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Climate Change and Solar Activity

By Martin Britton
January 2, 2015


 

Table of contents:

-  Attempting to pin Climate Change to Sun
- The Physics behind the Sun Keeping us warm
- Solar Cycles, the Mauder Minimum and Ice Age
Cosmic Rays driving the Cloud Factory
Global Warming, Cooling and the Sun – what's happening    now?
Converge, then diverge

 

Attempting to pin Climate Change to Sun

Imagine the planet Earth without our fiercely shining star to warm us, and you'd be looking at a dead, dark ball of frozen ice and rock. All the majestic variety of life, weather and climate of our home-world relies on the 1050 watts of energy that hits every square meter facing towards the sun. So it's small wonder that many, when casting around for an explanation for the rush of climate change seen over recent decades, point their fingers at the fiery ball in the sky.
 
Given that the sun's energy is known to vary over various long-term cycles – and that scientists have long pegged the Sun as a potential culprit in the onset of ice ages – looking for links between solar activity and recent climate change is more than just understandable. It's something that's completely necessary. Scientists have spent a considerable effort in trying to grasp how the sun can influence our planet's climate. If we are to get to the bottom of what's behind recent climate change, and where it's taking us, mapping that interaction between our star and us is crucial.
 
Interestingly, what the most recent scientific work has revealed is that the Sun could influence our climate in some surprisingly devious ways. But also, it has shown that any attempt to pin last 120 years of climate change to the Sun is far from straight-forward. Humankind, it seems, cannot dodge the bullet for global warming quite so easily.
 
 

The Physics behind the Sun keeping us warm

The Sun is actually something of an exceptional star, brighter than more than 80% of the stars in our galaxy[i]. It's made up of a ball of super-hot plasma, millions of degrees hot in its center, where a constant nuclear reaction pushes vast amounts of energy out through its outer layers. Once it's escaped the Sun's surface, that energy races towards the Earth, part of it reaching us as sunlight. As well as this 'sunshine', our star is also throwing out ultraviolet light, X-rays and even radio waves.
 
This spectrum of energy doesn't just flood into the space around our planet. It also has an effect on the wider radiation coming to us from the rest of the universe. Called galactic cosmic rays, this radiation is produced by the vast panoply of bodies making up our galaxy. The Sun's own boundless energy, however, helps protect us from much of this. We sit in the bounds of the Sun's so-called 'heliopause', which defects a good proportion of the galactic bombardment.  That's something we'll come back to, especially as some see changes in the heliopause as possibly having an effect on our climate.
 

Solar Cycles,  the Maunder Minimum and Ice Ages

The Sun's energy is, however, not felt by the Earth in a constant, never-changing stream. Two factors cause the amount of energy we receive to vary, especially when measured over decades and millennium. First, the Sun goes through natural 11-year cycles of higher and lower energy – the well-known Solar Cycle. This has been obvious to humans ever since we've been able to count sunspots on its surface.
 
When there are lots of sunspots (dark patches on its surface), scientists have measured increases in the sun's energy across the spectrum. But not by much – no more than 0.1% of extra brightness can be felt when sunspots are at their peak[ii]. There are possibly other cycles too, apparently showing themselves over longer periods, but these remain somewhat elusive. We simply haven't been tracking them long enough for their extent to be fully revealed.
 
One example is the long-term cycle suggested by the Maunder minimum of the 17th century. Then, the Sun underwent a prolonged period of  relative quiet. Virtually no sunspots were observed for more than 30 years, and at the same time, much of the Northern hemisphere was plunged into what was called the Little Ice Age. Some scientists[iii] believe the Sun's long slowdown had a real effect on the cold winters experienced then, though others point to volcanic eruptions at the same time as a possible cause.
 
The second thing that causes us to receive variable amounts of energy from the sun is the wobbliness of the Earth, as it circles round the solar system. Known as 'Milankovitch cycles', these long-term variations in the Earth's orbit change how much of the Sun's energy is scooped up by different parts of the planet. The variations are small, but seem to be enough to help trigger changes in the climate that bring on Ice Ages. These frozen epochs have appeared regularly, every 100,000 years or so, over the last million years[iv], matching one of the predicted Milankovitch cycles.
 
 

Cosmic Rays driving the Cloud Factory?

 
An even stranger effect comes from the heliopause mentioned earlier. This is the Sun's bubble of protection from the galaxy's cosmic rays. It has been seen to get pushed out further when the Sun is an energetic upswing. When there's a solar minimum, on the other hand, the heliopause retreats, and the Earth gets hit by more of these cosmic rays.
 
This matters because experimental evidence[v] has suggested that the ways that cloud form could be affected by the amount of cosmic rays bombarding the atmosphere. More cosmic rays cause the formation of more aerosol particles[vi] – the tiny 'seeds' that water condenses onto to make the clouds. And clouds have a big impact on the climate. The problem is that scientists aren't entirely sure how this works in the real world. Or what sort of clouds would be affected by the comic rays. That's very important to know, if we are to determine how these rays could possibly influence the climate.
 

Global Warming, Cooling and the Sun – what's happening now?

So putting all these things together, could it be the case that the Sun is having an effect on the pattern of climate change we've seen in the recent past? Well, it depends on how what you'd like to call 'recent'. Step back over global temperatures for the last 12,000 years, as a recent paper[vii] in Science did, and you'll see we came out of a severe cold period (a pulse of current Ice Age) 10,000 years ago. Temperatures were stable for a while, but then started cooling from around 4,000 BC.


Figure 1: From S.A. et al (2013) at
http://tamino.wordpress.com/2013/03/22/global-temperature-change-the-big-picture/
 
This global cooling has been shown by scientists[viii] to have been part of a reduced energy from the sun caused by the change in the Earth's orbit – the Milankovitch cycles. But the most recent sharp increase in global temperatures can't be related to such effects. Orbital changes are slow and steady. There's nothing slow or steady about that blue line. So what has caused the global cooling to reverse over the last century of two?
 

Converge, then diverge

 
Another possible solar explanation lies with long-term cycle of energy increase coming from the Sun (or total solar irradiance). That's something that has been mapped out by scientists, using the sunspot numbers so usefully recorded by astronomers over the last 3 centuries. Place their data on the Sun's energy output next to global temperatures, both measured over the last couple of hundred years, and there is something of a match.


 
Figure 2 : From NASA GISS (Temp) and  Krivova et al 2007 (TSI), adapted at http://www.skepticalscience.com/solar-activity-sunspots-global-warming-intermediate.htm
 
 
Total solar irradiance increases from 1880 until the mid-20th century, and so do global temperatures. Sadly for those looking to pin global warming to the Sun's erratic ways, the solar energy falls thereafter, while global temperatures soar, especially since 1970. In fact, the latest solar cycle has the Sun at one of the lowest ebbs in energy seen in many decades – and yet global temperatures remain high.
 
That leaves the 'cosmic ray cloud factory' hypothesis as the one remaining plank in the 'sun causes global warming' argument. Here, the jury remains firmly out. Scientist have struggled to agree on what exactly the mechanism could be, or whether there is any effect  measurable in the atmosphere. The IPCC, in their AR4 report[ix], stated that “evidence for a systematic indirect solar effect remains ambiguous” and possible mechanisms are “controversial”. They have simply not been able to find any relationship between the levels of cosmic rays, and measures of cloud cover.
 
So where does that leave the idea of solar activity as the main driver of climate change in the past, and today? As something of a sideshow, it would seem. That's not to say that the Sun hasn't, or won't continue to have, an effect on the climate. It's just that the effect is relatively small.
 
The IPCC[x] places changes in the Sun's energy as equivalent to +0.05 W per square meter of the planet. The effect of increased CO2 levels is put at over thirty times more, at +1.68 W per square meter.  The Sun may not be about to disappear and leave the Earth frigid – one worry to cross off the list. But if CO2 levels keep increasing, as they have for the last 100 years, it's the opposite extreme – of a bakingly hot planet Earth – that is the worry we should really be concerned about.

 

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[i]           "Astronomers Had it Wrong: Most Stars are Single". Space.com. Than, K. (2006).
[ii]                  “Ch. 8 Anthropogenic and Natural Radiative Forcing.” IPCC AR5, WG1.  IPCC (2013).
 
[ix]         “Ch. 2.7.1.3 Indirect Effects of Solar Variability” . IPCC AR4, WG1. IPCC (2007)
[x]            “Ch. 8 Anthropogenic and Natural Radiative Forcing.” IPCC AR5, WG1.  IPCC (2013).