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The Role of the Sun in 20th Century Climate Change

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Introduction

The energy that keeps the earth's surface warm originates from the sun. Although the energy that is emitted from the sun is almost constant, even small changes can have noticeable effects. As discussed earlier, there is no doubt that changes in the output of energy from the sun have caused climate change in the past. The mechanism by which even small changes in the sun's energy can cause substantial changes in the earth's temperature is the subject of some debate, although one hypothesis is that, as well as a direct effect, there are indirect effects caused by changes in the ozone layer or in the number of cosmic rays that hit the atmosphere (for more on cosmic rays, see below). However, assessments of the amount of energy coming from the sun shows no overall change in the second half of the twentieth century.

Solar Irradiance Measured From Space

Since 1979, a series of satellites have measured the energy that comes from the sun (solar irradiance) directly from space, and the results are shown in Figure 1. It can be seen that solar irradiance varies in a regular cycle of around 11 years in length. This cycle, the solar cycle, is matched by a regular cycle in the number of sunspots, the sunspot cycle (also shown in Figure 1).

It is thought that the solar cycle is too short to have a measurable effect on global temperatures, but long-term changes are thought to be more important. Unfortunately, because it has proved difficult to merge the data from consecutive satellites in the series that have watched the sun for the past 20 or so years, it is not certain whether the sun's output has increased over that period. One study found that there had been no increase over that time period. A second found that that there had been a small increase (around 0.1%). You can read more about this controversy in Under a Variable Sun

Solar irradiance data and sunspot data are provided by NOAA.

Irradiance measured by satellite compared with sunspot numbers
Figure 1. Solar irradiance compared with annual mean sunspot numbers since 1979

Sunspot Numbers

Before 1979, there are no direct measures of solar irradiance. However, sunspots have been counted for many hundreds of years and, as shown in Figure 1, the number of sunspots correlates well with overall solar irradiance.

Figure 2 shows the comparison between changes in surface temperatures over the 20th century with sunspot numbers. In order to smooth out the effects of the solar cycle. the 11-year average of both the sunspot number and the surface temperatures are shown (the yearly values are shown in faint blue and red lines, respectively).

The figure shows that sunspot numbers rose in the first half of the 20th century, along with temperatures. The rise in solar activity in the early part of the century is though to be connected with an 80 year cycle of solar activity known as the Gleissman cycle. The temperature increase in the second half of the twentieth century does not seem to linked with sunspot numbers.

Sunspot data are provided by NOAA. Surface temperature data are provided by CRU.

Sunspot numbers compared with Earth surface temperatures
Figure 2. Surface temperatures compared with sunspot numbers since 1858

Solar Irradiance Estimated From Sunspot Numbers

Although the correspondance with sunspot numbers is good, the relationship between sunspot numbers and solar irradiance is not straightforward. Several methods exist to calculate solar irradiance from sunspot numbers. One of the most recent is that of Solanki & Fligge (1999). When estimated irradiance is used instead of straight sunspot numbers, the correspondance with global temperatures, but it still cannot explain the increase in temperatures over the past 30 years.

Sunspot data are provided by NOAA. Surface temperature data are provided by CRU. The formula used to convert sunspot data to estimated solar irradiance was Equation 1 in Solanki & Fligge (Geophys Res Lett 1999;26:2465).

Estimated irradiance compared with Earth surface temperatures
Figure 3. Surface temperatures compared with sunspot numbers since 1858

Solar Irradiance Estimated From Solar Cycle Length

The energy coming from the sun can also be estimated using other techniques. In 1991, two scientists from the Danish Meteorological Institute published data showing a close correlation between the length of the solar cycle (which, although it averages 11 years, can in fact vary by several years) and the temperature at the Earth's surface (see Friis-Christensen and Lassen, 1991). This hypothesis is controversial for a number of reasons, one of which is that there appears to be no physical basis for a link between the length of the solar cycle and solar irradiance.

Since the original publication, the data have been updated, as discussed in review by the DMI (see also Thjell & Lassen, 2002). These updated data are shown in the Figure 4. The updated data show that, like other measures of solar activity, there is a good correlation with temperature rises in the earlier part of the 20th century, but little or no correlation in the second half of the 20th century. The authors describe these new results as 'The fingerprint of the anthropogenic greenhouse effect'.

Solar cycle length data are the L121 series of Thjell & Lassen (2000). Surface temperature data are provided by CRU.

Solar cycle length compared with Earth surface temperatures
Figure 4. Surface temperatures compared with solar cycle length since 1858

Solar Irradiance Estimated From Geomagnetic Activity

Changes in solar activity result in fluctuations of the Sun's magnetic field, which in turn cause fluctuations in the Earth's magnetic field. Since 1868, these fluctuations have been measured in Canberra, Australia, and Hartland, England (two sites that are on almost opposite sides of the earth). These recordings have allowed scientists to calculate changes in the sun's magnetic field over the past century, the AA Index.

Figure 4 shows that geomagnetic activity has changed similarly to other measurements of solar activity - an increase in the early part of the 20th century, followed by little overall change in the second half of the 20th century. These results are of interest, because changes in geomagnetic activity are thought to influence the number of cosmic rays that strike the earth.

AA index is provided by NOAA. Surface temperature data are provided by CRU.

Geomagnetic activity compared with Earth surface temperatures
Figure 4. Surface temperatures compared with geomagnetic activity (AA index) since 1858

Cosmic Rays

Cosmic rays, originating from deep space, have been proposed to affect the earth's climate by changing the amount of cloud cover (for more information, see the explanations written by Henrik Svensmark and Enric Palle). The number of cosmic rays that hit the earth depends upon the strength of the solar magnetic field - the stronger it is, the fewer cosmic rays strike the earth. It has been suggested that this effect may amplify the effects of changes in solar irradiance, thereby explaining why the small changes in solar irradiance, such as that observed in the first half of the 20th century, can affect the earth's climate (for more on this, see this explanation by Fangqun Yu).

Cosmic rays have been measured over the past few decades at a number of locations, and the results are shown in Figure 5. Most monitoring sites measure low energy cosmic rays only. These sites show strong fluctuations in the number of cosmic rays, fluctuations that mirror the 11-year solar cycle. This is because low-energy cosmic rays are strongly affected by solar activity. One site, Huancaryo in Peru, measures both high and low-energy cosmic rays. The results from this site show a similar response to the 11-year solar cycle, although the effect is weaker.

As with measures of solar irradiance, it's easier to see underlying trends when the 11-year running average is displayed, as shown in Figure 6. It can be seen that there have been no overall changes in number of cosmic rays striking the earth.

There are no direct measurements cosmic rays from before 1953. However, since it is thought that cosmic rays are strongly affected by the solar magnetic field, it is likely that the long-term changes in cosmic ray flux are similar to the long-term changes in the Geomagnetic AA Index (see above). In other words, changes in cosmic rays may help to explain changes in the Earth's surface temperature in the first half of the 20th century.

Cosmic ray data provided by NOAA.Surface temperature data are provided by CRU.

cosmic_rays.gif (18779 bytes)
Figure 5. Cosmic rays measured at a variety of sites around the globe
(% change from 1961-1990 average)

Cosmic rays compared with Earth surface temperatures

Figure 6. Cosmic rays (% change from 1961-1990 average) compared with surface temperatures. A decrease in cosmic rays is thought to increase surface temperatures.

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Last updated 21/02/05. By Tom Rees. Contact the author