The water vapor capacity of the atmosphere increases as it warms up so that is
a more interesting calculation.
The real issues are historical levels of CO2 versus temperature. Temperatures
are determined from proxy measurements of the ratio of oxygen 18 to oxygen 16.
1) The past 800,000 years from the Vostok ice core data.
http://nas-sites.org/americasclimatechoices/files/2012/10/Figure-14.png
Over the past 800,000 years, until about 1960 the CO2 levels were never above
280 parts per million.
Now, there are some global warming denialists who will point to the sharp
sawtooth changes and point out that there is a 600-800 year time lag between
temperature increase and CO2 (that CO2 leads temperature change). That is
thought to be because when CO2 is less than 400-600 ppm the temperatures are
driven by the Milankovich cycles. The Milankovich cycles are the periodic changes in the Earth's orbital parameters and Earth's axial tilt that
change the amount of sunlight reaching the Northern hemisphere.
The spike in CO2 is thought to have provided a positive feedback that raised
temperatures much more rapidly than Milankovich cycles would suggest.
2) Here are global temperatures for the past 5 million years
http://jonova.s3.amazonaws.com/graphs/lappi/Five_Myr_Climate_Change_Rev.jpg
You can see that for the past 3 million years, the earth was in an ice age,
with the Milankovich cycles dominating the climate picture. If we look back
further than 3 million years, then the CO2 levels were above 400ppm (not shown
on this graph) and the global temperatures really seem to be tied primarly
to CO2.
3) If we look at global temperatures for the past 65 million years, compared with estimates of CO2, then it seems that CO2 is very important regarding
temperature. However, as we go back beyond a few million years, the positions
of the continents, and ocean currents were different, so it is difficult
to understand the effects of these influences.
http://claudiocassardo.files.wordpress.com/2013/05/image007.jpg
Some milestones.
About 5 million years ago, the concentration of evaporites in the floor
of the Mediterranean Sea, due to the drying up of the Mediterranean from
the closure of the Straits of Gibraltar raised the temperature at which
sea ice forms, providing a mechanism for increasing the Earth's albedo (reflectivity).
33 million years ago - opening of the Drake passage, which permitted Antarctica
to have an ice cap. We are at or above 400ppm, and we don't see much of the
Milankovich cycle effect.
42 million years ago, the collision of the Indian subcontinent with Tibet
Note that CO2 is removed from the atmosphere by the weathering of rocks,
so the decline from 1000ppm to 450ppm from 42-33 million years ago is thought
to be because of increased erosion caused the uplift of the Tibetan plateau.
60 Million to 42 million years, this large triangular warming event is thought
to be the result of the subduction of the Tethys ocean basin under as India
was moving north to Asia.
The interesting spike at 55 million years is called the Paleocene Eocene Thermal Maximum (PETM). This is thought to have been due to outgasing of CO2
due to volcanic eruptions, combined with an outgasing of methane from the
from methane hydrates in the ocean floor. The amount of CO2 released is
thought to be more or less equivalent to what we will release due to the
burning of fossil fuels in the next century or so.
At any rate, these are important parts of the data. The question of modeling
this sort of stuff is important, but we do have to bow to the current facts
1) there has been warming in recent decades that is beyond normal climate
models to predict
2) the past 40 or 50 years are clearly different from the previous 3 million
years in terms of the amount of CO2. It's coming from human activities,
largest of which is the burning of fossil fuels. Fossil fuels account for
about 200 times as much carbon than from other sources.
3) when CO2 gets above about 450ppm it starts looking scary. The world was
a very different place, temperatures ruled by locations of continenents
Anyway we do live in interesting times.
-John