The blue graph shows the apparent percentage (not the absolute number) of marine animalgenera
becoming extinct during any given time interval. It does not represent
all marine species, just those that are readily fossilized. The labels
of the traditional "Big Five" extinction events and the more recently
recognised Capitanian mass extinction event
are clickable links. The two extinction events occurring in the
Cambrian (far left) are very large in percentage magnitude, but small in
absolute numbers of known taxa due to the relative scarcity of
fossil-producing life at that time.
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the background extinction rate and the rate of speciation.
Estimates of the number of major mass extinctions in the last 540
million years range from as few as five to more than twenty. These
differences stem from disagreement as to what constitutes a "major"
extinction event, and the data chosen to measure past diversity.
The "Big Five" mass extinctions
In a landmark paper published in 1982, Jack Sepkoski and David M. Raup identified five particular geological intervals with excessive diversity loss. They were originally identified as outliers on a general trend of decreasing extinction rates during the Phanerozoic,
but as more stringent statistical tests have been applied to the
accumulating data, it has been established that multicellular animal
life has experienced at least five major and many minor mass
extinctions.
The "Big Five" cannot be so clearly defined, but rather appear to
represent the largest (or some of the largest) of a relatively smooth
continuum of extinction events. An earlier (first) event at the end of the Ediacaran is speculated.
Ordovician–Silurian extinction events (End Ordovician or O–S): 445–444 Ma, just prior to and at the Ordovician–Silurian transition. Two events occurred that killed off 27% of all families, 57% of all genera and 85% of all species.
Together they are ranked by many scientists as the second-largest of
the five major extinctions in Earth's history in terms of percentage of genera that became extinct. In May 2020, studies suggested that the causes of the mass extinction were global warming, related to volcanism, and anoxia, and not, as considered earlier, cooling and glaciation. However, this is at odds with numerous previous studies, which have indicated global cooling as the primary driver.
Most recently, the deposition of volcanic ash has been suggested to be
the trigger for reductions in atmospheric carbon dioxide leading to the
glaciation and anoxia observed in the geological record.
Late Devonian extinctions: 372–359 Ma, occupying much of the Late Devonian up to the Devonian–Carboniferous
transition. The Late Devonian was an interval of high diversity loss,
concentrated into two extinction events. The largest extinction was the Kellwasser Event (Frasnian-Famennian,
or F-F, 372 Ma), an extinction event at the end of the Frasnian, about
midway through the Late Devonian. This extinction annihilated coral reefs and numerous tropical benthic (seabed-living) animals such as jawless fish, brachiopods, and trilobites. Another major extinction was the Hangenberg Event
(Devonian-Carboniferous, or D-C, 359 Ma), which brought an end to the
Devonian as a whole. This extinction wiped out the armored placoderm fish and nearly led to the extinction of the newly evolved ammonoids. These two closely-spaced extinction events collectively eliminated about 19% of all families, 50% of all genera and at least 70% of all species. Sepkoski and Raup (1982) did not initially consider the Late Devonian extinction interval (Givetian, Frasnian, and Famennian stages) to be statistically significant. Regardless, later studies have affirmed the strong ecological impacts of the Kellwasser and Hangenberg Events.
Trilobites were highly successful marine animals until the Permian–Triassic extinction event wiped them all out.Permian–Triassic extinction event (End Permian): 252 Ma, at the Permian–Triassic transition. Earth's largest extinction killed 53% of marine families, 84% of marine genera, about 81% of all marine species and an estimated 70% of terrestrial vertebrate species. This is also the largest known extinction event for insects. The highly successful marine arthropod, the trilobite, became extinct. The evidence regarding plants is less clear, but new taxa became dominant after the extinction. The "Great Dying" had enormous evolutionary significance: On land, it ended the primacy of early synapsids. The recovery of vertebrates took 30 million years, but the vacant niches created the opportunity for archosaurs to become ascendant. In the seas, the percentage of animals that were sessile
(unable to move about) dropped from 67% to 50%. The whole late Permian
was a difficult time, at least for marine life, even before the P–T
boundary extinction. More recent research has indicated that the End-Capitanian extinction event
that preceded the "Great Dying" likely constitutes a separate event
from the P–T extinction; if so, it would be larger than some of the "Big
Five" extinction events, and perhaps merit a separate place in this
list immediately before this one.
Triassic–Jurassic extinction event (End Triassic): 201.3 Ma, at the Triassic–Jurassic
transition. About 23% of all families, 48% of all genera (20% of marine
families and 55% of marine genera) and 70% to 75% of all species became
extinct. Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition. Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments. The Temnospondyl lineage of large amphibians also survived until the Cretaceous in Australia (e.g., Koolasuchus).
Badlands near Drumheller, Alberta, where erosion has exposed the Cretaceous–Paleogene boundary.Cretaceous–Paleogene extinction event (End Cretaceous, K–Pg extinction, or formerly K–T extinction): 66 Ma, at the Cretaceous (Maastrichtian) – Paleogene (Danian) transition.
The event was formerly called the Cretaceous-Tertiary or K–T extinction
or K–T boundary; it is now officially named the Cretaceous–Paleogene
(or K–Pg) extinction event. About 17% of all families, 50% of all genera and 75% of all species became extinct. In the seas all the ammonites, plesiosaurs and mosasaurs disappeared and the percentage of sessile animals was reduced to about 33%. All non-avian dinosaurs became extinct during that time. The boundary event was severe with a significant amount of variability in the rate of extinction between and among different clades. Mammals and birds, the former descended from the synapsids and the latter from theropod dinosaurs, emerged as dominant terrestrial animals.
Despite the popularization of these five events, there is no definite
line separating them from other extinction events; using different
methods of calculating an extinction's impact can lead to other events
featuring in the top five.
Older fossil records are more difficult to interpret. This is because:
Older fossils are harder to find as they are usually buried at a considerable depth.
Dating of older fossils is more difficult.
Productive fossil beds are researched more than unproductive ones, therefore leaving certain periods unresearched.
Prehistoric environmental events can disturb the deposition process.
The preservation of fossils varies on land, but marine fossils tend
to be better preserved than their sought after land-based counterparts.
It has been suggested that the apparent variations in marine
biodiversity may actually be an artifact, with abundance estimates
directly related to quantity of rock available for sampling from
different time periods. However, statistical analysis shows that this can only account for 50% of the observed pattern, and other evidence such as fungal spikes (geologically rapid increase in fungal
abundance) provides reassurance that most widely accepted extinction
events are real. A quantification of the rock exposure of Western Europe
indicates that many of the minor events for which a biological
explanation has been sought are most readily explained by sampling bias.
Sixth mass extinction
Research
completed after the seminal 1982 paper (Sepkoski and Raup) has
concluded that a sixth mass extinction event is ongoing due to human
activities:
Holocene extinction: currently ongoing. Extinctions have occurred at over 1000 times the background extinction rate since 1900, and the rate is increasing. The mass extinction is a result of human activity (an ecocide) driven by population growth and overconsumption of the earth's natural resources. The 2019 global biodiversity assessment by IPBES
asserts that out of an estimated 8 million species, 1 million plant and
animal species are currently threatened with extinction. In late 2021, WWF
Germany suggested that over a million species could go extinct within a
decade in the "largest mass extinction event since the end of the
dinosaur age." A 2023 study published in PNAS concluded that at least 73 genera
of animals have gone extinct since 1500. If humans had never existed,
it would have taken 18,000 years for the same genera to have disappeared
naturally, the report states.
Extinction events can be tracked by several methods, including
geological change, ecological impact, extinction vs. origination (speciation) rates, and most commonly diversity loss among taxonomic units. Most early papers used families as the unit of taxonomy, based on compendiums of marine animal families by Sepkoski (1982, 1992). Later papers by Sepkoski and other authors switched to genera, which are more precise than families and less prone to taxonomic bias or incomplete sampling relative to species.
These are several major papers estimating loss or ecological impact
from fifteen commonly-discussed extinction events. Different methods
used by these papers are described in the following section. The "Big
Five" mass extinctions are bolded.
Extinction proportions (diversity loss) of marine genera or ecological impact in estimates of mass extinction severity
Luis (left) and Walter Alvarez (right) at the K-Pg boundary in Gubbio, Italy
in 1981. This team discovered geological evidence for an asteroid
impact causing the K-Pg extinction, spurring a wave of public and
scientific interest in mass extinctions and their causes
For much of the 20th century, the study of mass extinctions was
hampered by insufficient data. Mass extinctions, though acknowledged,
were considered mysterious exceptions to the prevailing gradualistic
view of prehistory, where slow evolutionary trends define faunal
changes. The first breakthrough was published in 1980 by a team led by Luis Alvarez, who discovered trace metal evidence for an asteroid impact at the end of the Cretaceous period. The Alvarez hypothesis for the end-Cretaceous extinction gave mass extinctions, and catastrophic explanations, newfound popular and scientific attention.
Changes
in diversity among genera and families, according to Sepkoski (1997).
The "Big Five" mass extinctions are labelled with arrows, and taxa are
segregated into Cambrian- (Cm), Paleozoic- (Pz), and Modern- (Md) type
faunas.
Another landmark study came in 1982, when a paper written by David M. Raup and Jack Sepkoski was published in the journal Science. This paper, originating from a compendium of extinct marine animal families developed by Sepkoski,
identified five peaks of marine family extinctions which stand out
among a backdrop of decreasing extinction rates through time. Four of
these peaks were statistically significant: the Ashgillian (end-Ordovician), Late Permian, Norian (end-Triassic), and Maastrichtian (end-Cretaceous). The remaining peak was a broad interval of high extinction smeared over the later half of the Devonian, with its apex in the Frasnian stage.
Through the 1980s, Raup and Sepkoski continued to elaborate and
build upon their extinction and origination data, defining a
high-resolution biodiversity curve (the "Sepkoski curve") and successive evolutionary faunas with their own patterns of diversification and extinction.Though these interpretations formed a strong basis for subsequent
studies of mass extinctions, Raup and Sepkoski also proposed a more
controversial idea in 1984: a 26-million-year periodic pattern to mass
extinctions. Two teams of astronomers linked this to a hypothetical brown dwarf in the distant reaches of the solar system, inventing the "Nemesis hypothesis" which has been strongly disputed by other astronomers.
Around the same time, Sepkoski began to devise a compendium of marine animal genera,
which would allow researchers to explore extinction at a finer
taxonomic resolution. He began to publish preliminary results of this
in-progress study as early as 1986, in a paper which identified 29
extinction intervals of note.
By 1992, he also updated his 1982 family compendium, finding minimal
changes to the diversity curve despite a decade of new data.
In 1996, Sepkoski published another paper which tracked marine genera
extinction (in terms of net diversity loss) by stage, similar to his
previous work on family extinctions. The paper filtered its sample in
three ways: all genera (the entire unfiltered sample size),
multiple-interval genera (only those found in more than one stage), and
"well-preserved" genera (excluding those from groups with poor or
understudied fossil records). Diversity trends in marine animal families
were also revised based on his 1992 update.
Revived interest in mass extinctions led many other authors to
re-evaluate geological events in the context of their effects on life. A 1995 paper by Michael Benton
tracked extinction and origination rates among both marine and
continental (freshwater & terrestrial) families, identifying 22
extinction intervals and no periodic pattern. Overview books by O.H. Wallister (1996) and A. Hallam and P.B. Wignall (1997) summarized the new extinction research of the previous two decades.
One chapter in the former source lists over 60 geological events which
could conceivably be considered global extinctions of varying sizes.
These texts, and other widely circulated publications in the 1990s,
helped to establish the popular image of mass extinctions as a "big
five" alongside many smaller extinctions through prehistory.
New data on genera: Sepkoski's compendium
Major Phanerozoic extinctions tracked via proportional genera extinctions by Bambach (2006)
Though Sepkoski passed away in 1999, his marine genera compendium was
formally published in 2002. This prompted a new wave of studies into
the dynamics of mass extinctions. These papers utilized the compendium to track origination rates (the rate that new species appear or speciate) parallel to extinction rates in the context of geological stages or substages.
A review and re-analysis of Sepkoski's data by Bambach (2006)
identified 18 distinct mass extinction intervals, including 4 large
extinctions in the Cambrian.
These fit Sepkoski's definition of extinction, as short substages with
large diversity loss and overall high extinction rates relative to their
surroundings.
Bambach et al. (2004) considered each of the "Big Five"
extinction intervals to have a different pattern in the relationship
between origination and extinction trends. Moreover, background
extinction rates were broadly variable and could be separated into more
severe and less severe time intervals. Background extinctions were least
severe relative to the origination rate in the middle Ordovician-early
Silurian, late Carboniferous-Permian, and Jurassic-recent. This argues
that the Late Ordovician, end-Permian, and end-Cretaceous extinctions
were statistically significant outliers in biodiversity trends, while
the Late Devonian and end-Triassic extinctions occurred in time periods
which were already stressed by relatively high extinction and low
origination.
Computer models run by Foote (2005) determined that abrupt pulses
of extinction fit the pattern of prehistoric biodiversity much better
than a gradual and continuous background extinction rate with smooth
peaks and troughs. This strongly supports the utility of rapid, frequent
mass extinctions as a major driver of diversity changes. Pulsed
origination events are also supported, though to a lesser degree which
is largely dependent on pulsed extinctions.
Similarly, Stanley (2007) used extinction and origination data to
investigate turnover rates and extinction responses among different
evolutionary faunas and taxonomic groups. In contrast to previous
authors, his diversity simulations show support for an overall
exponential rate of biodiversity growth through the entire Phanerozoic.
Tackling biases in the fossil record
An illustration of the Signor-Lipps effect,
a geological bias which posits that increased fossil sampling would
help to better constrain the exact time when an organism truly goes
extinct.
As data continued to accumulate, some authors began to re-evaluate Sepkoski's sample using methods meant to account for sampling biases. As early as 1982, a paper by Phillip W. Signor and Jere H. Lipps noted that the true sharpness of extinctions was diluted by the incompleteness of the fossil record. This phenomenon, later called the Signor-Lipps effect,
notes that a species' true extinction must occur after its last fossil,
and that origination must occur before its first fossil. Thus, species
which appear to die out just prior to an abrupt extinction event may
instead be a victim of the event, despite an apparent gradual decline
looking at the fossil record alone. A model by Foote (2007) found that
many geological stages had artificially inflated extinction rates due to
Signor-Lipps "backsmearing" from later stages with extinction events.
Estimated extinction rates among genera through time. From Foote (2007), top, and Kocsis et al. (2019), bottom
Other biases include the difficulty in assessing taxa with high
turnover rates or restricted occurrences, which cannot be directly
assessed due to a lack of fine-scale temporal resolution. Many
paleontologists opt to assess diversity trends by randomized sampling
and rarefaction
of fossil abundances rather than raw temporal range data, in order to
account for all of these biases. But that solution is influenced by
biases related to sample size. One major bias in particular is the "Pull of the recent",
the fact that the fossil record (and thus known diversity) generally
improves closer to the modern day. This means that biodiversity and
abundance for older geological periods may be underestimated from raw
data alone.
Alroy (2010) attempted to circumvene sample size-related biases in diversity estimates using a method he called "shareholder
quorum subsampling" (SQS). In this method, fossils are sampled from a
"collection" (such as a time interval) to assess the relative diversity
of that collection. Every time a new species (or other taxon) enters the sample, it brings over all other fossils belonging to that species in the collection (its "share"
of the collection). For example, a skewed collection with half its
fossils from one species will immediately reach a sample share of 50% if
that species is the first to be sampled. This continues, adding up the
sample shares until a "coverage" or "quorum"
is reached, referring to a pre-set desired sum of share percentages. At
that point, the number of species in the sample are counted. A
collection with more species is expected to reach a sample quorum with
more species, thus accurately comparing the relative diversity change
between two collections without relying on the biases inherent to sample
size.
Alroy also elaborated on three-timer algorithms, which are meant
to counteract biases in estimates of extinction and origination rates. A
given taxon is a "three-timer" if it can be found before, after, and
within a given time interval, and a "two-timer" if it overlaps with a
time interval on one side. Counting "three-timers" and "two-timers" on
either end of a time interval, and sampling time intervals in sequence,
can together be combined into equations to predict extinction and
origination with less bias. In subsequent papers, Alroy continued to refine his equations to improve lingering issues with precision and unusual samples.
McGhee et al. (2013), a paper which primarily focused on
ecological effects of mass extinctions, also published new estimates of
extinction severity based on Alroy's methods. Many extinctions were
significantly more impactful under these new estimates, though some were
less prominent.
Stanley (2016) was another paper which attempted to remove two
common errors in previous estimates of extinction severity. The first
error was the unjustified removal of "singletons", genera unique to only
a single time slice. Their removal would mask the influence of groups
with high turnover rates or lineages cut short early in their
diversification. The second error was the difficulty in distinguishing
background extinctions from brief mass extinction events within the same
short time interval. To circumvent this issue, background rates of
diversity change (extinction/origination) were estimated for stages or
substages without mass extinctions, and then assumed to apply to
subsequent stages with mass extinctions. For example, the Santonian and Campanian stages were each used to estimate diversity changes in the Maastrichtian
prior to the K-Pg mass extinction. Subtracting background extinctions
from extinction tallies had the effect of reducing the estimated
severity of the six sampled mass extinction events. This effect was
stronger for mass extinctions which occurred in periods with high rates
of background extinction, like the Devonian.
Uncertainty in the Proterozoic and earlier eons
Because most diversity and biomass on Earth is microbial,
and thus difficult to measure via fossils, extinction events placed
on-record are those that affect the easily observed, biologically
complex component of the biosphere rather than the total diversity and abundance of life. For this reason, well-documented extinction events are confined to the Phanerozoic eon, before which all living organisms were either microbial or at most soft-bodied; the sole exception is the Great Oxidation Event in the Proterozoic. Perhaps due to the absence of a robust microbial fossil record, mass extinctions seem mainly to be a Phanerozoic phenomenon, with apparent extinction rates being low before large complex organisms arose.
The Great Oxidation Event, which occurred around 2.45 billion years ago in the Paleoproterozoic, was probably the first major extinction event. Since the Cambrian explosion,
five further major mass extinctions have significantly exceeded the
background extinction rate. The most recent and best-known, the Cretaceous–Paleogene extinction event,
which occurred approximately 66 Ma (million years ago), was a
large-scale mass extinction of animal and plant species in a
geologically short period of time. In addition to the five major Phanerozoic
mass extinctions, there are numerous minor ones as well, and the
ongoing mass extinction caused by human activity is sometimes called the
sixth extinction.
Mass extinctions have sometimes accelerated the evolution of life on Earth.
When dominance of particular ecological niches passes from one group of
organisms to another, it is rarely because the newly dominant group is
"superior" to the old but usually because an extinction event eliminates
the old, dominant group and makes way for the new one, a process known
as adaptive radiation.
For example, mammaliaformes ("almost mammals") and then mammals existed throughout the reign of the dinosaurs, but could not compete in the large terrestrial vertebrate niches that dinosaurs monopolized. The end-Cretaceous
mass extinction removed the non-avian dinosaurs and made it possible
for mammals to expand into the large terrestrial vertebrate niches. The
dinosaurs themselves had been beneficiaries of a previous mass
extinction, the end-Triassic, which eliminated most of their chief rivals, the crurotarsans.
Another point of view put forward in the Escalation hypothesis
predicts that species in ecological niches with more
organism-to-organism conflict will be less likely to survive
extinctions. This is because the very traits that keep a species
numerous and viable under fairly static conditions become a burden once
population levels fall among competing organisms during the dynamics of
an extinction event.
Furthermore, many groups that survive mass extinctions do not
recover in numbers or diversity, and many of these go into long-term
decline, and these are often referred to as "Dead Clades Walking".
However, clades that survive for a considerable period of time after a
mass extinction, and which were reduced to only a few species, are
likely to have experienced a rebound effect called the "push of the past".
Darwin was firmly of the opinion that biotic interactions, such
as competition for food and space – the 'struggle for existence' – were
of considerably greater importance in promoting evolution and extinction
than changes in the physical environment. He expressed this in The Origin of Species:
"Species are produced and exterminated by slowly acting causes
... and the most import of all causes of organic change is one which is
almost independent of altered ... physical conditions, namely the mutual
relation of organism to organism – the improvement of one organism
entailing the improvement or extermination of others".
Patterns in frequency
Various authors have suggested that extinction events occurred periodically, every 26 to 30 million years, or that diversity fluctuates episodically about every 62 million years. Various ideas, mostly regarding astronomical influences, attempt to explain the supposed pattern, including the presence of a hypothetical companion star to the Sun, oscillations in the galactic plane, or passage through the Milky Way's spiral arms.
However, other authors have concluded that the data on marine mass
extinctions do not fit with the idea that mass extinctions are periodic,
or that ecosystems gradually build up to a point at which a mass
extinction is inevitable. Many of the proposed correlations have been argued to be spurious or lacking statistical significance. Others have argued that there is strong evidence supporting periodicity in a variety of records,
and additional evidence in the form of coincident periodic variation in
nonbiological geochemical variables such as Strontium isotopes,
flood basalts, anoxic events, orogenies, and evaporite deposition. One
explanation for this proposed cycle is carbon storage and release by
oceanic crust, which exchanges carbon between the atmosphere and mantle.
Phanerozoic biodiversity as shown by the fossil record
Mass extinctions are thought to result when a long-term stress is compounded by a short-term shock. Over the course of the Phanerozoic, individual taxa appear to have become less likely to suffer extinction,
which may reflect more robust food webs, as well as fewer
extinction-prone species, and other factors such as continental
distribution.
However, even after accounting for sampling bias, there does appear to
be a gradual decrease in extinction and origination rates during the
Phanerozoic.
This may represent the fact that groups with higher turnover rates are
more likely to become extinct by chance; or it may be an artefact of
taxonomy: families tend to become more speciose, therefore less prone to
extinction, over time; and larger taxonomic groups (by definition) appear earlier in geological time.
It has also been suggested that the oceans have gradually become
more hospitable to life over the last 500 million years, and thus less
vulnerable to mass extinctions,
but susceptibility to extinction at a taxonomic level does not appear to make mass extinctions more or less probable.
Causes
There is
still debate about the causes of all mass extinctions. In general, large
extinctions may result when a biosphere under long-term stress
undergoes a short-term shock.
An underlying mechanism appears to be present in the correlation of
extinction and origination rates to diversity. High diversity leads to a
persistent increase in extinction rate; low diversity to a persistent
increase in origination rate. These presumably ecologically controlled
relationships likely amplify smaller perturbations (asteroid impacts,
etc.) to produce the global effects observed.
Identifying causes of specific mass extinctions
A good theory for a particular mass extinction should:
explain all of the losses, not just focus on a few groups (such as dinosaurs);
explain why particular groups of organisms died out and why others survived;
provide mechanisms that are strong enough to cause a mass extinction but not a total extinction;
be based on events or processes that can be shown to have happened, not just inferred from the extinction.
It may be necessary to consider combinations of causes. For example, the marine aspect of the end-Cretaceous
extinction appears to have been caused by several processes that
partially overlapped in time and may have had different levels of
significance in different parts of the world.
Arens and West (2006) proposed a "press / pulse" model in which
mass extinctions generally require two types of cause: long-term
pressure on the eco-system ("press") and a sudden catastrophe ("pulse")
towards the end of the period of pressure.
Their statistical analysis of marine extinction rates throughout the Phanerozoic
suggested that neither long-term pressure alone nor a catastrophe alone
was sufficient to cause a significant increase in the extinction rate.
Most widely supported explanations
MacLeod (2001)
summarized the relationship between mass extinctions and events that
are most often cited as causes of mass extinctions, using data from
Courtillot, Jaeger & Yang et al. (1996), Hallam (1992) and Grieve & Pesonen (1992):
Flood basalt events (giant volcanic eruptions): 11 occurrences, all associated with significant extinctions
But Wignall (2001) concluded that only five of the major extinctions
coincided with flood basalt eruptions and that the main phase of
extinctions started before the eruptions.
Sea-level falls: 12, of which seven were associated with significant extinctions.
Asteroid impacts:
one large impact is associated with a mass extinction, that is, the
Cretaceous–Paleogene extinction event; there have been many smaller
impacts but they are not associated with significant extinctions, or cannot be dated precisely enough. The impact that created the Siljan Ring either was just before the Late Devonian Extinction or coincided with it.
The most commonly suggested causes of mass extinctions are listed below.
Flood basalt events
The scientific consensus is that the main cause of the End-Permian extinction event was the large amount of carbon dioxide emitted by the volcanic eruptions that created the Siberian Traps, which elevated global temperatures.
Flood basalt events occur as pulses of activity punctuated by dormant
periods. As a result, they are likely to cause the climate to oscillate
between cooling and warming, but with an overall trend towards warming
as the carbon dioxide they emit can stay in the atmosphere for hundreds
of years.
These
are often clearly marked by worldwide sequences of contemporaneous
sediments that show all or part of a transition from sea-bed to tidal
zone to beach to dry land – and where there is no evidence that the
rocks in the relevant areas were raised by geological processes such as orogeny.
Sea-level falls could reduce the continental shelf area (the most
productive part of the oceans) sufficiently to cause a marine mass
extinction, and could disrupt weather patterns enough to cause
extinctions on land. But sea-level falls are very probably the result of
other events, such as sustained global cooling or the sinking of the mid-ocean ridges.
Sea-level falls are associated with most of the mass extinctions, including all of the "Big Five"—End-Ordovician, Late Devonian, End-Permian, End-Triassic, and End-Cretaceous, along with the more recently recognised Capitanian mass extinction of comparable severity to the Big Five.
A 2008 study, published in the journal Nature, established a relationship between the speed of mass extinction events and changes in sea level and sediment.
The study suggests changes in ocean environments related to sea level
exert a driving influence on rates of extinction, and generally
determine the composition of life in the oceans.
Extraterrestrial threats
Impact events
An artist's rendering of an asteroid
a few kilometers across colliding with the Earth. Such an impact can
release the equivalent energy of several million nuclear weapons
detonating simultaneously.
The impact of a sufficiently large asteroid or comet could have caused food chains to collapse both on land and at sea by producing dust and particulate aerosols and thus inhibiting photosynthesis. Impacts on sulfur-rich rocks could have emitted sulfur oxides precipitating as poisonous acid rain, contributing further to the collapse of food chains. Such impacts could also have caused megatsunamis and/or global forest fires.
The Permian-Triassic extinction event has also been hypothesised to have been caused by an asteroid impact that formed the Araguainha crater due to the estimated date of the crater's formation overlapping with the end-Permian extinction event. However, this hypothesis has been widely challenged, with the impact hypothesis being rejected by most researchers.
According to the Shiva hypothesis,
the Earth is subject to increased asteroid impacts about once every
27 million years because of the Sun's passage through the plane of the Milky Way
galaxy, thus causing extinction events at 27 million year intervals.
Some evidence for this hypothesis has emerged in both marine and
non-marine contexts.
Alternatively, the Sun's passage through the higher density spiral arms
of the galaxy could coincide with mass extinction on Earth, perhaps due
to increased impact events.
However, a reanalysis of the effects of the Sun's transit through the
spiral structure based on maps of the spiral structure of the Milky Way
in CO molecular line emission has failed to find a correlation.
A nearby nova, supernova or gamma ray burst
A nearby gamma-ray burst (less than 6000 light-years away) would be powerful enough to destroy the Earth's ozone layer, leaving organisms vulnerable to ultraviolet radiation from the Sun. Gamma ray bursts are fairly rare, occurring only a few times in a given galaxy per million years.
It has been suggested that a gamma ray burst caused the End-Ordovician extinction, while a supernova has been proposed as the cause of the Hangenberg event.
Global cooling
Sustained and significant global cooling could kill many polar and temperate species and force others to migrate towards the equator; reduce the area available for tropical
species; often make the Earth's climate more arid on average, mainly by
locking up more of the planet's water in ice and snow. The glaciation cycles of the current ice age
are believed to have had only a very mild impact on biodiversity, so
the mere existence of a significant cooling is not sufficient on its own
to explain a mass extinction.
It has been suggested that global cooling caused or contributed to the End-Ordovician, Permian–Triassic, Late Devonian
extinctions, and possibly others. Sustained global cooling is
distinguished from the temporary climatic effects of flood basalt events
or impacts.
This would have the opposite effects: expand the area available for tropical species; kill temperate species or force them to migrate towards the poles;
possibly cause severe extinctions of polar species; often make the
Earth's climate wetter on average, mainly by melting ice and snow and
thus increasing the volume of the water cycle. It might also cause anoxic events in the oceans (see below).
Global warming as a cause of mass extinction is supported by several recent studies.
Clathrates are composites in which a lattice of one substance forms a cage around another. Methane clathrates (in which water molecules are the cage) form on continental shelves.
These clathrates are likely to break up rapidly and release the methane
if the temperature rises quickly or the pressure on them drops
quickly—for example in response to sudden global warming or a sudden drop in sea level or even earthquakes. Methane is a much more powerful greenhouse
gas than carbon dioxide, so a methane eruption ("clathrate gun") could
cause rapid global warming or make it much more severe if the eruption
was itself caused by global warming.
The most likely signature of such a methane eruption would be a sudden decrease in the ratio of carbon-13 to carbon-12
in sediments, since methane clathrates are low in carbon-13; but the
change would have to be very large, as other events can also reduce the
percentage of carbon-13.
It has been suggested that "clathrate gun" methane eruptions were involved in the end-Permian extinction ("the Great Dying") and in the Paleocene–Eocene Thermal Maximum, which was associated with one of the smaller mass extinctions.
Anoxic events
Anoxic events
are situations in which the middle and even the upper layers of the
ocean become deficient or totally lacking in oxygen. Their causes are
complex and controversial, but all known instances are associated with
severe and sustained global warming, mostly caused by sustained massive
volcanism.
The bio-availability of essentialtrace elements (in particular selenium)
to potentially lethal lows has been shown to coincide with, and likely
have contributed to, at least three mass extinction events in the
oceans, that is, at the end of the Ordovician, during the Middle and
Late Devonian, and at the end of the Triassic. During periods of low
oxygen concentrations very soluble selenate (Se6+) is converted into much less soluble selenide (Se2-),
elemental Se and organo-selenium complexes. Bio-availability of
selenium during these extinction events dropped to about 1% of the
current oceanic concentration, a level that has been proven lethal to
many extant organisms.
British oceanologist and atmospheric scientist, Andrew Watson, explained that, while the Holocene epoch
exhibits many processes reminiscent of those that have contributed to
past anoxic events, full-scale ocean anoxia would take "thousands of
years to develop".
Oceanic overturn is a disruption of thermo-haline circulation
that lets surface water (which is more saline than deep water because
of evaporation) sink straight down, bringing anoxic deep water to the
surface and therefore killing most of the oxygen-breathing organisms
that inhabit the surface and middle depths. It may occur either at the
beginning or the end of a glaciation,
although an overturn at the start of a glaciation is more dangerous
because the preceding warm period will have created a larger volume of
anoxic water.
Unlike other oceanic catastrophes such as regressions (sea-level
falls) and anoxic events, overturns do not leave easily identified
"signatures" in rocks and are theoretical consequences of researchers'
conclusions about other climatic and marine events.
It has been suggested that oceanic overturn caused or contributed to the late Devonian and Permian–Triassic extinctions.
Geomagnetic reversal
One theory is that periods of increased geomagnetic reversals will weaken Earth's magnetic field long enough to expose the atmosphere to the solar winds, causing oxygen ions to escape the atmosphere in a rate increased by 3–4 orders, resulting in a disastrous decrease in oxygen.
Plate tectonics
Movement
of the continents into some configurations can cause or contribute to
extinctions in several ways: by initiating or ending ice ages;
by changing ocean and wind currents and thus altering climate; by
opening seaways or land bridges that expose previously isolated species
to competition for which they are poorly adapted (for example, the
extinction of most of South America's native ungulates and all of its large metatherians after the creation of a land bridge between North and South America).
Occasionally continental drift creates a super-continent that includes
the vast majority of Earth's land area, which in addition to the effects
listed above is likely to reduce the total area of continental shelf
(the most species-rich part of the ocean) and produce a vast, arid
continental interior that may have extreme seasonal variations.
Another theory is that the creation of the super-continent Pangaea contributed to the End-Permian
mass extinction. Pangaea was almost fully formed at the transition from
mid-Permian to late-Permian, and the "Marine genus diversity" diagram
at the top of this article shows a level of extinction starting at that
time, which might have qualified for inclusion in the "Big Five" if it
were not overshadowed by the "Great Dying" at the end of the Permian.
Other hypotheses
Many species of plants and animals are at high risk of extinction due to the destruction of the Amazon rainforest
Many other hypotheses have been proposed, such as the spread of a new
disease, or simple out-competition following an especially successful
biological innovation. But all have been rejected, usually for one of
the following reasons: they require events or processes for which there
is no evidence; they assume mechanisms that are contrary to the
available evidence; they are based on other theories that have been
rejected or superseded.
Scientists have been concerned that human activities could cause more
plants and animals to become extinct than any point in the past. Along
with human-made changes in climate (see above), some of these
extinctions could be caused by overhunting, overfishing, invasive
species, or habitat loss. A study published in May 2017 in Proceedings of the National Academy of Sciences argued that a "biological annihilation" akin to a sixth mass extinction event is underway as a result of anthropogenic causes, such as over-population and over-consumption.
The study suggested that as much as 50% of the number of animal
individuals that once lived on Earth were already extinct, threatening
the basis for human existence too.
The eventual warming and expanding of the Sun, combined with the
eventual decline of atmospheric carbon dioxide, could actually cause an
even greater mass extinction, having the potential to wipe out even
microbes (in other words, the Earth would be completely sterilized):
rising global temperatures caused by the expanding Sun would gradually
increase the rate of weathering, which would in turn remove more and
more CO2 from the atmosphere. When CO2 levels get
too low (perhaps at 50 ppm), most plant life will die out, although
simpler plants like grasses and mosses can survive much longer, until CO2 levels drop to 10 ppm.
With all photosynthetic organisms gone, atmospheric oxygen can no
longer be replenished, and it is eventually removed by chemical
reactions in the atmosphere, perhaps from volcanic eruptions. Eventually
the loss of oxygen will cause all remaining aerobic life to die out via
asphyxiation, leaving behind only simple anaerobic prokaryotes. When the Sun becomes 10% brighter in about a billion years,
Earth will suffer a moist greenhouse effect resulting in its oceans
boiling away, while the Earth's liquid outer core cools due to the inner
core's expansion and causes the Earth's magnetic field to shut down. In
the absence of a magnetic field, charged particles from the Sun will
deplete the atmosphere and further increase the Earth's temperature to
an average of around 420 K (147 °C, 296 °F) in 2.8 billion years,
causing the last remaining life on Earth to die out. This is the most
extreme instance of a climate-caused extinction event. Since this will
only happen late in the Sun's life, it would represent the final mass
extinction in Earth's history (albeit a very long extinction event).
Effects and recovery
The effects of mass extinction events varied widely. After a major extinction event, usually only weedy species survive due to their ability to live in diverse habitats. Later, species diversify and occupy empty niches. Generally, it takes millions of years for biodiversity to recover after extinction events. In the most severe mass extinctions it may take 15 to 30 million years.
The worst Phanerozoic event, the Permian–Triassic extinction,
devastated life on Earth, killing over 90% of species. Life seemed to
recover quickly after the P-T extinction, but this was mostly in the
form of disaster taxa, such as the hardy Lystrosaurus.
The most recent research indicates that the specialized animals that
formed complex ecosystems, with high biodiversity, complex food webs and
a variety of niches, took much longer to recover. It is thought that
this long recovery was due to successive waves of extinction that
inhibited recovery, as well as prolonged environmental stress that
continued into the Early Triassic. Recent research indicates that
recovery did not begin until the start of the mid-Triassic, four to six
million years after the extinction;
and some writers estimate that the recovery was not complete until 30
million years after the P-T extinction, that is, in the late Triassic.
Subsequent to the P-T extinction, there was an increase in
provincialization, with species occupying smaller ranges – perhaps
removing incumbents from niches and setting the stage for an eventual
rediversification.
The effects of mass extinctions on plants are somewhat harder to
quantify, given the biases inherent in the plant fossil record. Some
mass extinctions (such as the end-Permian) were equally catastrophic for
plants, whereas others, such as the end-Devonian, did not affect the
flora.
A flame detector is a sensor designed to detect and respond to the presence of a flame or fire, allowing flame detection.
Responses to a detected flame depend on the installation, but can
include sounding an alarm, deactivating a fuel line (such as a propane or a natural gas
line), and activating a fire suppression system. When used in
applications such as industrial furnaces, their role is to provide
confirmation that the furnace is working properly; it can be used to
turn off the ignition system though in many cases they take no direct
action beyond notifying the operator or control system. A flame detector
can often respond faster and more accurately than a smoke or heat detector due to the mechanisms it uses to detect the flame.
Optical flame detectors
Flame detector type regions
Ultraviolet detector
Ultraviolet
(UV) detectors work by detecting the UV radiation emitted at the
instant of ignition. While capable of detecting fires and explosions
within 3–4 milliseconds, a time delay of 2–3 seconds is often included
to minimize false alarms which can be triggered by other UV sources such
as lightning, arc welding, radiation, and sunlight. UV detectors typically operate with wavelengths shorter than 300 nm to minimize the effects of natural background radiation. The solar blind UV wavelength band is also easily blinded by oily contaminants.
Near IR array
Near infrared
(IR) array flame detectors (0.7 to 1.1 μm), also known as visual flame
detectors, employ flame recognition technology to confirm fire by
analyzing near IR radiation using a charge-coupled device
(CCD). A near infrared (IR) sensor is especially able to monitor flame
phenomena, without too much hindrance from water and water vapour. Pyroelectric sensors operating at this wavelength can be relatively cheap. Multiple channel or pixel
array sensors monitoring flames in the near IR band are arguably the
most reliable technologies available for detection of fires. Light
emission from a fire forms an image of the flame at a particular
instant. Digital image processing can be utilized to recognize flames through analysis of the video created from the near IR images.
Infrared
Infrared
(IR) or wideband infrared (1.1 μm and higher) flame detectors monitor
the infrared spectral band for specific patterns given off by hot gases.
These are sensed using a specialized fire-fighting thermal imaging camera (TIC), a type of thermographic camera. False alarms can be caused by other hot surfaces and background thermal radiation
in the area. Water on the detector's lens will greatly reduce the
accuracy of the detector, as will exposure to direct sunlight. A special
frequency range is 4.3 to 4.4 μm. This is a resonance frequency of CO2. During burning of a hydrocarbon (for example, wood or fossil fuels such as oil and natural gas) much heat and CO2 is released. The hot CO2
emits much energy at its resonance frequency of 4.3 μm. This causes a
peak in the total radiation emission and can be well detected. Moreover,
the "cold" CO2 in the air is taking care that the sunlight
and other IR radiation is filtered. This makes the sensor in this
frequency "solar blind"; however, sensitivity is reduced by sunlight. By
observing the flicker frequency of a fire (1 to 20 Hz) the detector is
made less sensitive to false alarms caused by heat radiation, for
example caused by hot machinery.
A severe disadvantage is that almost all radiation can be absorbed by water or water vapour;
this is particularly valid for infrared flame detection in the 4.3 to
4.4 μm region. From approx. 3.5 μm and higher the absorption by water or
ice is practically 100%. This makes infrared sensors for use in outdoor
applications very unresponsive to fires. The biggest problem is our
ignorance; some infrared detectors have an (automatic) detector window
self test, but this self test only monitors the occurrence of water or
ice on the detector window.
A salt film is also harmful, because salt absorbs water. However,
water vapour, fog or light rain also makes the sensor almost blind,
without the user knowing. The cause is similar to what a fire fighter
does if he approaches a hot fire: he protects himself by means of a
water vapour screen against the enormous infrared heat radiation. The
presence of water vapor, fog, or light rain will then also "protect" the
monitor causing it to not see the fire. Visible light will, however be
transmitted through the water vapour screen, as can easily been seen by
the fact that a human can still see the flames through the water vapour
screen.
The usual response time of an IR detector is 3–5 seconds.
Infrared thermal cameras
MWIR
infrared (IR) cameras can be used to detect heat and with particular
algorithms can detect hot-spots within a scene as well as flames for
both detection and prevention of fire and risks of fire. These cameras
can be used in complete darkness and operate both inside and outside.
UV/IR
These
detectors are sensitive to both UV and IR wavelengths, and detect flame
by comparing the threshold signal of both ranges. This helps minimize
false alarms.
IR/IR flame detection
Dual
IR (IR/IR) flame detectors compare the threshold signal in two infrared
ranges. Often one sensor looks at the 4.4 micrometer carbon dioxide (CO2), while the other sensor looks at a reference frequency. Sensing the CO2
emission is appropriate for hydrocarbon fuels; for non-carbon based
fuels, e.g., hydrogen, the broadband water bands are sensed.
IR3 flame detection
Multi-infrared
detectors make use of algorithms to suppress the effects of background
radiation (blackbody radiation), again sensitivity is reduced by this
radiation.
Triple-IR flame detectors compare three specific wavelength bands
within the IR spectral region and their ratio to each other. In this
case one sensor looks at the 4.4 micrometer range while the other
sensors look at reference wavelengths both above and below 4.4. This
allows the detector to distinguish between non-flame IR sources and
actual flames which emit hot CO2 in the combustion process.
As a result, both detection range and immunity to false alarms can be
significantly increased. IR3 detectors can detect a 0.1m2 (1 ft2)
gasoline pan fire at up to 65 m (215 ft) in less than 5 seconds. Triple
IRs, like other IR detector types, are susceptible to blinding by a
layer of water on the detector's window.
Most IR detectors are designed to ignore constant background IR
radiation, which is present in all environments. Instead they are
designed to detect suddenly changing or increasing sources of the
radiation. When exposed to changing patterns of non-flame IR radiation,
IR and UV/IR detectors become more prone to false alarms, while IR3
detectors become somewhat less sensitive but are more immune to false
alarms.
3IR+UV flame detection
Multi-Infrared
(Multi-IR/3IR) detectors use algorithms to determine the presence of
fire and tell them apart from background noise known to as black-body radiation,
which in generally reduce the range and accuracy of the detector.
Black-body radiation is constantly present in all environments , but is
given off especially strongly by objects at high temperature. this
makes high temperature environments, or areas where high temperature
material is handled especially challenging for IR only detectors. Thus,
one additional UV-C band sensor is sometimes included in flame detectors
to add another layer of confirmation, as black-body radiation does not
impact UV sensors unless the temperature is extremely high, such as the
plasma glow from an Arc welding machine.
Multi-wavelength detectors vary in sensor configuration. 1 IR+UV,
or UVIR being the most common and low cost. 2 IR + UV being a
compromise between cost and False alarm immunity and 3 IR + UV, which
combines past 3IR technology with the additional layer of identification
from the UV sensor.
Multi-Wavelength or Multi-spectral detectors such as 3IR+UV and
UVIR are an improvement over their IR-only detectors counterparts which
have been known to either false alarm or lose sensitivity and range in
the presence of strong background noise such as direct or reflected
light sources or even sun exposure. IR detectors have often relied on
Infrared bulk energy growth to as their primary determining factor for
fire detection, declaring an alarm when the sensors exceed a given range
and ratio. This approach however is prone to trigger from non-fire
noise. whether from blackbody radiation, high temperature environments,
or simply changes in the ambient lighting. alternatively in another
design approach, IR-only detectors may only alarm given perfect
conditions and clear signal matches, which results in missing the fire
when there is too much noise, such as looking into the sunset.
Modern Flame detectors may also make use of high speed sensors,
which allow the capture of the flickering movement of flame, and monitor
the pattern and ratios of the spectral output for patterns unique to
fire. Higher speed sensors allow for not only faster reaction times, but
also more data per second, increasing the level of confidence in fire
identification, or false alarm rejection.
Visible sensors
A
visible light sensor (for example a camera: 0.4 to 0.7 μm) is able to
present an image, which can be understood by a human being. Furthermore,
complex image processing analysis can be executed by computers, which
can recognize a flame or even smoke. Unfortunately, a camera can be
blinded, like a human, by heavy smoke and by fog. It is also possible to
mix visible light information (monitor) with UV or infrared
information, in order to better discriminate against false alarms or to
improve the detection range. The corona camera
is an example of this equipment. In this equipment the information of a
UV camera mixed with visible image information. It is used for tracing
defects in high voltage equipment and fire detection over high distances.
In some detectors, a sensor for visible radiation (light) is added to the design.
Video
Closed-circuit television or a web camera can be used for visual detection of (wavelengths between 0.4 and 0.7 μm). Smoke or fog can limit the effective range of these, since they operate solely in the visible spectrum.
Other types
Ionization current flame detection
The intense ionization within the body of a flame can be measured by means by the phenomena of flame rectification
whereby an AC current flows more easily in one direction when a voltage
is applied. This current can be used to verify flame presence and
quality.
Such detectors can be used in large industrial process gas heaters and
are connected to the flame control system. They usually act as both
flame quality monitors and for flame failure detection. They are also
common in a variety of household gas furnaces and boilers.
Problems with boilers failing to stay lit can often be due to
dirty flame sensors or to a poor burner surface with which to complete
the electrical circuit. A poor flame or one that is lifting off the
burner may also interrupt the continuity.
Flame igniter (top) and flame sensor
Thermocouple flame detection
Thermocouples
are used extensively for monitoring flame presence in combustion
heating systems and gas cookers. A common use in these installations is
to cut off the supply of fuel if the flame fails, in order to prevent
unburned fuel from accumulating. These sensors measure heat and
therefore are commonly used to determine the absence of a flame. This can be used to verify the presence of a pilot flame.
A fire emits radiation, which human eye
experiences as the visible yellow red flames and heat. In fact, during a
fire, relatively sparsely UV energy and visible light energy is
emitted, as compared to the emission of Infrared radiation. A
non-hydrocarbon fire, for example, one from hydrogen, does not show a CO2 peak on 4.3 μm because during the burning of hydrogen no CO2 is released. The 4.3 μm CO2
peak in the picture is exaggerated, and is in reality less than 2% of
the total energy of the fire. A multi-frequency-detector with sensors
for UV, visible light, near IR and/or wideband IR thus have much more
"sensor data" to calculate with and therefore are able to detect more
types of fires and to detect these types of fires better: hydrogen, methanol, ether or sulphur.
It looks like a static picture, but in reality the energy fluctuates,
or flickers. This flickering is caused by the fact that the aspirated
oxygen and the present combustible are burning and concurrently aspirate
new oxygen and new combustible material. These little explosions cause
the flickering of the flame.
Sunlight
Sunlight transmission
The sun
emits an enormous amount of energy, which would be harmful to human
beings if not for the vapours and gases in the atmosphere, like water (clouds), ozone, and others, through which the sunlight is filtered. In the figure it can clearly be seen that "cold" CO2
filters the solar radiation around 4.3 μm. An Infrared detector which
uses this frequency is therefore solar blind. Not all manufacturers of
flame detectors use sharp filters for the 4.3 μm radiation and thus
still pick up quite an amount of sunlight. These cheap flame detectors
are hardly usable for outdoor applications. Between 0.7 μm and approx.
3 μm there is relatively large absorption of sunlight. Hence, this
frequency range is used for flame detection by a few flame detector
manufacturers (in combination with other sensors like ultraviolet,
visible light, or near infrared). The big economical advantage is that
detector windows can be made of quartz instead of expensive sapphire. These electro-optical sensor
combinations also enable the detection of non-hydrocarbons like
hydrogen fires without the risk of false alarms caused by artificial
light or electrical welding.
Heat radiation
Heat radiation
Infrared flame detectors suffer from Infrared heat radiation which is
not emitted by the possible fire. One could say that the fire can be
masked by other heat sources. All objects which have a temperature
higher than the absolute minimum temperature (0 kelvins
or −273.15 °C) emit energy and at room temperature (300 K) this heat is
already a problem for the infrared flame detectors with the highest
sensitivity. Sometimes a moving hand is sufficient to trigger an IR
flame detector. At 700 K a hot object (black body) starts to emit
visible light (glow). Dual- or multi-infrared detectors suppress the
effects of heat radiation by means of sensors which detect just off the
CO2 peak; for example at 4.1 μm. Here it is necessary that
there is a large difference in output between the applied sensors (for
example sensor S1 and S2 in the picture). A disadvantage is that the
radiation energy of a possible fire must be much bigger than the present
background heat radiation. In other words, the flame detector becomes
less sensitive. Every multi infrared flame detector is negatively
influenced by this effect, regardless how expensive it is.
Cone of vision
Cone of Vision (Field of View)
The cone of vision of a flame detector is determined by the shape and
size of the window and the housing and the location of the sensor in
the housing. For infrared sensors also the lamination
of the sensor material plays a part; it limits the cone of vision of
the flame detector. A wide cone of vision does not automatically mean
that the flame detector is better. For some applications the flame
detector needs to be aligned precisely to take care that it does not
detect potential background radiation sources. The cone of vision of the
flame detector is three dimensional and is not necessarily perfectly
round. The horizontal angle of vision and the vertical angle of vision
often differ; this is mostly caused by the shape of the housing and by
mirroring parts (meant for the self test). Different combustibles can
even have a different angle of vision in the same flame detector. Very
important is the sensitivity at angles of 45°. Here at least 50% of the
maximum sensitivity at the central axis must be achieved. Some flame
detectors here achieve 70% or more. In fact these flame detectors have a
total horizontal angle of vision of more than 90°, but most of the
manufacturers do not mention this. A high sensitivity on the edges of
the angle of vision provides advantages for the projection of a flame
detector.
The detection range
Detection range
The range of a flame detector is highly determined by the mounting
location. In fact, when making a projection, one should imagine in what
the flame detector "sees". A rule of thumb is, that the mounting height
of the flame detector is twice as high as the highest object in the
field of view. Also the accessibility of the flame detector must be
taken into account, because of maintenance and/or repairs. A rigid
light-mast with a pivot point is for this reason recommendable. A "roof"
on top of the flame detector (30 x 30 cm, 1 x 1-foot) prevents quick
pollution in outdoor applications. Also the shadow effect must be
considered. The shadow effect can be minimized by mounting a second
flame detector in the opposite of the first detector. A second advantage
of this approach is, that the second flame detector is a redundant one,
in case the first one is not working or is blinded. In general, when
mounting several flame detectors, one should let them "look" to each
other not let them look to the walls. Following this procedure blind
spots (caused by the shadow effect) can be avoided and a better
redundancy can be achieved than if the flame detectors would "look" from
the central position into the area to be protected. The range of flame
detectors to the 30 x 30 cm, 1 x 1-foot industry standard
fire is stated within the manufacturers data sheets and manuals, this
range can be affected by the previously stated de-sensitizing effects of
sunlight, water, fog, steam and blackbody radiation.
The square law
Square Law
If the distance between the flame and the flame detector is large
compared to the dimension of the fire then the square law applies: If a
flame detector can detect a fire with an area A on a certain distance,
then a 4 times bigger flame area is necessary if the distance between
the flame detector and the fire is doubled. In short:
Double distance = four times bigger flame area (fire).
This law is equally valid for all optical flame detectors,
including video based ones. The maximum sensitivity can be estimated by
dividing the maximum flame area A by the square of the distance between
the fire and the flame detector: c = A/d2. With this constant c can, for the same flame detector and the same type of fire, the maximum distance or the minimum fire area be calculated:
A=cd 2 and d=√A/c
It must be emphasized, however, that the square root in reality
is not valid anymore at very high distances. At long distances other
parameters are playing a significant part; like the occurrence of water
vapour and of cold CO2 in the air. In the case of a very
small flame, on the other hand, the decreasing flickering of the flame
will play an increasing part.
A more exact relation - valid when the distance between the flame
and the flame detector is small - between the radiation density, E, at the detector and the distance, D, between the detector and a flame of effective radius, R, emitting energy density, M, is given by
E = 2πMR2/(R2+D2)
When R<<D then the relation reduces to the (inverse) square law
Artistic
rendition of the Earth's magnetopause. The magnetopause is where the
pressure from the solar wind and the planet's magnetic field are equal.
The position of the Sun would be far to the left in this image.
The magnetopause is the abrupt boundary between a magnetosphere and the surrounding plasma. For planetary science, the magnetopause is the boundary between the planet's magnetic field and the solar wind.
The location of the magnetopause is determined by the balance between
the pressure of the dynamic planetary magnetic field and the dynamic
pressure of the solar wind. As the solar wind pressure increases and
decreases, the magnetopause moves inward and outward in response. Waves
(ripples and flapping motion) along the magnetopause move in the
direction of the solar wind flow in response to small-scale variations
in the solar wind pressure and to Kelvin–Helmholtz instability.
The solar wind is supersonic and passes through a bow shock
where the direction of flow is changed so that most of the solar wind
plasma is deflected to either side of the magnetopause, much like water
is deflected before the bow of a ship. The zone of shocked solar wind
plasma is the magnetosheath.
At Earth and all the other planets with intrinsic magnetic fields, some
solar wind plasma succeeds in entering and becoming trapped within the
magnetosphere. At Earth, the solar wind plasma which enters the
magnetosphere forms the plasma sheet. The amount of solar wind plasma and energy that enters the magnetosphere is regulated by the orientation of the interplanetary magnetic field, which is embedded in the solar wind.
The Sun and other stars with magnetic fields and stellar winds have a solar magnetopause or heliopause where the stellar environment is bounded by the interstellar environment.
Characteristics
Schematic
representation of a planetary dipole magnetic field in a vacuum (right
side) deformed by a region of plasma with infinite conductivity. The Sun
is to the left. The configuration is equivalent to an image dipole
(green arrow) being placed at twice the distance from the planetary
dipole to the interaction boundary.
Prior to the age of space exploration, interplanetary space was
considered to be a vacuum. The coincidence of the first observation of a
solar flare and the geomagnetic storm of 1859 was evidence that plasma was ejected from the Sun during the flare event. Chapman and Ferraro
proposed that a plasma was emitted by the Sun in a burst as part of a
flare event which disturbed the planet's magnetic field in a manner
known as a geomagnetic storm. The collision frequency of particles in
the plasma in the interplanetary medium is very low and the electrical
conductivity is so high that it could be approximated to an infinite
conductor. A magnetic field in a vacuum cannot penetrate a volume with
infinite conductivity. Chapman and Bartels (1940)
illustrated this concept by postulating a plate with infinite
conductivity placed on the dayside of a planet's dipole as shown in the
schematic. The field lines on the dayside are bent. At low latitudes,
the magnetic field lines are pushed inward. At high latitudes, the
magnetic field lines are pushed backwards and over the polar regions.
The boundary between the region dominated by the planet's magnetic field
(i.e., the magnetosphere)
and the plasma in the interplanetary medium is the magnetopause. The
configuration equivalent to a flat, infinitely conductive plate is
achieved by placing an image dipole (green arrow at left of schematic)
at twice the distance from the planet's dipole to the magnetopause along
the planet-Sun line. Since the solar wind is continuously flowing
outward, the magnetopause above, below and to the sides of the planet
are swept backward into the geomagnetic tail as shown in the artist's
concept. The region (shown in pink in the schematic) which separates
field lines from the planet which are pushed inward from those which are
pushed backward over the poles is an area of weak magnetic field or
day-side cusp. Solar wind particles can enter the planet's
magnetosphere through the cusp region. Because the solar wind exists at
all times and not just times of solar flares, the magnetopause is a
permanent feature of the space near any planet with a magnetic field.
The magnetic field lines of the planet's magnetic field are not
stationary. They are continuously joining or merging with magnetic
field lines of the interplanetary magnetic field. The joined field
lines are swept back over the poles into the planetary magnetic tail.
In the tail, the field lines from the planet's magnetic field are
re-joined and start moving toward night-side of the planet. The physics
of this process was first explained by Dungey (1961).
If one assumed that magnetopause was just a boundary between a
magnetic field in a vacuum and a plasma with a weak magnetic field
embedded in it, then the magnetopause would be defined by electrons and
ions penetrating one gyroradius into the magnetic field domain. Since
the gyro-motion of electrons and ions is in opposite directions, an
electric current flows along the boundary. The actual magnetopause is
much more complex.
Estimating the standoff distance to the magnetopause
If
the pressure from particles within the magnetosphere is neglected, it
is possible to estimate the distance to the part of the magnetosphere
that faces the Sun. The condition governing this position is that the dynamic ram pressure from the solar wind is equal to the magnetic pressure from the Earth's magnetic field:
Since the dipole magnetic field strength varies with distance as the magnetic field strength can be written as , where is the planet's magnetic moment, expressed in .
Solving this equation for r leads to an estimate of the distance
The distance from Earth to the subsolar magnetopause varies
over time due to solar activity, but typical distances range from 6–15 R. Empirical models using real-time solar wind data can provide a real-time estimate of the magnetopause location. A bow shock
stands upstream from the magnetopause. It serves to decelerate and
deflect the solar wind flow before it reaches the magnetopause.
Research on the magnetopause is conducted using the LMN coordinate
system (which is set of axes like XYZ). N points normal to the
magnetopause outward to the magnetosheath, L lies along the projection
of the dipole axis onto the magnetopause (positive northward), and M
completes the triad by pointing dawnward.
Venus and Mars do not have a planetary magnetic field and do not
have a magnetopause. The solar wind interacts with the planet's
atmosphere
and a void is created behind the planet. In the case of the Earth's
moon and other bodies without a magnetic field or atmosphere, the body's
surface interacts with the solar wind and a void is created behind the
body.
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