Rising air temperatures bring heat waves, spread disease, shift plant and animal habitat and cause extreme weather events, from drought to blizzards.
How do we know the air is getting warmer? The evidence is very strong.
• The 2001-2010 decade is the warmest since 1880—the year when enough temperature records became available worldwide to calculate a global average.
• Over the last 50 years, the number of cold days and record low temperatures in various locations has declined, while the number of hot days and heat waves has risen most places worldwide.
• The best projections show that average global temperatures are likely to increase 3.1-7.2° F (1.8-4.0° C) by the end of the century depending on the amount of carbon emissions.
To document air temperature, scientists measure land surface temperatures a short distance above the ground at stations around the world. These researchers standardize the measurements by accounting for elevation, latitude, time of observation, and type of instrument, and then integrate the information to form a long-term record at a particular location.
Scientists combine measurements of land surface temperatures and sea surface temperatures to calculate the global average temperature. They report this average as the difference from a historical base period. For example, NASA compares the global average temperature each year to a base temperature of roughly 57.2° F (14° C)—an average derived from several decades.
Three major research centers regularly calculate the global average temperature. Although each center uses a slightly different technique, all the results show the same two trends. The first is that each of the last three decades has been hotter than the one before. The second is that the long-term average global temperature is rising.
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