Geology Reference
In-Depth Information
the absorption of carbon dioxide. It is almost certain that biodiversity in the oceans also
enhances this effect. Marine phytoplankton use carbon dioxide for photosynthesis much
as land plants do, drawing it out of the air and into their tiny bodies. Dead phytoplankton
sink, taking carbon that was once in the atmosphere with them to a muddy grave in the
sediments below. This is the biological pump that we encountered earlier, and it is al-
most certainly more effective at removing carbon dioxide from the atmosphere wherever
there is greater marine biodiversity, partly because the larger phytoplankton in more di-
verse communities increase the slow drift of carbon to the ocean depths.
Biodiversity may also influence the absorption and distribution of energy from the
sun. It could be that more diverse communities on land and in the ocean are better at
seeding clouds, but this remains to be seen. What is more certain is that a greater di-
versity of land plants could enhance cloud-making and energy distribution in two other
important ways: by transpiring more water from the soil through roots and out into the
air from pores on the undersides of leaves, and by providing more leaf surfaces from
which rainwater can evaporate directly.
A big rain storm has just finished watering several hundred square kilometres of
Amazon forest. The leaves are all wet, and those at the top of the canopy glisten in the
early afternoon sun. Some of the energy in the sunlight passes deep into the leaves where
it fuels photosynthesis, but a fairly large portion is absorbed directly by the recently ar-
rived film of water on the leaf surfaces. As the water molecules receive their gift of solar
energy they begin to gyrate like inspired dancers, and when sufficiently energised they
dance their way into the air as water vapour. This is evaporation. In the case of a leaf
drying in the sun, solar energy which might have heated the leaf is transferred to water
vapour, and as this is swept away by the wind, the leaf is kept cool, just as we are when
we sweat.
The energy held in water vapour can be released as heat whenever condensation con-
verts it back into liquid water. This energy is called 'latent heat' because it remains 'in-
visible' until condensation happens. On the other hand, any solar energy absorbed by
the surface of the leaf causes the molecules there to vibrate and to immediately re-emit
the energy as sensible heat, which you can detect directly with your skin or indirectly if
you have an infrared sensor.
But it is not just rainwater that evaporates from the surface of a leaf; so does water
that has travelled from the soil into the plant through tubes leading all the way from the
roots to the thousands of microscopic pores beneath a leaf's surface. This water, carry-
ing with it life-giving nutrients from the soil, eventually passes through the leaf pores
into the air, a process known as transpiration. Amazingly, plants keep the flow of wa-
ter going without the kind of muscular contraction seen in animal circulatory systems.
They do this by continually and deliberately leaking water through the pores, thereby
