We have seen that the Australian Bureau of Meteorology chart of mean temperatures over a century seem to suggest an inexorable rise (by decade) over the past 40 years or so. The chart roughly reflects measurements around the world. Before the 1970s, there seems to have been no discernable pattern.
The graph of atmospheric carbon dioxide from the Mauna Loa facility in Hawaii shows a virtual straight line increase in since recordings began there in 1958.
Is the phenomenon of increasing carbon dioxide in our atmosphere causing global temperatures to increase or vice versa? Is some external phenomenon driving these changes?
Dr Roy Spencer says that the increasing temperatures are natural. While he agrees that the planet is warming, his research indicates that it as a natural response to cloud changes associated with the Pacific Decadal Oscillation (PDO). On the other hand (and there is always one in this debate), others show that the PDO simply does that – it oscillates.
Could the Sun’s activity have an effect? Clearly, anything the Sun does is going to affect us as we rely on it – and its stability - for our existence.
Astronomer John Allen Eddy published an important paper about the effects of Sun’s activity on climate in the journal Science in 1976. In his paper “The Maunder Minimum” he pinpointed a period between 1645 and 1715 in which solar activity virtually stopped. That is not to say the Sun stopped shining. It was just that sunspot activity and radiativity was at a minimum. This period coincides with the coldest period in the “Little Ice Age” suggesting that variations in the Sun’s activity do indeed affect global temperature.
How do we know so much about Sun activity in that time? Telescopes were already in common use among scientists and observers at the time so there is a considerable record. Eddy and others were also able to demonstrate a relationship between carbon 14 and solar activity (the link above takes you to a particularly long page but it is worth reading as it shows the scientific struggle in action). The more carbon 14 in the environment – and it gets into everything – the lower the sunspot activity. Carbon 14 is unstable; it degrades over time and is therefore particularly important as it allows scientists to date objects up to 40,000 years old.
Did increasing Sun activity cause the so-called Medieval Warm Period? That period spans about 950 to 1250AD during the European Middle Ages. There is strong evidence in the record of warming in areas of the northern hemisphere. A paper published by Geophysical Research Letters in 2002 provides evidence of an unusually warm climate during the same period in New Zealand.
The Medieval Warming is taken among some to demonstrate that the planet has been warmer in the past than it is today at a time when humans were pumping far less carbon dioxide into the atmosphere. One of the most striking points is the evidence that Icelandic Vikings settled in Greenland during that time in a virtually ice-free environment. Chaucer’s Canterbury tales includes reference to a particularly pleasant environment in England during the Medieval Warming. The Greenland settlements collapsed around the early 15th century apparently by the onset of a much colder turn (the little ice age?).
So maybe the observed increases in global temperatures over the past few decades are all to do with current Sunspot activity. But recent reports show the Sun has been quiet lately.
Finally, was the planet’s climate warmer during the Middle Warming than it is today?
A graph at Wikimedia Commons is a comparison of 10 published reports on mean temperature over 1,000 years. It is a version of the renowned “hockey-stick” graph. If you really want to learn about a major element of the climate debate, Google “climate change hockey stick”. You’ll be busy reading for days
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