Well this is a pretty complicated question.
No one fully understands the causes because there
are many factors that contribute to drought.
The first point to note is that the atmosphere is
what we call a COMPLEX SYSTEM. Complex
systems are all around us; the human body is one.
An example: you go outside on a hot day; the sun
falls on your body and you begin the heat up. Now
the human body is a complex system. How does
the “system’ respond to the influx of radiant
Well you begin the SWEAT. Now
as the sweat evaporates from your body (since the
relative humidity is < 100%, it takes heat away
from your body. That is , heat FROM YOUR BODY is
used to vaporize the liquid water (sweat). Thus we
see the body tries to regulate its own temperature
by responding to a stimuli… the skin /body harts
up due to external factor (sun light) and the body
attempts to MODULATE this effect of body
heating, by engaging in an activity (SWEATING) that
OPPOSES the heating effect… By sweating, heat is
REMOVED FROM your body and is taken away by
What does this have to do with DROUGHTS?
Well, the Earth’s atmosphere is another example
of a vastly complicated COMPLEX SYSTEM. The
climate and weather we live with are
manifestations of the behavior of the Earth to
spread the heat energy that comes into the EARTH,
mainly at low latitudes around the globe.
This takes places by the movement of water
currents in the oceans and by the currents of air
we call winds. Atmosphere-ocean coupling affects
northern hemisphere climate including California,
mainly during the winter season, and the oceans
play the most important role . Because plants
put moisture BACK into the atmosphere (a
process called evapotranspiration) if a dry
spells occurs via random fluctuation in a complex
system, then this can be AMPLIFIED by a positive
feedback … Since without plants taking
groundwater and putting it into the atmosphere,
there is less moisture in the air and hence less
precipitation. This is an example of a POSITIVE
feedback … the drier it gets, the even more
dryer it becomes!!!!
So, it would be like the human body not
sweating when its hot; the skin and body would
get hotter and hotter !!!
Then the cause of drought can not be linked to
any one simple factor, the details are in how
the complex system we call climate,
we do know is that increasing the concentration
of carbon dioxide gas in the atmosphere means
that MORE of the sun's incoming RADIANT
energy remains WITHIN the atmosphere and
oceans; and that, while causing more
evaporation at low altitudes can actually dry out
the atmosphere at higher temperate
People that study the atmosphere and oceans
have their work cut out for them because the
system is very complicated. But it is also
rewarding because with enough study we can figure
out what is going on, and then take steps to
enhance the feedbacks that help keep the Earth in
a state that is good for us.