Solar Influence on the Earth System
The Sun varies over a wide range of timescales, from fluctuations associated with magnetic activity that can vary from minutes to years, to changes that may occur over longer term cycles. The most prominent solar cycle thus far observed by scientists is the 11-year solar cycle and its modulations. Related studies on the connection between solar activity and the Earth system include heliophysics (or solar physics), space weather, climate science, and the science of space-borne solar irradiance observatories.
Variations in the total solar irradiance (TSI) contribute to imbalances in Earth’s radiation budget that can induce temperature shifts near the surface. This is because Earth’s temperature can be understood to a first approximation as being controlled by the balance between the radiative energy received from the Sun and Earth’s thermal emission of radiative energy to space. Earth tends to be in an equilibrium state by adjusting its temperature so that its thermal radiation balances the solar energy absorbed by the planet. On this basis, an increase or a decrease in the TSI is expected to result in a proportional increase or decrease in the average temperature of Earth. For example, the TSI changes with an amplitude of nearly 0.1 percent over an 11-year cycle in step with the cycle of sunspots. The small temperature changes (perhaps with an amplitude of about 0.01°C) due to this variation can be detected, albeit with considerable imprecision, in the climate records. However, since the total energy that reaches Earth from Sun varies only by less than 0.1 percent over the 11‑year solar cycle, and varies even less when considered under longer time scales, such a small variation alone cannot possibly drive larger climate variability. On the other hand, connections of the solar cycle to the atmosphere and ocean are small but non-negligible in amplitude, and by working together they can induce processes that result in substantial climate change.
While it is challenging to define the role of the Sun in climate change through such complex interactions, the potential Sun-Climate connection can be better characterized through detailed analyses of the associated chemistry, dynamics, and radiation. Furthermore, attribution of such phenomena to natural and/or anthropogenic causes is necessary to understand specific forcing mechanisms as well as to develop appropriate mitigation and/or adaptation measures.
Solar spectral irradiance (SSI), which denotes solar radiation at different wavelengths (or colors of light), plays a key role in determining the chemistry and distribution of various atmospheric constituents. According to Planck’s law, the energy (E) of one photon (electromagnetic particle) of frequency v is hv, where h is Planck’s constant. Sometimes energy per ‘mole’ of photons is used to describe energy at frequency ν. Thus,
E = Lhν or E = L.hc/λ = 119625/λ k.J.mol-1 for wavelength λ,
where L is the Avogadro constant and c is the speed of light. In photochemistry, ‘hv’ is used in chemical equations as a shorthand for a photon that is a reactant, e.g.
O3 + hν -> O2 + O
At very short wavelengths (such as X-ray, extreme ultraviolet (EUV: 10nm – 120nm), and Hydrogen Lyman-alpha (121-122nm)), solar radiation has a high photon energy that is responsible for ionizing the upper atmosphere.
Longer-wavelength (>175nm) ultraviolet (UV) radiation can penetrate the stratosphere, where it affects ozone formation. Stratospheric ozone plays an important role in the Earth system, as it not only shields the Earth from the harmful UV radiation, but also is an important greenhouse gas that traps terrestrial thermal radiation from the surface. Here we focus on the chemistry of stratospheric ozone.
For a given atmospheric composition, change in spectral solar irradiance (SSI) leads to a proportional change in the atmospheric heating rate. However, change in SSI also induces change in atmospheric composition; causing the change in heating rate to occur in a non-linear fashion. Thus, photochemistry is particularly important in determining atmospheric composition as well as the temperature structure.
In the stratosphere, the main chemical reactions for ozone are:
O2 + hν -> O + O (1) O + O2 + M -> O3 + M (2) O3 + hν -> O2 + O (3) O + O3 -> 2O2 (4) O3 + X -> XO + O2 (5) XO + O -> X + O2 (6)
The first four reactions are referred to as oxygen-only chemistry, which was first proposed by Chapman in 1930 to explain the ozone layer. In reaction (1), molecular oxygen in the stratosphere is photodissociated by absorption in the Herzberg continuum (i.e., 180–240 nm wavelength). Then, in reaction (2), atomic oxygen attaches itself to O2 in the presence of an unspecified substrate, M, to form ozone. The third reaction is the photodissociation of ozone, mainly by radiation in the Hartley band (200–310 nm wavelength), into one atom and one molecule of oxygen. Note that this does not represent the fundamental destruction of the ozone because the oxygen atom produced can quickly recombine with an oxygen molecule. The destruction of ozone by its reaction with oxygen atoms is presented in reaction (4).
Reactions (5) and (6) represent catalytic processes that remove O or O3; where catalyst X may include OH, NO, and Cl. Thus, the ozone destruction may be caused by any catalyst X. The various destruction paths are important at different altitudes but the combined effect is an ozone concentration profile that peaks near 25 km in equatorial regions.
In summary, ozone is produced by short wavelength solar UV radiation and destroyed by UV radiation at longer wavelengths. Because the amplitude of solar cycle variability is greater in the short wavelength UV, ozone production likely dominates ozone destruction, leading to a higher net production of stratospheric ozone during solar maximum compared to solar minimum. From solar minimum to maximum, satellite observations suggest about 2% of ozone increase in the upper stratosphere with a secondary maximum in the lower stratosphere.
Although photochemistry produces atmospheric ozone, it does not explain the global ozone distribution. Photochemistry produces more ozone in the tropical stratosphere than in the mid-latitude and polar regions because the overhead sun in the tropics is more effective in breaking apart oxygen molecules into oxygen atoms, which quickly react with other oxygen molecules to form ozone. Ironically, higher concentrations of stratospheric ozone are found outside of the tropics. This higher ozone concentration in the higher latitudes is the result of the slow atmospheric circulation that moves ozone from the tropics into the mid-latitude and polar regions. This slow circulation is widely known as the Brewer-Dobson circulation.
This Brewer-Dobson circulation is not the result of solar heating in the tropics and cooling in the polar region, which causes a large equator-to-pole (meridional) overturning of air as warm (tropical) air rises and cold (polar) air sinks, similar to the so-called Hadley circulation. Rather, the Brewer-Dobson circulation results from wave motions in the extratropical stratosphere.
How solar energy interacts with Earth’s atmosphere depends on solar spectral irradiance (SSI). The coupling between solar forcing and atmospheric dynamics plays an important role in propagating solar signals from the upper stratosphere, where solar heating is strongest, to the lower stratosphere and troposphere: the so-called “top-down” mechanism. Over an 11-year solar cycle, the combined effects of UV irradiance changes and ozone feedback leads to a temperature anomaly of ~1-2K in the equatorial stratopause. This anomaly alters the meridional temperature gradient and hence the wind field through thermal wind balance, which in turn alters wave propagation and decelerates the Brewer-Dobson circulation near the solar maxima. The resulting changes in the stratosphere modify the tropical tropospheric circulation and thus contribute to an enhancement and poleward expansion of the tropical precipitation. Studies also show a broadening of the Hadley cell in response to the 11-year solar activity.
A second mechanism that can magnify the response to an initial small solar forcing involves air-sea coupling and interaction with incoming solar radiation at the surface in the relatively cloud-free areas of the subtropics. This so-called “bottom-up” mechanism suggests that the increase in solar irradiance during solar maxima over cloud-free regions in the subtropics leads to greater evaporation, moisture convergence, and precipitation; resulting in upward vertical motions that strengthen the Hadley and Walker circulations. This mechanism further leads to stronger subsidence in the subtropics, resulting in a positive feedback that reduces clouds and allows increased solar forcing.
Studies also suggest that these two mechanisms work in the same direction, acting together to produce an amplified SST, precipitation, and cloud response in the tropical Pacific, even for such a relatively small solar forcing.
Clouds and Solar Radiation
Solar radiation is the primary energy source for Earth. On a global, long-term scale, the incoming solar radiation is approximately balanced by the reflected (the difference between incident and absorbed) solar radiation and the emitted terrestrial radiation or outgoing longwave radiation (ORL). The radiative effective temperature (~255 K) of the Earth is fundamentally the result of the incident solar radiation at the top of the atmosphere (TOA) and the planetary albedo of the Earth. Earth’s radiative effective temperature is much smaller than the average surface temperature (~288 K) due to the atmospheric greenhouse effect. Accurate observation of Earth’s radiation budget is necessary for understanding the current climate and predicting future climate.
The total solar irradiance (TSI) at the mean Sun-Earth distance may be regarded as the “income” of Earth’s TOA radiation budget. The TSI at the TOA is also an important parameter for all climate models. Thus, it needs to be measured very accurately. Continuous observation of TSI from space started since 1979. The observations reveal that the TSI varies with the 11-year solar cycle. From solar minimum to solar maximum, there is an increase of about 0.1% in the TSI, indicating that solar energy flux is not a true constant, contrary to the use of the term “solar constant”. Although the variation of TSI with the 11-year solar activity has been well known for some time, based on observations from current spaceborne sensors, the most accurate TSI observation is 1360.8 ± 0.5 Wm-2, which is significantly lower than the value of 1365.4 ± 1.3 Wm-2 that was established in 1990.
Clouds are one of the most influential atmospheric variables of planet Earth that can change the amount of solar energy input to Earth’s climate system by altering its planetary albedo. Clouds cover about 70% of the globe and a small change in cloud planetary albedo can induce a significant imbalance in Earth’s energy budget. In the longwave region of the spectrum, clouds generally reduce the outgoing longwave radiation, resulting in the heating of the Earth. With solar (or shortwave) radiation, clouds are typically much more reflective than the underlying surface, resulting in the cooling of the planet. Thus, low, thick clouds (e.g., stratus clouds) primarily reflect solar radiation and cool the Earth, whereas high, thin clouds (i.e., cirrus clouds) that are transparent at solar radiation wavelengths, intercept and radiate some of the outgoing infrared radiation emanating from the Earth back downward to warm the Earth. On global average, the cooling and warming effects of clouds are very close, but cooling predominates.
Natural vs Anthropogenic Variability
There is no substantive scientific evidence that solar variability is the cause of climate change in the past 50 years, according to the National Research Council report on the Effects of Solar Variability on Earth's Climate [NRC, 2012], the Intergovernmental Panel on Climate Change Fourth Assessment report [IPCC, 2007], and the National Research Council report on climate choices [NRC, 2011]. This fact has been reiterated in the more recent IPCC Fifth Assessment report [IPCC, 2013, see Frequently Asked Questions FAQ 5.1: “Is the Sun a Major Driver of Recent Changes in Climate?”, Figure 1, pp. 392–393], where it is clearly illustrated that the rapidly increasing global surface temperatures over the last 50 years or so is dominated by anthropogenic influences relative to the combined known natural (solar, volcanic, internal) influences. However, the mechanisms by which solar variations can affect climate over longer timescales remain an open area of research.
Total and Spectral Solar Irradiances
The total solar input energy to Earth (i.e., TSI) consists of radiation from different wavelengths, with the primary contributions being from ultraviolet (UV), visible (VIS), and near infrared (NIR). The atmosphere and ocean respond differently to different wavelengths of solar radiation. The UV spectrum is responsible for stratospheric heating and ozone formation; the VIS spectrum heats the ocean mixed layer and drives the upper oceanic circulation; and the NIR directly heats the troposphere through water vapor absorption. Therefore, accurate observation of the spectral solar irradiance (SSI) is essential for better understanding of the associated physical processes in Sun-climate studies.
IPCC, 2007. Summary for Policymakers, Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, New York, 2007.
IPCC, 2013: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp, doi:10.1017/CBO9781107415324.
NRC (National Research Council), 2011. America’s Climate Choices. Washington, DC: The National Academies Press. doi: 10.17226/12781.
NRC (National Research Council), 2012. The Effects of Solar Variability on Earth's Climate: A Workshop Report. Washington, DC: The National Academies Press. doi: 10.17226/13519.