Temperature Increases
The direct evidence for positive climate change does not, contrary to popular opinion, equate to something as straightforward as perceptibly warmer summers. Rather, the empirical basis for a warming of the earth’s climate rests upon a global average of surface temperature readings, or mean surface temperature. Mean temperatures are derived from aggregate data collected from measuring stations around the world, the earliest consistent record beginning in 1861. Determination of temperature prior to this period is obtained from the measurement of certain trace elements recovered from ice cores that are known to correlate to surface temperature. The range of such average temperatures is very small—only a fraction of a degree Celsius—but it is known that major climatic events of the past, such as glaciation, were accompanied by only incremental changes in the global mean temperatures.
Records of global temperature are well established for the period, over the last century and a half, since consistent measurements have been taken. As the record is pushed further back in time, scarcity of data raises the degree of uncertainty, but temperature trends reconstructed from proxy evidence are largely uncontroversial. For temperatures prior to the mid-19th century, scientists make inferences on the basis of other variables known to correlate with temperature. Analysis of tree rings from exceptionally long-lived species, or from dead trees that have been somehow preserved, can extend the temperature record several thousand years into the past. Gas concentrations and trace elements frozen in the Antarctic and Greenland ice caps provide a record of atmospheric conditions extending back nearly a quarter of a million years; beyond this, seabed sediments and fossilized coral provide temperature indicators for climatic conditions that existed millions of years ago. Such long-term evidence is essential to determine the relative significance of more recent and comparatively brief warming. On this basis, paleoclimatic data suggest that, according to the IPCC Third Assessment Report, “the present CO2 concentration has not been exceeded during the past 20 million years,” and that “the current rate of increase is unprecedented during at least the past 20,000 years.”
It is acknowledged, however, that mean temperatures alone are insufficient for the attribution of human-induced climate changes. To bridge the inferential gap, throughout the 1990’s researchers have called for a wider array of experimental measurements of such phenomena as heat absorption by the oceans, and the cooling potential of ocean cloud-cover and atmospheric aerosols. Better knowledge of these processes would simultaneously reduce the speculative aspects of climate modeling (a controversial issue) and provide more direct evidence for the mechanics of climate change.
A call by NASA Goddard Institute researcher James Hansen for closer study of oceanic temperatures was recently answered by a project at the National Oceanic and Atmospheric Association (NOAA) to establish a database of ocean temperature measurements from 1948 to 1998. This recent effort demonstrated an average increase in ocean temperatures to a depth of 300 meters. Still another data set was recently compiled by researchers studying subsurface ground temperature measurements from “boreholes” on six continents. The results from this record, again, indicate a 20th century warming that is the greatest in 500 years.
In addition to enlarging the climate change database, much recent work has been devoted to refining one or another of the data sets that provide evidence for an abrupt warming during the last fifty years. For example, questions arose in the 1990’s as to whether thermometer readings used to calculate the global mean temperature are elevated by their location in urban areas, or heat islands, known to be hotter than the surrounding countryside. The temperature difference between cities and their surroundings is most notable at night—which would seem to offer one possible explanation for the observed global rise in nighttime minimum temperatures. Several considerations, however, have eliminated the possibility that urban areas are giving the illusion of a general warming trend. Studies carried out since the IPCC Second Assessment report separate urban from rural temperature series in order to isolate any statistically significant difference between the two trends, and found that “there is little difference in the long-term (1880 to 1998) rural… and full set of station temperature trends.” Even without separating urban from rural temperature readings, the average surface temperature record fits well with warming trends unaffected by urbanization: borehole temperatures, reduced terrestrial snow and ice cover, and changes in temperature of the ocean.
Records of global temperature are less well established than records of gas concentrations, but are still largely uncontroversial.