Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 25

Kasting and Ackerman (1986) investigated whether a runaway greenhouse
could have occurred on the early Earth. A runaway greenhouse was defined ''as
an atmosphere in which water is present entirely as steam or clouds; no oceans or
lakes are present at the surface.'' They concluded that the Earth is ''apparently
stable against the development of a runaway greenhouse.'' Their models indicated
that as the CO 2 pressure is increased up to 100 bar (about 300,000 PAL) the
temperature of the Earth reaches about 233 C, the water vapor pressure rises to
about 29 bar, and the atmospheric pressure is about 130 bar. Since 233 C is lower
than the boiling point of water at 130 bar, the oceans would not boil but would
remain as high-temperature liquids under high pressure. If the early Earth had a
pressure of 10 to 20 bar of CO 2 , the oceans would have been even more stable at
85 to 110 C with a vapor pressure of 0.6 to 1.5 bar, which is far less than the
atmospheric pressure.
While it may be comforting to know that the Earth will not go through a
runaway greenhouse in which the oceans boil, nevertheless, oceans at 233 C with
an atmospheric pressure of 130 bar are somewhat challenging to the imagination!
2.3 THE RELATION BETWEEN ANCIENT CLIMATES AND
CO 2 CONCENTRATION
2.3.1 Background
The current holy grail of climatology is to seek an estimate of how much the
global average temperature will increase if the CO 2 concentration doubles from
the pre-industrial value of 280 ppm to 560 ppm. Attempts to estimate this directly
are dicult due to uncertainties in secondary factors that accompany warming
from increased CO 2 (humidity, cloudiness, winds, ocean currents, glaciers, ice
sheets, etc.). Some climatologists have sought to estimate the dependence of the
climate on CO 2 concentration by analyzing paleoclimatic data on climate and
CO 2 concentration, with the intent of using the climate sensitivity derived from
this to estimate the global average temperature increase if the CO 2 concentration
doubles from the pre-industrial value of about 280 ppm.
Over the past couple of million years in which we have had alternating ice
ages and interglacials, there is fairly good evidence that the trigger to set the
cycles in motion is solar input to higher northern latitudes. But what does it mean
to set the cycle in motion? Presumably, it means that an albedo effect begins in the
60 N latitude range as snow and ice accumulate, causing a regional cooling. As
the ice sheet builds, sea ice expands, and the ocean drops, other albedo effects
occur and a greater regional cooling takes place. Dust is stirred up and this
further amplifies the cooling. Then as the cooling spreads, the CO 2 concentration
in the atmosphere decreases, producing additional negative forcing worldwide,
which lowers the worldwide temperature. However, changes in humidity and clou-
diness are unknown and may be very large factors. At the height of the Last
Glacial Maximum (LGM) some 20,000 years ago, the negative forcing produced a