Philip B Duffy, Senior Policy Analyst,
Office of Science and Technology Policy, Executive Office of the President
This week, the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA) released temperature data showing that 2011 was one of the warmest years since record-keeping began in 1880. Â The global temperature continued to be extremely warm even though at least two factors acted to push it downwards in the short term.
Despite a slight drop of about 0.2Â°F from 2010â€”which had tied with 2005 for the warmest year ever recordedâ€”the data show that Earth continues to experience warmer temperatures than a few decades ago. NOAAâ€™s analysis ranked 2011 as the 11th warmest year, while NASAâ€™s showed 2011 to be the 9th warmest (rankings are expected to differ slightly because temperature differences between the warmest years are extremely small). Both rankings are consistent with a clear trend of increasing global temperaturesâ€”with the 12 hottest years on record all occurring since 1997, and the decade that started in 2000 the hottest in recorded history.
Still, the data also show that temperatures have not changed much since 2005, the year that tied 2010 as the warmest ever recorded. If temperatures have not risen recently, does that mean climate change has stopped? Â Actually, no. Â Just as stock prices do not go up every day in a bull market, global temperatures do not rise every year in an era of warming. Â In both cases, to understand if an upward trend will continue, one must look at the underlying forces that drive it.
In the case of climate change, the most important underlying force is increasing amounts of greenhouse gases in the atmosphere. Â This results primarily from human burning of fossil fuels (coal, oil, and natural gas). Â Fossil fuel use not only continues, but is growing. Â And because carbon dioxide, the most important greenhouse gas, remains in the atmosphere for many decades, the amount in the atmosphere increases even in those unusual years when emissions are flat or decrease slightly. Â For example when global carbon dioxide emissions dipped by about 1% as a result of the recent global recession, the amount of carbon dioxide in the atmosphereâ€”which is the force that drives temperatureâ€”continued to increase. Â And in most years emissions increase, so the amount of carbon dioxide in the atmosphere increases at an even greater rate.
So why donâ€™t temperatures go up every year? Because although increasing greenhouse gases are the most important force driving the global temperature, they are not the only force. Â Weather variations, for example, push the global temperature up or down slightly every year. Â Powerful volcanic eruptions can cause noticeable cooling for a couple of years. Â And the energy output of the sun varies slightly (although since satellite measurements started in 1980 there has been no steady up or down trend in the sun). Â These and other forces cause global temperatures to vary slightly from year to year. Â To get a reliable indication of trends, one needs to consider at least a decade or two.
Itâ€™s noteworthy that 2011 was very warm despite two temporary cooling influences. The figure below shows that global temperatures were high in 2011 even though a weather variation known as La NiÃ±a caused colder than average temperatures over much of the Pacific Ocean. Â (La NiÃ±a is a temporary cooling of temperatures in the Eastern tropical Pacific region, which has widespread impacts, including a slight global cooling.) Â And between about 2005 and 2010 the sunâ€™s energy output was lower than ever recorded, which also pushed temperatures down a little. Â Both these forces are likely to reverseâ€”in fact the sunâ€™s energy output already hasâ€”which can be expected to drive future temperatures upwards in coming years.
Although the temperatures discussed above, which are measured just above the ground or ocean, are the most common way to gauge the global temperature, other data can be useful as well. Â In particular, the â€œocean heat content,â€ which involves ocean temperatures from the surface down to about 6,000 feet, is less susceptible to year-to-year variations. Â This shows a steady march upwards, with each of the last 10 years setting a new record.
The continued high temperaturesâ€”despite downward pressure from La NiÃ±a and the sun, together with ongoing increases in atmospheric greenhouse gases and ocean heat contentâ€”show that global warming continues.
Figure: Map showing 2011 temperatures as differences from the long-term average of 1971 through 2000. Â Red indicates warmer than average temperatures, blue colder than average. Â Colder than average temperatures in the Pacific Ocean region show the influence of La NiÃ±a, a common weather variation. Â Despite this strong temporary cooling influence, global temperatures remained very high in 2011.