Understanding Climate : #5 - Variations In Sunshine

Insolation variation

I am trying to explain in this series the most important aspects of the sciences most relevant to climate as a cross-disciplinary study.  This is the 5th part of the mainly astronomical section.  In part 2, I introduced the idea of, in a manner of speaking, building a model of our earth-moon-sun system.  In part 3 I continued with a discussion of seasons and their primary astronomical causes.  In part 4 I began to discuss the astronomical factors affecting annual insolation - how much heat the earth receives from the sun annually.

Here I continue the discussion of insolation.

New readers may wish to start at Understanding Climate : #1 - Components Of Climate.

Orbital variation

The earth's orbit around the sun is not quite a circle: this means that the earth is slightly closer to the sun at some times of the year than others. In 2010 the closest approach of the earth to the sun, perihelion, occurs on January 4, with earth at a distance of 91.402505 million miles 147.098074m km) from the sun.  Aphelion is on July 4, with the earth at 94.50913 million miles (152.097701m km) from the sun.

Due to precession, a very slow wobble in the earth's axis of rotation, which takes 22,000 years to complete one cycle, the earth's northern hemisphere points slightly towards the sun at perihelion at the moment.  11,000 years from now it will point away from the sun to about the same extent.  This seasonal effect is reversed for the southern hemisphere.

The "roundness", or eccentricity, of the earth's orbit varies over cycles of 100,000 and 400,000 years, and this affects how important the timing of perihelion is to the strength of the seasons.

Over thousands of years, the eccentricity of the Earth's orbit varies from nearly 0.0034 to almost 0.058 as a result of all planets exerting gravitational attractions on each other.

The eccentricity of the Earth's orbit is currently about 0.0167, nearly circular.  The semiminor axis is 98.6% of the semimajor axis.

The combination of the 41,000 year tilt cycle and the 22,000 year precession cycles, plus the smaller component due to eccentricity, affect the variation of summer and winter climate, and are thought to control the growth and retreat of ice sheets.  Frequent cooler summers in the northern hemisphere, where most of the earth's land mass is located, appear to occasionaly allow snow and ice to persist to the next winter, allowing the development of large ice sheets over hundreds to thousands of years. Conversely, frequent warmer summers shrink ice sheets by melting more ice than the amount accumulating during the winter.

Solar variation

The amount of radiation given out by the sun varies in different amounts over various timescales.  The most widely known cycle is probably the 11 year sunspot cycle.  The most widely know solar event is the Maunder Minimum.

It has long been believed intuitively that the sun and latitude between them affect climate.  Indeed, the word 'climate' comes from the Greek 'klimat', meaning a band or zone having a klimat - an angle to the sun - a zone of latitude.

William_Herschel, discoverer of Uranus and infrared radiation, speculated that solar variability might influence climate.  He collected data on the price of wheat, speculating that solar activity could be linked to relative crop abundance.  For example, he found periods in the 17th century, ranging from two decades to a few years, when hardly any sunspots had been observed, during which periods the price of wheat had been high.  His use of proxy data was not successful, but later researchers used similar methods to produce climate proxies.

Speculations multiplied about solar variation, particularly sunspot cycles, as possible causes of climate changes in historical records.  In order to make sense of historical data it was important to determine the level of accuracy of various records.  Over the years, statistical techniques were applied with various results.  Scientific views on the relationship of solar activity with climate raged from cautious to extreme.  So extreme were some views that by about the 1960s there were few scientists willing to seriously entertain the notion that solar variability was a main cause of short term climate variation.

As this is merely an introduction to the topic I will keep it short.  The 'solar variability' problem was this: some evidence from climate and sunspot researchers showed links between climate and solar activity.  Astrophysicists were inclined to believe that there had been no significant variation in the sun's output over the course of millions of years.

John_A._Eddy carried out a detailed investigation of the theories and records in order to determine whether or not solar output really varied enough to affect climate.  He found evidence of minimal sunspot activity during the so-called 'Little Ice Age' of the 16th-17th centuries.  This evidence had been recorded by William_Herschel, G.W. Spörer, and Edward_W._Maunder.

Following Eddy's studies, and with the advent of the satellite age, data continues to mount showing that the sun's output does indeed vary.  But of recent years it has varied in the opposite sense to global temperature trends.
There is considerable evidence for solar influence on the Earth's pre-industrial climate and the Sun may well have been a factor in post-industrial climate change in the first half of the last century. Here we show that over the past 20 years, all the trends in the Sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures.
There are many interesting palaeoclimate studies that suggest that solar variability had an influence on pre-industrial climate. There are also some detection–attribution studies using global climate models that suggest there was a detectable influence of solar variability in the first half of the twentieth century and that the solar radiative forcing variations were amplified by some mechanism that is, as yet, unknown. However, these findings are not relevant to any debates about modern climate change. Our results show that the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever of the mechanisms is invoked and no matter how much the solar variation is amplified.

Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature
Mike Lockwood and Claus Fröhlich

Concluding remarks

The earth's changing axial tilt and orbit, together with solar variation, can affect climate over timescales running from decades to thousands to millions of years.  This is fairly well understood and is accounted for in climate modelling.  It does not account for the general global warming trend since 1910.  Please note: the date cited above, 1985, is in reference to rapid rise.  The global warming trend has been on a steepening curve since about 1910.

Land, sea and ice absorb or reflect heat differently, thus they have different effects on climate.
The distribution of land in height and sea in depth, and the distribution of ice and snow have a major effect on climate.  Hypsometry, the measurement of these, will be the theme of the next article in this series.


Many articles on sunpots:
recommended: History of Sunspots: