Is half of the climate change in the past 110 years due to natural variation in the Sun's output?

Question

A NASA website says:

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 a measurable impact on the Earth’s climate in the foreseeable future. Many researchers believe the steady rise in sunspots and faculae since the late seventeenth century may be responsible for as much as half of the 0.6 degrees of global warming over the last 110 years (IPCC, 2001). Since pre-industrial times, it’s thought that the Sun has given rise to a global heating similar to that caused by the increase of carbon dioxide in the atmosphere. If the past is any indication of things to come, solar cycles may play a role in future global warming.

What is the evidence for and against the Sun being such a large factor in climate change?

Answer 1

Probably not.

Sunspots may be correlated with climate change. For example, the famous Maunder Minimum is correlated with the Little Ice Age. Causality is a bit trickier since the mechanism isn't well understood, but there is a hypothesis a rise in global temperatures might be partially caused by increased sunspot counts (and the corresponding increase in radiation).

The question then becomes - has sunspot activity been increasing over the last century or so?

Historically, there have been two sunspot counts. The Wolf Sunspot Number and the Group Sunspot Number. Problematically, these numbers did not agree. One indicated significantly different levels of solar activity before about 1885 and also around 1945 (source).

This meant that there was so called Modern Grand Maximum in sunspot count from the Group Sunspot Number. If you believe the above hypothesis, you could believe that this would cause an increase in global temperature.

Fortunately, the discrepancy between the sunspot counts was resolved in 2015. The Group Sunspot Number 1.0 was wrong - it was corrected by the Group Sunspot Number 2.0 and is now consistent with the Wolf Number. There is no more Modern Grand Maximum. To quote the IAU:

The Sunspot Number, the longest scientific experiment still ongoing, is a crucial tool used to study the solar dynamo, space weather and climate change. It has now been recalibrated and shows a consistent history of solar activity over the past few centuries. The new record has no significant long-term upward trend in solar activity since 1700, as was previously indicated. This suggests that rising global temperatures since the industrial revolution cannot be attributed to increased solar activity.

source

Answer 2

The latest word on this issue seems to be The Impact of the Revised Sunspot Record on Solar Irradiance Reconstructions Solar Physics (2016):

Climate sensitivity is uncertain by a factor of about two and may differ for different forcings, but it is generally considered to be in the range of 0.2 to 1 °C per [watt per square meter] forcing. The response time for the climate system to reach equilibrium is also uncertain. This depends primarily on the amount of energy transported through the near-surface ocean mixed layer to the deep ocean.

An increase in TSI [total solar irradiance] of 1 [watt per sq. m.] produces solar forcing [Delta]Fsol = 0.7×1/4 = 0.18 [watt per sq. m.], where the scale factors account for the Earth’s albedo and the geometric illumination of the entire Earth surface by the Sun. Assuming a climate sensitivity to solar forcing of 0.6 °C per [watt per sq. m.] (i.e. midway in the estimated range), an increase in TSI of about 1 [watt per sq. m.] from the seventeenth century Maunder Minimum to the present solar-cycle average produces global warming of 0.1 °C. This estimated warming is reduced by about 20 % to 0.08 °C in the NRLTSI2 model and is not reduced at all in the SATIRE model when using the new sunspot record to reconstruct TSI. The difference is small and within the uncertainty of the TSI reconstructions themselves (Figures 5 and 6). Furthermore, this possible solar-caused global surface-temperature increase is significantly lower than the net measured temperature increase of at least 0.8 °C over the same four-century time frame.

....

The net effect on climate according to this new sunspot-number record is at a nearly insignificant level. Using the SILSO record, global-warming estimates that are attributable to solar variability over the last four centuries may be up to 20%lower than current estimates suggest, perhaps causing only a 0.08 °C increase instead of a possible 0.1 °C increase in global surface temperature. These differences are negligible compared to the uncertainties, and either contribution to overall global warming remains much lower than those that are due to other climate influences.

So half of 0.6 °C being due to change in the Sun's output is ruled out.

On the other hand, even more recently, there is Solar activity has a direct impact on Earth's cloud cover and the corresponding journal article The response of clouds and aerosols to cosmic ray decreases Journal of Geophysical Research Volume 121, September 2016, Pages 8152–8181

which, quoting from the popular article, says:

"Earth is under constant bombardment by particles from space called galactic cosmic rays. Violent eruptions at the Sun's surface can blow these cosmic rays away from Earth for about a week. Our study has shown that when the cosmic rays are reduced in this way there is a corresponding reduction in Earth's cloud cover. Since clouds are an important factor in controlling the temperature on Earth our results may have implications for climate change", explains lead author on the study Jacob Svensmark of DTU

So overall the direct effect of total solar irradiance on the climate over the past 110 years has been shown to be small, but indirect effects, such as regulation of cloud formation by controlling incoming cosmic rays, have not been ruled out.

Answer 3

The Sun activity changes every 11 years or so. That's called solar cycle, and one of its manifestations are sunspots. Sunspots irradiate more energy so the more spots the more energy the Sun produces.

Here is what they look like

If we look at the data, we see that the first decade of the 21st century, the number of sun spots has been in decline, that's called a solar minimum.

_{(source: noaa.gov)}

This does not square with the rise in temperate we've seen during the same period:

While the second graph shows a much larger period, you can see that the slope between 2000 and 2015 does not change that much, which suggests that the solar activity is not the leading factor in temperature change.

This paper attributes 0.2K of the temperature change to sun variability, which is a very small portion ofthe total temperature change.