NASA - Jessica Merzdorf
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| Earth’s long-term warming trend can be seen in this visualization of
NASA’s global temperature record, which shows how the planet’s
temperatures are changing over time, compared to a baseline average from
1951 to 1980. The record is shown as a running five-year average.
Credit: NASA’s Scientific Visualization Studio/Kathryn Mersmann. Download related visualizations here. |
A new assessment of NASA's record of global temperatures revealed
that the agency's estimate of Earth's long-term temperature rise in
recent decades is accurate to within less than a tenth of a degree
Fahrenheit, providing confidence that past and future research is
correctly capturing rising surface temperatures.
The most complete assessment ever of statistical uncertainty within
the GISS Surface Temperature Analysis (GISTEMP) data product shows that
the annual values are likely accurate to within 0.09 degrees Fahrenheit
(0.05 degrees Celsius) in recent decades, and 0.27 degrees Fahrenheit
(0.15 degrees C) at the beginning of the nearly 140-year record.
This data record, maintained by NASA’s Goddard Institute for Space
Studies (GISS) in New York City, is one of a handful kept by major
science institutions around the world that track Earth's temperature and
how it has risen in recent decades. This global temperature record has
provided one of the most direct benchmarks of how our home planet's
climate has changed as greenhouse gas concentrations rise.
The
study
also confirms what researchers have been saying for some time now: that
Earth's global temperature increase since 1880 – about 2 degrees
Fahrenheit, or a little more than 1 degree Celsius – cannot be explained
by any uncertainty or error in the data. Going forward, this assessment
will give scientists the tools to explain their results with greater
confidence.
GISTEMP is a widely used index of global mean surface temperature
anomaly — it shows how much warmer or cooler than normal Earth’s surface
is in a given year. "Normal" is defined as the average during a
baseline period of 1951-80.
NASA uses GISTEMP in its annual global temperature update, in
partnership with the National Oceanic and Atmospheric Administration.
(In 2019, NASA and NOAA found that 2018 was the fourth-warmest year on
record, with 2016 holding the top spot.) The index includes land and sea
surface temperature data back to 1880, and today incorporates
measurements from 6,300 weather stations, research stations, ships and
buoys around the world.
Previously, GISTEMP provided an estimate of uncertainty accounting
for the spatial gaps between weather stations. Like other surface
temperature records, GISTEMP estimates the temperatures between weather
stations using data from the closest stations, a process called
interpolation. Quantifying the statistical uncertainty present in those
estimates helped researchers to be confident that the interpolation was
accurate.
“Uncertainty is important to understand because we know that in the
real world we don’t know everything perfectly,” said Gavin Schmidt,
director of GISS and a co-author on the study. “All science is based on
knowing the limitations of the numbers that you come up with, and those
uncertainties can determine whether what you’re seeing is a shift or a
change that is actually important.”
The study found that individual and systematic changes in measuring
temperature over time were the most significant source of uncertainty.
Also contributing was the degree of weather station coverage. Data
interpolation between stations contributed some uncertainty, as did the
process of standardizing data that was collected with different methods
at different points in history.
After adding these components together, GISTEMP’s uncertainty value
in recent years was still less than a tenth of a degree Fahrenheit,
which is “very small,” Schmidt said.
The team used the updated model to reaffirm that 2016 was very
probably the warmest year in the record, with an 86.2 percent
likelihood. The next most likely candidate for warmest year on record
was 2017, with a 12.5 percent probability.
“We’ve made the uncertainty quantification more rigorous, and the
conclusion to come out of the study was that we can have confidence in
the accuracy of our global temperature series,” said lead author Nathan
Lenssen, a doctoral student at Columbia University. “We don’t have to
restate any conclusions based on this analysis.”
Another recent study evaluated GISTEMP in a different way that also
added confidence to its estimate of long-term warming. A paper published
in March 2019, led by Joel Susskind of NASA's Goddard Space Flight
Center, compared GISTEMP data with that of the Atmospheric Infrared
Sounder (AIRS), onboard NASA's Aqua satellite.
GISTEMP uses air temperature recorded with thermometers slightly
above the ground or sea, while AIRS uses infrared sensing to measure the
temperature right at the Earth's surface (or “skin temperature”) from
space. The AIRS record of temperature change since 2003 (which begins
when Aqua launched) closely matched the GISTEMP record.
Comparing two measurements that were similar but recorded in very
different ways ensured that they were independent of each other, Schmidt
said. One difference was that AIRS showed more warming in the
northernmost latitudes.
“The Arctic is one of the places we already detected was warming the
most. The AIRS data suggests that it’s warming even faster than we
thought,” said Schmidt, who was also a co-author on the Susskind paper.
Taken together, Schmidt said, the two studies help establish GISTEMP
as a reliable index for current and future climate research.
“Each of those is a way in which you can try and provide evidence
that what you’re doing is real,” Schmidt said. “We’re testing the
robustness of the method itself, the robustness of the assumptions, and
of the final result against a totally independent data set.”
In all cases, he said, the resulting trends are more robust than what
can be accounted for by any uncertainty in the data or methods.
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