8
The only national-scale ecosystem assessment published at the
time of writing, the UK’s National Ecosystem Assessment,
2
ploughs
a similar track by focusing on key habitats – ‘mountains, moorlands
and heath’, ‘semi-natural grassland’, ‘enclosed farmland’, ‘woodland’,
‘freshwaters’, ‘urban’, ‘coastal margins’ and ‘marine’ – with no explicit
consideration of air.
Unseen oceans of gas
Omission of the airspace as a major habitat type is surprising as it
is not only important, it is also staggeringly big. Indeed, calculated up
to an altitude of 100 kilometres – and including the atmosphere, the
stratosphere, the troposphere and the mesosphere – the volume of air
surrounding the planet is about 51 thousand trillion (51 trilliard) cubic
kilometres, or 51,000 trillion trillion litres. This is about 38,000 times
the volume of all of the world’s oceans, which total about 1,347,000,000
cubic kilometres. The volume of the planet’s air is so great that it
eludes meaningful comparison in intuitive terms such as numbers of
bathtubs, swimming pools or shopping malls.
Each of us draws in substantially in excess of 20,000 breaths per
day, and we live our lives perpetually immersed in the bubble that
surrounds the Earth. Yet, for something so immense and familiar to
us, air remains incredibly easy to overlook.
That it is colourless, and also generally odourless and tasteless,
blinds at least three of our five primary senses to it. We acclimatise
to an ambient pressure of around 100 kilopascals (kPa) at sea level,
equivalent to a pressure of nearly 14 pounds per square inch (psi).
However, amazingly, we are unaware of this vast pressure except when
we climb steeply, as our ears ‘pop’ or we experience altitude sickness,
or when we immerse ourselves in the denser medium of water. And,
although rapid air movement can be hugely destructive and fast
movement through it can give rise to significant frictional drag, we
can walk through still air without the slightest sense of its viscosity.
Of the five major senses, that leaves only hearing and the longer
wavelength vibrations that we might sense as touch, both transmitted
as physical waves. The rush of air is audible as it interacts with hard
surfaces such as trees, buildings and ear lobes, and it is the physical
structure of air that we depend upon to convey the pressure waves
that we know as ‘sound’. However, still air is not exactly the most
raucous thing we ever experience.
To use a visual metaphor, the invisibility of air means that it
is all too easily and frequently taken for granted, both in our daily