SUN, SEA, AND AIR—REVELLE 255 
never been adequately explored and of phenomena that have never 
been adequately studied. 
To increase our understanding of climatic change we can ask first, 
what changes have occurred in the past and how did they happen? 
Second, because a change in climate is essentially a change of average 
air temperature, we need to examine the ways in which the heat con- 
tent of the air can vary. The heat content must be that required 
to give a balance between incoming and outgoing radiation; hence it 
can vary if there is a change in the amount of incoming radiation 
from the sun, in the proportion of sunlight reflected versus that ab- 
sorbed, or in the amount of the infrared back radiation absorbed 
by carbon dioxide, water vapor, and ozone. A 1-percent change in 
the intensity of the incoming sunlight or in the amount of sunlight 
reflected back to space would give about a 1° centigrade change in 
the average air temperature. 
The total solar radiation seems to be remarkably constant. The 
most recent continuous observations are those made at the Lowell 
Observatory since 1953. During this period of sunspot minimum 
no solar variations in the blue region of the spectrum greater than 
0.38 percent have occurred. lIonospheric observations show, however, 
that ultraviolet components of the solar radiation are larger during 
periods of sunspot maxima, and the visual spectrum observations 
must therefore be continued throughout at least one sunspot cycle 
before we can say definitely that solar radiation is virtually constant 
over decades. 
In contrast to the apparent constancy of the incoming radiation, the 
reflectivity of the earth would appear to be easily changeable. Clouds, 
snow, and ice reflect most of the sunlight that falls on them, whereas 
the ocean surface, vegetation, and bare ground are highly absorbing 
for visible light. At present about 50 percent of the earth’s surface is 
normally covered with clouds, while large areas are capped with snow 
and ice, particularly during winter. An average of 35 percent of the 
incoming sunlight is reflected back to space without being absorbed. 
The average air temperature would decrease by 1° centigrade if the 
reflection increased to 36 percent through increased cloudiness or a 
spreading of the snow- and ice-covered areas. 
Dust in the upper air also scatters and reflects sunlight before it can 
reach the ground and ocean surfaces. After the explosion of the vol- 
eano Krakatoa in 1883, the incoming radiation from the sun and sky 
decreased by 5 to 10 percent for three years. Changes in the water 
vapor, ozone, or carbon dioxide content of the air alter the amount 
and character of the infrared absorption. Calculations indicate that 
a 25-percent change in the carbon dioxide content of the air would 
change the average air temperature by 1° centigrade. 
