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Goddard Media Studios / NASA

By Alex Kirby

The Gulf Stream is slowing, the North Atlantic is cooling. An international scientific study has found new and harder evidence that one of the planet's key heat pumps, the currents which exchange warmth between the tropics and the Arctic, are weaker today than at any time in the last thousand years.

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Earth Policy Institute

By Janet Larsen and Sara Rasmussen

The global average temperature in 2011 was 14.52 degrees Celsius (58.14 degrees Fahrenheit). According to National Aeronautics and Space Administration (NASA) scientists, this was the ninth warmest year in 132 years of recordkeeping, despite the cooling influence of the La Niña atmospheric and oceanic circulation pattern and relatively low solar irradiance. Since the 1970s, each subsequent decade has gotten hotter—and 9 of the 10 hottest years on record have occurred in the twenty-first century.

Each year’s average temperature is determined by a number of factors, including solar activity and the status of the El Niño/La Niña phenomenon. But heat-trapping gases that have accumulated in the atmosphere, largely from the burning of fossil fuels, have become a dominant force, pushing the Earth’s climate out of its normal range. The planet is now close to 0.8 degrees Celsius warmer than it was a century ago. Hidden within annual averages and expected variability are startling instances of new temperature and rainfall records in many parts of the world—weather extremes that would once be considered anomalies but that now risk becoming the new norm as the Earth heats up.

Worldwide, 2011 was the second wettest year on record over land. (The record was set in 2010, which also tied 2005 as the warmest overall.) Heavier deluges are expected on a warmer planet. Each temperature rise of 1 degree Celsius increases the amount of moisture the atmosphere can hold by about 7 percent. Higher temperatures also can fuel stronger storms.

Brazil started the year with the deadliest natural disaster in its history—in January, a month’s worth of rain fell in a single day in Rio de Janeiro state, leading to floods and landslides that killed at least 900 people. That same month, flooding in eastern Australia covered an area nearly the size of France and Germany combined. Overall, it was the third wettest year in Australia since recordkeeping began in 1900.

The most expensive weather disaster of 2011 was the flooding in Thailand in the second half of the year, which ultimately submerged one third of the country’s provinces. At $45 billion worth of damage—equal to 14 percent of Thailand’s gross domestic product—it was also the costliest natural catastrophe the country ever experienced.

In October, more than 100 people died as two storms—one from the Pacific and the other from the Caribbean—pounded Central America with rain. In western El Salvador, nearly 1.5 meters of rain (almost 5 feet) fell over 10 days. And in December, Tropical Storm Washi hit the Philippines, creating flash floods that killed more than 1,200 people.

The 2011 Atlantic hurricane season had 19 named storms. Hurricane Irene brought extreme flooding to the northeastern U.S. in August, with total damages topping $7.3 billion. The year was the wettest on the books for seven states in the country, while it was among the driest for several others. Although the extremes appear to balance out, making for a near-average year, in fact a record 58 percent of the contiguous U.S. was either extremely wet or extremely dry in 2011.

Indeed, as is expected on a hotter planet, while some parts of the globe were overwhelmed by rain in 2011, others were distinguished by dryness. A severe drought in the Horn of Africa that began in 2010 devolved into a crisis situation in 2011, characterized by crop failure, exorbitant food prices, and widespread malnutrition. Exacerbated by chronic political instability and a belated humanitarian response, the death toll may have exceeded 50,000 people.

Back in North America, a drought that began in late 2010 and worsened over 2011 led hundreds of farmers from northern Mexico to march to that nation’s capital in January 2012 to draw the government’s attention to their suffering. Nearly 900,000 hectares of farmland (some 2.2 million acres) and 1.7 million head of livestock were lost due to the dryness—the worst in Mexico’s 70+ years of data collecting.

Scorching heat, drought, and wildfires across the U.S. Southern Plains and Southwest caused farm, ranch, and forestry damages that exceeded $10 billion in 2011. Wichita Falls, Texas, experienced 100 days over 100 degrees Fahrenheit—far more than the previous record of 79 days set in 1980. Oklahoma and Texas had the hottest summers of any states in history, breaking by a wide margin the record set in 1934 during the Dust Bowl. James Hansen, director of NASA’s Goddard Institute for Space Studies, writes that the likelihood of such extreme heat waves “was negligible prior to the recent rapid global warming.” Texas also had its lowest rainfall on record. Invigorated by the heat and drought, wildfires burned across an estimated 1.5 million hectares (3.7 million acres) in the state.

For the continental U.S., summer 2011 was the second warmest in history. Nearly three times more weather stations hit record highs than lows in 2011, in line with a trend of increasing heat extremes. Whereas in the middle of the 20th century there were close to the same number of record highs and lows—as would be expected absent a strong warming trend—in the 1990s highs began outpacing lows. In the first decade of this century, there were twice as many record highs as record lows.

Worldwide, seven countries set all-time temperature highs in 2011—Armenia, China, Iran, Iraq, Kuwait, Republic of the Congo, and Zambia. Interestingly, Zambia also was the only country to experience an all-time low temperature when it dropped to -9 degrees Celsius (16 degrees Fahrenheit) in June. Kuwait experienced the year’s highest temperature, with thermometers measuring a searing 53.3 degrees Celsius (127.9 degrees Fahrenheit), the highest temperature ever recorded on Earth during the month of August. Even more threatening to health than daytime highs are extra hot nighttime minimum temperatures, which do not allow any respite from the heat. The world’s hottest 24-hour minimum ever—41.7 degrees Celsius (107 degrees Fahrenheit)—was recorded in Oman in June 2011.

Even the Arctic had a notably warm year, with the 2011 temperature a record 2.2 degrees Celsius (4 degrees Fahrenheit) above the mean for 1951–80. Barrow, Alaska, the northernmost U.S. city, spent a record-breaking 86 consecutive days at or above freezing, far more than the previous record of 68 days set in 2009.

In fact, over the last 50 years temperatures in the Arctic have risen more than twice as fast as the global average, melting ice and thawing permafrost. Arctic sea ice has been shrinking more rapidly, falling to its lowest volume and second lowest area on record during the 2011 summer melt season. With the summertime ice loss outpacing wintertime recovery, Arctic sea ice has thinned, making it increasingly vulnerable to further melting. Scientists expect a completely ice-free summertime Arctic by 2030 or even earlier.

As the reflective ice disappears, it exposes the dark ocean, which more readily absorbs solar energy, further warming the region. This sets forth a climate cascade, accelerating ice loss both in the ocean as well as on nearby Greenland, which contains enough ice to raise global sea level by 7 meters (23 feet) if it completely melted. The warming also thaws Arctic permafrost, releasing carbon dioxide and methane, further accelerating global warming.

Even without fully incorporating such climate feedback, models show that continued reliance on fossil fuels could raise the global temperature by up to 7 degrees Celsius (over 12 degrees Fahrenheit) by the end of this century. Such an elevated temperature would amplify temperature and precipitation extremes enough to make the weather events of recent years look tame in comparison. Only a rapid, dramatic reduction of greenhouse gas emissions can hold future temperatures in a range bearing any resemblance to what civilization has known.

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National Aeronautics and Space Administration

The global average surface temperature in 2011 was the ninth warmest since 1880, according to National Aeronautics and Space Administration (NASA) scientists. The finding continues a trend in which nine of the 10 warmest years in the modern meteorological record have occurred since the year 2000.

NASA's Goddard Institute for Space Studies (GISS) in New York, which monitors global surface temperatures on an ongoing basis, released an updated analysis that shows temperatures around the globe in 2011 compared to the average global temperature from the mid-20th century. The comparison shows how Earth continues to experience warmer temperatures than several decades ago. The average temperature around the globe in 2011 was 0.92 degrees F (0.51 C) warmer than the mid-20th century baseline.

"We know the planet is absorbing more energy than it is emitting," said GISS Director James E. Hansen. "So we are continuing to see a trend toward higher temperatures. Even with the cooling effects of a strong La Niña influence and low solar activity for the past several years, 2011 was one of the 10 warmest years on record."

The difference between 2011 and the warmest year in the GISS record (2010) is 0.22 degrees F (0.12 C). This underscores the emphasis scientists put on the long-term trend of global temperature rise. Because of the large natural variability of climate, scientists do not expect temperatures to rise consistently year after year. However, they do expect a continuing temperature rise over decades.

The first 11 years of the 21st century experienced notably higher temperatures compared to the middle and late 20th century, Hansen said. The only year from the 20th century in the top 10 warmest years on record is 1998.

Higher temperatures today are largely sustained by increased atmospheric concentrations of greenhouse gases, especially carbon dioxide. These gases absorb infrared radiation emitted by Earth and release that energy into the atmosphere rather than allowing it to escape to space. As their atmospheric concentration has increased, the amount of energy "trapped" by these gases has led to higher temperatures.

The carbon dioxide level in the atmosphere was about 285 parts per million in 1880, when the GISS global temperature record begins. By 1960, the average concentration had risen to about 315 parts per million. Today it exceeds 390 parts per million and continues to rise at an accelerating pace.

The temperature analysis produced at GISS is compiled from weather data from more than 1,000 meteorological stations around the world, satellite observations of sea surface temperature and Antarctic research station measurements. A publicly available computer program is used to calculate the difference between surface temperature in a given month and the average temperature for the same place during 1951 to 1980. This three-decade period functions as a baseline for the analysis.

The resulting temperature record is very close to analyses by the Met Office Hadley Centre in the United Kingdom and the National Oceanic and Atmospheric Administration's National Climatic Data Center in Asheville, N.C.

Hansen said he expects record-breaking global average temperature in the next two to three years because solar activity is on the upswing and the next El Niño will increase tropical Pacific temperatures. The warmest years on record were 2005 and 2010, in a virtual tie.

"It's always dangerous to make predictions about El Niño, but it's safe to say we'll see one in the next three years," Hansen said. "It won't take a very strong El Niño to push temperatures above 2010."

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