'There's always an interest in the annual temperature numbers and on a given year's ranking, but usually that misses the point,' said James Hansen, the director of GISS, which conducts a similar analysis each year. 'There's substantial year-to-year variability of global temperature. But when we average temperature over five or ten years to minimize that variability, we find that global warming is continuing unabated.'
2008 was the coolest year of the decade, due to strong cooling of the tropical Pacific Ocean, but 2009 saw a return to near-record global temperatures. The past year’s temperatures were only barely surpassed by 2005, the warmest recorded year, and tied with a cluster of others — 1998, 2002, 2003, 2006 and 2007 — as second warmest. Through the last three decades, the GISS surface temperature record shows an upward trend of about 0.2°C (0.36°F) per decade. The greatest increases in 2000-2009 came in the Arctic and part of Antarctica. Except for a leveling off between the 1940s and 1970s, Earth’s surface temperatures have increased steadily since 1880, when modern scientific instrumentation became available to monitor temperatures precisely. In total, average global temperatures have increased by about 0.8°C (1.5°F) since 1880.
At least in the United States, the near-record temperature of 2009 would seem to be all the more striking because it includes a notably frigid December in much of North America, with heavy snowstorms and freezing of crops in the south. High air pressures in the Arctic decreased the east-west flow of the jet stream, while also increasing its tendency to blow from north to south. This resulted in an unusual effect, in which cold air from the Arctic rushed into North America and warmer mid-latitude air shifted toward the north. Of course, the contiguous 48 states cover only 1.5 percent of the world area, so the U.S. temperature does not affect the global temperature much,' said Hansen.
The long-term rise since 1880 is “the important number to keep in mind,' said Gavin Schmidt, another GISS climatologist. 'In contrast, the difference between, say, the second and sixth warmest years is trivial since the known uncertainty, or noise, in the temperature measurement is larger than some of the differences between the warmest years.'
Climate scientists agree that rising levels of carbon dioxide and other greenhouse gases trap incoming heat near the surface of the earth and are the key factors causing the rise in temperatures. However, these gases are not the only factors that can affect global temperatures. Other key factors--changes in the sun's irradiance, oscillations of tropical sea-surface temperature such as the El Niño-La Niña cycle, and changes in the levels of airborne particles — can also cause slight increases or decreases in the planet's temperature. But overall, the evidence suggests that these effects are not enough to account for the global warming observed since 1880.
El Niño and La Niña are prime examples of how the oceans can affect global temperatures. These terms describe abnormally warm or cool sea surface temperatures in the South Pacific, caused by changing ocean currents. Global temperatures tend to decrease in the wake of La Niña, which occurs when upwelling cold water off the coast of Peru spreads westward in the equatorial Pacific Ocean. La Niña lingered during the early months of 2009 and gave way to the beginning of an El Niño phase in October that is expected to continue in 2010. An especially powerful El Niño cycle in 1998 is thought to have contributed to the unusually high temperatures that year, and Hansen's group estimates that there is a good chance 2010 will be the warmest year on record if the current El Niño persists. At most, scientists estimate that El Niño and La Niña can cause global temperatures to deviate by about 0.2°C (0.36°F).
Warmer surface temperatures also tend to occur during particularly active parts of the solar cycle, known as solar maximums, while slightly cooler temperatures occur during lulls in activity, called minimums. A deep solar minimum has made sunspots a rarity in the last few years. Such lulls in solar activity, which can cause the total amount of energy given off by the sun to decrease by about a tenth of a percent, typically spur surface temperature to dip slightly. Overall, solar minimums and maximums are thought to produce no more than 0.1°C (0.18°F) of cooling or warming.
'In 2009, it was clear that even the deepest solar minimum in the period of satellite data hasn't stopped global warming from continuing,' said Hansen.
Small particles in the atmosphere, called aerosols, can also affect the climate. Volcanoes are powerful sources of sulfate aerosols that counteract global warming by reflecting incoming solar radiation back into space. In recent years, large eruptions at Mount Pinatubo in the Philippines and El Chichón in Mexico have caused global dips in surface temperature of as much as 0.3°C (0.54°F). But volcanic eruptions in 2009 did not have a significant impact. Meanwhile, other types of aerosols, often produced by burning fossil fuels, can change surface temperatures by either reflecting or absorbing incoming sunlight. Hansen's group estimates that aerosols probably counteract about half of the warming produced by manmade greenhouse gases, but he cautions that better measurements of these elusive particles are needed.
To conduct its analysis, GISS uses publicly available information from three sources: weather data from more than 1,000 meteorological stations around the world; satellite observations of sea-surface temperatures; and Antarctic research station measurements. These three data sets are loaded into a computer program (available for public download from the GISS website). The program calculates trends in temperature anomalies — not absolute temperatures, but rather changes relative to the average temperature for the same month during the period of 1951-1980. Other research groups also track global temperature trends, but use different analysis techniques. The Met Office Hadley Centre, based in the United Kingdom, uses similar input measurements as GISS, for example, but omits large areas of the Arctic and Antarctic, where monitoring stations are sparse. Although the two methods produce slightly different results in the annual rankings, the decade-long trends in the two records are essentially identical. Previously, the World Meteorological Organization issued a similar report in December 2009, saying that 2000-2009 would rank as the warmest decade on record, and 2009 would probably be among the warmest years.
'There's a contradiction between the results shown here and popular perceptions about climate trends,' Hansen said. 'In the last decade, global warming has not stopped.'