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95-575: Variations in the climate of the Sahara region can, at the simplest level, be attributed to the changes in insolation because of slow shifts in Earth's orbital parameters. The parameters include the precession of the equinoxes , obliquity , and eccentricity as put forth by the Milankovitch theory . The precession of the equinoxes is regarded as the most important orbital parameter in the formation of

190-1086: A , b and c are arc lengths, in radians, of the sides of a spherical triangle. C is the angle in the vertex opposite the side which has arc length c . Applied to the calculation of solar zenith angle Θ , the following applies to the spherical law of cosines: C = h c = Θ a = 1 2 π − φ b = 1 2 π − δ cos ⁡ ( Θ ) = sin ⁡ ( φ ) sin ⁡ ( δ ) + cos ⁡ ( φ ) cos ⁡ ( δ ) cos ⁡ ( h ) {\displaystyle {\begin{aligned}C&=h\\c&=\Theta \\a&={\tfrac {1}{2}}\pi -\varphi \\b&={\tfrac {1}{2}}\pi -\delta \\\cos(\Theta )&=\sin(\varphi )\sin(\delta )+\cos(\varphi )\cos(\delta )\cos(h)\end{aligned}}} This equation can be also derived from

285-511: A 19,000 to 23,000-year precession cycle occurs roughly 1500 to 2000 years after the June insolation maximum. Two other possible explanations for the observed lag in the data have been put forward. The first suggest that the development of the monsoons in the subtropics is tempered by the slow melting of polar ice sheets . Thus the full strength of the monsoonal pattern is not observed until the polar ice sheets have become so small that their impact on

380-481: A clear day. When 1361 W/m is arriving above the atmosphere (when the Sun is at the zenith in a cloudless sky), direct sun is about 1050 W/m , and global radiation on a horizontal surface at ground level is about 1120 W/m . The latter figure includes radiation scattered or reemitted by the atmosphere and surroundings. The actual figure varies with the Sun's angle and atmospheric circumstances. Ignoring clouds,

475-464: A consensus of observations or theory, Q ¯ day {\displaystyle {\overline {Q}}^{\text{day}}} can be calculated for any latitude φ and θ . Because of the elliptical orbit, and as a consequence of Kepler's second law , θ does not progress uniformly with time. Nevertheless, θ  = 0° is exactly the time of the March equinox, θ  = 90°

570-421: A day is the average of Q over one rotation, or the hour angle progressing from h = π to h = −π : Q ¯ day = − 1 2 π ∫ π − π Q d h {\displaystyle {\overline {Q}}^{\text{day}}=-{\frac {1}{2\pi }}{\int _{\pi }^{-\pi }Q\,dh}} Let h 0 be

665-404: A decrease thereafter. PMOD instead presents a steady decrease since 1978. Significant differences can also be seen during the peak of solar cycles 21 and 22. These arise from the fact that ACRIM uses the original TSI results published by the satellite experiment teams while PMOD significantly modifies some results to conform them to specific TSI proxy models. The implications of increasing TSI during

760-407: A deep solar minimum of 2005–2010) to be +0.58 ± 0.15 W/m , +0.60 ± 0.17 W/m and +0.85 W/m . Estimates from space-based measurements range +3–7   W/m . SORCE/TIM's lower TSI value reduces this discrepancy by 1   W/m . This difference between the new lower TIM value and earlier TSI measurements corresponds to a climate forcing of −0.8   W/m , which is comparable to

855-488: A global fully coupled atmosphere–ocean–sea ice climate model , which confirmed that precession and obliquity can combine to increase precipitation in North Africa through insolation feedbacks . Orbital eccentricity is a measure of the deviation of the Earth's orbit from a perfect circle. If the Earth's orbit was a perfect circle then the eccentricity would have a value of 0, and eccentricity value of 1 would indicate

950-1331: A more general formula: cos ⁡ ( Θ ) = sin ⁡ ( φ ) sin ⁡ ( δ ) cos ⁡ ( β ) + sin ⁡ ( δ ) cos ⁡ ( φ ) sin ⁡ ( β ) cos ⁡ ( γ ) + cos ⁡ ( φ ) cos ⁡ ( δ ) cos ⁡ ( β ) cos ⁡ ( h ) − cos ⁡ ( δ ) sin ⁡ ( φ ) sin ⁡ ( β ) cos ⁡ ( γ ) cos ⁡ ( h ) − cos ⁡ ( δ ) sin ⁡ ( β ) sin ⁡ ( γ ) sin ⁡ ( h ) {\displaystyle {\begin{aligned}\cos(\Theta )=\sin(\varphi )\sin(\delta )\cos(\beta )&+\sin(\delta )\cos(\varphi )\sin(\beta )\cos(\gamma )+\cos(\varphi )\cos(\delta )\cos(\beta )\cos(h)\\&-\cos(\delta )\sin(\varphi )\sin(\beta )\cos(\gamma )\cos(h)-\cos(\delta )\sin(\beta )\sin(\gamma )\sin(h)\end{aligned}}} where β

1045-466: A number of fixes for this problem. The most reasonable fix can be shown through a simple analog to today's climate. Currently the peak in solar radiation occurs on June 21, but the peak of the summer monsoon in North Africa occurs a month later in July. A one-month lag such as this should be represented by roughly a 1500 to 2000 year lag in the monsoonal circulation maximum, because a July insolation maximum in

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1140-438: A parabola. The Earth has two cycles of eccentricity that occur on cycles of 100,000 and 400,000 years. Over the years the Earth's eccentricity has varied between 0.005 and 0.0607, today the eccentricity of Earth's orbit is approximately 0.0167. While the value of eccentricity does impact the distance of the Earth from the Sun, its primary impact on insolation comes from its modulating effect on the procession cycle. For example, when

1235-432: A revolving body is the geometric plane in which its orbit lies. Three non- collinear points in space suffice to determine an orbital plane. A common example would be the positions of the centers of a massive body (host) and of an orbiting celestial body at two different times/points of its orbit. The orbital plane is defined in relation to a reference plane by two parameters : inclination ( i ) and longitude of

1330-426: A spectral graph as function of wavelength), or per- Hz (for a spectral function with an x-axis of frequency). When one plots such spectral distributions as a graph, the integral of the function (area under the curve) will be the (non-spectral) irradiance. e.g.: Say one had a solar cell on the surface of the earth facing straight up, and had DNI in units of W/m^2 per nm, graphed as a function of wavelength (in nm). Then,

1425-401: A surface is largest when the surface directly faces (is normal to) the sun. As the angle between the surface and the Sun moves from normal, the insolation is reduced in proportion to the angle's cosine ; see effect of Sun angle on climate . In the figure, the angle shown is between the ground and the sunbeam rather than between the vertical direction and the sunbeam; hence the sine rather than

1520-879: A time series for a Q ¯ d a y {\displaystyle {\overline {Q}}^{\mathrm {day} }} for a particular time of year, and particular latitude, is a useful application in the theory of Milankovitch cycles. For example, at the summer solstice, the declination δ is equal to the obliquity  ε . The distance from the Sun is R o R E = 1 + e cos ⁡ ( θ − ϖ ) = 1 + e cos ⁡ ( π 2 − ϖ ) = 1 + e sin ⁡ ( ϖ ) {\displaystyle {\frac {R_{o}}{R_{E}}}=1+e\cos(\theta -\varpi )=1+e\cos \left({\frac {\pi }{2}}-\varpi \right)=1+e\sin(\varpi )} For this summer solstice calculation,

1615-476: A timescale of decades to centuries. The onset and termination of the African Humid Period both occurred when the insolation cycle reached a value of roughly 4.2% higher than today. However, shifts in the insolation cycle are too gradual to cause abrupt climate transitions like those seen at the onset and termination of the African Humid Period all on their own. So to account for these rapid shifts in

1710-1003: Is sin ⁡ ( δ ) = sin ⁡ ( ε ) sin ⁡ ( θ ) {\displaystyle \sin(\delta )=\sin(\varepsilon )\sin(\theta )} . ) The conventional longitude of perihelion ϖ is defined relative to the March equinox, so for the elliptical orbit: R E = R o ( 1 − e 2 ) 1 + e cos ⁡ ( θ − ϖ ) {\displaystyle R_{E}={\frac {R_{o}(1-e^{2})}{1+e\cos(\theta -\varpi )}}} or R o R E = 1 + e cos ⁡ ( θ − ϖ ) 1 − e 2 {\displaystyle {\frac {R_{o}}{R_{E}}}={\frac {1+e\cos(\theta -\varpi )}{1-e^{2}}}} With knowledge of ϖ , ε and e from astrodynamical calculations and S o from

1805-405: Is π r , in which r is the radius of the Earth. Because the Earth is approximately spherical , it has total area 4 π r 2 {\displaystyle 4\pi r^{2}} , meaning that the solar radiation arriving at the top of the atmosphere, averaged over the entire surface of the Earth, is simply divided by four to get 340   W/m . In other words, averaged over

1900-447: Is a number of a day of the year. Total solar irradiance (TSI) changes slowly on decadal and longer timescales. The variation during solar cycle 21 was about 0.1% (peak-to-peak). In contrast to older reconstructions, most recent TSI reconstructions point to an increase of only about 0.05% to 0.1% between the 17th century Maunder Minimum and the present. However, current understanding based on various lines of evidence suggests that

1995-403: Is a primary cause of the higher irradiance values measured by earlier satellites in which the precision aperture is located behind a larger, view-limiting aperture. The TIM uses a view-limiting aperture that is smaller than the precision aperture that precludes this spurious signal. The new estimate is from better measurement rather than a change in solar output. A regression model-based split of

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2090-438: Is absorbed and the remainder reflected. Usually, the absorbed radiation is converted to thermal energy , increasing the object's temperature. Humanmade or natural systems, however, can convert part of the absorbed radiation into another form such as electricity or chemical bonds , as in the case of photovoltaic cells or plants . The proportion of reflected radiation is the object's reflectivity or albedo . Insolation onto

2185-916: Is an angle from the horizontal and γ is an azimuth angle . The separation of Earth from the Sun can be denoted R E and the mean distance can be denoted R 0 , approximately 1 astronomical unit (AU). The solar constant is denoted S 0 . The solar flux density (insolation) onto a plane tangent to the sphere of the Earth, but above the bulk of the atmosphere (elevation 100 km or greater) is: Q = { S o R o 2 R E 2 cos ⁡ ( Θ ) cos ⁡ ( Θ ) > 0 0 cos ⁡ ( Θ ) ≤ 0 {\displaystyle Q={\begin{cases}S_{o}{\frac {R_{o}^{2}}{R_{E}^{2}}}\cos(\Theta )&\cos(\Theta )>0\\0&\cos(\Theta )\leq 0\end{cases}}} The average of Q over

2280-408: Is at its weakest. The wind pattern that results from the continent/ocean insolation temperature gradient is known as a monsoon . Values of summer insolation are more important for a region's climate than winter values. This is because the winter phase of a monsoon is always dry. Thus the flora and fauna of a monsoonal climate are determined by the amount of rain that falls during the summer phase of

2375-629: Is exactly the time of the June solstice, θ  = 180° is exactly the time of the September equinox and θ  = 270° is exactly the time of the December solstice. A simplified equation for irradiance on a given day is: Q ≈ S 0 ( 1 + 0.034 cos ⁡ ( 2 π n 365.25 ) ) {\displaystyle Q\approx S_{0}\left(1+0.034\cos \left(2\pi {\frac {n}{365.25}}\right)\right)} where n

2470-503: Is found in the high latitudes. Two possible mechanisms for the existence of an obliquity tracer found in the Eastern Mediterranean Aeolian dust deposits have been proposed. The first of which suggests that at times of higher obliquity the temperature gradient between the poles and the equator in the southern hemisphere is greater during boreal summer (summer in the northern hemisphere). As a result of this gradient

2565-456: Is known as Milankovitch cycles . Distribution is based on a fundamental identity from spherical trigonometry , the spherical law of cosines : cos ⁡ ( c ) = cos ⁡ ( a ) cos ⁡ ( b ) + sin ⁡ ( a ) sin ⁡ ( b ) cos ⁡ ( C ) {\displaystyle \cos(c)=\cos(a)\cos(b)+\sin(a)\sin(b)\cos(C)} where

2660-2450: Is nearly constant over the course of a day, and can be taken outside the integral ∫ π − π Q d h = ∫ h o − h o Q d h = S o R o 2 R E 2 ∫ h o − h o cos ⁡ ( Θ ) d h = S o R o 2 R E 2 [ h sin ⁡ ( φ ) sin ⁡ ( δ ) + cos ⁡ ( φ ) cos ⁡ ( δ ) sin ⁡ ( h ) ] h = h o h = − h o = − 2 S o R o 2 R E 2 [ h o sin ⁡ ( φ ) sin ⁡ ( δ ) + cos ⁡ ( φ ) cos ⁡ ( δ ) sin ⁡ ( h o ) ] {\displaystyle {\begin{aligned}\int _{\pi }^{-\pi }Q\,dh&=\int _{h_{o}}^{-h_{o}}Q\,dh\\[5pt]&=S_{o}{\frac {R_{o}^{2}}{R_{E}^{2}}}\int _{h_{o}}^{-h_{o}}\cos(\Theta )\,dh\\[5pt]&=S_{o}{\frac {R_{o}^{2}}{R_{E}^{2}}}{\Bigg [}h\sin(\varphi )\sin(\delta )+\cos(\varphi )\cos(\delta )\sin(h){\Bigg ]}_{h=h_{o}}^{h=-h_{o}}\\[5pt]&=-2S_{o}{\frac {R_{o}^{2}}{R_{E}^{2}}}\left[h_{o}\sin(\varphi )\sin(\delta )+\cos(\varphi )\cos(\delta )\sin(h_{o})\right]\end{aligned}}} Therefore: Q ¯ day = S o π R o 2 R E 2 [ h o sin ⁡ ( φ ) sin ⁡ ( δ ) + cos ⁡ ( φ ) cos ⁡ ( δ ) sin ⁡ ( h o ) ] {\displaystyle {\overline {Q}}^{\text{day}}={\frac {S_{o}}{\pi }}{\frac {R_{o}^{2}}{R_{E}^{2}}}\left[h_{o}\sin(\varphi )\sin(\delta )+\cos(\varphi )\cos(\delta )\sin(h_{o})\right]} Let θ be

2755-449: Is roughly 23.5°. However, over long periods of time the tilt of Earth's axis of rotation changes because of the uneven distribution of mass across the planet and gravitational interactions with the Sun , Moon , and planets . Due to these interactions the tilt of Earth's axis of rotation varies between 22.2° and 24.5° on a 41,000-year cycle. Modulation of the precession driven insolation cycle

2850-484: Is simply a measure of the amount of solar radiation received on a given surface area in a given time period, is the fundamental factor behind the Orbital Monsoon Hypothesis. Due to variations in heat capacity , continents heat up faster than surrounding oceans during summer months when insolation is at its strongest and cool off faster than the surrounding oceans during winter months when insolation

2945-547: Is the primary impact of obliquity on the North African Monsoon. Evidence for the impact of obliquity on the intensity of the North African Monsoon has been found in records of dust deposits from ocean cores in the Eastern Mediterranean that occur as a result of Aeolian processes . This evidence requires complex feedback mechanisms to explain since the strongest impact of obliquity on insolation

North African climate cycles - Misplaced Pages Continue

3040-553: The Earth's surface after atmospheric absorption and scattering . Irradiance in space is a function of distance from the Sun, the solar cycle , and cross-cycle changes. Irradiance on the Earth's surface additionally depends on the tilt of the measuring surface, the height of the Sun above the horizon, and atmospheric conditions. Solar irradiance affects plant metabolism and animal behavior. The study and measurement of solar irradiance have several important applications, including

3135-494: The Moon's orbit as the angle between its orbital plane and the planet's equatorial plane . The coordinate system defined that uses the orbital plane as the x y {\displaystyle xy} plane is known as the perifocal coordinate system . For launch vehicles and artificial satellites, the orbital plane is a defining parameter of an orbit; as in general, it will take a very large amount of propellant to change

3230-635: The Tropic of Cancer . However, as the obliquity changes, the overall wandering path of the thermal equator shifts between 22.2° and 24.5° north and south. This wandering may affect the positioning of the North African Summer Monsoon Front and thus impact the perceived strength of the North African Monsoon. Further confirmation of the impacts of obliquity on the North African Monsoonal have been provided through

3325-559: The amplitude of the insolation maxima and minima that occur due to the precession cycle. Strong support for the modulation of the precession cycle by eccentricity can be found in Aeolian dust deposits in the Eastern Mediterranean. Upon close examination it can be shown that periods of low and high hematite fluxes correspond to both the 100,000-year and 400,000-year eccentricity cycles. It is believed that this evidence for

3420-472: The signal-to-noise ratio , respectively. The net effect of these corrections decreased the average ACRIM3 TSI value without affecting the trending in the ACRIM Composite TSI. Differences between ACRIM and PMOD TSI composites are evident, but the most significant is the solar minimum-to-minimum trends during solar cycles 21 - 23 . ACRIM found an increase of +0.037%/decade from 1980 to 2000 and

3515-467: The "green Sahara" and "desert Sahara" cycle. A January 2019 MIT paper in Science Advances shows a cycle from wet to dry approximately every 20,000 years. The idea that changes in insolation caused by shifts in the Earth's orbital parameters are a controlling factor for the long-term variations in the strength of monsoon patterns across the globe was first suggested by Rudolf Spitaler in

3610-469: The 23,000-year precession insolation cycle. The African Humid Period occurred between 14,800 and 5,500 years ago, and was the last occurrence of a "green Sahara". Conditions in the Sahara during the African Humid Period were dominated by a strong North African Monsoon, resulting in larger annual rainfall totals compared to today's conditions. With the increased rainfall, the vegetation patterns in North Africa were nothing like what we see today. The majority of

3705-631: The ACRIM III data that is nearly in phase with the Sun-Earth distance and 90-day spikes in the VIRGO data coincident with SoHO spacecraft maneuvers that were most apparent during the 2008 solar minimum. TIM's high absolute accuracy creates new opportunities for measuring climate variables. TSI Radiometer Facility (TRF) is a cryogenic radiometer that operates in a vacuum with controlled light sources. L-1 Standards and Technology (LASP) designed and built

3800-1040: The Earth Radiometer Budget Experiment (ERBE) on the Earth Radiation Budget Satellite (ERBS), VIRGO on the Solar Heliospheric Observatory (SoHO) and the ACRIM instruments on the Solar Maximum Mission (SMM), Upper Atmosphere Research Satellite (UARS) and ACRIMSAT . Pre-launch ground calibrations relied on component rather than system-level measurements since irradiance standards at the time lacked sufficient absolute accuracies. Measurement stability involves exposing different radiometer cavities to different accumulations of solar radiation to quantify exposure-dependent degradation effects. These effects are then compensated for in

3895-463: The Earth moving between its perihelion and aphelion , or changes in the latitudinal distribution of radiation. These orbital changes or Milankovitch cycles have caused radiance variations of as much as 25% (locally; global average changes are much smaller) over long periods. The most recent significant event was an axial tilt of 24° during boreal summer near the Holocene climatic optimum . Obtaining

North African climate cycles - Misplaced Pages Continue

3990-468: The North African Monsoon can be found in the deposits of freshwater diatoms in the tropical Atlantic. Ocean cores from the tropical Atlantic have been found to have distinct layers of the freshwater diatom Aulacoseira granulata , also known as Melosira granulata . These layers occur on a 23,000-year cycle that lags the maximum in precession insolation by roughly 5000 to 6000 years. To explain these cyclic freshwater diatom deposits we have to look inland at

4085-400: The Orbital Monsoon Hypothesis is that a detailed inspection of climate record indicates that there is a 1000 to 2000 year lag in the observed North African Monsoon maximum compared to the predicted maximum. This issue occurs because the Orbital Monsoon Hypothesis assumes that there is an instantaneous response by the climate system to changes in insolation from orbital forcing. However, there are

4180-401: The Sahara begin to dry up and expose potential freshwater diatom sources. One key factor that must be noted with freshwater diatom deposits is species identification. For instance some ocean cores directly off the western coast of Africa show a mix of freshwater lake and river diatom species. So for a core to accurately represent the diatom cycle of the Sahara it must be recovered from a region of

4275-651: The Sahara region for instance was characterized by expansive grasslands , also known as steppe . Meanwhile, the Sahel region south of the Sahara was mostly savanna. Today the Sahara region is mostly desert and the Sahel is characterized by savannah grasslands conditions. The African Humid Period was also characterized by a network of vast waterways in the Sahara, consisting of large lakes, rivers, and deltas. The four largest lakes were Lake Megachad , Lake Megafezzan , Ahnet-Mouydir Megalake , and Chotts Megalake . Large rivers in

4370-427: The Sahara region of Africa. Around the time of the insolation maximum in the precession cycle the North African Monsoon is at its strongest and the Sahara region becomes dominated by large monsoonal lakes. Then as time progress toward the insolation minima, these lakes begin to dry out due to weakening North African Monsoon. As the lakes dry up thin sediment deposits containing freshwater diatoms are exposed. Finally, when

4465-473: The TRF in both optical power and irradiance. The resulting high accuracy reduces the consequences of any future gap in the solar irradiance record. The most probable value of TSI representative of solar minimum is 1 360 .9 ± 0.5 W/m , lower than the earlier accepted value of 1 365 .4 ± 1.3 W/m , established in the 1990s. The new value came from SORCE/TIM and radiometric laboratory tests. Scattered light

4560-646: The TSI record is not sufficiently stable to discern solar changes on decadal time scales. Only the ACRIM composite shows irradiance increasing by ~1   W/m  between 1986 and 1996; this change is also absent in the model. Recommendations to resolve the instrument discrepancies include validating optical measurement accuracy by comparing ground-based instruments to laboratory references, such as those at National Institute of Science and Technology (NIST); NIST validation of aperture area calibrations uses spares from each instrument; and applying diffraction corrections from

4655-419: The abrupt shifts back and forth from the "green Sahara" to the "desert Sahara" are not entirely explained by long term changes in the insolation cycle. Precession of the equinoxes on Earth can be divided up into two distinct phases. The first phase is created by a wobbling of the Earth's axis of rotation and is known as axial precession . While the second phase is known as apsidal precession or procession of

4750-468: The ascending node (Ω). By definition, the reference plane for the Solar System is usually considered to be Earth's orbital plane, which defines the ecliptic , the circular path on the celestial sphere that the Sun appears to follow over the course of a year. In other cases, for instance a moon or artificial satellite orbiting another planet, it is convenient to define the inclination of

4845-442: The cavity, electronic degradation of the heater, surface degradation of the precision aperture and varying surface emissions and temperatures that alter thermal backgrounds. These calibrations require compensation to preserve consistent measurements. For various reasons, the sources do not always agree. The Solar Radiation and Climate Experiment/Total Irradiance Measurement ( SORCE /TIM) TSI values are lower than prior measurements by

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4940-407: The cavity. This design admits into the front part of the instrument two to three times the amount of light intended to be measured; if not completely absorbed or scattered, this additional light produces erroneously high signals. In contrast, TIM's design places the precision aperture at the front so that only desired light enters. Variations from other sources likely include an annual systematics in

5035-414: The climate of the Sahara, several nonlinear feedback mechanisms have been proposed. One of the most common sets of nonlinear feedback mechanisms considered, are vegetation-atmosphere interactions . Computer models looking at vegetation-atmosphere interactions and insolation across North Africa have shown the ability to simulate the rapid transitions between "green Sahara" and "desert Sahara" regimes. Thus

5130-451: The conventional polar angle describing a planetary orbit . Let θ  = 0 at the March equinox . The declination δ as a function of orbital position is δ = ε sin ⁡ ( θ ) {\displaystyle \delta =\varepsilon \sin(\theta )} where ε is the obliquity . (Note: The correct formula, valid for any axial tilt,

5225-401: The cosine is appropriate. A sunbeam one mile wide arrives from directly overhead, and another at a 30° angle to the horizontal. The sine of a 30° angle is 1/2, whereas the sine of a 90° angle is 1. Therefore, the angled sunbeam spreads the light over twice the area. Consequently, half as much light falls on each square mile. Orbital plane (astronomy) The orbital plane of

5320-426: The daily average insolation for the Earth is approximately 6 kWh/m = 21.6 MJ/m . The output of, for example, a photovoltaic panel, partly depends on the angle of the sun relative to the panel. One Sun is a unit of power flux , not a standard value for actual insolation. Sometimes this unit is referred to as a Sol, not to be confused with a sol , meaning one solar day . Part of the radiation reaching an object

5415-512: The development of yearly monsoons is minimal. The second alternative solution proposes that relatively cool tropical oceans left over from glaciation may initially slow the development of monsoons globally, since colder oceans are less potent sources of moisture. Sapropels are dark organic rich marine sediments that contain greater than 2% organic carbon by weight. In the Eastern Mediterranean layers of sapropels can be found in marine sediment cores that align with periods of maximum insolation in

5510-422: The eastern equatorial Atlantic remains strong and the waters in the pelagic zone are cooler. The proof that this pattern of periodic weakening of the eastern equatorial Atlantic upwelling exists is found in deposits of surface dwelling planktic organisms in ocean sediment cores. Such cores show that the relative abundance of warm and cold water planktic species vary with a consistent beat of 23,000 years, matching

5605-485: The eccentricity cycles in the dust record of the Eastern Mediterranean indicates a stronger northward progression of the North African Monsoonal Front during times when the eccentricity and precession insolation maxima coincide. The modulating effect of eccentricity on the precession cycle has also been shown using a global fully coupled atmosphere–ocean–sea ice climate model. One key issue with

5700-499: The electrical heating needed to maintain an absorptive blackened cavity in thermal equilibrium with the incident sunlight which passes through a precision aperture of calibrated area. The aperture is modulated via a shutter . Accuracy uncertainties of < 0.01% are required to detect long term solar irradiance variations, because expected changes are in the range 0.05–0.15   W/m per century. In orbit, radiometric calibrations drift for reasons including solar degradation of

5795-399: The ellipse and is related to the slow rotation of the Earth's elliptical orbit around the Sun. When combined these two phases create a precession of the equinoxes that has a strong 23,000-year cycle and a weak 19,000-year cycle. Variations in the strength of the North African Monsoon have been found to be strongly related to the stronger 23,000-year processional cycle. The relationship between

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5890-409: The energy imbalance. In 2014 a new ACRIM composite was developed using the updated ACRIM3 record. It added corrections for scattering and diffraction revealed during recent testing at TRF and two algorithm updates. The algorithm updates more accurately account for instrument thermal behavior and parsing of shutter cycle data. These corrected a component of the quasi-annual spurious signal and increased

5985-409: The equatorial Atlantic are strongly diverted toward the Sahara. This diversion weakens the equatorial upwelling zone in the eastern equatorial Atlantic, resulting in warmer waters in the pelagic . On the other end of the spectrum when insolation in North Africa is at a minimum due to the precession cycle, the diversion of the easterly trade winds is relatively weak. Due to this the region of upwelling in

6080-453: The final data. Observation overlaps permits corrections for both absolute offsets and validation of instrumental drifts. Uncertainties of individual observations exceed irradiance variability (~0.1%). Thus, instrument stability and measurement continuity are relied upon to compute real variations. Long-term radiometer drifts can potentially be mistaken for irradiance variations which can be misinterpreted as affecting climate. Examples include

6175-470: The geologic record. These lakes fill as the precession cycle approaches the insolation maximum and are then depleted as the precession cycle nears the insolation minimum. The largest of these paleolakes was Lake Megachad , which at its peak was 173 m deep and covered an area of roughly 400,000 km. Today the remnants of this once massive lake are known as Lake Chad , which has a maximum depth of 11 m and an area of only 1,350 km. Satellite imagery of

6270-533: The global warming of the last two decades of the 20th century are that solar forcing may be a marginally larger factor in climate change than represented in the CMIP5 general circulation climate models . Average annual solar radiation arriving at the top of the Earth's atmosphere is roughly 1361   W/m . The Sun's rays are attenuated as they pass through the atmosphere , leaving maximum normal surface irradiance at approximately 1000   W/m at sea level on

6365-844: The hour angle when Q becomes positive. This could occur at sunrise when Θ = 1 2 π {\displaystyle \Theta ={\tfrac {1}{2}}\pi } , or for h 0 as a solution of sin ⁡ ( φ ) sin ⁡ ( δ ) + cos ⁡ ( φ ) cos ⁡ ( δ ) cos ⁡ ( h o ) = 0 {\displaystyle \sin(\varphi )\sin(\delta )+\cos(\varphi )\cos(\delta )\cos(h_{o})=0} or cos ⁡ ( h o ) = − tan ⁡ ( φ ) tan ⁡ ( δ ) {\displaystyle \cos(h_{o})=-\tan(\varphi )\tan(\delta )} If tan( φ ) tan( δ ) > 1 , then

6460-574: The issue of the irradiance increase between cycle minima in 1986 and 1996, evident only in the ACRIM composite (and not the model) and the low irradiance levels in the PMOD composite during the 2008 minimum. Despite the fact that ACRIM I, ACRIM II, ACRIM III, VIRGO and TIM all track degradation with redundant cavities, notable and unexplained differences remain in irradiance and the modeled influences of sunspots and faculae . Disagreement among overlapping observations indicates unresolved drifts that suggest

6555-485: The key pieces of evidence linking the formation of sapropels to enhance discharge from the Nile River is the fact that they have occurred during both interglacial and glacial periods. Therefore, the formation of sapropels must be linked to fresh water discharge from the Nile River and not melt water from dissipating ice sheets. Evidence for the existence of large lakes in the Sahara can be found and interpreted from

6650-441: The large influx of nutrient rich fresh water causes a steep vertical salinity gradient . As a result, thermohaline convection is shut off and the water column becomes stably stratified. Once this stable stratification occurs, bottom waters in the Eastern Mediterranean quickly become depleted in oxygen and the large influx of pelagic organic matter from the nutrient rich surface waters is preserved as sapropel formations. One of

6745-470: The late nineteenth century, The hypothesis was later formally proposed and tested by the meteorologist John Kutzbach in 1981. Kutzbach's ideas about the impacts of insolation on global monsoonal patterns have become widely accepted today as the underlying driver of long term monsoonal cycles. Kutzbach never formally named his hypothesis and as such it is referred to here as the "Orbital Monsoon Hypothesis" as suggested by Ruddiman in 2001. Insolation , which

6840-508: The lower values for the secular trend are more probable. In particular, a secular trend greater than 2 Wm is considered highly unlikely. Ultraviolet irradiance (EUV) varies by approximately 1.5 percent from solar maxima to minima, for 200 to 300 nm wavelengths. However, a proxy study estimated that UV has increased by 3.0% since the Maunder Minimum. Some variations in insolation are not due to solar changes but rather due to

6935-526: The measuring instrument. Solar irradiance is measured in watts per square metre (W/m ) in SI units . Solar irradiance is often integrated over a given time period in order to report the radiant energy emitted into the surrounding environment ( joule per square metre, J/m ) during that time period. This integrated solar irradiance is called solar irradiation , solar exposure , solar insolation , or insolation . Irradiance may be measured in space or at

7030-510: The monsoon. Over periods of tens to hundreds of thousands of years the amount of insolation changes in a highly complex cycle that is based on orbital parameters. The result of this cycle of insolation is a waxing and waning in the strength of the monsoonal climates across the globe. A wide range of geologic evidence has shown that the North African Monsoon is particularly susceptible to insolation cycles, and long term trends in monsoonal strength can be linked to slow variations in insolation. However,

7125-452: The orbit of the Earth is highly elliptical one hemisphere will have hot summers and cold winters, corresponding to a larger than average yearly insolation gradient . At the same time the other hemisphere will have warm summers and cool winters due to a smaller than average yearly insolation gradient. Like obliquity, eccentricity is not considered to be a primary driver of the strength of the North African Monsoon. Instead eccentricity modulates

7220-419: The orbital plane of an object. Other parameters, such as the orbital period , the eccentricity of the orbit and the phase of the orbit are more easily changed by propulsion systems. Orbital planes of satellites are perturbed by the non-spherical nature of the Earth's gravity . This causes the orbital plane of the satellite's orbit to slowly rotate around the Earth, depending on the angle the plane makes with

7315-414: The precession cycle and the strength of the North African Monsoon exists because procession affects the amount of insolation received in a given hemisphere. The amount of insolation is maximized for the northern hemisphere when the precession cycle is aligned such that the northern hemisphere points toward the sun at perihelion . According to the Orbital Monsoon Hypothesis this maximum in insolation increases

7410-469: The precession cycle over Northern Africa. Such an alignment can be explained by a link to the North African Monsoon. During periods of high insolation the increased strength and northward progression of the North African Monsoonal Front causes very heavy rain along the upper and middle reaches of the Nile River basin. These rains then flow northward and are discharged into the Eastern Mediterranean, where

7505-437: The prediction of energy generation from solar power plants , the heating and cooling loads of buildings, climate modeling and weather forecasting, passive daytime radiative cooling applications, and space travel. There are several measured types of solar irradiance. Spectral versions of the above irradiances (e.g. spectral TSI , spectral DNI , etc.) are any of the above with units divided either by meter or nanometer (for

7600-404: The prevailing northeasterly winds arrive during winter, the freshwater diatom deposits in the dried lake beds are picked up as dust and carried thousands of kilometers out into the tropical Atlantic. From this series of events the reason for 5000 to 6000-year delay in the freshwater diatom deposits is evident, since the North African Monsoon must become sufficiently weak before the monsoonal lakes in

7695-630: The reference radiometer and the instrument under test in a common vacuum system that contains a stationary, spatially uniform illuminating beam. A precision aperture with an area calibrated to 0.0031% (1 σ ) determines the beam's measured portion. The test instrument's precision aperture is positioned in the same location, without optically altering the beam, for direct comparison to the reference. Variable beam power provides linearity diagnostics, and variable beam diameter diagnoses scattering from different instrument components. The Glory/TIM and PICARD/PREMOS flight instrument absolute scales are now traceable to

7790-492: The region included the Senegal River , Nile River , Sahabi River , and Kufra River . These river and lake systems provided corridors that allowed many animal species, including humans, to expand their range across the Sahara. Geologic evidence from the beginning and end of the African Humid Period suggests that both the onset and termination of the African Humid Period were abrupt. In fact both events likely occurred on

7885-497: The relative proportion of sunspot and facular influences from SORCE/TIM data accounts for 92% of observed variance and tracks the observed trends to within TIM's stability band. This agreement provides further evidence that TSI variations are primarily due to solar surface magnetic activity. Instrument inaccuracies add a significant uncertainty in determining Earth's energy balance . The energy imbalance has been variously measured (during

7980-411: The results from these models suggest the possible existence of a vegetation-insolation threshold, which if reached, allows the Sahara region to rapidly transition from "green Sahara" to "desert Sahara" and vice versa. Insolation Solar irradiance is the power per unit area ( surface power density ) received from the Sun in the form of electromagnetic radiation in the wavelength range of

8075-660: The role of the elliptical orbit is entirely contained within the important product e sin ⁡ ( ϖ ) {\displaystyle e\sin(\varpi )} , the precession index, whose variation dominates the variations in insolation at 65°   N when eccentricity is large. For the next 100,000 years, with variations in eccentricity being relatively small, variations in obliquity dominate. The space-based TSI record comprises measurements from more than ten radiometers and spans three solar cycles. All modern TSI satellite instruments employ active cavity electrical substitution radiometry . This technique measures

8170-413: The shorelines of ancient Lake Megachad reveal that the lake has existed under two distinctive wind regimes, one northeasterly and southwesterly. The northeasterly wind regime is consistent with today's wind patterns and is characteristic of weak monsoonal flow. Meanwhile, the southwesterly wind regime is characteristic of a stronger monsoonal flow. Another key piece of evidence for a processional control on

8265-527: The strength of monsoon circulations in the northern hemisphere. On the opposite end of the spectrum, when the Northern Hemisphere is pointed toward the sun during aphelion , there is a minimum in insolation and the North African Monsoon is at its weakest. Obliquity , otherwise known as (axial) tilt, refers to the angle that Earth's axis of rotation makes with a line that is perpendicular to Earth's orbital plane . The current tilt of Earth's axis

8360-486: The strength of the North African Monsoon increases. A second theory that may explain the existence of an obliquity signature in the North African climate record suggests that obliquity maybe related to changes in the latitude of the tropics. The latitudinal extent of the tropics is roughly defined by the maximum wandering path of the thermal equator . An area that today is located between the Tropic of Capricorn and

8455-392: The sun does not set and the sun is already risen at h = π , so h o = π . If tan( φ ) tan( δ ) < −1 , the sun does not rise and Q ¯ day = 0 {\displaystyle {\overline {Q}}^{\text{day}}=0} . R o 2 R E 2 {\displaystyle {\frac {R_{o}^{2}}{R_{E}^{2}}}}

8550-625: The system, completed in 2008. It was calibrated for optical power against the NIST Primary Optical Watt Radiometer, a cryogenic radiometer that maintains the NIST radiant power scale to an uncertainty of 0.02% (1 σ ). As of 2011 TRF was the only facility that approached the desired <0.01% uncertainty for pre-launch validation of solar radiometers measuring irradiance (rather than merely optical power) at solar power levels and under vacuum conditions. TRF encloses both

8645-409: The top of the Earth's atmosphere is about 1361   W/m . This represents the power per unit area of solar irradiance across the spherical surface surrounding the Sun with a radius equal to the distance to the Earth (1   AU ). This means that the approximately circular disc of the Earth, as viewed from the Sun, receives a roughly stable 1361   W/m at all times. The area of this circular disc

8740-425: The tropical Atlantic that has sufficient distance from the coast such that the impacts of river outflows are minimized. Observed variations in the strength of the eastern equatorial Atlantic upwelling zone can also be used to support a cycle of the North African Monsoon that is regulated by the precession cycle. When insolation in North Africa is at its peak during the precession cycle the easterly trade winds over

8835-443: The unit of the integral (W/m^2) is the product of those two units. The SI unit of irradiance is watts per square metre (W/m = Wm ). The unit of insolation often used in the solar power industry is kilowatt hours per square metre (kWh/m ). The Langley is an alternative unit of insolation. One Langley is one thermochemical calorie per square centimetre or 41,840   J/m . The average annual solar radiation arriving at

8930-471: The view-limiting aperture. For ACRIM, NIST determined that diffraction from the view-limiting aperture contributes a 0.13% signal not accounted for in the three ACRIM instruments. This correction lowers the reported ACRIM values, bringing ACRIM closer to TIM. In ACRIM and all other instruments but TIM, the aperture is deep inside the instrument, with a larger view-limiting aperture at the front. Depending on edge imperfections this can directly scatter light into

9025-469: The year and the day, the Earth's atmosphere receives 340   W/m from the Sun. This figure is important in radiative forcing . The distribution of solar radiation at the top of the atmosphere is determined by Earth's sphericity and orbital parameters. This applies to any unidirectional beam incident to a rotating sphere. Insolation is essential for numerical weather prediction and understanding seasons and climatic change . Application to ice ages

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