It is an easy and logical assertion to make: the main energy engine of the Earth’s climate system is the Sun. Therefore the most likely candidate for global warming is variation in the Sun’s energy. After all, the Sun has distinct cycles in solar output and is known to have affected historical climate. Problem solved – it’s so obvious, why are scientists spending so much time considering other explanations and discounting the notion that solar energy is currently warming the planet?
Historical changes – the case for solar
Let’s consider the past first. There are recorded episodes regional cold phases in temperature that appear to coincide with minima in sunspot numbers. The most well-known and cited cold episode of this type is the so-called ‘Little Ice Age’ period – between 1450 and 1800 – and characterised by a succession of cold winters, mediocre summers, rivers freezing over – and frost fairs on the Thames.
The argument goes that this cold episode coincided with the observed ‘Maunder Minimum’ of sunspot numbers during the 1600s – therefore the Earth was colder due to the lower solar output which happens when there are fewer sunspots. In turn, the theory continues, recent warming from the end of the 1800s to the present day must be therefore be due to more sunspots in the past few decades and a higher level of solar energy emission.
Historical changes – the problem
There is no evidence at all that the whole globe’s temperature was lower during the ‘Little Ice Age’. There is evidence for a regional cooling, over Europe at least. There are no direct temperature or written meterological accounts for other regions of the world in the 1600s. Therefore, it is conjecture to imply the globe was cooler – in the same way that a regional heatwave today does not imply the global temperature is higher. The term ‘Little Ice Age’ is a misnomer, and generally discounted by scientists as nothing more than a slang term for the European cold phase of the 1600s.
The bigger problem with the solar theory
If the Sun were to experience big enough fluctuations in energy output then the global surface temperature of the Earth will of course respond. As most science students will tell you, towards the end of the Sun’s life in billions of years time the solar output will fall to eventual levels that will commit the Earth to an icy death.
Modern solar output levels are accurately measured. Very accurately. Data from a number of NASA and ESA satellites show two things which which present problems to the hypothesis that solar variability is driving the global temperature increases. Firstly the level of energy variations associated with the well-known 11-year solar cycle is simply too small to account for the observed rise in Earth’s temperature – especially compared to compared to the changes in other factors which influence climate: volcanoes, oceanic variability and man-made atmospheric pollutants. The numbers for recent decades just simply do not add up.
Secondly, aside from the 11-year cycle, there is no longer-term rise in solar output in recent decades evident in the satellite record. How can this be, then, if the Sun is allegedly causing the planet to get hotter?
What about 1900-1950?
The satellite-observed record of solar output can be can be appended to observations of inferred solar output derived from visible Sun spot counts: When more sunspots are visible, total solar output rises. When the two records are joined together, an intriguing increase in solar output is evident between 1900 to about 1950.
Could global warming in this period, then, be due to solar changes? Well, the solar contribution to Earth’s temperature is indeed bigger then, than for more recent decades. Never-the-less, once again, the numbers just do not add up: the associated rise in solar output for this 50 year period is so slight it can only account for a maximum of 30% of the Earth’s temperature rise.
The inconsistencies for both the 1900-1950 phase – and the more recent decade of 1980 onwards – makes it incredibly difficult to accept that notion that solar variability is the primary cause of surface temperature change. On the other hand, however, if we calculate what the expected global temperature response is to the known augmentation of the atmosphere due to observed CO2, methane, and SO4 emissions, then the expected change is incredibly close to the actual observed temperature evolution. In fact, the energetic input to the global system (the so-called radiative forcing) attributable to man-made agents is about ten times more than the influence of the observed solar variability.
A further theory – at the forefront of some exciting research – is the notion that variability in the Sun’s output raises and lowers the ‘shield’ around our solar system. Lower solar activity lowers this shield (the magnetic heliosphere) and allows deep-space particles – emitted by distant galaxies – to interact with Earth and other planets. One suggested theory is that the waxing and waning of this particle stream can cause cloud amounts on Earth to also wax and wane, thus affecting Earth’s temperature by either reflecting or increasing the amount of sun energy that reaches the surface.
There is convincing suggestions that cloud amounts in some regions of Earth do respond variations in galactic ray arrival. What is problematic with the supposed connection with Earth’s recent temperature rise, however, is that there is no long-term decrease in ray intensity which would be needed to support the the idea that less clouds are existing over the Earth. In fact galactic ray measurements show a subtle increase in intensification, meaning, according to theory, the Earth’s temperature ought to have cooled in the last few decades. With the present evidence and understanding to hand, accepting intergalactic rays as the principal driver of the Earth’s temperature can not be entertained.
Directly measured variations in solar output are not large enough to account for the observed rise in surface global temperatures for the last 150 years. Solar output certainly contributes – and the level of contribution changes – but the energy calculations show that the recent increases in temperature far exceed the amount that can be attributed to solar activity.
The Sun continues to exert an affect on upper-atmosphere temperatures, will continue to affect climate on much longer time scales, and the general topic is at the forefront of exciting research that continues to question current thinking and understanding of the Earth system. It is entirely correct that these studies are given recognition and room in the global warming ‘debate’. Nonetheless, at present, there is little evidence that can support the idea that recent warming is due to solar properties.