Geology Reference
In-Depth Information
Suddenly you feel yourself caught in a wave that reaches skywards, and you are
pulled into an updraft of air. Up and up you go, buffeted by the winds, until you
reach the very centre of a large marine stratus cloud, which slowly moves north-
wards towards the pole.
For days you travel in your cloud, sensing how it helps to keep the Earth cool by
reflecting the sun's energy back to space from its dense white upper surface. You
look out beyond your cloud and see a whole host of similar clouds stretching off
in all directions. You feel grateful for anti-freeze molecules deep inside your uni-
cellular body that protect you from the intense cold.
Eventually, after many days, the water vapour around you begins to condense into
rain, and you find yourself tumbling seawards out of the sky in a rain drop. You
meet the sea with a gentle splash, happy once more to be in familiar surroundings.
You have been lucky, for the patch of sea you have landed in is rich in nutrients
brought up by currents from below. Your long fast is over, and you feast to your
heart's content on the nutritious sea water.
It is not just algae floating in the ocean surface that emit DMS; it has recently been dis-
covered that coral reefs do it too. Corals are sedentary, colonial animals related to sea
anemones and jellyfish that secrete the chalky armoured spires that make coral reefs
such architecturally complex, achingly beautiful places. Each coral animal is in fact
a small sea anemone, which feeds by capturing tiny planktonic creatures on tentacles
peppered with stinging cells. But closely packed inside cells of the coral animals are
symbiotic algae, called dinoflagellates, that provide their hosts with the sugary blessings
of photosynthesis, in return for protection and nutrients. Experiments have shown that
these symbiotic beings release DMS when they experience stress from increasing tem-
peratures or excessive ultraviolet light. Prodigious amounts of this premier cloud seed-
ing chemical have been detected over Australia's Great Barrier Reef, raising the exciting
possibility that corals may seed clouds that cool their immediate surroundings when the
sea water becomes too hot for comfort, or when ultraviolet radiation from the sun is too
intense. If so, we would have living proof that coral reefs could be operating a powerful
negative feedback that would counteract the dangerous effects of global warming.
Clouds teem with life: they are full of microbes of all kinds that use them as gigantic
dispersal 'buses'. Some of these microbes are so comfortable in their lofty homes that
they have been found happily reproducing thousands of feet above the surface. But it's
not just marine microbes that seed clouds; life on land does it too. Let's zoom down into
the spiky bracts of a bromeliad, a kind of aerial pineapple which lives on the branches
