Northern Hemisphere

Global ocean currents explain why Northern Hemisphere is the soggier one.

Posted by

0


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.

Global ocean currents explain why Northern Hemisphere is the soggier one

The findings, published Oct. 20 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  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  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  have been the accepted reason for the asymmetry in  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 ,” Frierson said. “The future will depend on air pollution and global warming, as well as  changes. That makes  particularly hard to predict.”



Global Average Temperatures Are Close to 11,000-Year Peak.

Posted by

0


global-average-temperatures-are-close-to-11000-year-peak_1

Global average temperatures are now higher than they have been for about 75% of the past 11,300 years, a study suggests. And if climate models are any indication, by the end of this century they will be the highest ever since the end of the most recent ice age.

Instrumental records of climate extend back to only the late nineteenth century. Beyond that, scientists depend on analyses of natural chronicles such as tree rings and isotope ratios in cave formations.

But even these archives have their limits: many detailed reconstructions of climate, particularly of temperature, apply to only limited regions or extend back at most a couple of millennia, says Shaun Marcott, a climate scientist at Oregon State University in Corvallis.

Marcott and his colleagues set about reconstructing global climate trends all the way back to 11,300 years ago, when the Northern Hemisphere was emerging from the most recent ice age. To do so, they collected and analyzed data gathered by other teams. The 73 overlapping climate records that they considered included sediment cores drilled from lake bottoms and sea floors around the world, along with a handful of ice cores collected in Antarctica and Greenland.

Each of these chronicles spanned at least 6,500 years, and each included a millennium-long baseline period beginning in the middle of the post-ice-age period at 3550 BC.

For some records, the researchers inferred past temperatures from the ratio of magnesium and calcium ions in the shells of microscopic creatures that had died and dropped to the ocean floor; for others, they measured the lengths of long-chain organic molecules called alkenones that were trapped in the sediments.

After the ice age, they found, global average temperatures rose until they reached a plateau between 7550 and 3550 BC. Then a long-term cooling trend set in, reaching its lowest temperature extreme between ad 1450 and 1850.

Since then, temperatures have been increasing at a dramatic clip: from the first decade of the twentieth century to now, global average temperatures rose from near their coldest point since the ice age to nearly their warmest, Marcott and his team report today in Science.

Climate context
The temperature trends that the team identified for the past 2,000 years are statistically indistinguishable from results obtained by other researchers in a previous study, says Marcott. “That gives us confidence that the rest of our record is right too,” he adds.

Marcott and his colleagues “have put together a pretty impressive set of climate proxies”, says Gavin Schmidt, a climate scientist at the NASA Goddard Institute for Space Studies in New York. “The overall climate picture has been clear for a long time, mostly from the Northern Hemisphere, but this compilation really puts the rest of the world in context,” he adds.

“Prior to this study, researchers could only guess whether global temperatures had exceeded the warmest part of the present interglacial period,” says Darrell Kaufman, a geologist at Northern Arizona University in Flagstaff. The latest findings show that the recent high temperatures are not necessarily the warmest, but they are unusually high, he notes.

The temperature trends during most of the post-ice-age period match those expected from natural factors such as the long-term variation in the tilt of Earth’s axis, says Marcott. But in the past century and a half, industrial emissions of the greenhouse gas carbon dioxide have increased — which helps to explain why global temperatures have risen so quickly in recent decades, he suggests.

Climate models from the Intergovernmental Panel on Climate Change suggest that by the end of this century, regardless of future carbon dioxide emissions, temperatures will be at their highest since the end of the most recent ice age, the researchers say.

Source: Scientific American.

Ozone Hole and The Global Climate Changes.

Posted by

0


The release of CFCs or chlorofluorocarbons into the atmosphere through human activities has caused a massive hole in the ozone layer right above Antarctica and if unchecked, melting icecaps may inundate several regions of the earth in the future.

What is Ozone and Where Is It Found in The Earth’s Atmosphere?

Ozone is a gas with a pungent odor whose molecule contains three oxygen atoms. At about 6–10 miles above the Earth’s surface and extending up to 30 miles, in a region of space called the stratosphere, you will find 90% ozone. The stratospheric region with the highest ozone concentration is commonly known as the “ozone layer”. The remaining ozone, about 10%, is found in the troposphere, which is the lowest region of the atmosphere, between Earth’s surface and the stratosphere.

Ozone at ground level in the troposphere is bad because it causes photochemical smog. The smog results when ultra-violet light falls on and reacts with nitrogen oxide from vehicle exhausts. Because of this, Ozone affects lung function, aggravates asthma and other chronic respiratory diseases.

On the other hand, ozone in the stratosphere performs a very useful function by acting as a blanket that blocks most of the sun’s high-frequency ultraviolet rays. These UV rays can cause skin cancer and cataracts in humans, as well as reproductive problems in several forms of life including even the single-celled phytoplankton at the bottom of the ocean food chain.

How Does Ozone Form in the Atmosphere?

When ultraviolet light strikes oxygen molecules containing two oxygen atoms (O2), it splits them into individual oxygen atoms (atomic oxygen), which then combines with unbroken O2 to create ozone, O3. Being unstable, this ozone once again splits into a molecule of O2 and an atom of atomic oxygen under the action of ultraviolet light. This continuing process called the ozone-oxygen cycle.

The ozone layer  is very effective at screening out UV-B; Nevertheless, some UV-B, particularly at its longest wavelengths, reaches the surface. Ozone cannot stop UV-A, the longer wavelength ultraviolet radiation which  reaches the earth’s surface. However, this type of UV radiation is significantly less harmful to DNA.

The thickness of the ozone layer varies widely throughout the world, being smaller near the equator and larger towards the poles. It also varies with season, being in general thicker during the spring and thinner during the autumn in the northern hemisphere.

Ozone ‘Hole’

In May 1985 scientists with the British Antarctic Survey announced the discovery of a huge hole in the ozone layer over Antarctica. They announced that Ozone levels over the northern hemisphere had been dropping by 4% per decade. They described the larger seasonal drops in the ozone levels around the south pole as a ozone hole.

The ozone hole is not technically a “hole” with no ozone is present, but is actually a region of exceptionally depleted ozone in the stratosphere over the Antarctic during the Southern Hemisphere spring (August–October).

Stratospheric temperatures in the Northern Hemisphere during winter/spring are generally slightly warmer than those in the Southern Hemisphere. Therefore ozone losses over the Arctic have been much smaller than over the Antarctic during the 1980s and early 1990s. However, the Arctic stratosphere has gradually cooled over the past few decades, and Ozone holes have been observed at the Arctic regions too recently. This is a dangerous trend, because unlike the Southern Polar hemisphere, the Northern Polar hemisphere is well populated.

Ozone hole is caused by chemicals called CFCs, or chlorofluorocarbons. CFCs escape into the atmosphere from refrigeration and propellant devices and processes. In the lower atmosphere, they are so stable that they persist for decades. Eventually, some of the CFCs reach the stratosphere where chemical reactions take place primarily on the surface of polar stratospheric clouds, ice particles, or liquid droplets, which form at high altitudes in the extreme cold of the polar regions. Ultraviolet light breaks the bond holding chlorine atoms (Cl) to the CFC molecule. Chlorine then destroys ozone molecules by “stealing” their oxygen atoms. The breakdown of ozone in the stratosphere makes it unable to absorb ultraviolet radiation. Consequently, the unabsorbed ultraviolet-B radiation is able to reach the Earth’s surface. The extent of ozone destruction is extremely sensitive to small changes in stratospheric temperature.

Another culprit responsible for the ozone depletion is nitrous oxide (N2O). The major sources of nitrous oxide are industrial processes and combustion engines of various vehicles. They are also emitted from livestock manure and sewage. Like CFCs, Nitrous oxide  is stable when emitted at ground level, but breaks down when it reaches the stratosphere to form nitrogen oxides that trigger ozone-destroying reactions.

In 1987 several UN countries gathered at Montreal, Canada, and signed a treaty to protect the stratospheric ozone layer. The Montreal Protocol stipulated that the production and consumption of compounds that deplete ozone in the stratosphere—chlorofluorocarbons, halons, carbon tetrachloride, and methyl chloroform—are to be phased out by 2005.

Chemical manufacturers soon created substitutes for CFCs with little added costs; thus, our life styles remained greatly unaffected by the switch-over from CFC’s. This has had the effect of putting a slow stopper on the Ozone hole.

Now, the issue of a possible connection between ozone hole and global warming is a controversial subject even among scientists. In fact, there is no unanimity in either of the assertions that Antarctica is warming or cooling. The British Antarctic Survey says categorically Antarctica to be both warming around the edges and cooling at the center at the same time. Thus it is not possible to say whether it is warming or cooling overall. Because there are too many parameters governing the global temperatures, it is difficult to correlate the theoretical temperature rise at the Antarctic caused by a thinner ozone layer with global climatic changes. It is useful to remember here that Ozone itself is a greenhouse gas and its thinning over the region reduces heat trapped over it and helps create sea spray that forms reflective, cooling clouds.

Source: http://scienceray.com