Th Century Climate

Considerable effort has been expended to analyse the current historical climate record over the past 150 years in order to establish whether there are any trends that could be attributed to greenhouse gas warming. For example, a number of global data sets of near-surface temperature have been developed and analysed (Nicholls et al., 1996). These data sets have been carefully corrected for errors or bias due to urban heat-island effects, non-homogeneities (such as instrument or location change) and changes in bucket types used to measure sea surface temperatures. One of the most carefully constructed data sets (Jones et al., 1994, 1999) indicates there has been a 0.3°C to 0.6°C warming of the earth's surface since the late 19th century (Fig. 2.2). This trend continued through 1998. The global average temperature from January to December 1998, was the warmest on record for the period 1880-1998 (National Climate Data Center web site, www.ncdc.noaa.gov). Specifically, the global average temperature for January until June 1998, was 0.6°C higher than the 1961-1990 global mean temperature. However, the distinct warming has not been regionally homogeneous, since some regions have experienced cooling during the 20th century.

The diurnal temperature range has primarily decreased in most regions, indicating that minimum temperatures have warmed more than maximum

Fig. 2.2. Estimated changes in annual global-mean temperatures (thin line) and carbon dioxide (thick line) over the past 138 years relative to the 1961-1990 average (horizontal solid line). Earlier values for carbon dioxide are from ice cores (dashed line), and for 1959-1996 from direct measurements made at Mauna Loa, Hawaii. The scale for carbon dioxide is in parts per million (ppmv) relative to a mean of 333.7 ppmv. (Source: Hurrell, 1998.)

Fig. 2.2. Estimated changes in annual global-mean temperatures (thin line) and carbon dioxide (thick line) over the past 138 years relative to the 1961-1990 average (horizontal solid line). Earlier values for carbon dioxide are from ice cores (dashed line), and for 1959-1996 from direct measurements made at Mauna Loa, Hawaii. The scale for carbon dioxide is in parts per million (ppmv) relative to a mean of 333.7 ppmv. (Source: Hurrell, 1998.)

temperatures, or that cloudiness has increased in these areas (Karl et al., 1993, 1996). A cooling of the lower stratosphere by 0.6°C has also occurred since 1979.

Sea level has increased on average between 10 and 25 cm over the past 100 years (Nicholls et al., 1996). This rise is related to the increase in near-surface temperatures, which has caused thermal expansion of the oceans and melting of glaciers and ice caps. Thermal expansion of the oceans has contributed between 2 and 7 cm to the total increase in sea level.

There has also been a small increase in global average precipitation over land during the 20th century (Dai et al., 1997). Recent investigations indicate that this mean increase has mainly influenced heavy precipitation rates (Groisman et al., 1999). Since the late 1970s, there have been increases in the percentage of the globe experiencing extreme drought or severe moisture surplus (Dai et al., 1998).

Many of the global climatic changes are analysed from the point of view of expected combinations of changes to different variables known as fingerprints. For example, the combination of cooling of the stratosphere, warming of the surface temperature and increased global mean precipitation is expected from increased greenhouse gas-induced climate change. These anticipated combinations of changes and their spatial patterns are based on our understanding of the physics of the earth/ocean/atmosphere system and results from climate models (Santer et al., 1996). Fingerprinting will be discussed further in section 2.6.4 on climate models.

In the 1990s, remotely sensed temperature data of the lower troposphere (700 millibar [mbar] height, about 2.5 km) have been analysed to compare trends with those from surface observation stations. This has led to a debate regarding the robustness of results from surface observations compared with remotely sensed data (Christy and Spencer, 1995; Hurrell and Trenberth, 1997, 1998). For example, Christy and Spencer used data from the microwave sounding units (MSU) on board the National Oceanic Atmospheric Administration (NOAA) polar orbiting satellites and investigated the time series from 1979 to 1995. They found a slight global cooling of -0.04°C over this time period. According to surface observations, the temperature has increased since 1979 by 0.14°C. This putative discrepancy has been used to question the surface temperature record and to provide evidence that global warming is not occurring.

The intense political debates concerning global warming tend to polarize results from scientific research on the subject and result in oversimplification of the methods/techniques and research results. In fact, the differences in these global mean trends can largely be explained based on the differences in the physical quantities being measured (Hurrell, 1998). The MSU measures lower tropospheric temperatures at an altitude of about 2.5 km, whereas the surface measurements are made on land at a screen height of about 2 m. Differences in the thermal characteristics of land and ocean affect the relative correlations of temperature with altitude. Over the ocean, the sea surface temperatures will not necessarily be highly correlated with temperatures at 700 mbar heights.

These and other issues, such as the myriad technical difficulties associated with using remotely sensed data to establish trends (Hurrell, 1998; Hurrell and Trenberth, 1998; Kerr, 1998), suggest that the intense debate is more political than scientific.

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