How reliable is our sun? We know it makes an appearance every morning, but how constant is its heat? Variation in solar output has been proposed as an important natural factor in causing earth's climate to vary over the life-time of the planet.
The sun can vary in its intensity or total irradiance, which would result in direct heating or cooling of the earth, but there are also other effects. Changes in the intensity of ultraviolet radiation may alter ozone production in the stratosphere, leading to STRATOSPHERIC heating and to poleward displacements in the stratospheric and TROPOSPHERIC wind systems. Changes in the solar wind and the sun's magnetic flux are important, because a more active solar wind and a stronger magnetic field reduce the cosmic rays striking the earth's atmosphere. The frequency of solar flares can also vary. The high-energy protons they produce can penetrate the upper atmosphere and cause increased levels of ionisation, which has been connected with the formation of clouds. Solar flaring also causes surges of energy in the radio wavelengths (radio flux), which can disrupt communication systems on earth.
All of these phenomena change in intensity over time and with differing periodicity. The most well-established cycle is the 11-year cycle of sunspot activity (see the figure below). The number of dark spots (known as sun spots) visible on the disc of the sun varies over roughly an 11-year period. When there are many spots, the sun's magnetic activity increases and the sun radiates more heat. These changes in total irradiance are very slight, and represent only a tiny percentage of the sun's total output, but even these small changes can produce a detectable change in the earth's heat balance.
Variations in solar irradiance since 1600
Reconstructions of the total solar irradiance time series starting as early as 1600. The upper envelope of the shaded regions shows irradiance variations arising from the 11-year activity cycle. The lower envelope is the total irradiance reconstructed by Lean (Geophys. Res. Lett., vol. 27, pp. 2425-2428, 2000), in which the long-term trend was inferred from brightness changes in Sun-like stars. In comparison, the recent reconstruction of Y. Wang et al. (Astrophys. J., vol. 625, pp. 522-538, 2005) is based on solar considerations alone, using a flux transport model to simulate the long-term evolution of the closed flux that generates bright faculae.
Source: Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 2 Changes in atmospheric constituents and in radiative forcing, Figure 2.17, p. 190.
The solar cycle, along with other solar variations, has been advanced as a significant influence on earth’s climate over time. When sunspot activity remained low for a prolonged time during the MAUNDER MINIMUM in the 1600s and the DALTON MINIMUM in the 1800s, the earth went through what are called the LITTLE ICE AGES. When solar activity was higher during the Medieval Period (500–1500 AD), Europe enjoyed a long warm period. However, as more data have been collected and analysed, climate correlations with the solar cycle have generally weakened.
Solar variation has been ruled out as a possible cause of most of the currently observed climate change. The Fourth Assessment Report of IPCC concluded that whilst there is a component of global warming related to solar activity, it amounts to around 0.12 watts per square metre, or about 7.5 per cent of the net impact from anthropogenic influences. Changes in solar activity and volcanic emissions may have caused the Maunder Minimum from 1645 to 1715 (a period of cooler climate), but solar activity appears to be only a minor contributor to global warming at the present time. The following figure shows the RADIATIVE FORCING of the earth's climate contributed by volcanic activity, solar irradiance, and all other factors (which are primarily due to human activities). This figure indicates that the influence of variations in the sun's output can cause warming or cooling trends in the earth's climate for several decades at a time, but that the recent warming trend is overwhelmingly due to anthropogenic forcing.
Radiative forcing due to volcanic activity, solar irradiance and all other factors during the last 1100 years
Radiative forcings during the last 1.1 kyr. Global mean radiative forcing (watts per square metre) used to drive climate model simulations due to (a) volcanic activity, (b) solar irradiance variations and (c) all other forcings (which vary between models, but always include greenhouse gases, and, except for those with dotted lines after 1900, tropospheric sulphate aerosols).
Source: Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 6 Palaeoclimate, Figure 6.13, p. 477.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 2 Changes in atmospheric constituents and in radiative forcing.
Intergovernmental Panel on Climate Change, Working Group I Contribution to the Fourth Assessment Report, Climate change 2007—the physical science basis, Chapter 6 Palaeoclimate.