A National Oceanic and Atmospheric Administration (NOAA) weather satellite. Image: NASA |
In
order to separate human-caused global warming from the “noise” of
purely natural climate fluctuations, temperature records must be at least 17
years long, according to climate scientists.
To
address criticism of the reliability of thermometer records of surface warming,
Lawrence Livermore National Laboratory (LLNL) scientists analyzed satellite
measurements of the temperature of the lower troposphere (the region of the
atmosphere from the surface to roughly five miles above) and saw a clear signal
of human-induced warming of the planet.
Satellite
measurements of atmospheric temperature are made with microwave radiometers,
and are completely independent of surface thermometer measurements. The
satellite data indicate that the lower troposphere has warmed by roughly 0.9 F
since the beginning of satellite temperature records in 1979. This increase is
entirely consistent with the warming of Earth’s surface estimated from
thermometer records.
Recently,
a number of global warming critics have focused attention on the behavior of
Earth’s temperature since 1998. They have argued that there has been little or
no warming over the last 10 to 12 years, and that computer models of the
climate system are not capable of simulating such short “hiatus periods”
when models are run with human-caused changes in greenhouse gases.
“Looking
at a single, noisy 10-year period is cherry picking, and does not provide
reliable information about the presence or absence of human effects on
climate,” says Benjamin Santer, a climate scientist and lead author on an
article in the Journal of Geophysical Research (Atmospheres).
Many
scientific studies have identified a human “fingerprint” in
observations of surface and lower tropospheric temperature changes. These
detection and attribution studies look at long, multi-decade observational
temperature records. Shorter periods generally have small signal to noise
ratios, making it difficult to identify an anthropogenic signal with high statistical
confidence, Santer says.
“In
fingerprinting, we analyze longer, multi-decadal temperature records, and we
beat down the large year-to-year temperature variability caused by purely
natural phenomena. This makes it easier to identify a slowly-emerging signal
arising from gradual, human-caused changes in atmospheric levels of greenhouse
gases,” Santer says.
The
LLNL-led research shows that climate models can and do simulate short, 10- to
12-year “hiatus periods” with minimal warming, even when the models
are run with historical increases in greenhouse gases and sulfate aerosol
particles. They find that tropospheric temperature records must be at least 17
years long to discriminate between internal climate noise and the signal of
human-caused changes in the chemical composition of the atmosphere.
“One
individual short-term trend doesn’t tell you much about long-term climate
change,” Santer says. “A single decade of observational temperature
data is inadequate for identifying a slowly evolving human-caused warming
signal. In both the satellite observations and in computer models, short,
10-year tropospheric temperature trends are strongly influenced by the large
noise of year-to-year climate variability.”
