Historically, many rulers have assumed titles such as the son of God, the son of a god or the son of heaven.
The term "Son of God" is used in the Hebrew Bible as another way to refer to humans who have a special relationship with God. In Exodus, the nation of Israel is called God's firstborn son. Solomon is also called "son of God". Angels, just and pious men, and the kings of Israel are all called "sons of God."
In the New Testament of the Christian Bible, "Son of God" is applied to Jesus on many occasions.
On two occasions, Jesus is recognized as the Son of God by a voice
which speaks from Heaven. Jesus explicitly and implicitly describes
himself as the Son of God and he is also described as the Son of God by
various individuals who appear in the New Testament. Jesus is called the "Son of God," and followers of Jesus are called, "Christians." As applied to Jesus, the term is a reference to his role as the Messiah, or Christ, the King chosen by God.
The contexts and ways in which Jesus' title, Son of God, means
something more or something other than the title Messiah remain the
subject of ongoing scholarly study and discussion.
The term "Son of God" should not be confused with the term "God the Son" (Greek: Θεός ὁ υἱός), the second person of the Trinity in Christian theology. The doctrine of the Trinity identifies Jesus as God the Son, identical in essence but distinct in person with regard to God the Father and God the Holy Spirit (the First and Third Persons of the Trinity). Nontrinitarian Christians accept the application to Jesus of the term "Son of God", which is found in the New Testament.
The title "Son of Heaven" i.e. 天子 (from 天 meaning sky/heaven/god and 子 meaning child) was first used in the Western Zhou dynasty (c. 1000 BC). It is mentioned in the Shijing book of songs, and reflected the Zhou belief that as Son of Heaven (and as its delegate) the Emperor of China was responsible for the well-being of the whole world by the Mandate of Heaven. This title may also be translated as "son of God" given that the word Tiān in Chinese may either mean sky or god. The Emperor of Japan was also called the Son of Heaven (天子 tenshi) starting in the early 7th century.
Among the Eurasian nomads,
there was also a widespread use of "Son of God/Son of Heaven" for
instance, in the third century BC, the ruler was called Chanyü and similar titles were used as late as the 13th century by Genghis Khan.
Examples of kings being considered the son of god are found throughout the Ancient Near East. Egypt in particular developed a long lasting tradition. Egyptian pharaohs
are known to have been referred to as the son of a particular god and
their begetting in some cases is even given in sexually explicit detail.
Egyptian pharaohs did not have full parity with their divine fathers
but rather were subordinate.Nevertheless, in the first four dynasties, the pharaoh was considered
to be the embodiment of a god. Thus, Egypt was ruled by direct
theocracy, wherein "God himself is recognized as the head" of the state. During the later Amarna Period, King Amenhotep IV/Akhenaten
redefined the pharaoh's godship. He taught "there was only one god and
only one person who now knew the god: Akhenaten himself" and assumed
position of the ḥm ntr tpy (first servant of god).
He eventually eliminated all representation on his behalf by the
priests of Amun as he also eliminated the god Amun, to solely lead
worship identifying as the Son of the God he called Father, the latter
which he recognized through the aten (sun), the vehicle through which
the power of the God manifested to him.
Within a few years of his first epiphany and becoming king, King
Akhenaten had dropped the priestly title of ḥm ntr tpy, but remained
serving as the sole cleric and son of the Father in his rule of the Two
Lands. Later still, the closest Egypt came to the Jewish variant of theocracy was during the reign of Herihor. He took on the role of ruler not as a god but rather as a high-priest and king.
According to the Bible, several kings of Damascus took the title son of Hadad. From the archaeological record a stela erected by Bar-Rakib for his father Panammuwa II contains similar language. The son of Panammuwa II a king of Sam'al referred to himself as a son of Rakib. Rakib-El is a god who appears in Phoenician and Aramaic inscriptions. Panammuwa II died unexpectedly while in Damascus. However, his son the king Bar-Rakib was not a native of Damascus but rather the ruler of Sam'al it is unknown if other rules of Sam'al used similar language.
In Greek mythology, Heracles (son of Zeus) and many other figures were considered to be sons of gods through union with mortal women. From around 360 BC onwards Alexander the Great may have implied he was a demigod by using the title "Son of Ammon–Zeus".
A denarius minted circa 18 BC. Obverse: CAESAR AVGVSTVS; reverse: DIVVS IVLIV(S)
In 42 BC, Julius Caesar was formally deified as "the divine Julius" (divus Iulius) after his assassination. His adopted son, Octavian (better known as Augustus, a title given to him 15 years later, in 27 BC) thus became known as divi Iuli filius (son of the divine Julius) or simply divi filius (son of the god). As a daring and unprecedented move, Augustus used this title to advance his political position in the Second Triumvirate, finally overcoming all rivals for power within the Roman state.
The word which was applied to Julius Caesar when he was deified was divus, not the distinct word deus. Thus, Augustus called himself Divi filius, not Dei filius.
The line between been god and god-like was at times less than clear to
the population at large, and Augustus seems to have been aware of the
necessity of keeping the ambiguity.
As a purely semantic mechanism, and to maintain ambiguity, the court of
Augustus sustained the concept that any worship given to an emperor was
paid to the "position of emperor" rather than the person of the
emperor. However, the subtle semantic distinction was lost outside Rome, where Augustus began to be worshiped as a deity. The inscription DF thus came to be used for Augustus, at times unclear which meaning was intended. The assumption of the title Divi filius
by Augustus meshed with a larger campaign by him to exercise the power
of his image. Official portraits of Augustus made even towards the end
of his life continued to portray him as a handsome youth, implying that
miraculously, he never aged. Given that few people had ever seen the
emperor, these images sent a distinct message.
Later, Tiberius (emperor from 14 to 37 AD) came to be accepted as the son of divus Augustus and Hadrian as the son of divus Trajan. By the end of the 1st century, the emperor Domitian was being called dominus et deus (i.e. master and god).
In the writings of the Baháʼí Faith, the term "Son of God" is applied to Jesus, but does not indicate a literal physical relationship between Jesus and God, but is symbolic and is used to indicate the very strong spiritual relationship between Jesus and God and the source of his authority. Shoghi Effendi,
the head of the Baháʼí Faith in the first half of the 20th century,
also noted that the term does not indicate that the station of Jesus is
superior to other prophets and messengers that Baháʼís name Manifestation of God, including Buddha, Muhammad and Baha'u'llah among others.
Shoghi Effendi notes that, since all Manifestations of God share the
same intimate relationship with God and reflect the same light, the term
Sonship can in a sense be attributable to all the Manifestations.
Another interpretation stems from the Judaic understanding of the
title, which describes all human beings as being Sons of God. In parts
of the Old Testament, historical figures like Jacob and Solomon are
referred to as Sons of God, referring to their descent from Adam.
Biblical scholars use this title as a way of affirming Jesus' humanity,
that he is fully human but, also sent from his father who is God
almighty alone as mentioned in John 3:16.
Islam rejects any kinship between God and any other being, including a son. Thus, rejecting the belief that Jesus is the begotten son of God, God himself or another god.
As in Christianity, Islam believes Jesus had no earthly father. In
Islam Jesus is believed to be born due to the command of God "be". God ordered the angel Jibrīl (Gabriel) to "blow" the soul of Jesus into Mary and so she gave birth to Jesus.
Although references to "sons of God", "son of God" and "son of the LORD" are occasionally found in Jewish literature, they never refer to physical descent from God. There are two instances where Jewish kings are figuratively referred to as a god. These terms are often used in the general sense in which the Jewish people were referred to as "children of the LORD your God".
When it was used by the rabbis, the term referred to Israel in particular or it referred to human beings in general, it was not used as a reference to the Jewish mashiach. In Judaism the term mashiach has a broader meaning and usage and can refer to a wide range of people and objects, not necessarily related to the Jewish eschaton.
Gabriel's Revelation, also called the Vision of Gabriel or the Jeselsohn Stone, is a three-foot-tall (one metre) stone tablet with 87 lines of Hebrew text written in ink, containing a collection of short prophecies written in the first person and dated to the late 1st century BC. It is a tablet described as a "Dead Sea scroll in stone".
The text seems to talk about a messianic figure from Ephraim who broke evil before righteousness by three days. Later the text talks about a "prince of princes" a leader of Israel who was killed by the evil king and not properly buried. The evil king was then miraculously defeated. The text seems to refer to Jeremiah Chapter 31.
The choice of Ephraim as the lineage of the messianic figure described
in the text seems to draw on passages in Jeremiah, Zechariah and Hosea.
This leader was referred to as a son of God.
The text seems to be based on a Jewish revolt recorded by Josephus dating from 4 BC. Based on its dating the text seems to refer to Simon of Peraea, one of the three leaders of this revolt.
Dead Sea Scrolls
In some versions of Deuteronomy the Dead Sea Scrolls refer to the sons of God rather than the sons of Israel, probably in reference to angels. The Septuagint reads similarly.
4Q174 is a midrashic text in which God refers to the Davidic messiah as his son.
4Q246
refers to a figure who will be called the son of God and son of the
Most High. It is debated if this figure represents the royal messiah, a
future evil gentile king or something else.
In 11Q13Melchizedek is referred to as god the divine judge. Melchizedek in the bible was the king of Salem. At least some in the Qumran community seemed to think that at the end of days Melchizedek would reign as their king. The passage is based on Psalm 82.
Pseudepigrapha
In both Joseph and Aseneth and the related text The Story of Asenath, Joseph is referred to as the son of God. In the Prayer of Joseph both Jacob and the angel are referred to as angels and the sons of God.
Scientific studies, based on evidence from climate models, have generally shown that some forms of SRM could in theory reduce global warming and therefore many effects of climate change.However, because warming from greenhouse gases and cooling from SRM would operate differently across latitudes and seasons,
a world where global warming would be offset by SRM would have a
different climate from one where this warming did not occur in the first
place. Furthermore, confidence in the current projections of how SRM
would affect regional climate and ecosystems is low. SRM would therefore pose environmental risks.
Governing SRM is challenging for multiple reasons, including that several countries would likely be capable of doing it alone.
For now, there is no formal international framework designed to
regulate SRM, although aspects of existing international law would be
applicable. Issues of governance and effectiveness are intertwined, as poorly governed use of SRM might lead to its highly suboptimal implementation.
Thus, many questions regarding the acceptable deployment of SRM, or
even its research and development, are currently unanswered. In 2022, a
dozen academics launched a campaign for national policies of "no public
funding, no outdoor experiments, no patents, no deployment, and no
support in international institutions... including in assessments by the
Intergovernmental Panel on Climate Change." As of December 2024, nearly 540 academics and 60 advocacy organizations have endorsed the proposal.
According to Bloomberg News, as of 2024 several American billionaires are funding research into SRM: "A growing number of Silicon Valley
founders and investors are backing research into blocking the sun by
spraying reflective particles high in the atmosphere or making clouds
brighter."
Potential
complementary responses to climate change: greenhouse gas emissions
abatement, carbon dioxide removal, SRM, and adaptation.
The
context for the interest in solar radiation modification (SRM) options
is continued high global emissions of greenhouse gases. Human's
greenhouse gas emissions have disrupted the Earth's energy budget. Due to elevated atmospheric greenhouse gas concentrations,
the net difference between the amount of sunlight absorbed by the Earth
and the amount of energy radiated back to space has risen from 1.7 W/m2 in 1980, to 3.1 W/m2 in 2019. This imbalance, or "radiative forcing," means that the Earth absorbs more energy than it emits, causing global temperatures to rise which will, in turn, have negative impacts on humans and nature.
In principle, net emissions could be reduced and even eliminated achieved through a combination of emission cuts and carbon dioxide removal (together called "mitigation").
However, emissions have persisted, consistently exceeding targets, and
experts have raised serious questions regarding the feasibility of
large-scale removals.The 2023 Emissions Gap Report from the UN Environment Programme
estimated that even the most optimistic assumptions regarding
countries' current conditional emissions policies and pledges has only a
14% chance of limiting global warming to 1.5 °C.
SRM would increase Earth's reflection of sunlight by increasing the albedo of the atmosphere or the surface. An increase in planetary albedo of 1% would reduce radiative forcing by 2.35 W/m2,
eliminating most of global warming from current anthropogenically
elevated greenhouse gas concentrations, while a 2% albedo increase would
negate the warming effect of doubling the atmospheric carbon dioxide concentration.
SRM could theoretically buy time by slowing the rate of climate
change or to eliminate the worst climate impacts until net negative
emissions reduce atmospheric greenhouse gas concentrations sufficiently. This is because SRM could, unlike the other responses, cool the planet within months after deployment.
SRM is generally intended to complement, not replace, emissions reduction and carbon dioxide removal. For example, the IPCCSixth Assessment Report
says: "There is high agreement in the literature that for addressing
climate change risks SRM cannot be the main policy response to climate
change and is, at best, a supplement to achieving sustained net zero or
net negative CO2 emission levels globally".
In 1974, Russian climatologistMikhail Budyko
suggested that if global warming ever became a serious threat, it could
be countered with airplane flights in the stratosphere, burning sulfur
to make aerosols that would reflect sunlight away. Along with carbon dioxide removal, SRM was discussed jointly as geoengineering in a 1992 climate change report from the US National Academies.
David Keith,
an American physicist, has worked on solar geoengineering since 1992,
when he and Hadi Dowlatabadi published one of the first assessments of
the technology and its policy implications, introducing a structured
comparison of cost and risk. Keith has consistently argued that
geoengineering needs a "systematic research program" to determine
whether or not its approaches are feasible. He has also appealed for international standards of governance and oversight for how such research might proceed.
The first modeled results of SRM were published in 2000. In 2006 Nobel LaureatePaul Crutzen
published an influential scholarly paper where he said, "Given the
grossly disappointing international political response to the required
greenhouse gas emissions, and further considering some drastic results
of recent studies, research on the feasibility and environmental
consequences of climate engineering [...] should not be tabooed."
Atmospheric methods
The
atmospheric methods for SRM include stratospheric aerosol injection
(SAI), marine cloud brightening (MCB) and cirrus cloud thinning (CCT).
For stratospheric aerosol injection (SAI) small particles would be injected into the upper atmosphere to cool the planet with both global dimming and increased albedo. Of all the proposed SRM methods, SAI has received the most sustained attention: The IPCC concluded in 2018 that SAI "is the most-researched SRM method, with high agreement that it could limit warming to below 1.5 °C." This technique would mimic a cooling phenomenon that occurs naturally by the eruption of volcanoes.
Sulfates are the most commonly proposed aerosol, since there is a
natural analogue with (and evidence from) volcanic eruptions.
Alternative materials such as using photophoretic particles, titanium dioxide, and diamond have been proposed. Delivery by custom aircraft appears most feasible, with artillery and balloons sometimes discussed.
This technique could give much more than 3.7 W/m2 of globally averaged negative forcing, which is sufficient to entirely offset the warming caused by a doubling of carbon dioxide.
The most recent Scientific Assessment of Ozone Depletion report in 2022 from the World Meteorological Organization
concluded "Stratospheric Aerosol Injection (SAI) has the potential to
limit the rise in global surface temperatures by increasing the
concentrations of particles in the stratosphere... . However, SAI comes
with significant risks and can cause unintended consequences."
A potential disadvantage of SAI is its potential to catalyze the destruction of the protective stratospheric ozone layer.
Marine cloud brightening (MCB) would involve spraying fine sea water
to whiten clouds and thus increase cloud reflectivity. It would work by
"seeding to promote nucleation, reducing optical thickness and cloud
lifetime, to allow more outgoing longwave radiation to escape into
space".
The extra condensation nuclei created by the spray would change
the size distribution of the drops in existing clouds to make them
whiter. The sprayers would use fleets of unmanned rotor ships
known as Flettner vessels to spray mist created from seawater into the
air to thicken clouds and thus reflect more radiation from the Earth. The whitening effect is created by using very small cloud condensation nuclei, which whiten the clouds due to the Twomey effect.
This technique can give more than 3.7 W/m2 of globally averaged negative forcing, which is sufficient to reverse the warming effect of a doubling of atmospheric carbon dioxide concentration.
Cirrus cloud thinning
(CCT) involves "seeding to promote nucleation, reducing optical
thickness and cloud lifetime, to allow more outgoing longwave radiation
to escape into space."Natural
cirrus clouds are believed to have a net warming effect. These could be
dispersed by the injection of various materials.
This method is strictly not SRM, as it increases outgoing longwave radiation instead of decreasing incoming shortwave radiation.
However, because it shares some of the physical and especially
governance characteristics as the other SRM methods, it is often
included.
The IPCC describes ground-based albedo modification (GBAM) as "whitening roofs, changes in land use management (e.g., no-till farming), change of albedo at a larger scale (covering glaciers or deserts with reflective sheeting and changes in ocean albedo)." It is a method of enhancing Earth's albedo, i.e. the ability to reflect the visible, infrared, and ultraviolet wavelengths of the Sun, reducing heat transfer to the surface.
Space-based
The
basic function of a space lens to mitigate global warming. The image is
simplified, as a 1000 kilometre diameter lens is considered sufficient
by most proposals, and would be much smaller than shown. Additionally, a
zone plate would only be a few nanometers thick.
There are some proponents who argue that unlike stratospheric aerosol
injection, space-based approaches are advantageous because they do not
interfere directly with the biosphere and ecosystems. However, space-based approaches would cost about 1000 times more than their terrestrial alternatives. In 2022, the IPCC Sixth Assessment Report discussed SAI, MCB, CCT and even attempts to alter albedo on the ground or in the ocean, yet completely ignored space-based approaches.
There has been a range of proposals to reflect or deflect solar
radiation from space, before it even reaches the atmosphere, commonly
described as a space sunshade. The most straightforward is to have mirrors orbiting around the Earth—an idea first suggested even before the wider awareness of climate change, with rocketry pioneer Hermann Oberth considering it a way to facilitate terraforming projects in 1923. and this was followed by other books in 1929, 1957 and 1978. By 1992, the U.S. National Academy of Sciences described a plan to suspend 55,000 mirrors with an individual area of 100 square meters in a Low Earth orbit. Another contemporary plan was to use space dust to replicate Rings of Saturn around the equator, although a large number of satellites
would have been necessary to prevent it from dissipating. A 2006
variation on this idea suggested relying entirely on a ring of
satellites electromagnetically tethered in the same location. In all
cases, sunlight exerts pressure which can displace these reflectors from
orbit over time, unless stabilized by enough mass. Yet, higher mass
immediately drives up launch costs.
In an attempt to deal with this problem, other researchers have proposed Inner lagrangian point
between the Earth and the Sun as an alternative to near-Earth orbits,
even though this tends to increase manufacturing or delivery costs
instead. In 1989, a paper suggested founding a lunar colony, which would produce and deploy diffraction grating made out of a hundred million tonnes of glass. In 1997, a single, very large mesh of aluminium wires "about one millionth of a millimetre thick" was also proposed.
Two other proposals from the early 2000s advocated the use of thin
metallic disks 50–60 cm in diameter, which would either be launched from
the Earth at a rate of once per minute over several decades, or be manufactured from asteroids directly in orbit.
When summarizing these options in 2009, the Royal Society
concluded that their deployment times are measured in decades and costs
in the trillions of USD,
meaning that they are "not realistic potential contributors to
short-term, temporary measures for avoiding dangerous climate change",
and may only be competitive with the other geoengineering approaches
when viewed from a genuinely long (a century or more) perspective, as
the long lifetime of L1-based approaches could make them cheaper than
the need to continually renew atmospheric-based measures over that
timeframe.
In 2021, researchers in Sweden considered building solar sails
in the near-Earth orbit, which would then arrive to L1 point over 600
days one by one. Once they all form an array in situ, the combined
1.5 billion sails would have total area of 3.75 million square
kilometers, while their combined mass is estimated in a range between 83
million tons (present-day technology) and 34 million tons (optimal
advancements). This proposal would cost between five and ten trillion
dollars, but only once launch cost has been reduced to US$50/kg, which
represents a massive reduction from the present-day costs of
$4400–2700/kg[56] for the most widely used launch vehicles.
In July 2022, a pair of researchers from MIT Senseable City Lab, Olivia Borgue and Andreas M. Hein, have instead proposed integrating nanotubes made out of silicon dioxide into ultra-thin polymeric films (described as "space bubbles" in the media ), whose semi-transparent nature would allow them to resist the pressure of solar wind
at L1 point better than any alternative with the same weight. The use
of these "bubbles" would limit the mass of a distributed sunshade
roughly the size of Brazil
to about 100,000 tons, much lower than the earlier proposals. However,
it would still require between 399 and 899 yearly launches of a vehicle
such as SpaceX Starship
for a period of around 10 years, even though the production of the
bubbles themselves would have to be done in space. The flights would not
begin until research into production and maintenance of these bubbles
is completed, which the authors estimate would require a minimum of
10–15 years. After that, the space shield may be large enough by 2050 to
prevent crossing of the 2 °C (3.6 °F) threshold.
In 2023, three astronomers revisited the space dust concept,
instead advocating for a lunar colony which would continuously mine the
Moon in order to eject lunar dust
into space on a trajectory where it would interfere with sunlight
streaming towards the Earth. Ejections would have to be near-continuous,
as since the dust would scatter in a matter of days, and about 10
million tons would have to be dug out and launched annually.
The authors admit that they lack a background in either climate or
rocket science, and the proposal may not be logistically feasible.
Costs
Cost estimates for SAI
A study in 2020 looked at the cost of SAI through to the year 2100.
It found that relative to other climate interventions and solutions, SAI
remains inexpensive. However, at about $18 billion per year per degree
Celsius of warming avoided (in 2020 USD), a solar geoengineering program
with substantial climate impact would lie well beyond the financial
reach of individuals, small states, or other non-state potential rogue
actors.
The annual cost of delivering a sufficient amount of sulfur to
counteract expected greenhouse warming is estimated at $5–10 billion US
dollars.
SAI is expected to have low direct financial costs of implementation, relative to the expected costs of both unabated climate change and aggressive mitigation.
Aspects of regional scales and seasonal timescales
A
moderate magnitude of SRM would bring important aspects of the
climate—for example, average and extreme temperature, water
availability, and cyclone intensity—closer to their preindustrial values
for most of the planet at a subregional resolution.
Furthermore, SRM's effect would occur rapidly, unlike those of other
responses to climate change. However, even under optimal implementation,
some climatic anomalies—especially regarding precipitation—would
persist, although mostly at lesser magnitudes than without SRM.
As well as imperfect and geographically uneven cancellation of
the climatic effect of greenhouse gases, SRM has other significant
technical problems. The IPCCSixth Assessment Report
explains some of the risks and uncertainties as follows: "[...] SRM
could offset some of the effects of increasing GHGs on global and
regional climate, including the carbon and water cycles. However, there
would be substantial residual or overcompensating climate change at the
regional scales and seasonal time scales, and large uncertainties
associated with aerosol–cloud–radiation interactions persist. The
cooling caused by SRM would increase the global land and ocean CO2 sinks, but this would not stop CO2 from increasing in the atmosphere or affect the resulting ocean acidification under continued anthropogenic emissions."
Likewise, a 2023 report from the UN Environment Programme
stated, "Climate model results indicate that an operational SRM
deployment could fully or partially offset the global mean warming
caused by anthropogenic GHG emissions and reduce some climate change
hazards in most regions. There could be substantial residual or possible
overcompensating climate change at regional scales and seasonal
timescales." The report also said: "An operational SRM deployment would introduce new risks to people and ecosystems".
SRM would imperfectly compensate for anthropogenic climate
changes. Greenhouse gases warm throughout the globe and year, whereas
SRM reflects light more effectively at low latitudes and in the hemispheric summer (due to the sunlight's angle of incidence)
and only during daytime. Deployment regimes could compensate for this
heterogeneity by changing and optimizing injection rates by latitude and
season.
Impacts on precipitation
Models indicate that SRM would compensate more effectively for temperature than for precipitation.[citation needed]
Therefore, using SRM to fully return global mean temperature to a
preindustrial level would overcorrect for precipitation changes. This
has led to claims that it would dry the planet or even cause drought,[citation needed] but this would depend on the intensity (i.e. radiative forcing) of SRM. Furthermore, soil moisture
is more important for plants than average annual precipitation. Because
SRM would reduce evaporation, it more precisely compensates for changes
to soil moisture than for average annual precipitation. Likewise, the intensity of tropical monsoons is increased by climate change and decreased by SRM.
A net reduction in tropical monsoon intensity might manifest at
moderate use of SRM, although to some degree the effect of this on
humans and ecosystems would be mitigated by greater net precipitation
outside of the monsoon system.[citation needed]
This has led to claims that SRM "would disrupt the Asian and African
summer monsoons", but the impact would depend on the particular
implementation regime.
Maintenance and termination shock
The direct climatic effects of SRM are reversible within short timescales. Models project that SRM interventions would take effect rapidly, but would also quickly fade out if not sustained.
If SRM masked significant warming, stopped abruptly, and was not
resumed within a year or so, the climate would rapidly warm towards
levels which would have existed without the use of SRM, sometimes known
as termination shock.
The rapid rise in temperature might lead to more severe consequences
than a gradual rise of the same magnitude. However, some scholars have
argued that this appears preventable because it would be in states'
interest to resume any terminated deployment regime, and because
infrastructure and knowledge could be made redundant and resilient.
SRM does not directly influence atmospheric carbon dioxide concentration and thus does not reduce ocean acidification. While not a risk of SRM per se, this points to the limitations of relying on it to the exclusion of emissions reduction.
Effect on sky and clouds
Managing
solar radiation using aerosols or cloud cover would involve changing
the ratio between direct and indirect solar radiation. This would affect
plant life and solar energy.
Visible light, useful for photosynthesis, is reduced proportionally
more than is the infrared portion of the solar spectrum due to the
mechanism of Mie scattering. As a result, deployment of atmospheric SRM would reduce by at least 2–5% the growth rates of phytoplankton, trees, and crops between now and the end of the century.
Uniformly reduced net shortwave radiation would hurt solar
photovoltaics by the same >2–5% because of the bandgap of silicon
photovoltaics.
Uncertainty regarding effects
Much uncertainty remains about SRM's likely effects. Most of the evidence regarding SRM's expected effects comes from climate models
and volcanic eruptions. Some uncertainties in climate models (such as
aerosol microphysics, stratospheric dynamics, and sub-grid scale mixing)
are particularly relevant to SRM and are a target for future research.
Volcanoes are an imperfect analogue as they release the material in the
stratosphere in a single pulse, as opposed to sustained injection.
Climate change has various effects on agriculture. One of them is the CO2 fertilization effect
which affects different crops in different ways. A net increase in
agricultural productivity from SRM has been predicted by some studies
due to the combination of more diffuse light and carbon dioxide's
fertilization effect. Other studies suggest that SRM would have little net effect on agriculture.
There have also been proposals to focus SRM at the poles, in order to combat sea level rise or regional marine cloud brightening (MCB) in order to protect coral reefs from bleaching. However, there is low confidence about the ability to control geographical boundaries of the effect.
SRM might be used in ways that are not optimal. In particular,
SRM's climatic effects would be rapid and reversible, which would bring
the disadvantage of sudden warming if it were to be stopped suddenly. Similarly, if SRM was very heterogenous, then the climatic responses could be severe and uncertain.
Governance and policy risks
Global governance issues
The
potential use of SRM poses several governance challenges because of its
high leverage, low apparent direct costs, and technical feasibility as
well as issues of power and jurisdiction. Because international law
is generally consensual, this creates a challenge of widespread
participation being required. Key issues include who will have control
over the deployment of SRM and under what governance regime the
deployment can be monitored and supervised. A governance framework for
SRM must be sustainable enough to contain a multilateral commitment over
a long period of time and yet be flexible as information is acquired,
the techniques evolve, and interests change through time.
Frank Biermann and other political scientists
argue that the current international political system is inadequate for
the fair and inclusive governance of SRM deployment on a global scale.
Other researchers have suggested that building a global agreement on
SRM deployment will be very difficult, and instead power blocs are
likely to emerge.
There are, however, significant incentives for states to cooperate in
choosing a specific SRM policy, which make unilateral deployment a
rather unlikely event.
Other relevant aspects of the governance of SRM include
supporting research, ensuring that it is conducted responsibly,
regulating the roles of the private sector and (if any) the military,
public engagement, setting and coordinating research priorities,
undertaking trusted scientific assessment, building trust, and
compensating for possible harms.
Although climate models of SRM rely on some optimal or consistent
implementation, leaders of countries and other actors may disagree as
to whether, how, and to what degree SRM be used. This could result in
suboptimal deployments and exacerbate international tensions. Likewise, blame for perceived local negative impacts from SRM could be a source of international tensions.
There is a risk that countries may start using SRM without proper
precaution or research. SRM, at least by stratospheric aerosol
injection, appears to have low direct implementation costs relative to
its potential impact, and many countries have the financial and
technical resources to undertake SRM.
Some have suggested that SRM could be within reach of a lone
"Greenfinger", a wealthy individual who takes it upon him or herself to
be the "self-appointed protector of the planet". Others argue that states will insist on maintaining control of SRM.
The existence of SRM may reduce the political and social impetus for climate change mitigation. This has often been called a potential "moral hazard",
although such language is not precise. Some modelling work suggests
that the threat of SRM may in fact increase the likelihood of emissions
reduction.
Advocacy for SRM
The
leading argument supportive of SRM research is that the risks of likely
anthropogenic climate change are great and imminent enough to warrant
research and evaluation of a wide range of responses, even one with
limitations and risks of its own. Leading this effort have been some
climate scientists (such as James Hansen), some of whom have endorsed one or both public letters that support further SRM research.
Scientific organizations that have called for further research in SRM include:
In 2024, Professor David Keith
stated that in the last year or so, there has been far more engagement
with SRM from senior political leaders than was previously the case.
Some nongovernmental organizations actively support SRM research and governance dialogues. The Degrees Initiative is a UK registered charity, established to build capacity in developing countries to evaluate SRM.
It works toward "changing the global environment in which SRM is
evaluated, ensuring informed and confident representation from
developing countries." However, the German NGO Geoengineering Monitor has criticized The Degrees Initiative
for "being an organisation based in the Global North imposing its
research agenda onto the Global South" as well as "normalising and
legitimising solar geoengineering as a viable mitigation strategy".
They also point out that it is "predominantly funded by foundations run
by technology and finance billionaires based in the Global North".
SilverLining is an American organization that advances SRM
research as part of "climate interventions to reduce near-term climate
risks and impacts." It is funded by "philanthropic foundations and individual donors focused on climate change".
The Alliance for Just Deliberation on Solar Geoengineering advances "just and inclusive deliberation" regarding SRM. The Carnegie Climate Governance Initiative catalyzed governance of SRM and carbon dioxide removal, although it ended operations in 2023.
Critics point out that some climate change deniers or former climate change deniers are now actively supporting research in SRM. One example is Danish author Bjorn Lomborg, who "poo-pooed the effects of climate change until he became a geoengineering proponent". Another example is Newt Gingrich, an American politician.
The fossil fuels lobby is among those who advocate for SRM research.
However, others say that "Concluding that advocacy of SRM research
originates from climate deniers and oil executives is perhaps
understandable but untrue".
Opposition to deployment and research
Opposition to SRM has come from various academics and NGOs.
The most common concern is that SRM could lessen climate change
mitigation efforts. Opponents of SRM research often emphasize that
reductions of greenhouse gas emissions would also bring co-benefits (for example reduced air pollution) and that consideration of SRM could prevent these outcomes.
The ETC Group, an environmental justice organization, has been a pioneer in opposing SRM research. It was later joined by the Heinrich Böll Foundation (affiliated with the German Green Party) and the Center for International Environmental Law. The German NGO Geoengineering Monitor
(funded through a collaboration between ETC Group, Biofuelwatch and the
Heinrich Boell Foundation) has the goal to "serve as a resource for
civil society, policy-makers, journalists and the wider public in order
to support advocacy work that opposes geoengineering and aims to address
the root causes of climate change instead".
In 2021, researchers at Harvard put plans for a SRM test on hold after Indigenous Sámi people objected to the test taking place in their homeland. Although the test would not have involved any atmospheric experiments, members of the Saami Council spoke out against the lack of consultation and SRM more broadly. Speaking at a panel organized by the Center for International Environmental Law and other groups, Saami Council Vice President Åsa Larsson Blind said, "This goes against our worldview that we as humans should live and adapt to nature."
By 2024, U.S. government agencies were operating an airborne
early warning system for detecting small concentrations of aerosols to
determine where other countries might be carrying out geoengineering attempts, thought to have unpredictable effects on climate.
The Climate Overshoot Commission is a group of global, eminent, and independent figures. It investigated and developed a comprehensive strategy to reduce climate risks.
The Commission is not supporting deployment of SRM. In fact, it
recommends a "a moratorium on the deployment of solar radiation
modification (SRM) and large-scale outdoor experiment". But it also says
that "governance of SRM research should be expanded".
Proposed international non-use agreement on solar geoengineering
In
2022, a dozen academics launched a campaign for national policies of
"no public funding, no outdoor experiments, no patents, no deployment,
and no support in international institutions... including in assessments
by the Intergovernmental Panel on Climate Change." The proponents call this an International Non-Use Agreement on Solar Geoengineering.
The advocates’ core argument is that, because SRM would be global
in effect and some countries are much more powerful than others, it is
“not governable in a globally inclusive and just manner within the
current international political system.”
They therefore oppose the “normalization” of SRM and call on countries,
intergovernmental organizations, and others to adopt the proposal’s
five elements.
On the day that the academic article was published, the authors
also launched a campaign calling for others to endorse the proposal. Their open letter
emphasized, in addition to the governance challenges, that SRM’s risks
are “poorly understood and can never be fully known” and that its
potential would threaten commitments to reducing greenhouse gas
emissions. As of December 2024, nearly 540 academics and 60 advocacy organizations have endorsed the proposal. Among the latter is Climate Action Network,
itself a coalition of more than 1900 political organizations. The
position from Climate Action Network included a footnote that excluded
the Environmental Defense Fund and the Natural Resources Defense
Council.
Research funding
As of 2018, total research funding worldwide remained modest, at less than 10 million US dollars annually. Almost all research into SRM has to date consisted of computer modeling or laboratory tests, and there are calls for more research funding as the science is poorly understood.
A study from 2022 investigated where the funding for solar
geoengineering (SG) came from globally. The authors concluded "the
primary funders of SG research do not emanate from fossil capital" and
that there are "close ties to mostly US financial and technological
capital as well as a number of billionaire philanthropists".
Country activities
Few countries have an explicit governmental position on SRM. Those that do, such as the United Kingdom and Germany. support some SRM research even if they do not see it as a current climate policy option. For example, the German Federal Government
does have an explicit position on SRM and stated in 2023 in a strategy
document climate foreign policy: "Due to the uncertainties, implications
and risks, the German Government is not currently considering solar
radiation management (SRM) as a climate policy
option". The document also stated: "Nonetheless, in accordance with the
precautionary principle we will continue to analyse and assess the
extensive scientific, technological, political, social and ethical risks
and implications of SRM, in the context of technology-neutral basic
research as distinguished from technology development for use at scale".
Major academic institutions, including Harvard University, have begun research into SRM, with NOAA alone investing $22 million from 2019 to 2022, though few outdoor tests have been run to date.
Some countries, such as the U.S., Germany, China, Finland,
Norway, and Japan, as well as the European Union, have funded SRM
research.
In 2021, the National Academies of Sciences, Engineering, and Medicine released their consensus study report Recommendations for Solar Geoengineering Research and Research Governance. The report recommended an initial investment into SRM research of $100–200 million over five years.
International collaborations
Under the World Climate Research Programme there is a Lighthouse Activity called Research on Climate Intervention as of 2024. This will include research on all possible climate interventions
(another term for climate engineering): "large-scale Carbon Dioxide
Removal (CDR; also known as Greenhouse Gas Removal, or Negative
Emissions Technologies) and Solar Radiation Modification (SRM; also
known as Solar Reflection Modification, Albedo Modification, or
Radiative Forcing Management)".
Philanthropic and venture capitalist activities
There are also research activities on SRM that are funded by philanthropy. According to Bloomberg News, as of 2024 several American billionaires are funding research into SRM: "A growing number of Silicon Valley
founders and investors are backing research into blocking the sun by
spraying reflective particles high in the atmosphere or making clouds
brighter." The article listed the following billionaires as being notable geoengineering research supporters: Mike Schroepfer, Sam Altman, Matt Cohler, Rachel Pritzker, Bill Gates, Dustin Moskovitz.
SRM research initiatives, or non-profit knowledge hubs, include for example SRM360 which is "supporting an informed, evidence-based discussion of sunlight reflection methods (SRM)". Funding comes from the LAD Climate Fund. David Keith, a long-term proponent of SRM, is one of the members of the advisory board.
Another example is Reflective, which is "a
philanthropically-funded initiative focused on sunlight reflection
research and technology development".
Their funding is "entirely by grants or donations from a number of
leading philanthropies focused on addressing climate change": Outlier
Projects, Navigation Fund, Astera Institute, Open Philanthropy, Crankstart, Matt Cohler, Richard and Sabine Wood.
Deployment activities
Some startups in the private sector have secured funding for potential SRM deployment. One such example is Make Sunsets, which began launching balloons containing helium and sulfur dioxide. The startup sells cooling credits, claiming that each US$10 credit would offset the warming effect of one ton of carbon dioxide warming for a year. Based in California, Make Sunsets
conducted some of its activities in Mexico. In response to these
activities, which were conducted without prior notification or consent,
the Mexican government announced measures to prohibit SRM experiments
within its borders. Even people who advocate for more research into SRM have criticized Make Sunsets' undertaking.
Mexico has announced that it will prohibit "experimental practices with solar geoengineering", although it remains unclear what this policy will include and whether the policy has actually been implemented.
Society and culture
There
have been a handful of studies into attitudes to and opinions of SRM.
These generally find low levels of awareness, uneasiness with the
implementation of SRM, cautious support of research, and a preference
for greenhouse gas emissions reduction. Although most public opinion studies have polled residents of developed countries, those that have examined residents of developing countries—which tend to be more vulnerable to climate change impacts—find slightly greater levels of support there.
The largest assessment of public opinion and perception of SRM,
which had over 30,000 respondents in 30 countries, found that "Global
South publics are significantly more favorable about potential benefits
and express greater support for climate-intervention technologies."
Though the assessment also found Global South publics had greater
concern the technologies could undermine climate-mitigation.
In popular culture
In the film Snowpiercer, as well as in the television spin-off,
an apocalyptic global ice-age is caused by the introduction of a
fictional substance, dubbed, CW-7 into the atmosphere, with the
intention of preventing global-warming by blocking out the light of the
sun.
In the novel TheMinistry for the Future by Kim Stanley Robinson, stratospheric aerosol injection is used by the Indian Government as a climate mitigation measure following a catastrophic and deadly heatwave.
The novel Termination Shock
by Neal Stephenson revolves around a private initiative by a
billionaire, with covert support or opposition from some national
governments, to inject sulfur into the stratosphere using recoverable
gliders launched with a gun.
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