Please note, this page has been archived as of Feb 2011 and
will not be updated.
There are four principal global temperature datasets. Three use
surface temperature measurements taken at land and sea, and record
a clear warming trend over the past century. One is collected by
the Met Office Hadley Centre jointly with the Climatic Research
Unit (HadCRU) in the United Kingdom. Two datasets are collected by
organisations in the United States: the NASA's Goddard Institute of
Space Studies (GISS) and NOAA's National Climatic Data Center
(NCDC).
The final dataset uses satellites to measure the troposphere -
the lower part of the Earth's Atmosphere. It is collected by the
University of Alabama in Huntsville (UAH), started in 1978, and
also records a clear warming trend.
The World Meteorological Organisation (WMO) of the
United Nations uses the datasets kept by GISS, NCDC and HadCRU to
calculate a single world average. These datasets are also quoted in
the IPCC's
Fourth Assessment Report (AR4).
Each year GISS, HadCRU and NCDC each announce a change in the
average global temperature. Each organisation uses subtly different
methodologies to analyse data, which means this announced statistic
can differ. For example, GISS ranks 2005 and 2010 as the joint
hottest years but HadCRU classifies 1998 as the warmest. The three
datasets record an increase in temperatures over the past century
of between about 0.6 and 0.8 degrees celcius.
Methodology
Surface temperatures on land and at sea have been collated since
1880 by HadCRU, GISS and NCDC. For the data used in the most recent
IPCC report, HadCRU used temperatures from 4,349 measuring
stations, while NCDC and GISS used more than 7,200. Papers by
GISS's Hansen
and Lebedeff and HadCRU's Phil Jones et
al provide blueprints for analysing data.
The basic technique for deriving temperatures involves mapping
the world into 5x5 degree grid boxes. Data from temperature
stations within the box is used to calculate an average for the
area.
However, stations are not evenly distributed around the world.
The Arctic, the Antarctic and the African interior have very few
stations and many grid boxes in these regions have no stations at
all. The different datasets deal with the lack of data from these
remote regions differently. GISS assumes that regions with no
stations have the same temperatures as the areas closest by that do
have stations. HadCRU classifies remote areas with no stations as
missing data. The NCDC extrapolates data, but not to areas in the
Arctic with sea ice.
The differences in GISS and HadCRU's published datasets as a
consequence of their extrapolation techniques is illustrated
below:
GISS on the left, HadCRU on the right. Source: NASA
Earth Observatory
The GISS methodology is more sensitive to unusual conditions in
areas with few temperature stations. Because HadCRU does not
extrapolate data to areas where there are no stations it can
underestimate the warming effect of climate change in the Arctic,
for example.
The rate of Arctic sea ice loss suggests temperatures are higher
then currently stated in the HadCRU data. Because temperatures were
cool in the Arctic during 1998 the GISS value was lower than that
given by HadCRU. During 2005 and 2010 the Arctic experienced warmer
conditions and thus GISS gave a higher global temperature than
HadCRU. This probably explains why HadCRU states that 1998 is the
warmest year on record while GISS states 2005 and 2010 are the
warmest years.
Temperature Anomalies
Scientists each year announce the change in global temperature
compared to a historic long term average. They describe this as the
"temperature anomaly" from a "base" period. We cannot calculate an
accurate absolute average temperature. This is because, among many
reasons, temperature stations are placed at different altitudes and
record data at different times, while some are placed in urban and
others in rural areas. Using anomalies allows scientists to
extrapolate data across geographical regions more accurately.
GISS, HadCRU and the NCDC each use different base periods: GISS
uses 1951-80, HadCRU uses 1961-90 and NCDC uses the whole of the
20th Century. This means that each organisation's annual figures
are different.
However, the annual changes in temperature recorded by the
different organisations are very similar.
How important is one year?
The difference between the 'warmest' or 'second warmest' year is
often just a few hundredths of a degree. Meanwhile, year on year
temperatures can fluctuate because of specific weather events (like
El Nino) and natural climate variability. Because of this,
scientists look at longer periods of time to track changes in the
climate.
James Hansen, the director of GISS, has said: "It's not
particularly important whether 2010, 2005, or 1998 was the hottest
year on record… It is the underlying trend that is important."
Long term temperature trends. Source: Nasa
Earth Observatory
A glance at the long-term temperature trends shows a clear
agreement between the datasets, with the warming trend also
apparent. Reto Ruedy, also of GISS, has said: "Official records
vary slightly because of subtle differences in the way we analyze
the data, but they agree extraordinarily well."
So despite their methodological differences, data from the
world's most respected temperature monitoring organisations shows,
with surprising harmony, that the world is warming.
References
Hansen, J.E., and S. Lebedeff, 1987: Global
trends of measured surface air temperature. J. Geophys. Res.,
92, 13345-13372, doi:10.1029/JD092iD11p13345.
Jones, P.D., New, M., Parker, D.E., Martin, S. and Rigor, I.G.,
1999: Surface air
temperature and its variations over the last 150 years.
Reviews of Geophysics 37, 173-199.
IPCC AR4 Working Group I Chapter 3.2.2 -
Temperature in the Instrumental Record for Land and Oceans
NASA Press release - NASA
Research Finds 2010 Tied for Warmest Year on Record
NASA Earth Observatory - Different
Records, Same Warming Trend
Please note - This material was last updated on 10/3/11
and has since been archived.
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