For several months, some NASA experts have alarmed the scientific world,
announcing that "If the Sun continues to show the total absence of sunspots on its surface,
a small ice age could soon occur on Earth". An absence that could have direct consequences
on our climate, even reversing the trend that was leading us towards a period of global warming.
The perspective may appear exaggerated and based on a hasty assessment of a trend
that has yet to be verified, however it must be admitted that in the recent past of human history
such a phenomenon has already occurred in the second half of the seventeenth century,
when a prolonged lack of sunspots, known as the Maunder Minimum, plunged Europe into an
era of intense cold characterized by famines and epidemics that decimated its population.
Could this happen again? To answer this question it is
first necessary to take a step back to learn more about the characteristics of our star.
Because obviously our Sun is a star: by far the closest to Earth,
since the second in order of distance, Proxima Centauri, is 270,000 times further away!
A star like so many in the Galaxy; all in all quite small,
although we are talking about a sphere of ionized gas of 1.4 million kilometers in diameter, or a
monster that radiates energy in every direction of space in the form of electromagnetic radiation.
All this thanks to a core where the temperature reaches values ​​of fifteen million degrees,
and where through the mechanism of nuclear fusion takes place the production of that immense amount
of energy that allows it to illuminate and heat the solar system for 4.6 billions of years.
In other words, the Sun is nothing more than a gigantic nuclear fusion reactor, wrapped in
a spherical shell called the photosphere, which is practically the visible surface of the Sun;
that is, the zone where the gases cease to be transparent to radiation. Above the photosphere
there is the chromosphere, a thin layer of gas where the solar protuberances originate
(gigantic gas eruptions with dimensions of tens of thousands of kilometers);
and even higher up the corona extends for millions of kilometers, a very hot region that represents
the outermost part of the solar atmosphere. The so-called solar wind emanates from the corona,
a continuous flow of charged particles that sweeps the entire solar system.
Fortunately, only a small fraction of the radiation - about half a billionth of the
total energy emitted - reaches the Earth's surface (and this by the way means that the Sun could heat
and keep alive two billion planets like ours!), Yet even this "modest" quantity can influence
the biological cycles and the Earth's climate. The Sun is in fact the almost exclusive source of
energy for the life forms of our planet and the main source for the activities of our species.
almost all of the energy used on earth by living beings and by our civilization is of solar origin.
Not only the energy that we obtain directly through the photovoltaic panels. But also,
for example, the energy extracted from fossil fuels, the result of the natural transformation
of plants and animals that lived tens of millions of years ago. Those organisms used solar energy by
means of photosynthesis and we reuse it today. The same can be said for hydroelectric energy, which
we could not exploit if the Sun did not keep the closed cycle of water thanks to the evaporation of
seas and oceans, and for wind energy, since the movements of air masses are produced by
a thermal gradient caused by differences in insolation. The chemical energy stored in our
organisms and used for the daily activities of our bodies is also ultimately of solar origin.
Only the sources of nuclear, geothermal and tidal energy are not directly derived from the sun. The
first two, in fact, exploit radioactive isotopes formed billions of years ago
in the nuclei of ancient generations of stars and incorporated within the Earth. The third,
however, is mainly produced by the gravitational influence of the Moon.
At this point, having established the importance of the Sun for maintaining the earth's ecosystem,
we must ask ourselves: can we trust this atomic furnace which is only 150 million kilometers away
from us? What guarantees do we have that one day solar activity will not increase so much
as to roast us all, or that on the contrary it will not decrease until the Earth freezes
as it has already happened in the past? A completely legitimate question,
considering the alarm raised by NASA ... Question immediately followed by another:
what do sunspots have to do with the variation of Earth's temperatures,
and why do they seem to be so important that they are cited by NASA as a trigger
for an upcoming glaciation? Let's try to understand this with a quick informative look.
Sunspots are structures of very variable dimensions, typically
from 5000 to 50,000 kilometers in diameter, which continuously form on the photosphere.
Generally in pairs or in small groups where they then grow in number and size. Although they
are inherently very bright, they appear darker because their temperature is a thousand degrees
lower than that of the surrounding gas. The suspicion that the number of spots on
the Sun could vary with a cyclical trend seems to have been the first to have the Danish astronomer
Christian Horrebow (1718 - 1776); Unfortunately, his works were only published in 1859
when the existence of a sunspot cycle had already been discovered by the German S. H. Schwabe
(1789 - 1875) whose work was disclosed in 1851. Statistical analyzes carried out on observed
values ​​starting from 1715 showed that the average duration of a solar cycle
is 11.4 years, while the longest period ever recorded was 17.1 years (from 1788 to 1805),
and that shorter by just 7.3 years (1829 to 1837). Unfortunately, a complete theory that explains in
detail the birth, evolution and disappearance of a sunspot and the existence of the eleven-year cycle
does not yet exist. What is known is that the presence of sunspots is
intimately linked to the rise from the inside towards the surface of tubular magnetic flows
"coiled" by the differential rotation of the star. At the points where these magnetic fields cross
the photosphere, spots form, which can also be interpreted as depressions on the surface.
Until the mid-nineteenth century it was thought that the intensity of solar activity
and the number of spots, although variable cyclically, could never reach extreme values,
in one direction or another. In 1893 Edward Maunder (1851 - 1928), superintendent for solar
research at the Royal Greenwich Observatory in London, however, noticed a decidedly curious fact,
which until then had gone unnoticed; Maunder had conducted a study on the number of spots
observed on the Sun starting from 1610, the year the telescope was introduced in Europe,
and he realized that in the period between 1645 and 1715 the sunspots had practically disappeared!
This was very strange because even during the minimum of a cycle
some spots are always present anyway; on the contrary, in the seventy years of that period
the number of spots present on the Sun was practically zero, to the point that in 1671
the appearance of a timid speck was treated by the press as an exceptional event.
Maunder published the results of his work in two articles (1894 and 1922) which, however,
were not taken into due consideration mainly for two reasons. First, the observations of
the number of sunspots are considered complete and reliable only since 1700;
secondly, there was a certain reluctance on the part of astronomers to accept the
existence of changes in solar activity other than the regular ones indicated by the Schwabe cycle.
Recent studies have instead re-evaluated Maunder's work as there are strong indications (not to
mention overwhelming evidence) that solar activity in the period from 1645 to 1715,
now known as the Maunder Minimum, actually underwent a drastic decrease. . This hypothesis
is also supported by the complete disappearance in those decades of the polar auroras,
a phenomenon also linked to the more or less intense activity of the Sun. This is impressive:
In a period of 30 years during the Maunder minimum, astronomers observed about 50 sunspots
instead of the normal 40,000 or 50,000! The fact is, however, that the Maunder
Minimum fell in the middle of the Little Ice Age, a period that goes from 1450 to 1850
with a climate characterized by a sharp drop in average temperatures
and a strong expansion of glaciers. In contrast, the so-called Medieval Maximum, an unusually
warm period from 1100 to 1250, has been shown to coincide with a period of strong solar activity.
All historical precedents, these, which have reinforced in some scientists
the belief that something very similar is about to happen even today.
In fact, from 2004 to 2011, there were 821 days without spots on the solar disk.
And in the last 100 years only between 1911 and 1914 had the Sun been so exceptionally lazy.
A trend that has been further consolidated in the last decade, when the solar cycle number 24
(the solar cycles recorded in historical times are numbered with a progressive number starting
with cycle number 1, started in 1755), which reached its maximum in 'April 2014,
it was classified as "the weakest of the last 200 years". And Solar Cycle 25,
which seems to have started just in recent weeks, is expected to be even weaker; on the whole,
a situation dangerously similar to that which occurred with the Maunder Minimum.
The consequences would clearly be a rather intense and long cooling period. A situation that,
although apparently worrying, could however prove to be good for the earth's climate,
which we continue to alter with emissions that tend to increase temperatures.
Thus continues the eternal scientific dispute between those who support the anthropic origin
of global warming and consider the contribution of the Sun to be irrelevant to climate change
underway, and those who instead always oppose new arguments in favor of solar influence,
citing physical phenomena never considered so far. , such as the rise of cosmic rays.
In fact, it seems that one of the effects of low solar activity may be the increase of up
to 20/25% of cosmic rays, which penetrating into the upper part of the earth's atmosphere
would favor the increase in cloud cover and the conspicuous drop in temperatures globally. .
In short, will we go back to skating on the frozen Thames like three centuries ago
or will we be forced to sow wheat and grow olive trees in Canada?
Will the equatorial regions of the Earth become depopulated, or will those in the north?
The answer, they say, will come within the next five years, the time to get to see the
maximum of Cycle 25. If in those days the Sun is still spotless, or nearly so, then we'll know.