detection OF current global temperature and reconstruction of past trends rely on data from four sources: surface temperature from weather or climate stations, weather balloons, satellite mounted remote sensors, and proxy sources. Data from the first three categories are known as the instrumental record.
Until recently, measurements of global air-temperature change were based entirely on measurements taken on the ground at weather stations. Three authorities that have taken responsibility for the combined surface record are the Climate Research Unit (CRU) of the University of East Anglia (UEA), the National Aeronautics and Space Administration's (NASA's) Goddard Institute for Space Studies (GISS), and the Global Historical Climate Network (GHCN) run by the U.S. National Oceanographic and Atmospheric Administration (NOAA).
Given that the data come from weather stations unevenly distributed over the Earth's surface, mostly in the Northern Hemisphere, mainly on land, close to towns and cities, the question arises as to the extent to which these temperatures can be taken as representative of the atmosphere as a whole.
Modification of the surface by human activity can have a significant effect on local climate near the ground. The best-documented example is the urban heat island effect, in which localized warming due to asphalt and concrete replacing grass and trees can influence data from urban stations. This can make an urban area several degrees C warmer than its rural surroundings.
Several researchers have demonstrated that only very small changes in population are enough to induce a statistically significant local warming effect. Many weather stations are located at airports, which originally were located in rural areas on the outskirts of urban areas. But with rapid population growth, airports have made a transition from rural to heat island and it is difficult for the national agencies responsible for archiving climate data to take these influences into account. The problem has been made worse by the fact that two-thirds of the weather stations operating around 1975, mainly rural, have been closed down. For example, the GHCN network showed a peak of about 15,000 stations around 1970, which declined to 5,000 as stations deemed non-essential for operational purposes were closed down. The loss was mainly of rural stations.
Over 70 percent of the Earth's surface is covered by water. Because of this, land-based surface temperature data are supplemented by measurements taken at sea, usually by ships. Scientists have assumed that there is a link between the surface layer temperature of seawater and that of the air above it. Sea surface temperatures are estimated from the temperature of seawater as it is taken aboard as an engine coolant. Sometimes, a bucket tethered to a rope thrown overboard serves the purpose. The bucket is then hauled aboard, and the water temperature is measured with a thermometer. Other sea surface data are taken by combining data from several rather different sources, such as buoys, satellite infrared data, nighttime marine air temperatures measured aboard ships, and measurements made at small island stations.
Each method has problems that result in errors or differences among the data sets. The mean global surface temperature of the Earth shows a warming of 0.3-0.7 degrees C over the past century; this is an increase statistically of about 0.003 to 0.007 degrees C per year, a rate not far from the standard error for the data. The data sets from the various sources (CRU, GISS and GHCN) show slightly dissimilar trends as the data are processed in different ways.
weather balloon data
Weather balloon data originate from a package of instruments called a radiosonde attached to a balloon that sends pressure, humidity, temperature, and wind data by radio transmitter to a ground-based receiving station as the balloon ascends. NOAA and the U.S. National Climate Data Center have assembled data on temperatures through the lower atmosphere (troposphere) based on radiosondes since 1958, and have assembled it into the Radiosonde Atmospheric Temperature Products for Assessing Climate (RATPAC) data set.
RATPAC consists of station and spatially averaged monthly mean temperature from 1958 to present derived from radiosonde observations taken at 87 weather stations across the globe. The data set consists of a timed series for 13 pressure levels from the surface to 30 mbar pressure level in the atmosphere and a seasonal time series for three layers of the atmosphere (850-300, 300-100, and 100-50 mbar). Although more limited in time and space, these data are free of the ground-based contamination that affect the surface temperature record. RATPAC data shows a global temperature increase of 0.17±0.05 degrees C per decade between 1979 and 2004 compared with the GHCN of 0.16±0.04 degrees C, GISS of 0.16±0.04 degrees C and CRU of 0.17±0.04 degrees C per decade for the same period.
NOAA polar-orbiting Tiros-N satellites have made temperature measurements of the atmosphere
(troposphere and stratosphere) since 1979, using microwave radiometry called Microwave Sounding Units (MSU).
MSU instruments measure the microwave emissions of oxygen molecules, which are related to air temperature. By monitoring microwave emissions at different frequencies, it is possible to identify temperature changes in various layers of the atmosphere. Initial analyses of the MSU data were conducted by scientists at the University of Alabama, Huntsville (UAH), and later by scientists at the Remote Sensing Systems (RSS) facility in Santa Rosa, California.
The MSU temperature data set is the only one that is truly global, highly accurate, and uses a completely homogeneous measurement over the entire planet. It also measures the part of the atmosphere that, according to the climate models, should be experiencing the greatest warming due to the enhanced greenhouse effect. The accuracy of the MSU measurements is 0.1 degrees C, which is considerably better than the accuracy of surface data.
Both the MSU UAH, and MSU RSS estimates indicate the middle part of the lower atmosphere (mid-troposphere) has warmed since 1979, but the RSS estimate at 0.18 degrees C per decade is higher than that of the UAH estimate at 0.14 degrees C per decade, both for the period extending from January 1979 to June 2006.
SEE ALSO: Climatic Data, Nature of the Data; Climatic Data, Oceanic Observations; National Oceanic and Atmospheric Administration (NOAA); National Aeronautics and Space Administration (NASA).
BIBLIOGRApHY. Melissa Free, D.J. Seidel, and J.K. Angell, "Radiosonde Atmospheric Temperature Products for Assessing Climate (RATPAC): A New Data Set of Large-Area Anomaly Time Series," Journal of Geophysical Research (v.110, 2005); D.E. Paker, et al., "Climate Observations—The Instrumental Record," Space Science Reviews (v.94, 2000); P.W. Thorne, et al., "Revisiting Radiosonde Upper Air Temperatures from 1958 to 2002," Journal of Geophysical Research (v.110, 2005).
C.R. de Freitas University of Auckland, New Zealand
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