Spectral intensity of sunlight (average at top of atmosphere) and thermal radiation emitted by Earth's surface.
In climate science, longwave radiation (LWR) is electromagneticthermal radiation emitted by Earth's surface, atmosphere, and clouds. It is also referred to as terrestrial radiation. This radiation is in the infrared portion of the spectrum, but is distinct from the shortwave (SW) near-infrared radiation found in sunlight.
Outgoing longwave radiation (OLR) is the longwave radiation emitted to space from the top of Earth's atmosphere. It may also be referred to as emitted terrestrial radiation. Outgoing longwave radiation plays an important role in planetary cooling.
Longwave radiation generally spans wavelengths ranging from 3–100
micrometres (μm). A cutoff of 4 μm is sometimes used to differentiate
sunlight from longwave radiation. Less than 1% of sunlight has
wavelengths greater than 4 μm. Over 99% of outgoing longwave radiation
has wavelengths between 4 μm and 100 μm.
The flux of energy transported by outgoing longwave radiation is typically measured in units of watts per metre squared (W⋅m−2). In the case of global energy flux, the W/m2
value is obtained by dividing the total energy flow over the surface of
the globe (measured in watts) by the surface area of the Earth, 5.1×1014 m2 (5.1×108 km2; 2.0×108 sq mi).
Emitting outgoing longwave radiation is the only way Earth loses energy to space, i.e., the only way the planet cools itself. Radiative heating from absorbed sunlight, and radiative cooling to space via OLR power the heat engine that drives atmospheric dynamics.
The balance between OLR (energy lost) and incoming solar shortwave radiation (energy gained) determines whether the Earth is experiencing global heating or cooling (see Earth's energy budget).
Planetary energy balance
The growth in Earth's energy imbalance from satellite and in situ measurements (2005–2019). A rate of +1.0 W/m2 summed over the planet's surface equates to a continuous heat uptake of about 500 terawatts (~0.3% of the incident solar radiation).
Outgoing longwave radiation (OLR) constitutes a critical component of Earth's energy budget.
The principle of conservation of energy
says that energy cannot appear or disappear. Thus, any energy that
enters a system but does not leave must be retained within the system.
So, the amount of energy retained on Earth (in Earth's climate system)
is governed by an equation:
[change in Earth's energy] = [energy arriving] − [energy leaving].
Energy arrives in the form of absorbed solar radiation
(ASR). Energy leaves as outgoing longwave radiation (OLR). Thus, the
rate of change in the energy in Earth's climate system is given by Earth's energy imbalance (EEI):
.
When energy is arriving at a higher rate than it leaves (i.e., ASR
> OLR, so that EEI is positive), the amount of energy in Earth's
climate increases. Temperature
is a measure of the amount of thermal energy in matter. So, under these
circumstances, temperatures tend to increase overall (though
temperatures might decrease in some places as the distribution of energy
changes). As temperatures increase, the amount of thermal radiation
emitted also increases, leading to more outgoing longwave radiation
(OLR), and a smaller energy imbalance (EEI).
Similarly, if energy arrives at a lower rate than it leaves
(i.e., ASR < OLR, so than EEI is negative), the amount of energy in
Earth's climate decreases, and temperatures tend to decrease overall. As
temperatures decrease, OLR decreases, making the imbalance closer to
zero.
In this fashion, a planet naturally constantly adjusts its
temperature so as to keep the energy imbalance small. If there is more
solar radiation absorbed than OLR emitted, the planet will heat up. If
there is more OLR than absorbed solar radiation the planet will cool. In
both cases, the temperature change works to shift the energy imbalance
towards zero. When the energy imbalance is zero, a planet is said to be
in radiative equilibrium. Planets natural tend to a state of approximate radiative equilibrium.
In recent decades, energy has been measured to be arriving on
Earth at a higher rate than it leaves, corresponding to planetary
warming. The energy imbalance has been increasing.
It can take decades to centuries for oceans to warm and planetary
temperature to shift sufficiently to compensate for an energy imbalance.
Emission
Thermal radiation is emitted by nearly all matter, in proportion to the fourth power of its absolute temperature.
In particular, the emitted energy flux, (measured in W/m2) is given by the Stefan–Boltzmann law for non-blackbody matter:
The
emissivity of Earth's surface has been measured to be in the range 0.65
to 0.99 (based on observations in the 8-13 micron wavelength range)
with the lowest values being for barren desert regions. The emissivity
is mostly above 0.9, and the global average surface emissivity is
estimated to be around 0.95.
Atmosphere
The
most common gases in air (i.e., nitrogen, oxygen, and argon) have a
negligible ability to absorb or emit longwave thermal radiation.
Consequently, the ability of air to absorb and emit longwave radiation
is determined by the concentration of trace gases like water vapor and
carbon dioxide.
According to Kirchhoff's law of thermal radiation, the emissivity of matter is always equal to its absorptivity, at a given wavelength. At some wavelengths, greenhouse gases absorb 100% of the longwave radiation emitted by the surface.
So, at those wavelengths, the emissivity of the atmosphere is 1 and the
atmosphere emits thermal radiation much like an ideal blackbody would.
However, this applies only at wavelengths where the atmosphere fully
absorbs longwave radiation.
Although greenhouse gases in air have a high emissivity at some
wavelengths, this does not necessarily correspond to a high rate of
thermal radiation being emitted to space. This is because the atmosphere
is generally much colder than the surface, and the rate at which
longwave radiation is emitted scales as the fourth power of temperature.
Thus, the higher the altitude at which longwave radiation is emitted,
the lower its intensity.
Atmospheric absorption
The atmosphere is relatively transparent to solar radiation, but it is nearly opaque to longwave radiation. The atmosphere typically absorbs most of the longwave radiation emitted by the surface. Absorption of longwave radiation prevents that radiation from reaching space.
At wavelengths where the atmosphere absorbs surface radiation,
some portion of the radiation that was absorbed is replaced by a lesser
amount of thermal radiation emitted by the atmosphere at a higher
altitude.
When absorbed, the energy transmitted by this radiation is transferred to the substance that absorbed it. However, overall, greenhouse gases in the troposphere emit more thermal radiation than they absorb, so longwave radiative heat transfer has a net cooling effect on air.
Atmospheric window
Assuming no cloud cover, most of the surface emissions that reach space do so through the atmospheric window.
The atmospheric window is a region of the electromagnetic wavelength
spectrum between 8 and 11 μm where the atmosphere does not absorb
longwave radiation (except for the ozone band between 9.6 and 9.8 μm).
The absorption of longwave radiation by gases depends on the specific absorption bands of the gases in the atmosphere. The specific absorption bands are determined by their molecular structure and energy levels. Each type of greenhouse gas has a unique group of absorption bands that correspond to particular wavelengths of radiation that the gas can absorb.
Clouds
The
OLR balance is affected by clouds, dust, and aerosols in the
atmosphere. Clouds tend to block penetration of upwelling longwave
radiation, causing a lower flux of long-wave radiation penetrating to
higher altitudes.
Clouds are effective at absorbing and scattering longwave radiation,
and therefore reduce the amount of outgoing longwave radiation.
Clouds have both cooling and warming effects. They have a cooling effect insofar as they reflect sunlight (as measured by cloud albedo),
and a warming effect, insofar as they absorb longwave radiation. For
low clouds, the reflection of solar radiation is the larger effect; so,
these clouds cool the Earth. In contrast, for high thin clouds in cold
air, the absorption of longwave radiation is the more significant
effect; so these clouds warm the planet.
Details
The
interaction between emitted longwave radiation and the atmosphere is
complicated due to the factors that affect absorption. The path of the
radiation in the atmosphere also determines radiative absorption: longer
paths through the atmosphere result in greater absorption because of
the cumulative absorption by many layers of gas. Lastly, the temperature
and altitude of the absorbing gas also affect its absorption of
longwave radiation.
OLR is affected by Earth's surface skin temperature (i.e, the
temperature of the top layer of the surface), skin surface emissivity,
atmospheric temperature, water vapor profile, and cloud cover.
Day and night
The net all-wave radiation is dominated by longwave radiation during the night and in the polar regions.
While there is no absorbed solar radiation during the night,
terrestrial radiation continues to be emitted, primarily as a result of
solar energy absorbed during the day.
Relationship to greenhouse effect
Outgoing
radiation and greenhouse effect as a function of frequency. The
greenhouse effect is visible as the area of the upper red area, and the
greenhouse effect associated with CO2 is directly visible as the large dip near the center of the OLR spectrum.
The reduction of the outgoing longwave radiation (OLR), relative to
longwave radiation emitted by the surface, is at the heart of the greenhouse effect.
More specifically, the greenhouse effect may be defined
quantitatively as the amount of longwave radiation emitted by the
surface that does not reach space. On Earth as of 2015, about 398 W/m2 of longwave radiation was emitted by the surface, while OLR, the amount reaching space, was 239 W/m2. Thus, the greenhouse effect was 398−239 = 159 W/m2, or 159/398 = 40% of surface emissions, not reaching space.
the fraction of surface emissions that are absorbed is
increased, decreasing OLR (unless 100% of surface emissions at that
wavelength are already being absorbed);
the altitude from which the atmosphere emits that that wavelength to
space increases (since the altitude at which the atmosphere becomes
transparent to that wavelength increases); if the emission altitude is
within the troposphere, the temperature of the emitting air will be
lower, which will result in a reduction in OLR at that wavelength.
The size of the reduction in OLR will vary by wavelength. Even if OLR
does not decrease at certain wavelengths (e.g., because 100% of surface
emissions are absorbed and the emission altitude is in the
stratosphere), increased greenhouse gas concentration can still lead to
significant reductions in OLR at other wavelengths where absorption is
weaker.
When OLR decreases, this leads to an energy imbalance, with
energy received being greater than energy lost, causing a warming
effect. Therefore, an increase in the concentrations of greenhouse gases
causes energy to accumulate in Earth's climate system, contributing to global warming.
Surface budget fallacy
If
the absorptivity of the gas is high and the gas is present in a high
enough concentration, the absorption at certain wavelengths becomes
saturated.
This means there is enough gas present to completely absorb the
radiated energy at that wavelength before the upper atmosphere is
reached.
It is sometimes incorrectly argued that this means an
increase in the concentration of this gas will have no additional effect
on the planet's energy budget. This argument neglects the fact that
outgoing longwave radiation is determined not only by the amount of
surface radiation that is absorbed, but also by the altitude (and temperature) at which longwave radiation is emitted
to space. Even if 100% of surface emissions are absorbed at a given
wavelength, the OLR at that wavelength can still be reduced by increased
greenhouse gas concentration, since the increased concentration leads
to the atmosphere emitting longwave radiation to space from a higher
altitude. If the air at that higher altitude is colder (as is true
throughout the troposphere), then thermal emissions to space will be
reduced, decreasing OLR.
False conclusions about the implications of absorption being "saturated" are examples of the surface budget fallacy,
i.e., erroneous reasoning that results from focusing on energy exchange
at the surface, instead of focusing on the top-of-atmosphere (TOA)
energy balance.
Measurements
Example wavenumber spectrum of Earth's infrared emissions (400-1600 cm−1) measured by IRIS on Nimbus 4 in year 1970.
Measurements of outgoing longwave radiation at the top of the
atmosphere and of longwave radiation back towards the surface are
important to understand how much energy is retained in Earth's climate
system: for example, how thermal radiation cools and warms the surface,
and how this energy is distributed to affect the development of clouds.
Observing this radiative flux from a surface also provides a practical
way of assessing surface temperatures on both local and global scales. This energy distribution is what drives atmospheric thermodynamics.
OLR
Outgoing
long-wave radiation (OLR) has been monitored and reported since 1970 by
a progression of satellite missions and instruments.
Longwave radiation at the surface (both outward and inward) is mainly measured by pyrgeometers. A most notable ground-based network for monitoring surface long-wave radiation is the Baseline Surface Radiation Network (BSRN), which provides crucial well-calibrated measurements for studying global dimming and brightening.
Data
Data on surface longwave radiation and OLR is available from a number of sources including:
NASA GEWEX Surface Radiation Budget (1983-2007)
NASA Clouds and the Earth's Radiant Energy System (CERES) project (2000-2022)
OLR calculation and simulation
Simulated wavenumber spectrum of the Earth's outgoing longwave radiation (OLR) using ARTS. In addition the black-body radiation for a body at surface temperature Ts and at tropopause temperature Tmin is shown.Simulated wavelength spectrum of Earth's OLR under clear-sky conditions using MODTRAN.
Many applications call for calculation of long-wave radiation quantities. Local radiative cooling
by outgoing longwave radiation, suppression of radiative cooling (by
downwelling longwave radiation cancelling out energy transfer by
upwelling longwave radiation), and radiative heating through incoming
solar radiation drive the temperature and dynamics of different parts of
the atmosphere.
Another common approach is to estimate values using surface temperature and emissivity, then compare to satellite top-of-atmosphere radiance or brightness temperature.
There are online interactive tools that allow one to see the
spectrum of outgoing longwave radiation that is predicted to reach space
under various atmospheric conditions.
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 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.
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, 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.
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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