Our sun really is a breath-taking miracle of nature. Essentially it's a massive nuclear
fusion reactor. Every single second of every single day it converts about 600 million tons of
hydrogen into helium. And every single second of every single day that fusion reaction results in
about 4 million tons of matter being converted to energy. You could fit a million earths inside it,
and it's got about 330,000 times the mass of the planet we live on. In fact,
it's so massive that it actually makes up about ninety-nine point eight six percent of all the
mass that exists in our solar system. It's about 4.6 billion years old right now, which is about
halfway through its life cycle. It hasn't changed dramatically in about four billion years and it's
on course to say pretty much stable for another five billion or so. It is of course by far the
most important source of energy for life on Earth, but every 11 years or so it goes through a cycle
of activity that includes what science refers to as a 'solar minimum' and a 'solar maximum'. Some
people regard these cycles as the most significant influence on changes in Earth's climate, and right
now the Sun is approaching one of its minimum periods of activity. In fact this one is expected
to be so quiet that it may fall into a special category called 'Grand Solar Minimum'. So is it
time for us to stop talking about global warming and start talking about global cooling instead?
Hello and welcome to Just Have a Think. So what's all this about Grand Solar Minimums then? Well,
ever since Galileo first got the hang of a new-fangled contraption called a telescope
back in the early 17th century, scientists have been observing little black spots that seem to
come and go on the sun's surface. In 1775 a guy called Christian Horrebow wrote "it appears that
after the course of a certain number of years the appearance of the Sun repeats itself with respect
to the number and size of the spots". We now know that those cycles repeat themselves about once
every 11 years or so. In the 19th century another scientist by the name of Rudolf Wolf tracked all
these observations right back to 1745 and put them together in a numbering system that's still
known today as the Wolf Index. The first cycle in Wolf's index occurred between 1755 and 1766,
so in the time-honoured way that these meticulous science types always seem to work, Wolfe called
this cycle 'Cycle Number One'. Unsurprisingly science was obsessed with discovering what caused
these periodic black spots and whether they were of any consequence to the sun's output. In 1908,
a group led by George Ellery Hale managed to show that the sunspots were strongly magnetized,
and that was apparently a fairly big deal in the astronomical world because it was the first time
anyone had detected any kind of magnetism outside of our own planets field. Modern science tells
us that the Sun is in fact a massive globe of extremely hot ionized gas called plasma,
and that it's the flow of this electrically charged gas that causes the movement of the
sun's magnetic field, which in turn results in these solar cycles and which also governs the
number of sunspots, as this slightly insane NASA animation shows us. You can see how the purple
and green lines representing the opposite poles of the magnetic field are constantly shifting
over the 4-year period that this animation covers, and every 11 years or so the sun's magnetic field
completely flips, so the North Pole becomes the south Pole and the South Pole becomes the North
Pole. Here's a slightly clearer representation of the last full solar cycle - Cycle Number 23. This
one started with the solar minimum in 1996. You can see there's very little sunspot activity at
all at that point, and as the cycle moves towards the solar maximum in 2001 things clearly get a lot
busier, before dying back down to the end of the cycle and another solar minimum in 2006. This is
an image of sunspots taken by NASA on July the 19th 2000 towards the maximum of the last cycle,
and there's quite a few spots across the surface. This is the same image in the ultraviolet light
range showing the corresponding increase in solar activity at those spots. Compare that to
this image taken in March 2009 around the time of the last solar minimum and you can see that
the sun's surface is literally spotless, with the corresponding ultraviolet image showing a
very quiet Sun indeed. Right now, in 2019, we're pretty much coming to the end of Cycle Number 24
and we're approaching another solar minimum. Here's some commentary on Solar Cycle Number
25 from Wikipedia. "A preliminary consensus of the Solar Cycle 25 Prediction Panel was made in
early 2019. The panel, which was organized by NOAA's Space Weather Prediction Centre, NASA,
and the International Solar Energy Society, concluded that Solar Cycle 25 will be very
similar to Solar Cycle 24. They anticipate that the solar cycle minimum before Cycle 25 will be
long and deep, just as the minimum that preceded Cycle 24." So the obvious question is - how big
an influence do these solar cycles have on the climate of our planet, and would a Grand Solar
Minimum cause major cooling on the planet and bring about the onset of a mini ice age,
as some folks seem to be suggesting? Well, to get an answer on that one we need to take
a quick look at something called 'irradiance'. Here's a quick rundown on radiation units from
a presentation published by Blaise Norton, which I'll link in the description box below. The basic
unit of radiant energy is the Joule. Radiant flux is how much energy is provided over time,
with the time usually measured in seconds - so for example, one Joule per second. And 'one
Joule per second' is one Watt. You may remember from school that the WATT is a unit of power,
so power is how much energy any given thing can give to any other given thing over a given period
of time. You getting this?? So then we come to radiant flux density, which is better known as
irradiance. And irradiance is the amount of energy over a given period of time over a given area,
usually measured in square metres. So those units come out as joules per second per square metre,
otherwise known as Watts per square metre. So when scientists measure the amount of energy from the
Sun hitting the surface of our planet, which they call solar irradiance, they measure it in Watts
per square metre. There's a great NASA article on solar radiance which again I'll link in the
description box below. These two charts from that article show how the actual sunspots from solar
cycles between 1975 and 2015 corresponded with total solar irradiance and you can clearly see
that the total solar irradiance goes up and down in lockstep with the number of sunspots. But as
with all charts and graphs, it's pretty important to understand the scale on each axis. If you check
out the left-hand side of the top chart you can see that it tells us that, sure enough,
it's measuring solar irradiance in Watts per square metre, exactly as we've just established.
But look at the actual numbers. The range bounded by the top and bottom horizontal dotted blue lines
goes from about thirteen hundred and sixty point four, up to around thirteen hundred
and sixty one point eight, which is a variance of one point four Watts per square metre. Now here's
another tricky bit to do with something called 'radiative forcing'. Wikipedia explains that
radiative forcing can be used to estimate a change in surface temperature arising from that forcing.
For solar irradiance the radiative forcing is the average amount of solar energy absorbed
per square metre of the Earth's area. Because the earth is a sphere, the area exposed to the Sun is
only a quarter of the total surface area of the earth, so the total input per unit area is only
one-quarter the change in solar intensity. And because of the Earth's reflectivity, or 'albedo',
a little bit more of that energy has to be factored out the equation. Wikipedia tells us it's
actually a multiple of nought point seven. So one point four divided by 4 is 0.35, and nought point
three five times point seven is nought point two four five. So on the right-hand side of the chart
it tells us that the variance in solar forcing over the period is nought point two five watts
per square metre. So let's just have a think about that for a second. Nought point two five watts per
square metre as a proportion of 1361 watts per square metre works out at about zero point zero
one eight percent. This chart was produced as part of the Intergovernmental Panel on Climate Change
SR5 report in 2013. It shows the effect of all the main radiative forcing factors that our atmosphere
experiences. The horizontal bars represent the effect that each factor is predicted to have
relative to 1750 levels. Once again measured in Watts per square metre. Right at the top we've
got good old CO2 - by far the most significant of all the factors, causing as much as two Watts per
square metre of extra warming, predicted with a very high level of confidence, as the far
right-hand column indicates. By the way you can also see that some of our human or anthropogenic
activities caused emissions that actually have a cooling effect on our atmosphere - things like
black carbon and sulphates. And then way down at the bottom of the chart we get to 'natural
radiative forcing factors', which is basically the Sun. At an overall additional forcing over the
timescale of the chart of no more than 0.1 watts per square metre, it barely registers! And just to
hammer home the point about human greenhouse gas emissions, the three red bars at the bottom show
how anthropogenic radiative forcing has increased from 1950 to 2011. But nevertheless there are
those who insist that solar activity does exert some kind of disproportionate and scientifically
unexplainable extra influence on the climate of our planet. A chart that's often produced to
suggest a large cooling effect of a Grand Solar Minimum is this one, showing the solar cycles
going right back to 1600. Between 1645 and 1715 there was an event called the Maunder Minimum. The
Maunder Minimum was a prolonged period when very few sunspots were observed. It corresponds with
a period of lower winter temperatures in Europe and North America, and as a result it's sometimes
cited as being the cause of what's known as the 'Little Ice Age'. But according to paleoclimate
reconstruction, the onset of the Little Ice Age occurred between 1560 and 1600 - at least 50
years before the beginning of the Maunder Minimum. Winter temperatures didn't go down significantly
more when the Maunder Minimum did kick in, and actually the summer temperatures throughout the
Maunder Minimum were very similar to summer temperatures in subsequent years. In fact,
according to this 2012 paper by Miller et al, the best current hypothesis is that the Little Ice Age
most probably came about largely as a result of volcanic activity. This report from Carbon Brief
tells us 'at first glance solar forcing seems to make intuitive sense. The Sun is a massive nuclear
fusion reactor and million times larger than Earth. It is responsible for almost all the energy
reaching our planet, and in the past few decades scientists have learned that solar activity varies
significantly over time. Indeed changes in the distribution of sunlight reaching the earth
clearly change the temperature dramatically on a daily and annual timescale. However, since 1970,
global temperatures have shot up by almost nought point seven degrees Celsius, while
the amount of solar energy reaching the earth has actually declined. Similarly the upper atmosphere
is cooling, while the lower atmosphere warms - a clear fingerprint of warming from greenhouse gases
rather than the Sun'. This chart based on a paper by George Filner and Stefan Rahmstorf, published
in Geophysical Research Letters in 2010 shows how our planets temperature would most likely
be affected by a Grand Solar Minimum as severe as the Maunder Minimum. The red line shows predicted
temperature change for the current level of solar activity and the blue line shows the predicted
temperature change if solar activity went all the way down to the level of the Maunder Minimum. The
black line shows observed temperatures from the NASA GIS data set through 2010. The authors found
that the average global surface temperature will be diminished by no more than 0.3 degrees Celsius
even at the very lowest solar activity. So even the grandest of grand solar minimums comes nowhere
close to compensating for the warming that humans CO2 and other greenhouse gas emissions will cause.
And you'll find the link to the paper in the description box below if you want to have a look
at their full commentary. It's fair to say that there's a growing level of fear and uncertainty
about the weather patterns our planet seems to have been experiencing in recent times. Major cold
snaps and record heat waves all over the planet, devastating forest fires, not just in California
but all the way up into the Arctic Circle, extended droughts in many parts of the world often
followed by extreme flooding in the same areas giving people very little chance to recover from
one event to the next, as the cereal crop farmers of the United States know only too well this year.
It's understandably tempting to blame these chaotic weather patterns on the Sun. The great
thing about blaming the Sun is that we can delude ourselves that climate change has nothing to do
with human beings, that our climate has always changed, that it's all just down to nature, and so
we don't need to accept any culpability for what's happening or take any extra responsibility for our
own actions. But the reality is that for several decades now climate scientists have been warning
us that precisely these sorts of extreme weather events would occur if we didn't make wholesale
reductions in our greenhouse gas emissions. The increase in these types of weather events
is entirely predictable and explainable within the context of human-induced global warming,
and the change that that warming is causing to our climates basic physics and chemistry show
us that it's carbon dioxide and other greenhouse gases, and not the Sun, that are the main drivers
of changes to our atmospheric temperatures. And those temperatures are increasing at a rate that's
so fast it's completely unprecedented in our planet's history. And we also know that it's
our own human activity that's producing massive quantities of these greenhouse gases as a result
of the way we burn fossil fuels to power our modern lifestyles and the way we're reducing
forests to make way for unsustainable levels of single crop agriculture and massive intensive
livestock farming. Our planet's atmosphere will continue to warm throughout the course of this
century and beyond and it's got very little to do with whereabouts we are in a solar cycle grand or
otherwise. Blaming the Sun is a cop-out! We've caused climate change, and we all need to change
our lifestyles radically if we're to stand any chance at all of fixing the problem. That's it
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