I read Phillip Morrison's review of Spencer
R. Weart's book, The Discovery of Global Warming,
in the June 2004 issue of Physics Today(page
60). Weart's book contains four graphics
and other evidence that apparently convinces Morrison
of global warming's causes.
There is evidence of increasing global temperatures
and increasing atmospheric carbon dioxide concentrations.
Morrison is convinced that one causes the other but
never mentions whether Weart says which is the cause
and which the effect, or whether he gives evidence
to support either case. Solid science, though, does
support one case.
It is widely known that the largest single repository
of CO2 on Earth is the oceans, and that
the solubility of CO2 in water drops as
the water temperature increases. So clearly a mechanism
exists whereby increasing ocean water temperatures
(which is where most of the solar energy goes) causes
increased outgassing of CO2 into the atmosphere.
Furthermore, Arctic permafrost zones revert to marshy
peat bogs when the Arctic warms, and then bacterial
activity takes hold and converts decaying ancient
vegetation into atmospheric CO2. Both of
those processes are happening right now.
The Russian Vostok ice cores going back 420 000
years and the Dome-C ice cores going back 730 000
years show that the Antarctic ice sheet has not melted
during that time frame, even in the warmest interglacial
periods. The ice cores also show periods of rapid
global warming accompanied by rapidly increasing atmospheric
CO2.
Now we know that our sport-utility vehicles did
not cause all those CO2 increases back
then, but we do understand how global warming causes
them. So perhaps Weart can tell us conclusively which
of the two is the cause and which the effect; the
ice cores seem to give us the answer.
By the way, when floating sea ice melts, Archimedes
would insist that the level does not change; in particular,
it does not go up. That takes care of gravitational
energy, but the melting of all that sea ice extracts
astronomical quantities of latent heat from the surrounding
ocean water and lowers the mean ocean temperature;
so the level will go down, not up. And I can suggest
a very illuminating experiment for anyone who believes
that heat to melt sea ice does not come from the surrounding
ocean.
Weart replies: Earth's climate system
involves many basic phenomena—science teachers
should note how that could be used to spark interest!
George Smith's letter shows some ways a temperature
rise can cause CO2 emissions. Such feedbacks
are worrisome, because they could accelerate warming
once it is initiated.
What initiated the current warming? It took many
decades for scientists to agree on the most likely
answer.1 The crucial
observation was the recent atmospheric CO2
increase, whose rate and magnitude are vastly beyond
anything in the ice core record. The steep climb
neatly matches calculations of the rise expected
from the known consumption of fossil fuels. The
calculations include estimates of gas exchanges
with the oceans, tundra, forests, and so forth:
estimates checked through many measurements—for
example, of carbon isotopes. The oceans are found
to be a net absorber, transporting carbon into their
depths. Net biosphere output, although harder to
estimate, is certainly dominated at present by emissions
due to human activities.2
Sea ice will melt provided the greenhouse effect
adds enough energy to the planet to warm the seawater
even while the ice melts. A temperature increase
has been observed, and will bring sea-level rise
through thermal expansion.
References
1. Spencer Weart, The Discovery
of Global Warming, Harvard U. Press, Cambridge,
MA (2003). For additional material, see http://www.aip.org/history/climate.
2. Intergovernmental Panel on Climate
Change, Climate Change 2001: The Scientific Basis,
J. T. Houghton et al., eds., Cambridge U. Press,
New York (2001), online at http://www.grida.no/climate/ipcc_tar.
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