Re: population growth & physics ed

[palmer@sfu.ca (Leigh Palmer)]

> During the year without summer the temperature supposedly dropped
> around 2 degrees Celsius.           This was the direct result of the eruption
> of Krakatoa in Indonesia in 1883 and can not be compared to a long term global
> trend in terms of significance.   Changes of as little as 4-5 deg C mark the
> difference between ice ages and interglacials.  In terms of melting of glaciers
> changes in weather patterns, rain fall, and crop production I think most of us
> would see a half a degree as significant.

OK, how about the period which was called "the little ice age"? That must
be the one I was thinking about if 1883 was the year without a summer.
Here is a clip from the Washington Post which seems germane (reproduced
without permission but with apology to the author and the Washington Post):

                      'Little Ice Age' Supported
                          BY Boyce Rensberger

  (C) 1994 The Washington Post (LEGI-SLATE Article No. 215202)

    An analysis of tooth enamel from Vikings who lived in Greenland centuries
  ago supports the theory that their colonies on the huge island died out
  because of a major period of global cooling known as the Little Ice Age.

   Henry C. Fricke, a University of Michigan graduate student in geology, and
  James R. O'Neill, his professor, reported the findings yesterday at a meeting
  of the Geological Society of America in Seattle.
    Historians have long known that Vikings from Norway were living in
  Greenland at least as long ago as 1000 A.D., along with the indigenous Inuit
  people. But by 1850 the villages of both groups had been abandoned. It has
  generally been thought that the habitations died out because Earth's climate
  underwent a pronounced cooling that began around 1300 and ended about 1850.
  The planet has been warming ever since.
    Fricke and O'Neill sought to confirm that the cooling affected the
  Greenlanders by testing their tooth enamel for the ratio of two forms of the
  oxygen atom that are known to vary according to the prevailing temperature.
  Oxygen in enamel comes from that in water, which, in turn, comes from rain or
  snow that formed at some particular temperature.
   The two scientists obtained teeth of Greenlanders from Norwegian
  anthropologists, tested for their oxygen-isotope ratios and found that the
  ratio did change over time, confirming a cooling in the local climate.
    The researchers say their method should prove valuable in measuring
  climatic changes that affected human populations in many parts of the world.
  But there is one drawback. To test the enamel, they have to destroy the tooth
  and anthropologists are famously reluctant to part with their specimens.

It is thought that this global temperature change was greater than the one
that has been marginally detected over the last century. That is my point.
Here is a large effect (by the standards of the last two or three millenia)
which is greater in duration and magnitude than the global warming so
prominent in the news.

> The correlation between Carbon dioxide levels and temperature are pretty well
> established.

Hold on right there! The carbon dioxide level has been rising over this
period; that is well established. The global temperature has been rising
over this period; that is likely, but less well established because the
effect *is* small compared to uncertaintiy in the measurement. The Dow
Jones Industrial Average has also rise over the same period; that too is
well established. Would you claim that the latter two time series are
also correlated? Unless that is what you mean by "correlated" (and it is
a perfectly acceptable meaning) then I believe that you are implying that
the causal connection is well established. That, of course, would be a
fallacy. Qualifying is by later statements may seem to relieve you of
guilt for doing so, but it has been my experience that when anyone starts
with that statement you can be sure that there is an underlying intent to
use it to persuade that carbon dioxide is causing global warming.

It should always be pointed out that the rise in carbon dioxide levels
over the years of monitoring has been very much larger than the rise in
temperature over the same period.

> The exact mechanisms are less well understood since the carbon
> cycle is pretty complex.  What are really speculative are the economic issues.
> What people want to know is how many dollars will it cost if we ignore the
> problem and how much will it cost to solve the problem and at what level of
> solution does the ratio  become 1:1.

Mechanisms? Mechanisms for what? Are you implying a causal relationship
exists? Was my initial prejudice reinforced by this instance?

> What should we teach in a Physics class?  We can certainly teach how the
> greenhouse effect works and the role of various atmospheric gasses in
> controlling our climate.  We can also expose students to data showing
> carbon dioxide and possibly ave temp rising.  Should we mention projections
> when we don't know how they were made?  Should we discuss economic issues?
> How do we leave students with the understanding that science can contribute
> to our understanding of environmental problems, but usually can not give us
> nice neat solutions?

Well, we might start by talking about something we understand, and that
would be physics. That's what I do in my physics classes. Climate I don't
understand. I understand that some experts in the field believe they do
understand climate, but these seem to be split into sizable camps that have
different understandings, a significant departure from the stodgier
physicist's meaning of "understanding", which requires a large concensus.
I could say similar things about economists, but I might be even less kind.

Stick to physics. I'll try to show you that you can do so valuably.

R. W. Wood, a physicist, did an experiment back in the 'teens to measure
the greenhouse effect in greenhouses. He constructed two small enclosures,
one of glass and the other of rocksalt, and situated them in direct
sunlight in order to measure the temperature rise in each. Glass as you
all know is opaque to near infrared, and rock salt (sodium chloride) is
not. Both enclosures heated to temperatures above the outside ambient
temperature, and those temperatures were insignificantly different from
each other. Woods's conclusion was that the infrared transmittance of the
windows in his enclosures was unimportant to the greenhouse effect. Today
greenhouses are often constructed from polyethylene sheet and other pastics
which, like rock salt, are quite transparent to the infrared in the region
of terrestrial thermal radiation.

Changes your outlook when you know the physics, doesn't it? You are not
likely to teach the greenhouse effect the same way next time, are you?

OK, I'll drop the other shoe. What *is* the physical mechanism responsible
for the real greenhouse effect? Actually I shouldn't have to tell you
because you live in the real world, and physics reflects Nature in a
mathematical mirror. Look at a real greenhouse. sometimes it gets hotter
than you want it to get. What do you see on the greenhouse that might help
make it cooler? Your car is like a greenhouse on a sunny day. If it is not
to get too hot while parked in the sun, what can you do to prevent it?

Right! There's a little door at the top of many greenhouses. Opening that
and the main door will allow air to circulate, bringing in cooler air from
outside. To keep your car cooler, crack the windows a bit. The change in
area of IR opaque material is negligible, but it isn't important anyway.
Greenhouses work by supression of convection. Their interiors are not
heated more by the sun than the outside; they are cooled less because the
most important cooling mechanism (by orders of magnitude above radiation),
convection, is less inside than outside.

How about planetary atmospheres? That is a different story entirely.
Planetary atmospheres (like those of Venus and Earth)) work by the
greenhouse effect, not the real greenhouse effect! It would be less
confusing if we called it "the planetary atmosphere effect", but I can't
change that.

None of this is new physics, folks; it's been around for decades. Instead
of teaching "socially relevant" garbage from the newspapers let's just
teach the physics we know. Leave economics and climatology to others.

Leigh