It is due to the remarkable progress in climate science that
we are now able to have this knowledge. It is no coincidence that this knowledge comes
at the same time as many other advances in science as they are ultimately
connected through the same laws of physics and chemistry.
When we look back through eons of paleo-climate there are
two opposing questions that come to mind.
How can the Earth have remained stable enough
to maintain life? This is in spite of solar activity increasing by nearly
5% over the last five or 6 hundred million years over a period of time known as
the phanerozoic when life evolved large enough to view the fossilized remains
without the aid of microscopes. (This 5% is about 50 times greater than the
plus or minus swings that we associate with a typical 11year solar cycle.)
And.....
How can Earth have
gone through large swings in temperature over shorter time scales? These
swings would make life for human civilization as we know it impossible.
Feedbacks.
The observations that
the Earth’s temperatures have remained within limits leads us to the first
question above. This further correctly indicates that there must be some
stabilizing effects, or negative feedbacks, but incorrectly lead some to
believe that these stabilizing effects imply there are no concerns and therefore
present day greenhouse gases can have no effect. These limits are not constrained enough for
human civilization.
The observations that
the Earth has gone through massive swings correctly lead us to see that
there are dangerous positive or amplifying feedbacks due to some factors, or
forcings, that have destabilized the climate on many occasions. Without an
amplifying feedback the magnitudes in the swings can’t be numerically explained
by the forcings alone. (On much shorter time scales, that are only really
resolvable in more recent history, we also see swings in the climate over
periods of about hundred thousand years, although not as extreme these also cannot
be explained without some positive feedbacks at play. These swings are
associated with the Milankovitch cycles due to the changes in the Earth’s tilt
and orbit around the sun, but are not the swings that are discussed in here.)
Triggers and
feedbacks can account for these changes.
If some large enough trigger (or forcing) for change occurs
then positive feedbacks step in amplifying that change in the direction of the
initial trigger, then on longer time scales negative feedbacks step in
ultimately bringing the swing to an end.
This whole process may have been in the order of tens of millions of
years. Eventually we get a trigger of the opposite sign; one that has almost been
inevitable.
These triggers occur on a relatively short time scale at the
onset of a new change in direction and it is unlikely to be a change of solar
activity as the solar activity is remarkably constant over these short time
scales. (We must take account of the differences of solar activity however when
comparing different eras of climate.)
Positive forcing
>>>>>>> Positive
feedbacks>>>>>>>>>>.negative feedback...........
Negative forcing
>>>>>>> Positive
feedbacks>>>>>>>>>>.negative feedback
So a positive forcing would increase global temperatures,
positive feedbacks would amplify this increase and eventually some negative
feedback would ultimately limit this increase. Later a negative
forcing would reduce temperatures, some positive feedback would amplify this
temperature reduction and eventually some negative feedback would limit this
reduction.
Examples of forcings.
A positive forcing causing increased temperature could be a
massive volcanic output of CO2.
A negative forcing could be..... reduced volcanic activity,
increased albedo due to continents moving over the poles, increased weathering
of rocks removing carbon dioxide or a combination of these with contributing
changes in the Earth’s orbit and tilt.
Positive feedbacks:-
These would inevitably be at least..... oceans changing
amount of CO2 released or absorbed, water vapour feedback and albedo changes.
What could the negative feedbacks be over these extremely long time scales?
When applying the laws of physics there is an automatic
feedback effect included whereby the warmer an object is the more it releases
heat limiting the final temperature reached. This is the same physics applied
if for example one continuously adds heat to say an iron bar in a flame. It
will ultimately reach a temperature whereby it is absorbing and emitting energy
at the same rate. (This however works on all time scales and it is
automatically included when one applies the Stefan- Boltzmann equation.)
Also greenhouse gases have been the control for the Earth’s
temperature. On shorter time scales increased CO2 increases the temperature and
this in turn increases the water vapour but this is a positive feedback....on
longer time scales with increasing CO2 the resulting melting ice reduces the
Earth’s albedo and further CO2 is released from the oceans. These are also
positive feedbacks. On even longer time
scales CO2 acts as a negative feedback.
So it is the greenhouse effect that could
help us explain not only the extreme swings in temperature but also why the
Earth has remained within limits (in spite of solar activity that gradually increases over
eras of time.)
CO2 as a negative
feedback over tens of millions of years.
The weathering of rocks removes CO2 from the air. This
weathering increases when land is exposed or temperatures increases or when
there is more rain.
When temperatures increase more ice melts, more rain falls
and more weathering of rocks occur. Gradually over millions of years the atmospheric
CO2 reduces as it falls behind CO2 emission from volcanoes.
When temperatures decrease more ice is formed and with lower
temperatures, less rain and less exposed land all cause weathering to decrease.
This allows CO2 to build up at a faster rate than the CO2 from volcanic
activity.
Looking to the future.
Everything is NOT merely cyclic. There has never been this
much CO2 in our atmosphere since humans have been on this planet and if we look
further back hundreds of millions of years ago the solar activity was weaker.
It will be many tens of millions of years into the future before an increasing
sun will warm our planet noticeably but within just several decades we risk
much more warming from rising CO2 with all the impacts warned by
climatologists. How much more warming will mainly depend on how we react to the
knowledge we presently have.
See also:-
