The Climate Surprise

Recent global temperature trends
by Roy W. Spencer
Our measurements of global average
temperature are fundamental to
establishing whether global warming
is occurring, how strong it is, and whether its
magnitude agrees with global warming theory
as embodied in computer climate model predicons.
Since those model predicons are the
basis for energy policy, it is important that they
be tested with actual observaons.
But scienfic measurements always have
errors. And the main reason why global temperature
measurements are so controversial—
and somemes even contradictory—is that
none of our temperature monitoring systems
were designed to measure the small signal of
global warming, which is expected to be on the
order of . degrees cengrade per decade.
That is only . degrees cengrade per year.
In contrast, most of us are used to experiencing
tens of degrees of temperature variaon,
from day to night, and from season to season.
Our backyard thermometers can be off by one
or two degrees and it really doesn’t maer to us
when we are used to tens of degrees of varia-
on. But for global warming, one to two degrees
is the enre warming that has been alleged to
have occurred over the last century.
Despite the uncertaines in the measurements,
the importance of the global warming
issue to energy policy, agricultural producvity,
ecosystem health, and so on necessitates
that we use whatever data we have in order to
determine whether climate really is changing.
Three Temperature Measurement Methods
The only truly global measurement strategy
is with Earth-orbing satellites. Satellite instruments
measure the intensity of microwave radiaon
emied by oxygen in the atmosphere,
and that intensity is directly proporonal to
temperature. The satellite instruments are
connuously calibrated with views of the cosmic
background radiaon (near absolute zero
in temperature) and on-board precision planum
resistance thermometers that have themselves
been calibrated in a laboratory. While using radia-
on to measure temperature might seem rather
indirect, this is how the nurse now takes your
body temperature—by the intensity of infrared
radiaon, usually measured in your ear.
The second and most familiar method of
monitoring global temperatures is with groundbased
thermometers. Many years ago these instruments
were liquid-in-glass (either alcohol or
mercury), which required a person to esmate
the temperature visually and record the data
manually. These have been largely replaced with
electronic thermometers, called thermistors,
which measure electrical resistance, which is
then converted to a temperature—like radia-
on, an indirect measure. Most thermometer
measurements are made on land, and tend to
be located where people have replaced natural
vegetaon with buildings and pavement, leading
to a spurious long-term warming signal (the
“urban heat island” effect) that is difficult to
correct for. There are large land areas of the
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