Time To Reassess The Role Of The Sun In Climate Change

Sun spot numbers (in decades) since 1150 BC. Note that the maximum  in the second half of the 20th century. The solid line is the average. Sunspot numbers directly observed since 1610 AD are shown in red.  Prior to this the estimates are reconstructed from proxy data. From the paper "Evidence for distinct modes of solar activity" by I. G. Usoskin, G. Hulot, Y. Gallet, R. Roth, A. Licht, F. Joos, G. A. Kovaltsov, E. Thébaultand A. Khokhlov published in Astronomy & Astrophysics. Courtesy: Astronomy & Astrophysics and authors.

Putting the sun in the frame… Sun spot numbers (in decades) since 1150 BC. Note the maximum in the second half of the 20th century and the rapid fall since. Note too the Maunder Minimum around 1645.  The solid line is the average. Sun spot numbers directly observed since 1610 AD are shown in red. Prior to this the estimates are reconstructed from proxy data with the gray shading representing the statistical 95 per cent confidence interval. From the paper “Evidence for distinct modes of solar activity” by I. G. Usoskin, G. Hulot, Y. Gallet, R. Roth, A. Licht, F. Joos, G. A. Kovaltsov, E. Thébaultand A. Khokhlov published in Astronomy & Astrophysics. Courtesy: Astronomy & Astrophysics and the authors.

As the so called pause in global warming continues, space scientists may be giving climate scientists some pause for thought.

Global surface temperatures have remained statistically flat for over a decade following a rapid rise in the second half of the 20th century despite the fact that the long-term increase in carbon dioxide associated with this rapid global warming has continued throughout the whole of the pause period.

Two pieces of research published this year suggest that the sun has played a bigger role in these events than is widely accepted by climate scientists and they imply, as a result, that the role of greenhouse gases may be less significant than climate scientists currently believe.

The research shows that the sun is far more variable than we had previously thought and that variations in solar activity correlate very closely with changes in global surface temperature. This challenges the prevailing orthodoxy in climate science that our star plays no significant role in global warming.

Minuscule solar impact on climate change

Most climate scientists believe that the sun is a stable star and contributes relatively little to climate change compared with the massive warming impact – or radiative forcing – of human greenhouse gas emissions. And, for a quarter of a century, this has been the settled view of the international climate science establishment led by the Intergovernmental Panel on Climate Change (IPCC).

The most recent IPCC scientific report published last year – the AR5 WG 1 – states that the total natural contribution to global warming “from solar irradiance changes and stratospheric volcanic aerosols made only a small contribution to the net radiative forcing throughout the last century”. And the evidence they cite would appear to support this contention.

Solar heating on the Earth is measured by what is called total solar irradiance – the average amount of solar energy falling on the Earth. Total solar irradiance is measured in watts of energy per square meter (W/m2) and data in the last IPCC report shows that solar irradiance between 1755 and 2012 varied in a narrow band throughout this period by only around 0.02 per cent… Minuscule.

Indeed, the IPCC report states that the warming effect due to human greenhouse gas emissions – what it calls the effective radiative forcing – is 2.3 W/m2 compared with a radiative forcing due to the small variations in solar irradiance of around 0.05 W/m2. In other words, the impact of the sun on the climate is only around 2 per cent of the scale of the impact of human greenhouse gas emissions.

Strong correlation between the sun and climate

However, research from an international team of space scientists shows that the second half of the last century – the very time when global temperatures started to tick up significantly – coincided with a once in 3,000 year record high in solar activity (see the graph at the top of the page). The paper, published in a specialist space science journal called Astronomy & Astrophysics, makes only passing and understated reference to the implications of the research on climate science reporting that: “These results provide important constraints for both dynamo models of Sun-like stars and investigations of possible solar influence on Earth’s climate”.

But another group of space science researchers from China took a much more combative stance earlier this year when they published a paper in the Chinese Science Bulletin which showed a strong correlation between the variation in the temperature of the surface of the Earth and variations in solar activity. Their paper explicitly stated solar activity has a “non-negligible” effect on the temperature change of the Earth”. Furthermore, they issued a press release to coincide with the appearance of their paper stating: “The climate models of IPCC seem to underestimate the impact of natural factors on the climate change, while overstate that of human activities. Solar activity is an important ingredient of natural driving forces of climate”. In the rarefied atmosphere of scientific debate, this constitutes fighting talk.

A mechanism connecting sun spot activity with the atmosphere

If there is a link between solar activity and the climate then it would need to be more subtle than the simple heating impact of solar radiation which appears to vary little. One possibility, suggested by NASA, is that solar magnetism associated with sun spots plays a role. In 2010 research published in Nature reported that NASA’s Solar Radiation and Climate Experiment (SORCE) satellite had measured significant falls in ultraviolet emissions linked to reduced sun spot activity.

As NASA notes on a specialist web page dedicated to solar activity: “Although sunspots themselves produce only minor effects on solar emissions, the magnetic activity that accompanies the sunspots can produce dramatic changes in the ultraviolet and soft x-ray emission levels. These changes over the solar cycle have important consequences for the Earth’s upper atmosphere.”

Sun spots have been implicated with climate change in the past. The lack of sun spots between 1645 and 1715, known as the Maunder Minimum, has been blamed for the so called Little Ice Age. Worryingly, sun spot numbers appear to be in a long term decline at the moment with a number of solar scientists speculating that if this continues then, far from global warming, we may be in for a repeat of the Little Ice Age.

Furthermore, the sun is currently going through a particularly quiet cycle phase – the quietest in over a century – and this has led some climate change sceptics to argue that the sun is in fact the cause of the global warming pause. Now, sceptics would say that, wouldn’t they, and climate scientists stress that changes in solar output are just too small to affect the climate.

However, the space physics research on solar activity is not the work of climate sceptics and it is showing a strong correlation between solar activity and global temperature.

It is important to state that correlation does not imply causation and none of this solar research alters the fact that carbon dioxide and other greenhouse gases undoubtedly have a warming effect on the atmosphere.

But the circumstantial evidence that the sun plays a bigger role in climate change than hitherto thought is growing and a potential mechanism connecting sun spot related magnetic activity with upper atmospheric changes has been hypothesised.

It is now high time for climate scientists to set the established orthodoxy to one side and to reassess the contribution of the sun to climate change.

Sources

Read the Astronomy & Astrophysics paper here.

Read the English language abstract of the Chinese Science Bulletin paper here.

Read accompanying press release to Chinese Science Bulletin paper here.

See news reports on both pieces of research here and here.

Most recent IPCC scientific report (AR5 WG 1) report here.  See The Summary For Policy makers and Section 8 on Anthropogenic and Natural Radiative Forcing

Nature paper on sunspot activity and solar UV output here.

NASA comment on the link between sun spots and the upper atmosphere here.

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  1. […] orthodoxy in climate science that our star plays no significant role in global warming. – Click here to read the full article […]



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