Let's start by looking at the concept of solar activity which is generally measured using sunspots. Sunspots are regions on the solar surface that appear dark because they are somewhat cooler than the solar surface with temperatures measuring about 4000 degrees Kelvin compared to 5700 degrees Kelvin like the sun's photosphere. They have diameters up to 50000 kilometres and are sometimes visible to the naked eye. Their persistence is quite variable, ranging from hours to months. They were first observed by Chinese astronomers in 800 B.C. but were not easily observed until the telescope was developed. According to NASA, in March 1611, a German medical student, Johannes Fabricius became the first to publish a scientific paper on sunspots. Sunspots generally occur near the solar equator in the region between 5 degrees and 40 degrees north and south of the equator. They have diameters up to 50000 kilometres, move across the solar surface and are sometimes visible to the naked eye. Their persistence is quite variable, ranging from hours to months.
A 2010 paper by Ilya Usoskin examines the history of solar activity over the past twelve thousand years using indirect methodology through the examination of the levels of Carbon 14 and Beryllium 10 isotopes in tree rings, ice cores and other stratified systems. Usoskin's research found that the first part of the new millennium was a period of high solar activity when measured using sunspot activity as shown on this graphic:
Please note that red areas denote grand maxima and blue areas denote grand minima.
The most famous grand minima took place during the late 17th century. This grand minima, known as the Maunder minimum, occurred when sunspots almost completely vanished from the sun's surface. The Maunder minimum occurred between the years 1645 and 1715 with a complete absence of sunspots between 1645 and 1700. It was during this timeframe that the United Kingdom and continental Europe experienced a climatic period that is referred to as the "Little Ice Age". Normally ice-free rivers like the Thames were frozen and snow fields remained present year-round at lower than normal altitudes. This suggests that the earth's climate is related to solar activity, however, temperature records show that the winter of 1685 – 1686 was the fifth warmest on record and occurred in the middle of the Maunder minimum showing that low solar activity does not guarantee cold conditions.
Here is a graphic showing the number of sunspots between 1600 and the early part of the 21st century showing the Maunder minimum:
Here is a table showing the approximate dates of grand minima going back 12000 years to the beginning of the Holocene:
As you can see, sunspot/solar activity has varied significantly over the past 12000 years with solar minima occurring every 20 to 160 years.
If we look more closely at the data since 800 CE, we can see quite clearly that we are currently living in a period of extremely high solar activity:
Now that we have that background, let's go back to the paper by Ineson et al. The authors note that the very rapid decline (the most rapid decline in the last 9300 years) in the level of solar activity in recent years suggests that there is 15 to 20 percent chance of returning to a Maunder-like solar activity minimum within the next 40 years. The paper goes on to look at the impact of this lower solar activity on earth's climate and how it will impact global warming by using a model that simulated conditions between the years 2050 and 2099 with a Maunder-like minimum in place. Let's now look at the results.
As a result of the decrease in solar activity, the authors found that the relative annual global mean surface temperature over the period from 2050 to 2099 would decline by between 0.12 and 0.13 degrees Celsius as shown on these maps which represent the lower and upper bounds:
As well, changing solar activity will have an impact on the number of frost days as shown on this graphic which shows the decreasing annual number of frost days between 2050 and 2099 for the control (changes in frost days between 1971 and 2000 compared to 2050 to 2099) and the changes in the number of frost days (increases in green and blue) for both upper and lower bounds:
Note that in both models, the number of frost days increases across most of Europe and the southern United States, however the increase in the number of frost days is generally less than four.
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