In 1801, astronomer William Herschall noted a relationship between sunspots and the price of wheat.
He saw that the more sunspots there were, the lower the price of wheat. Fewer sunspots and the price rose.
The Sun acts to shield the Solar System from the full effects of cosmic rays
The suggestion was that Earth's weather depends on the Sun's natural variability - and 200 years later, scientists are still debating this effect.
Though few now dispute that humans are contributing to global warming through car and industrial emissions, there are healthy arguments over just how important this influence is - and whether the Sun's impact may have been underestimated.
One popular theory - currently being tested in Denmark - is that variation in solar activity acts on climate indirectly through its effect in limiting cosmic rays hitting the Earth as they journey across our galaxy.
"I knew that people had been seeing correlations between solar activity and the climate on Earth, and I knew that people dismissed it as being entirely accidental," Professor Henrik Svensmark, of the Danish Space Research Institute (DSRI), told BBC World Service's Discovery programme.
"However when I looked at the correlations at that time, I saw that there was too much to be really accidental,"
This way, that way
Professor Svensmark's theory, first put forward six years ago, was that the link between solar variation and climate change was to do with particle ionisation.
The more cosmic rays reaching Earth, the more clouds would be formed, and therefore the cooler the climate would be.
This is because when solar activity is high, the Earth gets better shielding from cosmic rays - magnetic fields carried in the solar wind make it harder for the rays to make it through to Earth.
Conversely, when solar activity declines, the influence of cosmic rays increases.
However, the theory depends on the ability of cosmic rays to activate cloud formation.
Do cosmic rays trigger cloud formation - and therefore cool the Earth?
Cosmic rays are not really rays - they consist of particles, mainly high-energy protons, released when a star explodes at the end of its life.
"They are important in this context because they ionise the atmosphere," Professor Svensmark explained.
When these particles collide with the gas molecules that make up the atmosphere, the molecules are given either a positive or negative charge.
According to Professor Svensmark, these particles then become the nuclei of tiny droplets of water - that go on to form clouds.
The DSRI has been testing the theory with a small transparent-sided box, containing atmosphere gases, through which natural cosmic rays are allowed to pass.
"These ideas are so developed that we can begin testing them," Professor Svensmark said.
Light and bright
Svensmark's theory has gained support from the Europe's particle physics research centre (Cern) in Geneva.
"If you actually increase the amount of aerosol - these cloud condensation nuclei - then you don't increase the amount of water vapour in the sky, but you do distribute it over more droplets and smaller droplets," said Jasper Kirkby, a physicist at Cern.
"The amount of water is the same, but it's spread out over more droplets.
"The net effect is that the cloud actually lives longer, because it doesn't 'rain out' so easily - the smaller droplets don't sediment out so quickly.
Humans impact climate - but by how much?
"It's also brighter and very much more reflective, which very much affects the radiative energy balance on Earth."
So far, the necessary funding for a bigger, more conclusive experiment - to be conducted either in Geneva or at Stanford University in the US - has not been made available.
Meanwhile, other researchers in Denmark believe the Sun could have a much more straightforward influence on climate.
Among them is Dr Peter Thejll, of the Danish Meteorological Institute, who is attempting to quantify the variation in the amount of direct energy we receive from the Sun.
Dr Thejll claims to have brought together various data collected from weather balloons, aircraft and ships since 1950.
"You can interpolate that [data] into a big model - that'll give you a nice smooth data set," he said.
Effects like El Nino cloud the statistics
After the effects of greenhouse gases, El Nino and volcanic activity were subtracted, a "strong, clear solar signal in that data" emerged, he added.
"In the temperature, in the atmospheric pressure, in winds and circulation system, [the signal is] changing its behaviour in time with the solar cycle," he said.
"To us, it's a sign that it's caused by the solar cycle."
Although these results are convincing, they are not conclusive.
Dr Thejll conceded that the global weather system was so complicated it was very hard to be sure of anything.
"[The climate] is complex - meaning all the parts interconnect. That's why so many results from statistical research into the climate produce confusing results.
"One person will say, 'look, we can show it gets warmer in Bristol', and another chap says 'no, it gets colder in Berlin'. But wait a minute - they might both be the same phenomenon."
Mix and muddle
This concession is crucial, highlighting the key problem in this area of research - the various data sets are far from conclusive and often conflict with each other.
"The analysis of the existing data is statistically not very robust - it's a bit flaky," Professor Joanna Hague, of Imperial College London, told Discovery.
"As in a lot of atmospheric studies, there's just not long enough, or good quality enough, data for us to put our fingers on precisely what the mechanisms are."
Professor Hague, who supports a theory that ultraviolet radiation is the key element of the Sun's impact on Earth's climate, added that there were also great problems with Professor Svensmark's indirect link theory.
In the end, it seems impossible to be sure on the sun's effect
"The other drawback is the actual physical or chemical mechanisms," she said.
"Cosmic rays do affect the atmosphere, and it's quite plausible that the ionisation can affect aerosol formation.
"But in order for them to produce cloud, these very tiny particles need to coalesce and expand until they become large enough to become cloud condensation nuclei.
"The mechanisms involved in each of the stages in that chain are not established - either in the atmosphere or in a laboratory."