Read the first posting in this series: Tale and a description of the figure to the left. I believe the apparent 0.8ºC increase in Global Temperature over the past 150 years is due to three major causes and one minor one, as indicated by the parts of the "tiger". (The second posting details Data Bias.)
This posting is about NATURAL CYCLES that I estimate are responsible for about 40% of the apparent warming. In other words, 0.3ºC to 0.4ºC of the apparent 0.8ºC temperature increase is due to natural cycles not under human control.
You are familiar with three of the natural cycles that affect the energy input and heat balance of the Earth: 1) diurnal - the daily rotation of the Earth, 2) seasons - Earth's yearly orbit around the Sun, and 3) sunspots - 9 to 13-year magnetic cycles on the Sun. Sunrise to afternoon temperatures vary by 10ºC or more and seasonal temperatures by 40ºC or more.
However, these cycles have no long-term effect on surface temperatures. They are not the cause of Global Warming. However, when scientists are trying to detect long-term temperature variations of fractions of a degree, multi-degree daily and yearly variations complicate the task.
Individual sunspot cycles are not long enough to have significant effects on global temperatures. However, when a multi-decadal series of especially short-strong cycles, or long-weak cycles occur, there are significant effects, see (7) below.
Three more natural variations are called Milankovitch cycles: 3) eccentricity of Earth orbit around the Sun, 4) axial tilt, and 5) precession. These changes do not increase or decrease the total amount of solar radiation falling on the Earth. However, they change the relative distribution between the polar and equatorial regions. It turns out that the more energy that falls on the polar regions, the more the Earth warms, and vice-versa. These cycles run from 19,000 to 400,000 years and they are the most likely cause of the approximate 100,000 year glaciation cycles seen in the ice core records. Over the past 20 thousand years or so, according to ice core data, the Earth has warmed by over 10ºC. During most of that time, human activity had no effect on global temperatures. The Milankovitch cycles may have contributed 0.1ºC or more to the 0.8ºC apparent Global Warming over the past 150 years.
Another set of cycles of interest are: 6) multi-decadal oscillations of the oceans, including the Pacific Decadal Oscillation (20-30 years), Interdecadal Pacific Oscillation (15-20 years) PDO and IDO, El Nino-Southern Oscillation (3-8 years) ENSO, Atlantic Multidecadal Oscillation (70 years) AMO, and others. These cycles are associated with temperature variations of 1ºC or more, and they may have contributed 0.1ºC or more to the 0.8ºC apparent Global Warming over the past 150 years.
The last cycle of interest is: 7) multi-decadal magnetic activity variations on the Sun. As I noted above, individual sunspot cycles, per se, are not associated with significant global temperature variations. However, a series of especially short and strong cycles may be associated with multi-decadal warming trends. Conversely, a series of especially long and weak cycles may be associated with multi-decadal cooling trends. See Solar variation for an excellent discusion.
CORRELATION OF TEMPERATURE WITH SUNSPOT NUMBER TRENDS
The figure (from Solar variation, with annotations in green and pink by Ira) compares global temperature with CO2 and Sunspot Number. [Click the diagram for a larger version.]
The upper dark red curve shows how temperatures have increased by an apparent 0.8ºC over the past 150 years. Note that there was a dip around 1860, another around 1910, and a third around 1950. A small, relatively short rise followed the 1860 dip, a larger, longer rise followed the 1910 dip, and the 1950 dip was followed by a very long and strong rise. That last rise has triggered current Global Warming fears.
The middle blue curve shows how CO2 has increased steadily since 1850, with a particularly rapid rise from 1960 to the present. (The reason the blue curve gets brush-like after 1960 is better measurement equipment that captured seasonal variations.) The correlation between CO2 rise and temperature rise has lent credence to the theory that rising CO2 levels are the main cause of Global Warming.
The lower yellow curve shows sunspot number variations. The thin line shows the individual 9 to 13-year sunspot cycles and the thicker line is the multi-cycle average.
My annotation shows, in green, the sunsport cycles that peaked below 80 sunspots/day. Note the correlation between those low cycles and the dips in global temperature experienced about a decade later. Annotations in pink indicate the sunspot cycles that peaked above 110 sunspots/day. Note that the peaks are correlated with those more active sunspot cycles.
In particular, note that six of the last seven sunspot cycles have peaked above 110 and that correllates with the global temperature rise from 1950 to 2000. However, since 1999 there has been a stabilization of global temperatures and, since 2005, a dip of nearly 0.2ºC in the non-smoothed data. That dip is NOT correlated with any dip in CO2 level rise - indeed CO2 levels are rising faster than ever.
Therefore, it is plausable that CO2 levels, while significant, are NOT the primary cause of Global Warming.
So, what is the main cause? Well, look at the thick yellow smoothed sunspot curve. It has been on the downswing for the past decade! The latest averages are below 80!
Couple that with the two-year (and counting) delay in the expected start of sunspot cycle #24. In 2006, NASA experts predicted cycle #24 would start at the end of 2006 or early 2007 and that it would be a doozy, peaking over 150! But, here at mid-2009 #23 has probably not ended yet nor has #24 started. So, NASA's latest prediction is that #24 will be be a weak kitten, peaking at 90. (I predicted, back in January that it would peak even lower, at 80. Also see an "Inconvenient" Minimum.)
Sunspot activity is better correlated with global temperature than CO2 levels. It is probably responsible for about 40% of the apparent Global Warming we have experienced over the past 150 years and over 0.3ºC of the actual warming. If the coming sunspot cycle is further delayed and if it is as weak as expected, that could stabilize global temperatures for a decade or more and give us breathing room to control CO2 levels in a conservative way.