oxford spelling recognizes the lowercase form as the most common, with the capitalized form as a variant of it. Another convention that is very common is to spell the name with a capital when occurring absolutely (e.g. Earth's atmosphere) and lowercase when preceded by "the" (e.g. the atmosphere of the earth). The term almost exclusively exists in lowercase when appearing in common phrases, even without "the" preceding it (e.g. "It does not cost the earth.", "What on earth are you doing?").^[32]

Chronology

Formation

The earliest material found in the Solar System is dated to 4.5666-4.5678 bya;^[33] therefore, it is inferred that the Earth, must have formed around this time. By 4.50-4.58 bya^[22] the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–20 myr.^[34] The Moon formed shortly thereafter, about 4.53 bya.

The Moon's formation remains a mystery. The working hypothesis is that it formed by accretion from material loosed from the Earth after a Mars-sized object, dubbed Theia, had a giant impact with Earth,^[36] but the model is not self-consistent. In this scenario the mass of Theia is 10% of the Earth's mass,^[37] it impacts with the Earth in a glancing blow,^[38] and some of its mass merges with the Earth. Between approximately 3.8 and 4.1 bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to the Earth.
Earth's atmosphere and oceans formed by volcanic activity and outgassing that included water vapor. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, proto-planets, and comets.^[39] In this model, atmospheric "greenhouse gases" kept the oceans from freezing while the newly forming Sun was only at 70% luminosity.^[40] By 3.5 bya, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.^[41]
A crust formed when the molten outer layer of the planet Earth cooled to form a solid as the accumulated water vapor began to act in the atmosphere. The two models^[42] that explain land mass propose either a steady growth to the present-day forms^[43] or, more likely, a rapid growth^[44] early in Earth history^[45] followed by a long-term steady continental area.^[46][47][48] Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from the earth's interior. On time scales lasting hundreds of millions of years, the supercontinents have formed and broken up three times. Roughly 750 mya (million years ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which also broke apart 180 mya.

Evolution of life

Main article: Evolutionary history of life

For more details on the current eon, see Geological history of Earth.

Highly energetic chemistry is believed to have produced a self-replicating molecule around 4 bya and half a billion years later the last common ancestor of all life existed.^[50] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed a layer of ozone (a form of molecular oxygen [O₃]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.^[51] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.^[52]
Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.
Following the Cambrian explosion, about 535 mya, there have been five major mass extinctions.^[54] The most recent such event was 65 mya, when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews. Over the past 65 myr, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.^[55] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,^[56] affecting both the nature and quantity of other life forms.
The present pattern of ice ages began about 40 mya and then intensified during the Pleistocene about 3 mya. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last continental glaciation ended 10,000 years ago.

Future

The life cycle of the Sun

The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 byr and by 40% over the next 3.5 byr.^[58] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the loss of the planet's oceans.^[59]
The Earth's increasing surface temperature will accelerate the inorganic CO₂ cycle, reducing its concentration to levels lethally low for plants (10 ppm) for C4 photosynthesis) in approximately 500-900 myr.^[24] The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.^[60] After another billion years all surface water will have disappeared^[25] and the mean global temperature will reach 70 °C^[60] (158 °F). The Earth is expected to be effectively habitable for about another 500 myr from that point,^[24] although this may be extended up to 2.3 byr if the nitrogen is removed from the atmosphere.^[26] Even if the Sun were eternal and stable, 27% of the water in the modern oceans will descend to the mantle in one billion years due to reduced steam venting from mid-ocean ridges.^[61]
The Sun, as part of its evolution, will become a red giant in about 5 byr. Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km).^[58][62] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius. The planet was therefore initially expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life would have been destroyed by the Sun's increased luminosity (peaking at about 5000 times its present level). A 2008 simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be vaporized. After that, the Sun's core will collapse into a white dwarf, as its outer layers are ejected into space as a planetary nebula. The matter that once made up the Earth will be released into interstellar space, where it will one day become incorporated into a new generation of planets and other celestial bodies.

Cultural viewpoint

The first photograph ever taken by astronauts of an "Earthrise", from Apollo 8

The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle.
Unlike the rest of the planets in the Solar System, humankind did not begin to view the Earth as a moving object in orbit around the Sun until the 16th century. Earth has often been personified as a deity, in particular a goddess. In many cultures a mother goddess is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism or Islam, assert that their interpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life. Such assertions are opposed by the scientific community and by other religious groups.^[A prominent example is the creation-evolution controversy.
In the past there were varying levels of belief in a flat Earth, but this was displaced by the concept of a spherical Earth due to observation and circumnavigation. The human perspective regarding the Earth has changed following the advent of spaceflight, and the biosphere is now widely viewed from a globally integrated perspective. This is reflected in a growing environmental movement that is concerned about humankind's effects on the planet.

The Story Of Venus

Venus is the second planet from the Sun, orbiting it every 224.7 Earth days.^[11] The planet is named after the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows.^[13] Because Venus is an inferior planet from Earth, it never appears to venture far from the Sun: its elongation reaches a maximum of 47.8°. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, for which reason it has been referred to by ancient cultures as the Morning Star or Evening Star.

Venus is classified as a terrestrial planet and is sometimes called Earth's "sister planet" owing to their similar size, gravity, and bulk composition (Venus is both the closest planet to Earth and the planet closest in size to Earth). However, it has been shown to be very different from Earth in other respects. Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. It has the densest atmosphere of the four terrestrial planets, consisting mostly of carbon dioxide. The atmospheric pressure at the planet's surface is 92 times that of Earth's. With a mean surface temperature of 735 K (462 °C; 863 °F), Venus is by far the hottest planet in the Solar System. It has no carbon cycle to lock carbon back into rocks and surface features, nor does it seem to have any organic life to absorb it in biomass. Venus is believed to have previously possessed oceans, but these vaporized as the temperature rose due to the runaway greenhouse effect. The water has most probably photodissociated, and, because of the lack of a planetary magnetic field, the free hydrogen has been swept into interplanetary space by the solar wind. Venus's surface is a dry desertscape interspersed with slab-like rocks and periodically refreshed by volcanism.

Physical characteristics

Venus is one of the four solar terrestrial planets, meaning that, like the Earth, it is a rocky body. In size and mass, it is similar to the Earth, and is often described as Earth's "sister" or "twin".^[17] The diameter of Venus is 12,092 km (only 650 km less than the Earth's) and its mass is 81.5% of the Earth's. Conditions on the Venusian surface differ radically from those on Earth, owing to its dense carbon dioxide atmosphere. The mass of the atmosphere of Venus is 96.5% carbon dioxide, with most of the remaining 3.5% being nitrogen.^[18]

Geography

The Venusian surface was a subject of speculation until some of its secrets were revealed by planetary science in the 20th century. It was finally mapped in detail by Project Magellan in 1990–91. The ground shows evidence of extensive volcanism, and the sulfur in the atmosphere may indicate there have been some recent eruptions.^[19][20]
About 80% of the Venusian surface is covered by smooth, volcanic plains, consisting of 70% plains with wrinkle ridges and 10% smooth or lobate plains.^[21] Two highland "continents" make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra, after Ishtar, the Babylonian goddess of love, and is about the size of Australia. Maxwell Montes, the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11 km above the Venusian average surface elevation. The southern continent is called Aphrodite Terra, after the Greek goddess of love, and is the larger of the two highland regions at roughly the size of South America. A network of fractures and faults covers much of this area.^[22]
The absence of evidence of lava flow accompanying any of the visible caldera remains an enigma. The planet has few impact craters, demonstrating the surface is relatively young, approximately 300–600 million years old.^[23][24] In addition to the impact craters, mountains, and valleys commonly found on rocky planets, Venus has a number of unique surface features. Among these are flat-topped volcanic features called "farra", which look somewhat like pancakes and range in size from 20–50 km across, and 100–1,000 m high; radial, star-like fracture systems called "novae"; features with both radial and concentric fractures resembling spider webs, known as "arachnoids"; and "coronae", circular rings of fractures sometimes surrounded by a depression. These features are volcanic in origin.^[25]
Most Venusian surface features are named after historical and mythological women.^[26] Exceptions are Maxwell Montes, named after James Clerk Maxwell, and highland regions Alpha Regio, Beta Regio and Ovda Regio. The former three features were named before the current system was adopted by the International Astronomical Union, the body that oversees planetary nomenclature.^[27]
The longitudes of physical features on Venus are expressed relative to its prime meridian. The original prime meridian passed through the radar-bright spot at the center of the oval feature Eve, located south of Alpha Regio.^[28] After the Venera missions were completed, the prime meridian was redefined to pass through the central peak in the crater Ariadne.^[29][30]

Surface geology

Global radar view of the surface from Magellan radar imaging between 1990–1994

Main article: Geology of Venus

Much of the Venusian surface appears to have been shaped by volcanic activity. Venus has several times as many volcanoes as Earth, and it possesses some 167 large volcanoes that are over 100 km across. The only volcanic complex of this size on Earth is the Big Island of Hawaii.^[25] This is not because Venus is more volcanically active than Earth, but because its crust is older. Earth's oceanic crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years,^[31] while the Venusian surface is estimated to be 300–600 million years old.^[23][25]
Several lines of evidence point to ongoing volcanic activity on Venus. During the Soviet Venera program, the Venera 11 and Venera 12 probes detected a constant stream of lightning, and Venera 12 recorded a powerful clap of thunder soon after it landed. The European Space Agency's Venus Express recorded abundant lightning in the high atmosphere.^[32] While rainfall drives thunderstorms on Earth, there is no rainfall on the surface of Venus (though it does rain sulfuric acid, in the upper atmosphere, which evaporates around 25 km above the surface). One possibility is ash from a volcanic eruption was generating the lightning. Another piece of evidence comes from measurements of sulfur dioxide concentrations in the atmosphere, which were found to drop by a factor of 10 between 1978 and 1986. This may imply the levels had earlier been boosted by a large volcanic eruption.^[33]

Impact craters on the surface of Venus (image reconstructed from radar data)

Almost a thousand impact craters on Venus are evenly distributed across its surface. On other cratered bodies, such as the Earth and the Moon, craters show a range of states of degradation. On the Moon, degradation is caused by subsequent impacts, while on Earth, it is caused by wind and rain erosion. On Venus, about 85% of the craters are in pristine condition. The number of craters, together with their well-preserved condition, indicates the planet underwent a global resurfacing event about 300–600 million years ago,^[23][24] followed by a decay in volcanism.^[34] Whereas Earth's crust is in continuous motion, Venus is thought to be unable to sustain such a process. Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which mantle temperatures rise until they reach a critical level that weakens the crust. Then, over a period of about 100 million years, subduction occurs on an enormous scale, completely recycling the crust.^[25]
Venusian craters range from 3 km to 280 km in diameter. No craters are smaller than 3 km, because of the effects of the dense atmosphere on incoming objects. Objects with less than a certain kinetic energy are slowed down so much by the atmosphere, they do not create an impact crater.^[35] Incoming projectiles less than 50 meters in diameter will fragment and burn up in the atmosphere before reaching the ground.^[36]

Internal structure

Without seismic data or knowledge of its moment of inertia, little direct information is available about the internal structure and geochemistry of Venus.^[37] The similarity in size and density between Venus and Earth suggests they share a similar internal structure: a core, mantle, and crust. Like that of Earth, the Venusian core is at least partially liquid because the two planets have been cooling at about the same rate.^[38] The slightly smaller size of Venus suggests pressures are significantly lower in its deep interior than Earth. The principal difference between the two planets is the lack of evidence for plate tectonics on Venus, possibly because its crust is too strong to subduct without water to make it less viscous. This results in reduced heat loss from the planet, preventing it from cooling and providing a likely explanation for its lack of an internally generated magnetic field.^[39] Instead, Venus may lose its internal heat in periodic major resurfacing events.

Magnetic field and core

Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars in true-color.

In 1967, Venera-4 found the Venusian magnetic field is much weaker than that of Earth. This magnetic field is induced by an interaction between the ionosphere and the solar wind,^[60][61] rather than by an internal dynamo in the core like the one inside the Earth. Venus's small induced magnetosphere provides negligible protection to the atmosphere against cosmic radiation. This radiation may result in cloud-to-cloud lightning discharges.^[62]
The lack of an intrinsic magnetic field at Venus was surprising given it is similar to Earth in size, and was expected also to contain a dynamo at its core. A dynamo requires three things: A conducting liquid, rotation, and convection. The core is thought to be electrically conductive and, while its rotation is often thought to be too slow, simulations show it is adequate to produce a dynamo.^[63][64] This implies the dynamo is missing because of a lack of convection in the Venusian core. On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much hotter than the top. On Venus, a global resurfacing event may have shut down plate tectonics and led to a reduced heat flux through the crust. This caused the mantle temperature to increase, thereby reducing the heat flux out of the core. As a result, no internal geodynamo is available to drive a magnetic field. Instead, the heat energy from the core is being used to reheat the crust.^[65]
One possibility is Venus has no solid inner core,^[66] or its core is not currently cooling, so the entire liquid part of the core is at approximately the same temperature. Another possibility is its core has already completely solidified. The state of the core is highly dependent on the concentration of sulfur, which is unknown at present.^[65]
The weak magnetosphere around Venus means the solar wind is interacting directly with the outer atmosphere of the planet. Here, ions of hydrogen and oxygen are being created by the dissociation of neutral molecules from ultraviolet radiation. The solar wind then supplies energy that gives some of these ions sufficient velocity to escape the planet's gravity field. This erosion process results in a steady loss of low-mass hydrogen, helium, and oxygen ions, while higher-mass molecules, such as carbon dioxide, are more likely to be retained. Atmospheric erosion by the solar wind most probably led to the loss of most of the planet's water during the first billion years after it formed. The erosion has increased the ratio of higher-mass deuterium to lower-mass hydrogen in the upper atmosphere by a multiple of 150 times the ratio in the lower atmosphere.^[67]

Orbit and rotation

Venus orbits the Sun at an average distance of about 108 million kilometers (about 0.7 AU) and completes an orbit every 224.65 days. Venus is the second planet from the Sun and it revolves round the Sun approximately 1.6 times (yellow trail) in Earth's 365 days (blue trail)

Venus orbits the Sun at an average distance of about 0.72 AU (108,000,000 km; 67,000,000 mi), and completes an orbit every 224.65 days. Although all planetary orbits are elliptical, Venus's orbit is the closest to circular, with an eccentricity of less than 0.01.^[2] When Venus lies between the Earth and the Sun, a position known as inferior conjunction, it makes the closest approach to Earth of any planet at an average distance of 41 million km.^[2] The planet reaches inferior conjunction every 584 days, on average.^[2] Owing to the decreasing eccentricity of Earth's orbit, the minimum distances will become greater over tens of thousands of years. From the year 1 to 5383, there are 526 approaches less than 40 million km; then there are none for about 60,158 years.^[68] During periods of greater eccentricity, Venus can come as close as 38.2 million km.^[2]
All the planets of the Solar System orbit the Sun in a counter-clockwise direction as viewed from above the Sun's north pole. Most planets also rotate on their axis in a counter-clockwise direction, but Venus rotates clockwise (called "retrograde" rotation) once every 243 Earth days—the slowest rotation period of any planet. The equator of the Venusian surface rotates at 6.5 km/h, while on Earth rotation speed at the equator is about 1,670 km/h.^[69] Venus's rotation has slowed down by 6.5 minutes per day since the Magellan spacecraft visited it 16 years ago.^[70] A Venusian sidereal day thus lasts longer than a Venusian year (243 versus 224.7 Earth days). Because of the retrograde rotation, the length of a solar day on Venus is significantly shorter than the sidereal day, at 116.75 Earth days (making the Venusian solar day shorter than Mercury's 176 Earth days); one Venusian year is about 1.92 Venusian (solar) days long.^[12] To an observer on the surface of Venus, the Sun would rise in the west and set in the east.^[12]
Venus may have formed from the solar nebula with a different rotation period and obliquity, reaching to its current state because of chaotic spin changes caused by planetary perturbations and tidal effects on its dense atmosphere, a change that would have occurred over the course of billions of years. The rotation period of Venus may represent an equilibrium state between tidal locking to the Sun's gravitation, which tends to slow rotation, and an atmospheric tide created by solar heating of the thick Venusian atmosphere.^[71][72] A curious aspect of the Venusian orbit and rotation periods is the 584-day average interval between successive close approaches to the Earth is almost exactly equal to five Venusian solar days.^[73] However, the hypothesis of a spin–orbit resonance with Earth has been discounted.^[74]
Venus has no natural satellite,^[75] though the asteroid 2002 VE₆₈ presently maintains a quasi-orbital relationship with it.^[76] In the 17th century, Giovanni Cassini reported a moon orbiting Venus, which was named Neith and numerous sightings were reported over the following 200 years, but most were determined to be stars in the vicinity. Alex Alemi's and David Stevenson's 2006 study of models of the early Solar System at the California Institute of Technology shows Venus likely had at least one moon created by a huge impact event billions of years ago.^[77][78] About 10 million years later, according to the study, another impact reversed the planet's spin direction and caused the Venusian moon gradually to spiral inward^[79] until it collided and merged with Venus. If later impacts created moons, these also were absorbed in the same way. An alternative explanation for the lack of satellites is the effect of strong solar tides, which can destabilize large satellites orbiting the inner terrestrial planets.^[75]

Observation

Venus is always brighter than the brightest stars outside our solar system, as can be seen here over the Pacific Ocean

Phases of Venus and evolution of its apparent diameter

Venus is always brighter than any star (apart from the Sun). The greatest luminosity, apparent magnitude −4.9,^[9] occurs during crescent phase when it is near the Earth. Venus fades to about magnitude −3 when it is backlit by the Sun.^[8] The planet is bright enough to be seen in a mid-day clear sky,^[80] and the planet can be easy to see when the Sun is low on the horizon. As an inferior planet, it always lies within about 47° of the Sun.^[10]
Venus "overtakes" the Earth every 584 days as it orbits the Sun.^[2] As it does so, it changes from the "Evening Star", visible after sunset, to the "Morning Star", visible before sunrise. While Mercury, the other inferior planet, reaches a maximum elongation of only 28° and is often difficult to discern in twilight, Venus is hard to miss when it is at its brightest. Its greater maximum elongation means it is visible in dark skies long after sunset. As the brightest point-like object in the sky, Venus is a commonly misreported "unidentified flying object". U.S. President Jimmy Carter reported having seen a UFO in 1969, which later analysis suggested was probably the planet. Countless other people have mistaken Venus for something more exotic.^[81]
As it moves around its orbit, Venus displays phases in a telescopic view like those of the Moon: In the phases of Venus, the planet presents a small "full" image when it is on the opposite side of the Sun. It shows a larger "quarter phase" when it is at its maximum elongations from the Sun, and is at its brightest in the night sky, and presents a much larger "thin crescent" in telescopic views as it comes around to the near side between the Earth and the Sun. Venus is at its largest and presents its "new phase" when it is between the Earth and the Sun. Its atmosphere can be seen in a telescope by the halo of light refracted around the planet.