Jökull - 01.01.2005, Blaðsíða 6

Wallace S. Broecker
Figure 4. Mean annual earth temperature as recorded by thermometers scattered about the planet (Hansen et al.,
1999). –Meðalárshiti við yfirborð jarðar frá 1860, frávik frá meðaltali fyrir tímabilið 1961–1990.
THE INSTRUMENTAL RECORD
In Figure 4, the trend in mean annual earth surface
temperature is shown (Hansen et al., 1999). Several
features jump out. The warming during the last 140
years is only about 1.0◦C. Further, about half of this
warming took place prior to 1940 (i.e., before there
had been an appreciable increase in any of the green-
house gases). Further, between 1940 and 1975, the
temperature remained nearly constant. Only follow-
ing 1975 has there been a steady increase in tempera-
ture which might be attributed to manmade pollutants.
This comparison tells us in no uncertain terms that
our activities have yet to produce a global temperature
change larger than those which the planet has been
undergoing on its own. Clearly, the warming which
occurred during the latter half of the 19 th century and
the first part of the 20th century must be attributed to
natural causes. If, indeed, the models have it right,
then the pause in warming during the middle of the
20th century suggests a natural cooling during this
time interval. By chance, this natural cooling counter-
acted the man-induced tendency to warm. Only after
1975 did our influence take hold.
Clearly, this is music to critics’ ears. Aha, they can
say, perhaps the entire warning is natural. Lindzen has
it right. A water vapor deficit in desert air has largely
nulled the impact of CO2 and other greenhouse gases.
The proponents of action would counter by cry-
ing wait! Clearly the man-made warming has yet to
push Earth’s temperature out of the range of natural
“noise.” As we have no way to determine how the
Earth’s temperature would have changed in the ab-
sence of the Industrial Revolution, this test is not a
valid one. Further, because the atmosphere can warm
no faster than the upper ocean, there is a significant
lag in response. Finally, the reflection of sunlight by
light-colored aerosols and by the aerosol-brightened
clouds may be counteracting a sizable portion of the
warming by greenhouse gases.
6 JÖKULL No. 55, 2005