How we know that the sun changes the Climate. Part I: The past

From Climate Etc.

by Javier Vinós

Part I of a three part series.

The Sun is a variable star and the amount of energy it emits varies from month to month, year to year, and century to century. One of the manifestations of these variations are sunspots, which are more common when the Sun is more active and disappear when it is less active. These spots follow a solar cycle of about 11 years, but sometimes there is a longer period, decades or centuries, when the Sun’s activity is so low that there are no spots. These periods are called grand solar minima. There are also periods of decades or centuries when the activity is higher. These are called grand solar maxima.

The Sun provides 99.9% of the energy that the climate system receives. So, there have always been scientists who thought that variations in the Sun were the cause of climate change. The problem is that they never had enough evidence to prove it. Until now.

1. The IPCC and NASA say…

The IPCC and NASA are convinced that changes in the Sun have very little effect on climate. They rely on two arguments. The first is that changes in solar activity are very small. We measure them with satellites because they cannot be measured from the surface, and we know that the radiant energy coming from the Sun varies by only 0.1%. The magnitude of the changes is better appreciated when we use the full scale. Many scientists believe that such a small change can only produce small changes in climate.

The second argument is that the evolution of temperature does not coincide with the evolution of solar activity. Since the 1990s, solar activity has decreased while warming has continued.[i]

Actually, this argument is not valid because it does not say that the Sun does not affect temperature, but that it is not the only factor in doing so, something we already knew because temperature responds to many factors such as El Niño, volcanoes, the polar vortex, or changes in the Earth’s orbit. There are many natural causes that change the climate, and what we need to know is whether the Sun is one of the main ones.

To find out, we don’t need to care what the IPCC and NASA think, we need to ask the climate itself. It doesn’t matter how small the changes in the Sun are if it turns out that the climate responds strongly to them by causing big changes.

2. Climate during the Holocene

And the best way to find out is to look at what has happened to the climate over the last 11,000 years, the interglacial period we call the Holocene. The advantage of doing this is that the Holocene climate changes could not have been caused by changes in CO₂. They must have been caused by something else.

To study the climate of the past, scientists use various climate proxies that they collect in different parts of the world. A major study published in Science used 73 of these proxies to reconstruct Holocene climate.[ii] I have used the same proxies, with a slight modification in the way they are mixed.

What we see, and what a large number of studies also support, is that there was a warm period of thousands of years, called the Climate Optimum, followed by a long period of cooling, called Neoglaciation.

How do we know that this reconstruction is correct? Another study reconstructed the progress of the Earth’s glaciers over the past 11,000 years.[iii] They divided the globe into 17 regions, and this graph shows the number of regions whose glaciers increased in size during each century of the Holocene.

Since glaciers grow when it is colder, we can invert their figure and compare it to the temperature reconstruction graph so that its meaning is the same. We find a high degree of agreement. The glaciers confirm what the temperature reconstruction shows. We also know that CO₂ has done the opposite of temperature, but that is a story for another day.

Note: y-axis is the Z factor, which is related to temperature anomaly.

Both graphs also show some severe cooling episodes that were accompanied by increased glacier growth. These abrupt climate events of the past have been studied and identified by paleoclimatologists. Of all of them, we will focus on four of the most important ones. The Boreal Oscillation, the 5.2 kiloyear event, the 2.8 kiloyear event, and the Little Ice Age.

The four are separated by multiples of 2,500 years and form a cycle that I have called the Bray cycle because that was the name of the scientist who discovered it in 1968.[iv]

Now that we know the climate of the past, we need to talk about the activity of the Sun in the past.

3. Past solar activity

The Sun’s activity is recorded in the tree rings through the action of cosmic rays. A constant stream of cosmic rays from the galaxy reaches the solar system. Some interact with the atmosphere. Some collide with nitrogen in the atmosphere, converting it to carbon-14, which is heavier than normal carbon-12 and radioactive. This carbon-14 combines with oxygen to form radioactive CO₂, which is breathed by trees. The carbon is used in photosynthesis to make cellulose, which allows the tree trunk to grow in diameter. When the tree dies, the carbon-14 in the wood slowly decays over centuries and millennia. You just have to measure how much carbon-14 is left in the wood to know how much time has passed since the tree died.

Each growth ring of a tree records the carbon-14 that was in the atmosphere that year, and scientists have used millennia-old trees and preserved logs to construct a calibration curve that spans tens of thousands of years. This allows them to determine the age of any organic remains, even if it is not a tree trunk, just by knowing the carbon-14 it contains. This is known as radiocarbon dating.

The only problem is that the production of carbon-14 by cosmic rays is not constant. The Sun’s magnetic field deflects the path of cosmic rays, causing many to miss the Earth, and changes in the Sun’s activity affect its magnetic field.

As the Sun’s activity increases, fewer cosmic rays arrive, less carbon-14 is produced, and organic remains appear older because they contain less of it. When the Sun’s activity becomes weaker, more cosmic rays arrive, more carbon-14 is produced, and the organic remains look younger because they contain more of it.

This produces deviations in the calibration curve that allow us to know what the Sun’s activity was in the past.

4. Spörer-type solar minima

When we analyze the radiocarbon curve over the last 11,000 years, we observe large deviations that indicate long periods of low solar activity. These extended periods of low solar activity are called grand solar minima and increase carbon-14 production by 2%. The most common ones last about 75 years, and there have been about twenty in the last 11,000 years. The most recent was the Maunder Minimum in the late 17th century. But there are other types of grand solar minima that are much more severe because they last twice as long, about 150 years. The last of these severe solar minima was the Spörer Minimum, which occurred in the 15th and 16th centuries.

There have been only four such Spörer-type grand minima in the entire Holocene. 2,800 years ago, there was the Homer Minimum, 5,200 years ago the Sumerian Minimum, and 10,300 years ago the Boreal Minimum. We know when they occurred thanks to tree rings.

If the dates sound familiar, it is because the four grand Spörer-type Holocene minima coincide exactly with the four major climatic events on the graph we saw earlier. We know that during each of these grand solar minima, when the Sun’s activity dropped for 150 years, the climate experienced a tremendous cooling that had a major effect on climate proxies around the globe.

We also know that low solar activity during the grand minima has had a major impact on human populations. Past human settlements and their component structures can be radiocarbon dated. When humans were doing well in the past, the population grew and they built more, and when they were doing poorly, usually because there was less food, the population decreased and they built less. Scientists have estimated the evolution of the human population of the British Isles by analyzing the radiocarbon dates of thousands and thousands of remains from hundreds of archaeological excavations.[v]

What they have found is that the population increased greatly with the advent of agriculture, but every time there was a severe deterioration in the climate, the human population suffered from diminishing resources. And the largest declines occurred when grand Spörer-type solar minima took place. Other population declines also coincide with other cooling periods, confirming our reconstruction.

This tells us that the worst climate changes in the past have been caused by changes in solar activity. It also tells us that what is bad for humanity is cooling, not warming.

Now we can respond to the IPCC and NASA. Never mind that solar irradiance changes very little, and never mind that temperature does not always do the same thing as solar activity. Clearly there are other factors at play. But we can state emphatically that changes in solar activity affect the climate because that is what the climate says. The study of past climate leaves no room for doubt. The Sun changes the climate. And if we don’t know how it does it, we should study it.

5. The 20th century solar maximum

Since low solar activity causes cooling, it stands to reason that high activity must cause warming. Solar activity in the 20th century was very high, in the top 10% of the last 11,000 years.

If we count the number of sunspots in each solar cycle over the last 300 years and divide by the length of each cycle, we can see how much solar activity has deviated from the average. Since the Maunder Minimum, during the Little Ice Age, solar activity has been increasing and was well above average between 1933 and 1996, a period of six cycles of increased solar activity that formed the 20th century solar maximum.

Although we cannot know how much of the 20th century warming is due to this modern solar maximum, there is no denying that it is a significant part, because as we have seen, the Sun has been the cause of much of the major climate change over the past 11,000 years.

6. Conclusions

There are two pieces of good news. The first is that solar activity cannot rise above the 20th century maximum. It is not like CO₂, which can keep going up. The Sun’s activity can stay high or go down, but it cannot go up, so the warming should not accelerate and should not be dangerous.

In 2016, I developed a model to predict solar activity in the 21st century. At the time, some scientists believed that solar activity would continue to decline until a new grand solar minimum and mini-ice age. But my model predicts that solar activity in the 21st century will be similar to that of the 20th century. It also predicted that the current solar cycle, the 25th, would have more activity than the previous one, and it was right.

The second piece of good news is that if much of the 20th century warming is due to the Sun, then there is no climate emergency. Believing that all climate change is due to our emissions is one of those errors that sometimes occur in science, like believing that the Earth is the center of the solar system, that interplanetary space is full of ether, or that stomach ulcers are caused by stress, not bacteria.

This article can also be

[i] NASA. Is the Sun causing global warming?

[ii] Marcott, S.A., et al., 2013. A reconstruction of regional and global temperature for the past 11,300 years. science, 339 (6124), pp.1198-1201.

[iii] Solomina, O.N., et al., 2015. Holocene glacier fluctuations. Quaternary Science Reviews, 111, pp.9-34.

[iv] Bray, J.R., 1968. Glaciation and solar activity since the Fifth Century BC and the solar cycle. Nature, 220 (5168).

[v] Bevan, A., et al., 2017. Holocene fluctuations in human population demonstrate repeated links to food production and climate. PNAS, 114 (49), pp.E10524-E10531.