]]]]]]]]     CARBON DIOXIDE -- AN ALTERNATIVE VIEW   [[[[[[[[[[[[ 
                        Sherwood B. Idso               (27/11/88)
       (From New Scientist, 12 November 1981, pp. 444-446)

             [Kindly uploaded by Freweman 10602PANC]

Many experts  argue that rising  levels of carbon  dioxide in the
atmosphere will  lead to  a climatological  catastrophe, but they
base  their  opinions on  computer  models.  Here  we  present an
alternative view, based on "natural experiments", suggesting that
the effects of carbon dioxide will not be so drastic.

["Dr Sherwood  B. Isdo  is a  research physicist  with the United
States Department of  Agriculture's Water Conservation Laboratory
in Phoenix, Arizona." (p. 444:1)]

   Carbon  dioxide  is  only   a  trace  constituent  of  Earth's
atmosphere  -- with  a concentration  of about  0.03 per  cent by
volume.  Yet  CO2 stands  in the  centre of  a raging controversy
about climate.  The gas is fairly transparent to solar radiation,
but rather opaque  to some wavelengths  of thermal radiation, and
so acts as a classic "greenhouse  gas": it allows the rays of the
Sun to  pass through  the atmosphere and  heat the  Earth, but it
absorbs a significant proportion of the heat radiated by the land
and sea,  and radiates  some of this  energy back  to the earth's
surface.  Thus the Earth is warmer than it would be if there were
no CO2  in the  atmosphere.  But  how much  warmer?  That  is the
question that now divides the world's meteorologists.
   At  the end  of the  last century  two Swedish  physicists, S.
Arrhenius   and   T.C.   Chamberlin,   estimated   that   if  the
concentration of CO2  were doubled, the Earth  would be warmer by
8oC.  Later, scientists came to agree on a figure of 2-3oC.  This
latter figure is also predicted by the most sophisticated general
circulation models (GCMs) of the atmosphere but it is an order of
magnitude   greater  than   several  recent   experiments  imply.
Consequently, the views of  scientists have become polarised.  At
one extreme many physicists look  to the computer models and view
the steadily rising concentration of atmospheric CO2 as a prelude
to  climatological catastrophe,  while, at  the other  extreme, a
group is beginning to emerge  that concentrates on more empirical
data.  This group suggests that increases in atmospheric CO2, far
from being detrimental, is actually beneficial.  These scientists
feel that any change  in the climate caused  by the rising levels
of   CO2  will   be   indistinguishable  from   natural  climatic
fluctuations.  But  more significantly  they foresee  that higher
concentrations of CO2 will  tend to stimulate photosynthesis, and
so increase  the productivity  of crops  and the  efficiency with
which  they  use   water,  thus  helping   to  feed  the  world's
   The divergence of  opinion is becoming  more acute because the
burning of  fossil fuels,  such as coal  and oil,  is causing the
concentration of CO2 in the atmosphere to increase at such a rate
that experts estimate that by AD 2025  it may be twice as high as
it  was before  the Industrial  Revolution.   This date  is close
enough  to  prompt   the  scientists  who   belong  to  the  "CO2
catastrophe camp" to urge the  world's governments to curtail the
use  of  fossil  fuels.   But  if  the  emerging  group  of  more
empirically-minded scientists is correct, such pressures can only
be  counter-productive  to  our  future  well-being.   Given  the
seriousness of the problem (or  non-problem, depending on how one
views  it),  along  with its  many  political  ramifications (New
Scientist, vol 90, p 82), we must endeavour to break the impasse.
   As an advocate of what  is currently the minority viewpoint --
that increasing CO2  in the atmosphere will  not lead to imminent
catastrophe -- I would  like to set forth  the alternative to the
long-unchallenged majority position, to help those in power reach
a decision in an open-minded manner.   To this end I will present
the case  against CO2 as  a significant modulator  of the Earth's
   First,  I  should  like to  make  a  philosophical distinction
between  the  two  approaches.    Those  who  work  with  general
circulation models calculate on theoretical grounds the effect of
a two-fold increase of  CO2 on the radiation  passing both to and
from the Earth.  They then feed that information into a computer,
which calculates the resulting change in global temperatures.
   The empirical approach, on the  other hand, depends on finding
some natural event -- such as the passage of a dust cloud through
a  particular locality  --  which temporarily  disturbs  the heat
balance of the atmosphere.  By monitoring the temperature changes
and the flow of radiative heat  during such natural events, it is
possible  to  measure  the  response of  the  real  world  to the
perturbation.   I call  this  response the  Earth's  "surface air
temperature  response   function";  we  observe   the  change  in
temperature of  the lower  atmosphere in  the face  of a measured
change in heat flow and extrapolate from this the expected global
effect of a doubling of CO2.
   The  first  investigators  to  publish  an empirically-derived
value for  this response function  within this  context were R.E.
Newell  of the  Massachusetts  Institute of  Technology  and T.G.
Dopplick of  Scott Air  Force Base  in Illinois.   Writing in the
June  1979  issue of  the  Journal of  Applied  Meteorology, they
concluded   from   studies  of   the   temperature   of  tropical
sea-surfaces,  and  the  way energy  is  transferred  between the
oceans and the  atmosphere, that for every  extra watt applied to
the  surface  of the  Earth  the mean  surface  temperature would
increase by  0.1oC; that  is, that the  mean global  value of the
Earth's  surface  temperature  sensitivity  was  about  0.1oC per
watt/sq.m.   I  applied  these  values  to  calculations  of  the
radiative  perturbation  --  changes  in  the  passage  of energy
radiating to and from the Earth  -- caused by doubling the amount
of  atmospheric   CO2.   From  this   I  was   able  to  conclude
independently  that the  resulting  increase in  the  mean global
surface air temperature  if CO2 were doubled  would be only about
   In coming  to this  conclusion, I had  drawn upon  more than a
dozen  years of  field  experience related  to  three independent
"natural experiments".   The first of  these concerns  the way in
which   dust   in   the   atmosphere   above   Phoenix,  Arizona,
redistributes itself in altitude between summer and winter.  In a
series of papers  in Science, Nature and  elsewhere, I wrote that
dust  low  in   the  atmosphere  exerts   a  significant  thermal
blanketing effect on  the Earth, much like  a greenhouse gas, but
that dust at higher levels has a much smaller effect.  Thus, from
measurements of radiation  and temperature, I  could evaluate the
radiative perturbation  that the vertical  redistribution of dust
produced, and the  effect it had on  surface air temperature.  By
dividing the change in temperature by the radiative perturbation,
I produced a local value for the surface air temperature response
function --  that is,  the amount  the temperature  changed for a
given change in local radiation.  It was 0.173oC per watt/sq.m.

                     The effects of moisture

   The second natural experiment I used was the annual arrival of
the  summer  monsoon season  at  Phoenix.  From  equations  I had
developed that relate atmospheric  thermal radiation with surface
air  temperature  and  vapour  pressure,  I  could  calculate the
changes caused by the influx of moisture over the city.  Checking
long-term weather records,  I found that in  going from a surface
vapour pressure of 4 to 20 millibars, the surface air temperature
just  before dawn  increased  by approximately  11.4oC.  Dividing
this change in temperature by the change in energy that caused it
yielded  a second  value, 0.196oC  per  watt/sq.m, for  the local
surface air temperature response function.  This was very similar
to the figure calculated from the data on dust storms.
   The third natural experiment dealt  with the change in surface
air temperature caused by the annual variation in solar radiation
received at the  Earth's surface.  I made  this evaluation at 105
stations  scattered  across  the United  States  --  and  for all
interior  locations   the  mean   result  for   the  surface  air
temperature response function was 0.185oC per watt/sq.m, the same
as the average of  the two "local" results  from Phoenix.  For 15
stations  on the  West Coast,  however,  where the  Pacific Ocean
greatly  influences  the climate,  the  result was  only  half as
great.  I took  this number to  represent an upper  limit for the
world's seas, and therefore calculated an approximate upper limit
for the whole globe,  taking account of the  area covered by land
and by sea, of 0.113oC per watt/sq.m.
   The  good  agreement  among the  separate  evaluations  of the
surface air  temperature response  function gave  me considerable
confidence, as they involve three different perturbing mechanisms
(variations in  altitude of dust,  a temporal  variation in water
vapour, and  the movement  of the Earth  in its  orbit around the
Sun), two different wavelengths of radiation (solar and thermal),
and two  different time-scales  (days to  months).  All  that was
lacking was a demonstration that  this common result also applied
to time-scales of the order of  decades to centuries, and that it
incorporated effects due to the  thermal inertia of the oceans --
that is,  the fact  that the  temperature of  the oceans responds
only sluggishly to changes in energy input.
   I approached  this task  by considering  the earth  without an
atmosphere.   A  simple calculation  gives  the  mean equilibrium
temperature of  an airless  globe as  -18.6oC.  The  current mean
temperature of the  globe is about 15oC,  so the total greenhouse
effect  of the  entire  atmosphere is  to  raise the  surface air
temperature by about 33.6oC.
   The heat the  atmosphere radiates back  to the Earth's surface
is  348 watts/sq.m.   Dividing the  temperature change  of 33.6oC
(the difference between an airless Earth to the present state) by
this  radiative   energy  yields   a  mean   global  surface  air
temperature response  function of  approximately 0.1oC/watt/sq.m;
and this result must  have included within it  the effects of all
significant feedback processes between  the Earth, the oceans and
atmosphere that operate over large time scales.
   This  number  is   just  slightly  less   than  the  value  of
0.113oC/watt/sq.m,  which I  had  acknowledged must  be  an upper
limit.  It  is identical  to the  value that  Newell and Dopplick
determined, so it  seems to be  the value of  the Earth's surface
air  temperature  response  function   that  should  be  used  in
evaluating the climatic  effects of CO2.  And  this value gives a
temperature   increase  of   0.25oC   for  a   doubling   of  CO2
concentrations in the atmosphere.
   The computer modeling  and empirical approaches  both make the
same assumptions about radiative changes  that would be caused by
an increase  in atmospheric  CO2, so  I find  no recourse  but to
reject as erroneous the  great temperature increases predicted by
the GCMs, as  generally run.  However, when  these models are run
with the constraints  on sea-surface temperature  that Newell and
Dopplick have noted, they too  give an increase in temperature of
0.3oC for  a doubling in  the CO2 concentration,  as has recently
been demonstrated at Oregon State University.
   So  where do  we go  from  here?  The  case for  the empirical
evaluation of the surface  air temperature response function seem
complete; and it cannot be significantly in error.  Let me use it
to make further extrapolation concerning the climate.
   At present  the atmosphere reflects  about 90 per  cent of the
energy from the Earth back to the ground: that is, its emissivity
is 90 per  cent.  Suppose the  atmosphere were to  send even more
radiation back.  I have shown that  in going from a value of zero
(assuming the  Earth had no  atmosphere) to the  current value of
about 90 per  cent the surface  air temperature response function
has  averaged the  same as  its  current value  -- that  is about
0.1oC/watt/sq.m.  Thus, there  is good reason  to believe that it
would  not change  significantly  if the  atmosphere's emissivity
were  to increase  by 10  percent, to  unity --  that is,  if the
atmosphere reflected  all radiation  from the  Earth back  to the
ground.  This being the case, it is easy to show that the maximum
temperature  increase  that  this  change  would  cause  is  just
slightly  over  4oC.   With  a  response  function  an  order  of
magnitude  greater,  however,  the  GCMs  predict  a  temperature
increase on the order of 40oC.  Which estimate is more realistic?
   One way to approach this question is to look at past climates.
We do not know for sure how the composition of the atmosphere has
changed in the distant past, but  there is reason to believe that
the  proportions  of  certain greenhouse  gases  may  have varied
considerably; and it is possible that  at some time over the past
several million  years the  atmospheric emissivity  may have been
significantly greater (or smaller) than it is now.
   But how  has the temperature  varied?  At the  meeting on past
climates  at the  "Carbon  dioxide and  climate  research program
conference"  held in  Washington DC  in  April 1980,  W. Broecker
noted that over  the past few  million years the  Earth may never
have been more than  a couple of degrees  warmer than at present;
and this seemed to be the consensus  of most of the people at the
meeting.  Also, R.K. Matthews and  R.Z. Poore, writing in Geology
(vol  8,  p  501),  have   suggested  from  data  on  changes  in
concentration of one  isotope of oxygen,  O18, that ocean surface
temperatures  have over  that  entire period  remained relatively
constant at about 28oC.  Thus, whereas the GCMs lead one possibly
to  expect significant  temperature  excursions from  the present
mean  global value,  the  available empirical  evidence indicated
that this is not the case for at least the past 50 million years,
and possibly  the past 200  million years  (Geological Society of
America  Bulletin,  vol  88,   p  390).   Consequently,  the  GCM
predictions that  temperature would  increase by  2-3oC due  to a
mere doubling of the atmospheric concentration of CO2 seem highly
   But let us  consider again the temperature  rise that could be
caused by an increase in the atmosphere's emissivity.  How big is
the effect on  emissivity of doubling CO2  in the atmosphere?  In
reality, very little.  If the atmospheric CO2 were to increase by
a factor of 10, it would still  fill in only about 20 per cent of
the  atmosphere's "window"  to electromagnetic  radiation.  Thus,
even  an  order of  magnitude  increase in  the  concentration of
atmospheric CO2 would increase the mean global air temperature by
only  about  0.8oC;  and such  an  increase  in  concentration is
considerably greater than any current predictions.
   There  is one  more  point we  can  learn from  past climates.
Recent studies of air  trapped in ice cores  show that the amount
of CO2 in the atmosphere 18 000  years ago was about half what it
is now (Nature, vol  284, p 155).  And  other work has shown that
incoming solar radiation at that time was not much different from
now  (Meteorological Monographs,  No 34,  American Meteorological
Society).  The GCMs  predict a drop in  temperature for that time
of more than 2oC  (Nature, vol 209, p  9), but data obtained from
three cores in the subtropical gyre  -- a current of the Atlantic
at  20oN --  show essentially  no  temperature difference  at all
between than and now (Geological Society of America Memoranda, no
145, p 43).  This again indicates  that the results of the models
are at odds with reality.
   Considering all the  available evidence, then,  there seems no
reason  to  suppose  that  carbon  dioxide  gas,  present  in the
atmosphere only as a "trace", has  any more than a "trace effect"
on Earth's surface  air temperature.  Thus,  we should not regard
potential increases in concentration due to continued utilisation
of  fossil  fuels  as  bad.   Quite  to  the  contrary,  such  an
enrichment of  CO2 in the  atmosphere is desirable  -- because of
its helpful effects on the growth of plants.

                         *     *     *

Return to the ground floor of this tower
Return to the Main Courtyard
Return to Fort Freedom's home page