Environment - Global Climate Change

Global Climate Change

Climate Change Facts
October 3, 1997

Issue 3 U.S. House of Representatives

Minority Staff, Committee on Government Reform and Oversight

Has Global Warming Already Begun?
Human activities such as burning of fossil fuels have significantly increased levels of greenhouse gases in the atmosphere. Two commonly asked questions are: (1) what effects have these increases already had on the earth's climate? and (2) what effects are these increases likely to have in the future? This fact sheet addresses the first question. The next fact sheet will address future predictions.

Evidence of Recent Warming: Surface Temperature Records. Surface air temperature records have been kept since approximately 1860. Surface data is collected from both land surface and sea surface stations. There are about 1,000-2,000 land surface monitoring stations worldwide. Sea surface data is collected by thousands of ships, and more recently from satellite recordings. Adjustments are made to account for any excess warming that could be attributed to measurement stations located near urban areas that could be effected by "heat island" effects.

Combined land and sea global surface temperature records show that average temperatures have risen approximately one degree Fahrenheit over the past 100 years. Warming patterns are not globally uniform, so variations occur between regions, with the greatest warming occurring over mid-latitude continents in winter and spring. The ten warmest years this century have all occurred since 1980. The warmest year on record was in 1995, and the 1990s so far have been the warmest decade on record. According to NASA's Goddard Institute for Space Studies, the global warmth of the 1990s is "particularly remarkable" because it occurs during "the decade cooled by the greatest volcanic aerosol veil of the century and record depletion of ozone." Both volcanic and manmade aerosols, which reflect incoming light, and depletion of ozone, a strong greenhouse gas, are thought to "mask" warming and temporarily offset warming trends. The chart on the reverse shows the observed surface air temperature trends from the late 1800s projected through the year 2000.

Other Evidence of Recent Warming. In the past few decades, satellites and balloons have been used to record temperature in the upper atmosphere. Regular upper air balloon measurements began at scattered locations in the 1940s. Satellite data collection began in the 1970s. Initial analysis of the data suffered from calibration problems, complications with the sensors due to humidity, and nonuniform geographic distribution of stations. In addition, since satellites measure temperature at higher altitudes than the land surface measurements, the temporary cooling effects of aerosols have had a more significant impact on satellite temperature readings. When the satellite readings are calibrated properly, the eruption of Mt. Pinatubo is accounted for, and records are compared with balloon temperature readings, they reveal a similar record to the land surface measurements.

Records of glacial mass and balance fluctuations offer additional evidence of global warming trends. Calculations of changes in glacial mass and length demonstrate a coherent pattern of glacial retreat over the past 100 years. Tropical mid-latitude glaciers have been melting for at least two decades, with the rate of melting increasing over time. This evidence of glacial retreat is consistent with climate model projections of more rapid warming in interior alpine regions and at high tropical elevations.

Scientific Conclusions. The Intergovernmental Panel on Climate Change (IPCC), an international scientific body assembled by the United Nations Environment Program and the World Meteorological Organization, concluded in their 1995 Summary Report that "global mean surface temperature has increased by between about .3 and .6 degrees Celsius since the late 19th century, a change that is unlikely to be entirely natural in origin." The National Academy of Sciences reached the same conclusion in 1991: "The most comprehensive assessment of the record of surface temperature reveals a warming since the late nineteenth century of between .3 and .6 degrees Celsius."

There is also emerging scientific evidence that warming is due in part to human-induced emissions of greenhouse gases. In its most recent report, the IPCC concluded that "the balance of evidence suggests a discernible human influence on global climate." Recently 1,496 scientists, including 102 Nobel laureates and 60 U.S. National Medal of Science winners, signed a statement recognizing "[a] broad consensus among the world's climatologists that there is now a discernible human influence on global climate."