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Global ocean currents explain why Northern Hemisphere is the soggier one

By PressRelease, | Science | 0 Comments

A quick glance at a world precipitation map shows that most tropical rain falls in the Northern Hemisphere. The Palmyra Atoll, at 6 degrees north, gets 175 inches of rain a year, while an equal distance on the opposite side of the equator gets only 45 inches. Scientists long believed that this was a quirk of the Earth’s geometry – that the ocean basins tilting diagonally while the planet spins pushed tropical rain bands north of the equator. But a new University of Washington study shows that the pattern arises from ocean currents originating from the poles, thousands of miles away.

The findings, published in Nature Geoscience, explain a fundamental feature of the planet’s climate, and show that icy waters affect seasonal rains that are crucial for growing crops in such places as Africa’s Sahel region and southern India.

In general, hotter places are wetter because hot air rises and moisture precipitates out.

“It rains more in the Northern Hemisphere because it’s warmer,” said corresponding author Dargan Frierson, a UW associate professor of atmospheric sciences. “The question is: What makes the Northern Hemisphere warmer? And we’ve found that it’s the ocean circulation.”

Frierson and his co-authors first used detailed measurements from NASA’s Clouds and Earth’s Radiant Energy System, or CERES, satellites to show that sunlight actually provides more heat to the Southern Hemisphere – and so, by atmospheric radiation alone, the Southern Hemisphere should be the soggier one.

After using other observations to calculate the ocean heat transport, the authors next used computer models to show the key role of the huge conveyor-belt current that sinks near Greenland, travels along the ocean bottom to Antarctica, and then rises and flows north along the surface. Eliminating this current flips the tropical rain bands to the south.

The reason is that as the water moves north over many decades it gradually heats up, carrying some 400 trillion (that’s four with 14 zeroes after it) watts of power across the equator.

For many years, slanting ocean basins have been the accepted reason for the asymmetry in tropical rainfall.

“But at the same time, a lot of people didn’t really believe that explanation because it’s kind of a complicated argument. For such a major feature there’s usually a simpler explanation,” Frierson said.

The ocean current they found to be responsible was made famous in the 2004 movie “The Day After Tomorrow,” in which the premise was that the overturning circulation shut down and New York froze over. While a sudden shutdown like in the movie won’t happen, a gradual slowing – which the recent United Nations report said was “very likely” by 2100 – could shift tropical rains south, the study suggests, as it probably has in the past.

The slowdown of the currents is predicted because increasing rain and freshwater in the North Atlantic would make the water less dense and less prone to sinking.

“This is really just another part of a big, growing body of evidence that’s come out in the last 10 or 15 years showing how important high latitudes are for other parts of the world,” Frierson said.

Frierson’s earlier work shows how the changing temperature balance between hemispheres influences tropical rainfall. A recent study by Frierson and collaborators looked at how pollution from the industrial revolution blocked sunlight to the Northern Hemisphere in the 1970s and ’80s and shifted tropical rains to the south.

“A lot of the changes in the recent past have been due to air pollution,” Frierson said. “The future will depend on air pollution and global warming, as well as ocean circulation changes. That makes tropical rainfall particularly hard to predict.”

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Co-authors of the paper are Yen-Ting Hwang, Elizabeth Maroon, Xiaojuan Liu and David Battisti at the UW; Neven Fuckar at the University of Hawaii; Richard Seager at Columbia University; Sarah Kang at South Korea’s Ulsan National Institute of Science and Technology; and Aaron Donohoe at the Massachusetts Institute of Technology.

The research was funded by the National Science Foundation, the Japan Agency for Marine-Earth Science and Technology, the National Aeronautic and Space Administration, the National Oceanic and Atmospheric Administration and the U.S. Department of Defense.

By Samantha-Rae Tuthill, AccuWeather.com Staff Writer

November 27, 2013; 8:28 PM

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With the holiday season upon the United States, food donations are garnering more attention. But for the 49 million Americans who lived without enough food in 2012, hunger is not just a seasonal struggle. It is also compounded by economic downturns, natural disasters and complex climates.

According to the United Nations World Food Programme (WFP), there is enough food in the world today to feed the planet’s population. Yet across the globe, 842 million people do not have enough to eat, which results in the deaths of 3.1 million children under the age of five each year.

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“It’s a very complex problem,” Richard Choularton, Chief of Program Innovations Service for the WFP, said. “A lot has to do with the distribution of food. At the moment, we have enough food in the world to feed everybody, but not everybody has access to it. Either they’re too poor to afford it or the markets aren’t developed well enough to get it to them.”

Most food-insecure people live in rural areas and depend on rain-fed agriculture in fragile and hazard-prone areas, Choularton said. This includes the 650 million people living in arid regions of Africa, where their food supply depends on rain and is vulnerable to floods and droughts.

Droughts and floods account for as much as 80 percent of economic losses across the African continent.

To compensate for what cannot be grown or what is lost in adverse weather, food must be brought in. For some, the added cost of this transportation makes it too expensive to buy.

A woman holds her child at a local hospital where children receive treatment for malnutrition at the border town of Dadaab, Kenya. People who can barely stay on their feet due to hunger walk for days or even weeks through parched wasteland to find a meal and water. Many of them also set out to seek help for their ailing children. The drought in the Horn of Africa and the famine in Somalia has left more than two million children at risk of starvation. (AP Photo/Schalk van Zuydam)

Some of the most vulnerable people to food scarcity and poverty actually make their income working on farms to grow and tend to food they cannot afford to eat, Choularton said. As quinoa‘s trendy food status increases its supply across America and Europe, for example, it is becoming too expensive for some of the farmers in Bolivia who grow it to continue eating this grain that has been a staple in local diets for centuries.

“The places with the highest levels of food insecurity and hunger are the places most exposed to climate risk, and that’s really because most food-insecure people don’t have access to good irrigation systems, definitely not greenhouses, and they barely have access to high-yielding seeds and agricultural inputs like fertilizers. So their crop yields are almost entirely determined by the level of labor they put into their soil and how much it rains, when it rains and the distribution of the rains,” Choularton said.

This graphic by Met Office and the WFP shows, from yellow to dark red, which areas face the most risk of climate-related disasters, such as droughts or floods, that can affect food supplies.

Weather and natural disasters impact crops by determining how well they grow or by destroying existing product. Floods, cyclones and other damaging events can also deposit materials, such as sands, onto agricultural fields and into irrigation systems. As a result, the area’s ability to rebound following a major weather event is severely inhibited.

Church volunteers pack boxes of donations for the survivors of Typhoon Haiyan in the Philippines at a church in Hong Kong Sunday, Nov. 17, 2013. (AP Photo/Kin Cheung)

Nationally, Feeding America is the largest networks of food banks and the largest hunger-fighting organization in the U.S.

One of their branches, the Central PA Food Bank, is headquartered in Harrisburg, Pa., and services 27 of the state’s counties, including 121,000 local children under the age of 18 who suffer from hunger and malnutrition. Feeding America food banks work with community networks and food pantries, soup kitchens and low-incoming housing centers to provide for those in need on a local level.

“Our job is to find them the most food, and the most nutritious food, at the lowest cost,” Joe Arthur, executive director of Central PA Food Bank, said. “Primarily that is donated food, and most of that donated food actually comes from food companies, large farms, food producers, USDA commodity foods, really a whole host of sources.”

Arthur said that his food bank serves about 54,000 people per week, which is about 75 percent more than they were serving in 2007 before the economic downtown in the U.S.

Financial hardships that direct people to food banks may only be temporary, such as immediately following the loss of a job. For others, however, even working full time may not be enough to keep adequate food on the table. Poverty, unemployment and hunger are interconnected for many people.

Mississippi, for example, has the lowest median household income of any state at $37,095. It also has the highest percentage of food-insecure residents than any other state, with 20.9 percent of the state’s households struggling to get the food they need.

“Hunger in the United States isn’t really centered around our ability to grow food or produce food,” Arthur said. “So it’s not really our capacity as much as people’s access in their communities.”

In urban areas, as well some rural and even suburban areas, there are areas known as “food deserts,” where low-income families do not have reasonable access to grocery stores or markets with a good selection of food. For many, high-quality and nutritious food is too expensive, or fresh produce is too hard to access, resulting in diets that consist of foods with lower nutritional values or more unhealthy fat content.

When a natural disaster strikes, such as the recent tornado outbreak across the Midwest, the demand grows rapidly.

An overturned car rests on top of tree branches and other rubble near the destroyed home of Curt Zehr, about a mile northeast of Washington, Ill. (AP Photo/David Mercer)

“There are layers of need that start to happen as people are displaced from their homes or are not able to afford or get to grocery stores. Our network steps up to serve that need that develops related to the storm, over and above the normal amount of food we provide to people in need. Depending on where you are, there will be a significant increase in demand for communities affected by storms,” Arthur said.

Arthur emphasized that the contributions and volunteer efforts from the general public are a huge part of what the food banks are able to accomplish. He said that without volunteers’ time or food and monetary donations, they would not be able to operate.

While natural disasters and holidays may inspire some of the more publicized food drives, there are people in need in every community year round.

•Visit FeedAmerica.org to find a food bank location near you.
•Find out if you are eligible for assistance.
•Find ways to volunteer to help in your community.
•Organize your own food drive.
•Donate in-demand food staples, such as peanut butter, stews and baby food. For the holidays, frozen turkeys are especially sought out.
•To help abroad, donate to reputable organizations, such as WFP or the Red Cross.
•Buy sustainable, fair-trade imports to help improve livelihood opportunities for people around

EPA may have underestimated U.S. methane emissions by 50 percent

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U.S. emissions of methane — a potent greenhouse gas — could be significantly higher than indicated in estimates by the US Environmental Protection Agency, according to a new study published Monday.

The study found the EPA numbers could underestimate by as much as 50 percent the true amount of the gas being produced by the United States.

The most striking discrepancy, the researchers said, was in the oil-producing south-central United States, where their results were nearly three times higher than EPA estimates.

“It will be important to resolve that discrepancy in order to fully understand the impact of these industries on methane emissions,” said lead author Scot Miller, a doctoral student at Harvard University.

Methane is produced by livestock, landfills, coal mining, and natural gas production and distribution, among other natural and man-made activities, the authors explained, adding that humans are thought to contribute around 60 percent of the total.

The researchers explained their figures differ from the government ones because of a difference in methodology.

The EPA, they explained, uses a “bottom-up” approach that multiplies amounts typically released, for example, by each cow, per unit of coal, or per unit of natural gas sold.

But in this new study, researchers took the opposite “top-down” approach, calculating how much methane is actually present in the atmosphere and then tracing it to its sources using meteorological and statistical analysis.

“When we measure methane gas at the atmospheric level, we’re seeing the cumulative effect of emissions that are happening at the surface across a very large region,” said Steven Wofsy, a Harvard professor and co-author of the PNAS study.

“That includes the sources that were part of the bottom-up inventories, but maybe also things they didn’t think to measure,” he explained.

For the analysis, the researchers used observational data from 2007-2008, when the US sharply increased its natural gas production, and compared it with the EPA figures from the same period.

They intend to repeat the analysis using present-day data.

“Now that we know the total does not equal the sum of the parts, that means that either some of those parts are not what we thought they were, or there are some parts that are simply missing from the inventories,” said co-author Anna Michalak of the Carnegie Institution for Science.

“It really offers an opportunity for governments to reexamine the inventories in light of what we now know.”

Methane is the second-most important greenhouse gas after carbon dioxide, emphasized the researchers, who also hailed from the University of Michigan, NOAA’s Earth System Research Laboratory, Lawrence Berkeley National Laboratory, Atmospheric and Environmental Research, the European Commission Joint Research Centre in Italy, and the University of Colorado Boulder.

It traps 70 times more heat than CO2 in the atmosphere, but it only lasts 10 years in the atmosphere, compared to 100 years for carbon dioxide.

The study was published in the Proceedings of the National Academy of Sciences.

[Image via Agence France-Presse]

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