An introduction to climate change - Climate Change: Biological and Human Aspects - page 23

Milankovitch concluded that a drop in the amount of sunlight falling on the northern
hemisphere at the end of both the precession and tilt cycles would make it more likely
for there to be a glacial and that when the opposite happened it would coincide with
the timing of an interglacial. For many years Milankovitch's theory did not have
much currency. This was due to two main things. First, for much of this time there
was no real understanding of when in the past glacials and interglacials had actually
taken place, so the theory could not be checked. Second, the variations in the solar
energy falling on a square metre in the northern hemisphere that Milankovitch was
talking about were of the order of 0.7 W m − 2 ; in other words less than one-tenth of
1% of the sunlight (the solar constant is close to 1400 W m − 2 ) bathing the planet.
What turned things around was the palaeo-evidence from Antarctica's ice cores in
the 1970s and 1980s, showing when glacials and interglacials had taken place. This
confirmed Milankovitch's timings and, as we shall see below, there is a considerable
body of other biotic evidence corroborating the timing of past climates and climatic
change.
If Milankovitch's theory simply provides a glacial-interglacial pacemaker but does
not account for sufficient energy changes needed to instigate and terminate glacials,
then what is amplifying this signal? The answer lies in the complexity of the global
biogeosphere system (from here on simply referred to as the biosphere system). There
are numerous factors determining the global climate. Some, such as silicate erosion
(see Chapter 3), affect the planet over long timescales. Others, such as the burning of
fossil fuels (a major factor), stratospheric and tropospheric ozone (medium factors)
and biomass burning, mineral-dust aerosols and variations in the Sun's energy output
(very low factors), affect the climate on timescales of far less than a century. Other
factors we still know little about and so their climatic effects are hard to quantify
(such as aircraft condensation trails, or 'con trails'; see Chapter 5). Complicating
matters further still, there are many factors that conspire, or interact synergistically,
to affect the climate with positive or negative feedback. These feedbacks either
amplify climate change or have a stabilising effect. This text focuses on the biology
of climate change but it is important for life scientists interested in climate change
to have at least a basic appreciation that such feedbacks exist. Figure 1.8 illustrates
three such feedbacks (there are many). Figure 1.8a and 1.8b are physical systems and
might operate on a lifeless planet. These are both examples of positive feedback that
add to, reinforce or amplify any forcing of the climate. That is to say, if something (be
it the release of a greenhouse gas, either human-made or natural) forces the climate
to warm up, then these feedbacks will serve to amplify the net warming.
Figure 1.8c represents a biophysical feedback system of a different kind. This is an
example of a negative-feedback system that dampens any net change in the climate.
We have already referred to iron that can fertilise the oceans, which allows more algae
to grow that in turn draws down carbon dioxide from the atmosphere, so reducing
the greenhouse effect and cooling the planet. Take this a step further. Consider a
world slightly warmer than ours. Being slightly warmer there is more evaporation
from the oceans; more evaporation means more rainfall (and/or snow), which in
turn means more geological erosion. This increased erosion increases the amounts
of iron (eroded from minerals) transported to the oceans that in turn encourages
algae, which draws down atmospheric carbon dioxide. Of course, the timescale and