Solar irradiance is the solar energy flux density outside Earth's atmosphere at a distance from the Sun of 1 Astronomical Unit (AU), given in SI units of Watts per square meter (W/m²). The sun's total energy input reaching Earth is called total solar irradiance, or TSI. It comes in many different color bands or wavelengths. The distribution of the Sun’s energy input across ultraviolet, visible, infrared, and other wavelengths of light is called solar spectral irradiance, or SSI.

The total solar irradiance, or TSI, is the key energy input to Earth, essential for assessing the radiative energy balance in the Earth's climate system.  Though TSI was historically referred to by the term 'solar constant', TSI is known to vary over many time scales, ranging from minutes to months, decades, and longer times up to solar evolutionary times on the stellar main sequence.

 The absorption of the total solar irradiance (TSI), the total energy input to the Earth, determines the Earth’s radiation budget and mean temperature. The TSI comes in many different color bands or wavelengths. The distribution of the Sun’s energy input across ultraviolet, visible, and infrared wavelengths of light is called solar spectral irradiance, or SSI.  SSI identify the irradiance of the Sun by characterizing the Sun's energy and emissions in the form of colors, i.e. wavelengths, that are absorbed by certain molecules in Earth's atmosphere, land and oceans.  Ozone, water vapor and liquid, carbon dioxide, methane, etc, each absorb their own set of characteristic wavelengths. Where this radiative energy is deposited into the different layers of the climate system will be determined by how the incoming solar radiation is distributed with wavelength, and how much variation occurs at each wavelength. So, it is important to know how the energy in each wavelength varies, since that tells us how the heating varies in different parts of Earth’s atmosphere.

Sun changes all the time, with different time scales.  A large part of the variations in the solar output can be attributed to the presence of sunspot regions, as well as larger scale faculae and network, and associated changes in the surface magnetic field.  Variations in TSI are due to a balance between dimming caused by sunspots and brightening caused by bright areas called faculae which surround sunspots. Sunspots are dark area on the Sun in which magnetic forces are very strong, and these forces block the hot solar plasma, and as a result sunspots are cooler and darker than their surroundings. Faculae, the bright area the surface of the Sun, put out more radiation than other areas and increase the solar irradiance. They too are the result of magnetic storms, and their numbers increase and decrease in concert with sunspots. The effects of the faculae tend to beat out those of the sunspots, in general. The variations in the solar output can be transported to solar energy input within Earth's view, by solar rotation. Therefore the TSI is larger during the maximum of the 11 year cycle when there are more sunspots, even though the individual spots themselves cause a decrease in TSI when facing Earth.

Sunspots, dark areas on the solar surface, occur when strong magnetic fields emerge through the solar surface and allow the area to cool, from a background value of 6000 ° C down to about 4200 ° C.  This area appears as a dark spot in contrast with the very bright photosphere of the sun. The rotation of these sunspots can be seen on the solar surface, and they take about 27 days to make a complete rotation as seen from Earth. Near the solar equator the surface rotates at a faster rate than near the solar poles.  The 11-year solar cycle modulations of sunspot numbers were observed, over the last 300 years. Galileo observed them with his telescope in early 1600. 

The sun goes through periodic variations or cycles of high and low activity that repeat approximately every 11 years. Solar minimum refers to a period of several Earth years when the TSI is lowest; solar maximum refers the years when TSI is highest. During solar maximum, activity on the Sun and the effects of space weather on our terrestrial environment are high.  

 Astronomers observed that no sunspots appeared on the Sun’s surface during the Maunder Minimum, time period from 1650 to 1715 AD. This lack of solar activity, which some scientists attribute to a low point in a multiple-century-long cycle, may have been partly responsible for the Little Ice Age in Europe. During this period, winters in Europe were much longer and colder than they are today. Scientists believe that since this minimum in solar energy output, there has been a slow increase in the overall sunspots and solar energy throughout regular 11-year cycle. 

Will the next cycle be lower than cycle 24 as predicted? Already cycle 24 is the weakest cycle in the past 90 years. Recent Solar Radiation and Climate Experiment, or SORCE, observation measured an unusually extended minimum at the end of solar cycle 23, and low solar maximum during solar cycle 24, which is the least active cycle in 90 years.  There was a period from mid-2008 to mid-2009 when the Sun was without sunspots for many days. It was probably the quietest period we’ve seen since the first total solar irradiance measurements. But we didn’t go into a prolonged minimum because the Sun still had a few active regions—not sunspots, but small bright faculae regions—and we could see the irradiance continue to fluctuate throughout this quiet period. We are expecting a solar minimum within a few years after the launch of TSIS-1, in December 2017. No one knows what the next solar cycle is going to bring. However, we need to continue to monitor the Sun’s measure energy input to Earth.

The impacts of solar UV radiation may be substantial. Since UV radiation modulate ozone in the stratosphere, the oscillation in UV levels may also affect the size of the ozone hole. Absorption of UV radiation by the ozone also heats up the stratosphere. Many scientists suspect that changes in stratospheric temperatures may alter weather patterns in the troposphere. Finally, an increase in the amount of UV radiation could impact human health, increasing the incidence of skin cancer, cataracts, and other Sun-exposure-related maladies

No. Scientists who study the link between solar activity and climate say there is no evidence that solar irradiance has changed enough over the last century to explain the intensity and speed of warming trends seen on Earth during the last century (https://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=30615).

Of the many trends that appear to cause fluctuations in the Sun’s energy, those that last decades to centuries are the most likely to have little impact on the Earth’s climate in the foreseeable future. Studies of the link between solar activity and climate show that variations in the solar irradiance have been too small to explain the Earth’s warming during the last century. Though complex feedbacks between different components of the climate system (clouds, ice, oceans, etc.) make detailed climate predictions difficult and highly uncertain, most scientists predict the release of greenhouse gases from the burning of fossil fuels will continue to block a larger and larger percentage of outgoing thermal radiation emanating from the Earth. According to the 2013 report of the Intergovernmental Panel on Climate Change (IPCC), the resulting imbalance between incoming solar radiation and outgoing thermal radiation will likely cause the Earth to heat up over the next century, possibly melting polar ice caps, causing sea levels to rise, creating violent global weather patterns, and increasing vegetation density (IPCC, 2013).