Because the burning of fossil fuels over the last 200 years has increased carbon dioxide concentrations in the atmosphere, Earth should be warming via the greenhouse effect. Indeed, four climate research groups—one in Japan, one in the United Kingdom, and two in the United States—have shown that global temperatures have increased significantly since 1880 by about 0.85°C (1.5°F) (Figure 14.19).

The relatively smooth increase in atmospheric concentration of CO₂ (see Figure 14.18) contrasts sharply with the considerable variability in global temperatures shown in Figure 14.19. This contrast suggests that factors other than greenhouse gases significantly affect global temperatures, at least in the short term. One of the main sources of this discrepancy is the El Niño Southern Oscillation (ENSO), which we reviewed in Chapter 6 (see page 161). For instance, the peak in global temperature around 1942 was associated with a strong El Niño, and the subsequent declines that followed were associated with several La Niña episodes in the 1950s. Similarly, the exceptionally strong El Niño of 1998 brought about a spike in global temperatures and was followed by a La Niña episode, which reduced global temperature. Such influences are not surprising, since ENSO events involve the massive transport of heat stored in the Pacific Ocean. However, these short-term variations do not affect the long-term trends of increasing global temperature that have been documented.

The increase in global temperature becomes even more apparent in a plot of temperatures averaged by decades (Figure 14.20). By removing the higher-frequency, year-to-year variation in temperatures, the plot becomes smooth. Figure 14.20 shows that record high temperatures were recorded from the 1980s through the 2000s, with each succeeding decade setting a new record for high temperatures. Global temperatures for 2010 to 2014 continued to be well above average, despite a prolonged La Niña at the beginning of this new decade. In fact, the average global temperature in 2014 was the warmest recorded since 1880.

Consequently, the warming of the globe shows no signs of flagging. By the end of the 21st century, climate models suggest that the temperature of Earth’s surface is expected to rise between 2° and 3°C, according to the IPCC.

Although it may seem like a trivial matter to measure air temperature in a single spot over time, several criticisms have been leveled at attempts to test whether Earth as a whole is really warming.

Some critics have pointed out that weather stations are mostly located in urban areas, which have become “urban heat islands” over time because asphalt and cement absorb a lot of heat. In addition, the quality of the monitoring process varies from station to station, which raises the possibility that these records are inaccurate. Some critics argue that only data from the best sites should be included in global estimates. Others say the opposite: Climate researchers have been too selective in their choice of data sources. If the records from more weather stations were included, they argue, the warming pattern would disappear.

To settle the question, Richard A. Muller at the University of California at Berkeley organized the Berkeley Earth Surface Temperature (BEST) study. Muller, who was himself a skeptic of global temperature estimates, compiled weather records from 36,000 land-based weather stations, approximately 5 times the number of stations included in other temperature analyses. These records included 1.6 billion temperature measurements from around Earth, going back to 1753, which extended the analysis over 100 years farther back in time than the plot in Figure 14.20. In 2011 Muller reported that temperatures on land have risen 1.5°C in the past 250 years and 0.9°C in the most recent 50 years. “I was not expecting this, but as a scientist,” Muller later told the newspaper The Telegraph when he published his findings in 2012, “I feel it is my duty to let the evidence change my mind” (Figure 14.21). He went on to say that the best statistical fit to the rise in Earth temperatures was the human-caused increase in atmospheric CO₂.

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