An introduction to climate change - Climate Change: Biological and Human Aspects - page 29

is a relationship. However, there is also another larger, longer-term (over a timescale
of centuries) solar component, the exact historic magnitude of which is a matter for
speculation but which is roughly five times that of the solar variation reflected by
sunspot activity. Furthermore, there does seem to be a correlation between global
temperature and solar output (as we shall see in Chapter 4, which might account for
the Little Ice Age) but is this relationship real, partial (with other factors determining
climate), weak or just a coincidence?
In 2004 a team of five European researchers from Germany, Finland and
Switzerland used the carbon isotope 14 C from tree rings going back 11 000 years
(Solanki et al., 2004). It is usual to think of 14 C as a means of dating objects using
radioactivity, because this isotope is produced in the upper atmosphere at roughly
a constant rate. The 14 C isotope is then absorbed into living things and begins to
decay slowly at a known exponential rate. So the amount of 14 C left compared to
the stable 12 C enables one to date objects, albeit with a certain amount of error.
Ye t , 14 C is not produced at an exactly constant rate. Carbon-14 is produced in the
upper atmosphere due to the action of cosmic rays from the Sun on nitrogen and
carbon atoms; the level of cosmic rays is an indication of the Sun's output. Count-
ing tree rings from overlapping samples of wood enables each ring to be dated in
a different, more accurate, way to carbon dating. It is therefore possible to deduce
the amount of 14 C produced compared to what would have been produced if solar
activity was constant. The research team's 14 C-determined calculation of solar output
was corroborated by 10 Be (a beryllium isotope) from Antarctic and Greenland ice
cores, as this isotope also relates to solar output. The researchers found that there
was indeed unusually high solar activity at the end of the 20th century and that this
would have certainly contributed to some of the global warming experienced then.
However, it could not account for it all and was 'unlikely to be the prime cause'
(Solanki et al., 2004). Given that it has been so controversial early in the 21st century,
should you want more detail a good review of this topic is given by Foukal et al.
(2006).
What seems to be happening is this: there are many factors affecting the climate.
Some 'force' it in a positive (warming) way and others in a negative way. Furthermore,
some climate forcing factors are strong and some weak. Greenhouse gases are strong
forcers. Variations in the Sun's output over tens of thousands of years do occur but are
comparatively small. Their effects may be superimposed on the climate change that is
determined by the sum total of all other forcing agents and as such they may account
for small changes in the climate (and possibly even the Little Ice Age). However,
it is difficult for small climate forcing (such as the small increase in 20th-century
solar output) to account for the large temperature changes measured. Remember, the
Milankovitch variations in energy reaching the northern hemisphere in the summer
are small. As we shall see in Chapters 3 and 4, these small changes in solar radiation
help trigger larger changes in carbon dioxide and methane as the Earth moves between
glacial and interglacial climate modes. One should not be surprised that greenhouse
gases are not the only positive climate forcing factors contributing to current warming.
There are others, both positive and negative, and increases in solar output are but
one. These others include volcanic activity, marine release of methane and (with
regional effects on climate) oceanic and atmospheric circulation. We shall return to
them later in the topic. Although these other factors can play an important part in