Global Sea Level Rise Dampened by Australia Floods

 When enough raindrops fall over land instead of the ocean, they begin to add up. New research led by the National Center for Atmospheric Research (NCAR) shows that when three atmospheric patterns came together over the Indian and Pacific oceans, they drove so much precipitation over Australia in 2010 and 2011 that the world's ocean levels dropped measurably. Unlike other continents, the soils and topography of Australia prevent almost all of its precipitation from running off into the ocean.

The 2010-11 event temporarily halted a long-term trend of rising sea levels caused by higher temperatures and melting ice sheets.

Now that the atmospheric patterns have snapped back and more rain is falling over tropical oceans, the seas are rising again. In fact, with Australia in a major drought, they are rising faster than before.

"It's a beautiful illustration of how complicated our climate system is," says NCAR scientist John Fasullo, the lead author of the study. "The smallest continent in the world can affect sea level worldwide. Its influence is so strong that it can temporarily overcome the background trend of rising sea levels that we see with climate change."

The study, with co-authors from NASA's Jet Propulsion Laboratory and the University of Colorado at Boulder, will be published next month in Geophysical Research Letters. It was funded by the National Science Foundation, which is NCAR's sponsor, and by NASA.

Consistent rising, interrupted

As the climate warms, the world's oceans have been rising in recent decades by just more than 3 millimeters (0.1 inches) annually. This is partly because the heat causes water to expand, and partly because runoff from retreating glaciers and ice sheets is making its way into the oceans.

But for an 18-month period beginning in 2010, the oceans mysteriously dropped by about 7 millimeters (about 0.3 inches), more than offsetting the annual rise.

Fasullo and his co-authors published research last year demonstrating that the reason had to do with the increased rainfall over tropical continents. They also showed that the drop coincided with the atmospheric oscillation known as La Niña, which cooled tropical surface waters in the eastern Pacific and suppressed rainfall there while enhancing it over portions of the tropical Pacific, Africa, South America, and Australia.

But an analysis of the historical record showed that past La Niña events only rarely accompanied such a pronounced drop in sea level.

Using a combination of satellite instruments and other tools, the new study finds that the picture in 2010-11 was uniquely complex. A rare combination of two other semi-cyclic climate modes came together to drive such large amounts of rain over Australia that the continent, on average, received almost one foot (300 millimeters) of rain more than average.

The initial effects of La Niña were to cool surface waters in the eastern Pacific Ocean and push moisture to the west. A climate pattern known as the Southern Annular Mode then coaxed the moisture into Australia's interior, causing widespread flooding across the continent. Later in the event, high levels of moisture from the Indian Ocean driven by the Indian Ocean Dipole collided with La Niña-borne moisture in the Pacific and pushed even more moisture into the continent's interior. Together, these influences spurred one of the wettest periods in Australia's recorded history.

Australia's vast interior, called the Outback, is ringed by coastal mountains and often quite dry. Because of the low-lying nature of the continent's eastern interior and the lack of river runoff in its western dry environment, most of the heavy rainfall of 2010-11 remained inland rather than flowing into the oceans. While some of it evaporated in the desert sun, much of it sank into the dry, granular soil of the Western Plateau or filled the Lake Eyre basin in the east.

"No other continent has this combination of atmospheric set-up and topography," Fasullo says. "Only in Australia could the atmosphere carry such heavy tropical rains to such a large area, only to have those rains fail to make their way to the ocean."

Measuring the difference

To conduct the research, the scientists turned to three cutting-edge observing instrument systems:

• NASA's Gravity Recovery and Climate Experiment (GRACE) satellites, which make detailed measurements of Earth's gravity field. The satellites enable scientists to monitor changes in the mass of continents.

• The Argo global array of 3,000 free-drifting floats that measure the temperature and salinity of the upper 6,000 feet of the world's oceans.

• Satellite-based altimeters that are continuously calibrated against a network of tide gauges. Scientists subtract seasonal and other variations to closely estimate global sea level changes.

Using these instruments, the researchers found that the mass in Australia and, to a lesser extent, South America began to increase in 2010 as the continents experienced heavy and persistent rain. At the same time, sea levels began to measurably drop.

Since 2011, when the atmospheric patterns shifted out of their unusual combination, sea levels have been rising at a faster pace of about 10 millimeters (0.4 inches) per year.

Scientists are uncertain how often the three atmospheric events come together to cause such heavy rains over Australia. Fasullo believes there may have been a similar event in 1973-74, which was another time of record flooding in that continent. But modern observing instruments did not exist then, making it impossible to determine what took place in the atmosphere and whether it affected sea level rise.

"Luckily we've got great observations now," Fasullo says. "We need to maintain these observing platforms to understand what is a complicated climate system."

The University Corporation for Atmospheric Research (UCAR) operates NCAR under primary sponsorship by the National Science Foundation. Opinions, findings, conclusions, or recommendations expressed in this release do not necessarily reflect the views of the National Science Foundation.

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Newly Discovered Ocean Plume Could Be Major Source of Iron

Scientists have discovered a vast plume of iron and other micronutrients more than 1,000 km long billowing from hydrothermal vents in the South Atlantic Ocean. The finding, soon to be published in the journal Nature Geoscience, calls past estimates of iron abundances into question, and may challenge researchers' assumptions about iron sources in the world's seas.

"This study and other studies like it are going to force the scientific community to reevaluate how much iron is really being contributed by hydrothermal vents and to increase those estimates, and that has implications for not only iron geochemistry but a number of other disciplines as well," says Mak Saito, a WHOI associate scientist and lead author of the study.

Saito and his team of collaborators -- which includes WHOI researchers and a colleague affiliated with the University of Liverpool (U.K.) -- didn't set out to find iron plumes in the South Atlantic. They set sail aboard the R/V Knorr in 2007 as part of the Cobalt, Iron and Micro-organisms from the Upwelling zone to the Gyre (or CoFeMUG, pronounced "coffee mug") expedition, which intended to map chemical composition and microbial life along the ship's route between Brazil and Namibia. As the scientists traveled the route, they sampled the seawater at frequent intervals and multiple depths along the way, and then stored the samples for in-depth analysis back on land.

Their route passed over the Mid-Atlantic Ridge, a band of mountains and valleys running along the Atlantic Ocean floor from the Arctic to the Antarctic where several of Earth's major tectonic plates are slowly spreading apart. Hydrothermal vents, or fissures in Earth's crust, are found along the ridge, but they haven't been extensively studied because slow-spreading ridges are thought to be less active than fast-spreading ones. Past studies using helium, which is released from Earth's mantle through hydrothermal vents and is routinely used as an indicator of vent activity, have found little coming from mid-Atlantic vents, and researchers have assumed that means the vents spew little iron as well.

So Saito and his colleagues were surprised by what their samples revealed when later studied in the lab. Once filtered and analyzed, some of the seawater showed unexpectedly high levels of iron and manganese. When Abigail Noble, then a WHOI graduate student, and Saito plotted the sites where the iron-rich samples were taken, they realized the samples formed a distinct plume -- a cloud of nutrients ranging in depth from 1,500 to 3,500 meters that spanned more than 1,000 km of the South Atlantic Ocean.

"We had never seen anything like it," Saito says. "We were sort of shocked -- there's this huge bull's-eye right in the middle of the South Atlantic Ocean. We didn't quite know what to do with it, because it went contrary to a lot of our expectations."

The plume's ratio of iron to helium was 80-fold higher than ratios reported for faster-spreading ridges in the southeastern Pacific Ocean.

The serendipitous discovery casts doubt on the assumption that slow-spreading ridges are iron-poor, and it raises questions about the use of helium as an indicator for iron flux in hydrothermal vents, Saito says.

"We've assumed that low helium means low iron, and our study finds that that's not true," Saito says. "There's actually quite a lot of iron coming out of these slow-spreading regions in the Atlantic, where people thought there would be little to none."

And that has profound implications, because iron is a critical element for ocean life. Iron is known to spur the growth of phytoplankton in many marine habitats, especially those important in the ocean's carbon cycle, which, in turn, impacts atmospheric carbon dioxide levels and Earth's climate. Because more than half the world's seafloor ridges are slow-spreading, the team's discovery suggests there may be far more iron from these locations than previously estimated.

"We need to understand where iron is in the ocean and where it's coming from to understand the role of iron in the marine carbon cycle with any confidence," Saito says.

Saito and his colleagues hope future studies will reveal the exact shape and extent of the plume, and just how much of its iron and other micronutrients persist and rise to the surface. Answering these lingering questions will help researchers truly understand how hydrothermal vents affect the ocean as a whole, Saito says.

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Why an Ice Age Occurs Every 100,000 Years: Climate and Feedback Effects Explained

Science has struggled to explain fully why an ice age occurs every 100,000 years. As researchers now demonstrate based on a computer simulation, not only do variations in insolation play a key role, but also the mutual influence of glaciated continents and climate.

Ice ages and warm periods have alternated fairly regularly in Earth's history: Earth's climate cools roughly every 100,000 years, with vast areas of North America, Europe and Asia being buried under thick ice sheets. Eventually, the pendulum swings back: it gets warmer and the ice masses melt. While geologists and climate physicists found solid evidence of this 100,000-year cycle in glacial moraines, marine sediments and arctic ice, until now they were unable to find a plausible explanation for it.

Using computer simulations, a Japanese, Swiss and American team including Heinz Blatter, an emeritus professor of physical climatology at ETH Zurich, has now managed to demonstrate that the ice-age/warm-period interchange depends heavily on the alternating influence of continental ice sheets and climate.

"If an entire continent is covered in a layer of ice that is 2,000 to 3,000 metres thick, the topography is completely different," says Blatter, explaining this feedback effect. "This and the different albedo of glacial ice compared to ice-free earth lead to considerable changes in the surface temperature and the air circulation in the atmosphere." Moreover, large-scale glaciation also alters the sea level and therefore the ocean currents, which also affects the climate.

Weak effect with a strong impact

As the scientists from Tokyo University, ETH Zurich and Columbia University demonstrated in their paper published in the journal Nature, these feedback effects between Earth and the climate occur on top of other known mechanisms. It has long been clear that the climate is greatly influenced by insolation on long-term time scales. Because Earth's rotation and its orbit around the sun periodically change slightly, the insolation also varies. If you examine this variation in detail, different overlapping cycles of around 20,000, 40,000 and 100,000 years are recognisable.

Given the fact that the 100,000-year insolation cycle is comparatively weak, scientists could not easily explain the prominent 100,000-year-cycle of the ice ages with this information alone. With the aid of the feedback effects, however, this is now possible.

Simulating the ice and climate

The researchers obtained their results from a comprehensive computer model, where they combined an ice-sheet simulation with an existing climate model, which enabled them to calculate the glaciation of the northern hemisphere for the last 400,000 years. The model not only takes the astronomical parameter values, ground topography and the physical flow properties of glacial ice into account but also especially the climate and feedback effects. "It's the first time that the glaciation of the entire northern hemisphere has been simulated with a climate model that includes all the major aspects," says Blatter.

Using the model, the researchers were also able to explain why ice ages always begin slowly and end relatively quickly. The ice-age ice masses accumulate over tens of thousands of years and recede within the space of a few thousand years. Now we know why: it is not only the surface temperature and precipitation that determine whether an ice sheet grows or shrinks. Due to the aforementioned feedback effects, its fate also depends on its size. "The larger the ice sheet, the colder the climate has to be to preserve it," says Blatter. In the case of smaller continental ice sheets that are still forming, periods with a warmer climate are less likely to melt them. It is a different story with a large ice sheet that stretches into lower geographic latitudes: a comparatively brief warm spell of a few thousand years can be enough to cause an ice sheet to melt and herald the end of an ice age.

The Milankovitch cycles

The explanation for the cyclical alternation of ice and warm periods stems from Serbian mathematician Milutin Milankovitch (1879-1958), who calculated the changes in Earth's orbit and the resulting insolation on Earth, thus becoming the first to describe that the cyclical changes in insolation are the result of an overlapping of a whole series of cycles: the tilt of Earth's axis fluctuates by around two degrees in a 41,000-year cycle. Moreover, Earth's axis gyrates in a cycle of 26,000 years, much like a spinning top. Finally, Earth's elliptical orbit around the sun changes in a cycle of around 100,000 years in two respects: on the one hand, it changes from a weaker elliptical (circular) form into a stronger one. On the other hand, the axis of this ellipsis turns in the plane of Earth's orbit. The spinning of Earth's axis and the elliptical rotation of the axes cause the day on which Earth is closest to the sun (perihelion) to migrate through the calendar year in a cycle of around 20,000 years: currently, it is at the beginning of January; in around 10,000 years, however, it will be at the beginning of July.

Based on his calculations, in 1941 Milankovitch postulated that insolation in the summer characterises the ice and warm periods at sixty-five degrees north, a theory that was rejected by the science community during his lifetime. From the 1970s, however, it gradually became clearer that it essentially coincides with the climate archives in marine sediments and ice cores. Nowadays, Milankovitch's theory is widely accepted. "Milankovitch's idea that insolation determines the ice ages was right in principle," says Blatter. "However, science soon recognised that additional feedback effects in the climate system were necessary to explain ice ages. We are now able to name and identify these effects accurately."

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Earth's Core Affects Length of Day

Researchers studied the variations and fluctuations in the length of day over a one to 10 year period between 1962 and 2012

Research at the University of Liverpool has found that variations in the length of day over periods of between one and 10 years are caused by processes in Earth's core.
Earth rotates once per day, but the length of this day varies. A year, 300million years ago, lasted about 450 days and a day would last about 21 hours.
Length of day increases
As a result of the slowing down of Earth's rotation the length of day has increased.
The rotation of Earth on its axis, however, is affected by a number of other factors -- for example, the force of the wind against mountain ranges changes the length of the day by plus or minus a millisecond over a period of a year.
Professor Richard Holme, from the School of Environmental Sciences, studied the variations and fluctuations in the length of day over a one to 10 year period between 1962 and 2012. The study took account of the effects on Earth's rotation of atmospheric and oceanic processes to produce a model of the variations in the length of day on time scales longer than a year.
Professor Holme said: "The model shows well-known variations on decadal time scales, but importantly resolves changes over periods between one and 10 years.
"Previously these changes were poorly characterised; the study shows they can be explained by just two key signals, a steady 5.9 year oscillation and episodic jumps which occur at the same time as abrupt changes in the Earth's magnetic field, generated in the Earth's core.
He added: "This study changes fundamentally our understanding of short-period dynamics of the Earth's fluid core. It leads us to conclude that the Earth's lower mantle, which sits above the Earth's outer core, is a poor conductor of electricity giving us new insight into the chemistry and mineralogy of the Earth's deep interior."
The research was conducted in partnership with the Université Paris Diderot and is published in Nature.
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