Ancient shellfish remains rewrite 10,000-year history of El Nino cycles

The middens are ancient dumping sites that typically contain a mix of mollusk shells, fish and bird bones, ceramics, cloth, charcoal, maize and other plants. -  M. Carré / Univ. of Montpellier
The middens are ancient dumping sites that typically contain a mix of mollusk shells, fish and bird bones, ceramics, cloth, charcoal, maize and other plants. – M. Carré / Univ. of Montpellier

The planet’s largest and most powerful driver of climate changes from one year to the next, the El Niño Southern Oscillation in the tropical Pacific Ocean, was widely thought to have been weaker in ancient times because of a different configuration of the Earth’s orbit. But scientists analyzing 25-foot piles of ancient shells have found that the El Niños 10,000 years ago were as strong and frequent as the ones we experience today.

The results, from the University of Washington and University of Montpellier, question how well computer models can reproduce historical El Niño cycles, or predict how they could change under future climates. The paper is now online and will appear in an upcoming issue of Science.

“We thought we understood what influences the El Niño mode of climate variation, and we’ve been able to show that we actually don’t understand it very well,” said Julian Sachs, a UW professor of oceanography.

The ancient shellfish feasts also upend a widely held interpretation of past climate.

“Our data contradicts the hypothesis that El Niño activity was very reduced 10,000 years ago, and then slowly increased since then,” said first author Matthieu Carré, who did the research as a UW postdoctoral researcher and now holds a faculty position at the University of Montpellier in France.

In 2007, while at the UW-based Joint Institute for the Study of the Atmosphere and Ocean, Carré accompanied archaeologists to seven sites in coastal Peru. Together they sampled 25-foot-tall piles of shells from Mesodesma donacium clams eaten and then discarded over centuries into piles that archaeologists call middens.

While in graduate school, Carré had developed a technique to analyze shell layers to get ocean temperatures, using carbon dating of charcoal from fires to get the year, and the ratio of oxygen isotopes in the growth layers to get the water temperatures as the shell was forming.

The shells provide 1- to 3-year-long records of monthly temperature of the Pacific Ocean along the coast of Peru. Combining layers of shells from each site gives water temperatures for intervals spanning 100 to 1,000 years during the past 10,000 years.

The new record shows that 10,000 years ago the El Niño cycles were strong, contradicting the current leading interpretations. Roughly 7,000 years ago the shells show a shift to the central Pacific of the most severe El Niño impacts, followed by a lull in the strength and occurrence of El Niño from about 6,000 to 4,000 years ago.

One possible explanation for the surprising finding of a strong El Niño 10,000 years ago was that some other factor was compensating for the dampening effect expected from cyclical changes in Earth’s orbit around the sun during that period.

“The best candidate is the polar ice sheet, which was melting very fast in this period and may have increased El Niño activity by changing ocean currents,” Carré said.

Around 6,000 years ago most of the ice age floes would have finished melting, so the effect of Earth’s orbital geometry might have taken over then to cause the period of weak El Niños.

In previous studies, warm-water shells and evidence of flooding in Andean lakes had been interpreted as signs of a much weaker El Niño around 10,000 years ago.

The new data is more reliable, Carré said, for three reasons: the Peruvian coast is strongly affected by El Niño; the shells record ocean temperature, which is the most important parameter for the El Niño cycles; and the ability to record seasonal changes, the timescale at which El Niño can be observed.

“Climate models and a variety of datasets had concluded that El Niños were essentially nonexistent, did not occur, before 6,000 to 8,000 years ago,” Sachs said. “Our results very clearly show that this is not the case, and suggest that current understanding of the El Niño system is incomplete.

Warm US West, cold East: A 4,000-year pattern

<IMG SRC="/Images/485889256.jpg" WIDTH="350" HEIGHT="262" BORDER="0" ALT="University of Utah geochemist Gabe Bowen led a new study, published in Nature Communications, showing that the curvy jet stream pattern that brought mild weather to western North America and intense cold to the eastern states this past winter has become more dominant during the past 4,000 years than it was from 8,000 to 4,000 years ago. The study suggests global warming may aggravate the pattern, meaning such severe winter weather extremes may be worse in the future. – Lee J. Siegel, University of Utah.”>
University of Utah geochemist Gabe Bowen led a new study, published in Nature Communications, showing that the curvy jet stream pattern that brought mild weather to western North America and intense cold to the eastern states this past winter has become more dominant during the past 4,000 years than it was from 8,000 to 4,000 years ago. The study suggests global warming may aggravate the pattern, meaning such severe winter weather extremes may be worse in the future. – Lee J. Siegel, University of Utah.

Last winter’s curvy jet stream pattern brought mild temperatures to western North America and harsh cold to the East. A University of Utah-led study shows that pattern became more pronounced 4,000 years ago, and suggests it may worsen as Earth’s climate warms.

“If this trend continues, it could contribute to more extreme winter weather events in North America, as experienced this year with warm conditions in California and Alaska and intrusion of cold Arctic air across the eastern USA,” says geochemist Gabe Bowen, senior author of the study.

The study was published online April 16 by the journal Nature Communications.

“A sinuous or curvy winter jet stream means unusual warmth in the West, drought conditions in part of the West, and abnormally cold winters in the East and Southeast,” adds Bowen, an associate professor of geology and geophysics at the University of Utah. “We saw a good example of extreme wintertime climate that largely fit that pattern this past winter,” although in the typical pattern California often is wetter.

It is not new for scientists to forecast that the current warming of Earth’s climate due to carbon dioxide, methane and other “greenhouse” gases already has led to increased weather extremes and will continue to do so.

The new study shows the jet stream pattern that brings North American wintertime weather extremes is millennia old – “a longstanding and persistent pattern of climate variability,” Bowen says. Yet it also suggests global warming may enhance the pattern so there will be more frequent or more severe winter weather extremes or both.

“This is one more reason why we may have more winter extremes in North America, as well as something of a model for what those extremes may look like,” Bowen says. Human-caused climate change is reducing equator-to-pole temperature differences; the atmosphere is warming more at the poles than at the equator. Based on what happened in past millennia, that could make a curvy jet stream even more frequent and-or intense than it is now, he says.

Bowen and his co-authors analyzed previously published data on oxygen isotope ratios in lake sediment cores and cave deposits from sites in the eastern and western United States and Canada. Those isotopes were deposited in ancient rainfall and incorporated into calcium carbonate. They reveal jet stream directions during the past 8,000 years, a geological time known as middle and late stages of the Holocene Epoch.

Next, the researchers did computer modeling or simulations of jet stream patterns – both curvy and more direct west to east – to show how changes in those patterns can explain changes in the isotope ratios left by rainfall in the old lake and cave deposits.

They found that the jet stream pattern – known technically as the Pacific North American teleconnection – shifted to a generally more “positive phase” – meaning a curvy jet stream – over a 500-year period starting about 4,000 years ago. In addition to this millennial-scale change in jet stream patterns, they also noted a cycle in which increases in the sun’s intensity every 200 years make the jet stream flatter.

Bowen conducted the study with Zhongfang Liu of Tianjin Normal University in China, Kei Yoshimura of the University of Tokyo, Nikolaus Buenning of the University of Southern California, Camille Risi of the French National Center for Scientific Research, Jeffrey Welker of the University of Alaska at Anchorage, and Fasong Yuan of Cleveland State University.

The study was funded by the National Science Foundation, National Natural Science Foundation of China, Japan Society for the Promotion of Science and a joint program by the society and Japan’s Ministry of Education, Culture, Sports, Science and Technology: the Program for Risk Information on Climate Change.

Sinuous Jet Stream Brings Winter Weather Extremes

The Pacific North American teleconnection, or PNA, “is a pattern of climate variability” with positive and negative phases, Bowen says.

“In periods of positive PNA, the jet stream is very sinuous. As it comes in from Hawaii and the Pacific, it tends to rocket up past British Columbia to the Yukon and Alaska, and then it plunges down over the Canadian plains and into the eastern United States. The main effect in terms of weather is that we tend to have cold winter weather throughout most of the eastern U.S. You have a freight car of arctic air that pushes down there.”

Bowen says that when the jet stream is curvy, “the West tends to have mild, relatively warm winters, and Pacific storms tend to occur farther north. So in Northern California, the Pacific Northwest and parts of western interior, it tends to be relatively dry, but tends to be quite wet and unusually warm in northwest Canada and Alaska.”

This past winter, there were times of a strongly curving jet stream, and times when the Pacific North American teleconnection was in its negative phase, which means “the jet stream is flat, mostly west-to-east oriented,” and sometimes split, Bowen says. In years when the jet stream pattern is more flat than curvy, “we tend to have strong storms in Northern California and Oregon. That moisture makes it into the western interior. The eastern U.S. is not affected by arctic air, so it tends to have milder winter temperatures.”

The jet stream pattern – whether curvy or flat – has its greatest effects in winter and less impact on summer weather, Bowen says. The curvy pattern is enhanced by another climate phenomenon, the El Nino-Southern Oscillation, which sends a pool of warm water eastward to the eastern Pacific and affects climate worldwide.

Traces of Ancient Rains Reveal Which Way the Wind Blew

Over the millennia, oxygen in ancient rain water was incorporated into calcium carbonate deposited in cave and lake sediments. The ratio of rare, heavy oxygen-18 to the common isotope oxygen-16 in the calcium carbonate tells geochemists whether clouds that carried the rain were moving generally north or south during a given time.

Previous research determined the dates and oxygen isotope ratios for sediments in the new study, allowing Bowen and colleagues to use the ratios to tell if the jet stream was curvy or flat at various times during the past 8,000 years.

Bowen says air flowing over the Pacific picks up water from the ocean. As a curvy jet stream carries clouds north toward Alaska, the air cools and some of the water falls out as rain, with greater proportions of heavier oxygen-18 falling, thus raising the oxygen-18-to-16 ratio in rain and certain sediments in western North America. Then the jet stream curves south over the middle of the continent, and the water vapor, already depleted in oxygen-18, falls in the East as rain with lower oxygen-18-to-16 ratios.

When the jet stream is flat and moving east-to-west, oxygen-18 in rain is still elevated in the West and depleted in the East, but the difference is much less than when the jet stream is curvy.

By examining oxygen isotope ratios in lake and cave sediments in the West and East, Bowen and colleagues showed that a flatter jet stream pattern prevailed from about 8,000 to 4,000 years ago in North America, but then, over only 500 years, the pattern shifted so that curvy jet streams became more frequent or severe or both. The method can’t distinguish frequency from severity.

The new study is based mainly on isotope ratios at Buckeye Creek Cave, W. Va.; Lake Grinell, N.J.; Oregon Caves National Monument; and Lake Jellybean, Yukon.

Additional data supporting increasing curviness of the jet stream over recent millennia came from seven other sites: Crawford Lake, Ontario; Castor Lake, Wash.; Little Salt Spring, Fla.; Estancia Lake, N.M.; Crevice Lake, Mont.; and Dog and Felker lakes, British Columbia. Some sites provided oxygen isotope data; others showed changes in weather patterns based on tree ring growth or spring deposits.

Simulating the Jet Stream

As a test of what the cave and lake sediments revealed, Bowen’s team did computer simulations of climate using software that takes isotopes into account.

Simulations of climate and oxygen isotope changes in the Middle Holocene and today resemble, respectively, today’s flat and curvy jet stream patterns, supporting the switch toward increasing jet stream sinuosity 4,000 years ago.

Why did the trend start then?

“It was a when seasonality becomes weaker,” Bowen says. The Northern Hemisphere was closer to the sun during the summer 8,000 years ago than it was 4,000 years ago or is now due to a 20,000-year cycle in Earth’s orbit. He envisions a tipping point 4,000 years ago when weakening summer sunlight reduced the equator-to-pole temperature difference and, along with an intensifying El Nino climate pattern, pushed the jet stream toward greater curviness.

Ancient Indonesian climate shift linked to glacial cycle

Using sediments from a remote lake, researchers from Brown University have assembled a 60,000-year record of rainfall in central Indonesia. The analysis reveals important new details about the climate history of a region that wields a substantial influence on the global climate as a whole.

The Indonesian archipelago sits in the Indo-Pacific Warm Pool, an expanse of ocean that supplies a sizable fraction of the water vapor in Earth’s atmosphere and plays a role in propagating El Niño cycles. Despite the region’s importance in the global climate system, not much is known about its own climate history, says James Russell, associate professor of geological sciences at Brown.

“We wanted to assess long-term climate variation in the region,” Russell said, “not just to assess how global climate influences Indonesia, but to see how that feeds back into the global climate system.”

The data are published this week in the Proceedings of the National Academy of Sciences.

The study found that the region’s normally wet, tropical climate was interrupted by a severe dry period from around 33,000 years ago until about 16,000 years ago. That period coincides with peak of the last ice age, when glaciers covered vast swaths of the northern hemisphere. Climate models had suggested that glacial ice could shift the track of tropical monsoons, causing an Indonesian dry period. But this is the first hard data to show that was indeed the case.

It’s also likely, Russell and his colleagues say, that the drying in Indonesia created a feedback loop that amplified ice age cooling.

“A very large fraction of the Earth’s water vapor comes from evaporation of the ocean around Indonesia, and water vapor is the Earth’s most important greenhouse gas,” Russell said. “As you start varying the hydrological cycle of Indonesia, you almost have to vary the Earth’s water vapor concentration. If you reduce the water vapor content it should cool the climate globally. So the fact that we have this very strong drying in the tropics during glaciation would argue for a strong feedback of water vapor concentration to the global climate during glacial-interglacial cycles.”

Surprisingly absent from the data, Russell says, is the influence of other processes known to drive climate elsewhere in the tropics. In particular, there was no sign of climate change in Indonesia associated with Earth’s orbital precession, a wobble caused by Earth’s axis tilt that generates differences in sunlight in a 21,000-year cycle.

“There’s very little indication of the 21,000-year cycle that dominates much of the tropics,” he said. “Instead we see this very big set of changes that appear linked to the amount of ice on earth.”

To arrive at those conclusions, the researchers used sediment cores from Lake Towuti, an ancient lake on the island of Sulawesi in central Indonesia. By looking at how concentrations of chemical elements in the sediment change with depth, the researchers can develop a continuous record of how much surface runoff poured into the lake. The rate of runoff is directly related to the rate of rainfall.

In this case, Russell and his colleagues looked at titanium, an element commonly used to gauge surface runoff. They found a marked dip in titanium levels in sediments dated to between 33,000 and 16,000 years ago – a strong indicator that surface runoff slowed during that period.

That finding was buttressed by another proxy of rainfall: carbon isotopes from plant leaf wax. Leaves are covered with a carbon-based wax that protects them from losing too much water to evaporation. Different plants have different carbon isotopes in their leaf wax. Tropical grasses, which are adapted for dryer climates, tend to have the C-13 isotope. Trees, which thrive in wetter environs, use the C-12 isotope. The ratio of those two isotopes in the sediment cores is an indicator of the relative abundance of grass versus trees.

The cores showed an increase in abundance of grass in the same sediments that showed a decrease in surface runoff. Taken together, the results suggest a dry period strong enough to alter the region’s vegetation that was closely correlated with the peak glaciation in the northern hemisphere.

The next step for Russell and his colleagues is to see if this pattern is repeated in multiple glacial cycles. Glacial periods run on cycles of about 100,000 years. Core samples from deeper in the Lake Towuti sediment will show whether this drying evident during the last ice age also happened in previous ice ages. It’s estimated that Lake Tuwuti sediments record up to 800,000 years of climate data, and Russell recently received funding to take deeper cores.

Ultimately, Russell hopes his work will help to predict how the region might be influenced by human-forced global warming.

“This provides the kind of fundamental data we need to understand how the climate of this region operates on long timescales,” he said. “That can then anchor our understanding of how it might respond to global warming.”

Study explains Pacific equatorial cold water region

A new study published this week in the journal Nature reveals for the first time how the mixing of cold, deep waters from below can change sea surface temperatures on seasonal and longer timescales.

Because this occurs in a huge region of the ocean that takes up heat from the atmosphere, these changes can influence global climate patterns, particularly global warming.

Using a new measurement of mixing, Jim Moum and Jonathan Nash of the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University have obtained the first multi-year records of mixing that permit assessment of seasonal changes. This is a significant advance beyond traditional shipboard measurements that are limited to the time that a ship can be away from port. Small instruments fueled by lithium batteries were built to be easily deployed on deep-sea equatorial moorings.

Moum employs a simple demonstration to show how mixing works.

He pours cold, white cream into a clear glass mug full of hot, black coffee, very carefully, using a straw to inject the heavier cream at the bottom of the mug, where it remains.

“Now we can wait until the cream diffuses into the coffee, and we’ll have a nice cuppa joe,” Moum says. “Unfortunately, the coffee will be cold by then. Or, we can introduce some external energy into the system, and mix it.”

A stirring spoon reveals motions in the mug outlined by the black/white contrasts of cream in coffee until the contrast completely disappears, and the color achieves that of café au lait.

“Mixing is obviously important in our normal lives, from the kitchen to the dispersal of pollutants in the atmosphere, reducing them to levels that are barely tolerable,” he said.

The new study shows how mixing, at the same small scales that appear in your morning coffee, is critical to the ocean. It outlines the processes that create the equatorial Pacific cold tongue, a broad expanse of ocean near the equator that is roughly the size of the continental United States, with sea surface temperatures substantially cooler than surrounding areas.

Because this is a huge expanse that takes up heat from the atmosphere, understanding how it does so is critical to seasonal weather patterns, El Nino, and to global climate change.

In temperate latitudes, the atmosphere heats the ocean in summer and cools it in winter. This causes a clear seasonal cycle in sea surface temperature, at least in the middle of the ocean. At low latitudes near the equator, the atmosphere heats the sea surface throughout the year. Yet a strong seasonal cycle in sea surface temperature is present here, as well. This has puzzled oceanographers for decades who have suspected mixing may be the cause but have not been able to prove this.

Moum, Nash and their colleagues began their effort in 2005 to document mixing at various depths on an annual basis, which previously had been a near-impossible task.

“This is a very important area scientifically, but it’s also quite remote,” Moum said. “From a ship it’s impossible to get the kinds of record lengths needed to resolve seasonal cycles, let alone processes with longer-term cycles like El Nino and La Nina. But for the first time in 2005, we were able to deploy instrumentation to measure mixing on a NOAA mooring and monitor the processes on a year-round basis.”

The researchers found clear evidence that mixing alone cools the sea surface in the cold tongue, and that the magnitude of mixing is influenced by equatorial currents that flow from east to west at the surface, and from west to east in deeper waters 100 meters beneath the surface.

“There is a hint – although it is too early to tell – that increased mixing may lead, or have a correlation to the development of La Niña,” Moum said. “Conversely, less mixing may be associated with El Niño. But we only have a six-year record – we’ll need 25 years or more to reach any conclusions on this question.”

Nash said the biggest uncertainty in climate change models is understanding some of the basic processes for the mixing of deep-ocean and surface waters and the impacts on sea surface temperatures. This work should make climate models more accurate in the future.

Continuous satellite monitoring of ice sheets needed to better predict sea-level rise

The findings, published in Nature Geoscience, underscore the need for continuous satellite monitoring of the ice sheets to better identify and predict melting and the corresponding sea-level rise.

The ice sheets covering Antarctica and Greenland contain about 99.5 per cent of the Earth’s glacier ice which would raise global sea level by some 63m if it were to melt completely. The ice sheets are the largest potential source of future sea level rise – and they also possess the largest uncertainty over their future behaviour. They present some unique challenges for predicting their future response using numerical modelling and, as a consequence, alternative approaches have been explored. One common approach is to extrapolate observed changes to estimate their contribution to sea level in the future.

Since 2002, the satellites of the Gravity Recovery and Climate Experiment (GRACE) detect tiny variations in Earth’s gravity field resulting from changes in mass distribution, including movement of ice into the oceans. Using these changes in gravity, the state of the ice sheets can be monitored at monthly intervals.

Dr Bert Wouters, currently a visiting researcher at the University of Colorado, said: “In the course of the mission, it has become apparent that ice sheets are losing substantial amounts of ice – about 300 billion tonnes each year – and that the rate at which these losses occurs is increasing. Compared to the first few years of the GRACE mission, the ice sheets’ contribution to sea level rise has almost doubled in recent years.”

Yet, there is no consensus among scientists about the cause of this recent increase in ice sheet mass loss observed by satellites. Beside anthropogenic warming, ice sheets are affected by many natural processes, such as multi-year fluctuations in the atmosphere (for example, shifting pressure systems in the North Atlantic, or El Niño and La Niña events) and slow changes in ocean currents.

“So, if observations span only a few years, such ‘ice sheet weather’ may show up as an apparent speed-up of ice loss which would cancel out once more observations become available,” Dr Wouters said.

The team of researchers compared nine years of satellite data from the GRACE mission with reconstructions of about 50 years of mass changes to the ice sheets. They found that the ability to accurately detect an accelerating trend in mass loss depends on the length of the record.

At the moment, the ice loss detected by the GRACE satellites is larger than what we would expect to see just from natural fluctuations, but the speed-up of ice loss over the last years is not.

The study suggests that although there may be almost enough satellite data to detect a speed-up in mass loss of the Antarctic ice sheet with a reasonable level of confidence, another ten years of satellite observations is needed to do so for Greenland. As a result, extrapolation of the current contribution to sea-level rise of the ice sheets to 2100 may be too high or low by as much as 35 cm. The study, therefore, urges caution in extrapolating current measurements to predict future sea-level rise.

Tree rings tell a 1,100-year history of El Niño

This graph shows El Niño amplitude derived from North American tree rings (blue) and instrumental measurements (red). The green curve represents the long-term trend in El Nino strength. (Individual El Niño events occur typically at intervals of 2-7 years.) Periods of strong El Niño activity are indicated by amplitudes above 1.0. Superimposed on a general rising trend, cycles of strong activity occurred about every 50-90 years. -  International Pacific Research Center
This graph shows El Niño amplitude derived from North American tree rings (blue) and instrumental measurements (red). The green curve represents the long-term trend in El Nino strength. (Individual El Niño events occur typically at intervals of 2-7 years.) Periods of strong El Niño activity are indicated by amplitudes above 1.0. Superimposed on a general rising trend, cycles of strong activity occurred about every 50-90 years. – International Pacific Research Center

El Niño and its partner La Niña, the warm and cold phases in the eastern half of the tropical Pacific, play havoc with climate worldwide. Predicting El Niño events more than several months ahead is now routine, but predicting how it will change in a warming world has been hampered by the short instrumental record. An international team of climate scientists has now shown that annually resolved tree-ring records from North America, particularly from the US Southwest, give a continuous representation of the intensity of El Niño events over the past 1100 years and can be used to improve El Niño prediction in climate models. The study, spearheaded by Jinbao Li, International Pacific Research Center, University of Hawai’i at Manoa, is published in the May 6 issue of Nature Climate Change.

Tree rings in the US Southwest, the team found, agree well with the 150-year instrumental sea surface temperature records in the tropical Pacific. During El Niño, the unusually warm surface temperatures in the eastern Pacific lead to changes in the atmospheric circulation, causing unusually wetter winters in the US Southwest, and thus wider tree rings; unusually cold eastern Pacific temperatures during La Niña lead to drought and narrower rings. The tree-ring records, furthermore, match well existing reconstructions of the El Niño-Southern Oscillation and correlate highly, for instance, with δ18O isotope concentrations of both living corals and corals that lived hundreds of years ago around Palmyra in the central Pacific.

“Our work revealed that the towering trees on the mountain slopes of the US Southwest and the colorful corals in the tropical Pacific both listen to the music of El Niño, which shows its signature in their yearly growth rings,” explains Li. “The coral records, however, are brief, whereas the tree-ring records from North America supply us with a continuous El Niño record reaching back 1100 years.”

The tree rings reveal that the intensity of El Niño has been highly variable, with decades of strong El Niño events and decades of little activity. The weakest El Niño activity happened during the Medieval Climate Anomaly in the 11th century, whereas the strongest activity has been since the 18th century.

These different periods of El Niño activity are related to long-term changes in Pacific climate. Cores taken from lake sediments in the Galapagos Islands, northern Yucatan, and the Pacific Northwest reveal that the eastern-central tropical Pacific climate swings between warm and cool phases, each lasting from 50 to 90 years. During warm phases, El Niño and La Niña events were more intense than usual. During cool phases, they deviated little from the long-term average as, for instance, during the Medieval Climate Anomaly when the eastern tropical Pacific was cool.

“Since El Niño causes climate extremes around the world, it is important to know how it will change with global warming,” says co-author Shang-Ping Xie. “Current models diverge in their projections of its future behavior, with some showing an increase in amplitude, some no change, and some even a decrease. Our tree-ring data offer key observational benchmarks for evaluating and perfecting climate models and their predictions of the El Niño-Southern Oscillation under global warming.”

Antarctic Climate: Short-Term Spikes, Long-Term Warming Linked to Tropical Pacific





When a strong El Niño develops across the tropical Pacific, it can influence weather and climate as far away as the southern polar region. This occurs via a “wave train” of areas with unusually high or low pressure in the upper atmosphere (H’s and L’s) that leads to warmer-than-normal temperatures in West Antarctica. Bright reds near the equator show the unusually warm sea-surface temperatures (SSTs) associated with an El Niño during 1940-41. There are no SST data for that period for the portions of the Southern Ocean shown here. Analysis of ice cores drilled in West Antarctica (red dots) reveals that air temperatures there warmed by as much as 10° Fahrenheit as this three-year-long El Niño unfolded, then dropped by as much as 13° F afterward. [ENLARGE] (Image by Steve Deyo, ©UCAR.)

Dramatic year-to-year temperature swings and a century-long warming trend across West Antarctica are linked to conditions in the tropical Pacific Ocean, according to a new analysis of ice cores conducted by scientists at the National Center for Atmospheric Research (NCAR) and the University of Washington (UW). The findings show the connection of the world’s coldest continent to global warming, as well as to periodic events such as El Niño.



“As the tropics warm, so too will West Antarctica,” says NCAR’s David Schneider, who conducted the research with UW’s Eric Steig. “These ice cores reveal that West Antarctica’s climate is influenced by atmospheric and oceanic changes thousands of miles to the north.”



The research appears this week in the online Early Edition of Proceedings of the National Academy of Sciences. The work was supported by the National Science Foundation, NCAR’s sponsor.



Scientists are keenly interested in whether warming will destabilize the West Antarctic ice sheet over a period of decades or centuries. The ice sheet covers an area the size of Mexico, averages about 6,500 feet deep, and, if melted, would raise global sea levels by about 8 to 16 feet (2.5-5 meters).



Antarctica’s climate is difficult to study, partly because there are few observations of this vast and remote region and partly because the cold, dry atmosphere is unlike that of the other six continents. Scientists previously determined that Antarctica overall probably warmed by about 0.4 degrees Fahrenheit (0.2 degrees Celsius) in the last century. But it has not been apparent until now that low-lying West Antarctica is more responsive to global warming trends than East Antarctica, where wind patterns have largely kept out comparatively warm air.



Schneider and Steig estimate that West Antarctica warmed about 1.6 degrees F (0.9 degrees C) over the 20th century. That is slightly more than the global average of about 1.3 degrees F (0.7 degrees C). Because of the large swings in annual temperature during the 1930s and 1940s, there is a considerable margin of uncertainty in the century-long estimate, says Schneider. He notes that there is increased confidence that warming has occurred since 1950, averaging about 0.8 degree F (0.4 degrees C) per decade.


The new set of cores analyzed by Schneider and Steig comes from a relatively snowy part of the continent. This provides enough detail for scientists to infer year-to-year temperature changes. The data show that the Antarctic climate is highly responsive to changes in the Pacific. For example, during a major El Niño event from 1939 to 1942, temperatures in West Antarctica rose by about 6 to 10 degrees F (3-6 degrees C), and then dropped by an estimated 9 to 13 degrees F (5-7 degrees C) over the next two years. El Niño is a periodic shift in air pressure accompanied by oceanic warming in the tropical Pacific.



Although the heart of El Niño’s oceanic warming is in the tropical Pacific, it often fosters a circulation pattern that pushes relatively mild, moist air toward West Antarctica, where it can temporarily displace much colder air. As a result, West Antarctica has one of the world’s most variable climates.



“These results help put Antarctica’s recent climate trends into a global context,” says Schneider.



Steig adds that while the influence of tropical climate on West Antarctica climate was not unknown, “these results are the first to demonstrate that we can unambiguously detect that influence in ice core records.”


Decoding the climate record



Ice-core analysis is critical for understanding the climate of West Antarctica. Few weather stations existed before the 1950s, and even satellite readings can be unreliable because of the difficulty in distinguishing clouds from snow cover.



To reconstruct climate trends over the last century, Schneider and Steig analyzed ice cores collected from eight locations across West Antarctica. They measured heavy and light stable isotopes of oxygen and hydrogen, the elements that make up the ice itself. During warm episodes, the heavy isotopes are more common because of a number of processes, such as a reduction in condensation that would otherwise remove them.



The ice cores for the study were collected from 2000 to 2002 during the U.S. International Trans-Antarctic Scientific Expedition, which Schneider and Steig participated in. The expedition and subsequent ice core analysis was sponsored by the National Science Foundation’s Office of Polar Programs.

2007 Was Earth’s Second Warmest Year in a Century





Temperature changes in 2007 from the previous year
Temperature changes in 2007 from the previous year

Climatologists at the NASA Goddard Institute for Space Studies (GISS) at Columbia University have found that 2007 tied with 1998 for Earth’s second warmest year in a century.



Goddard Institute researchers used temperature data from weather stations on land, satellite measurements of sea ice temperature since 1982 and data from ships for earlier years.



The greatest warming in 2007 occurred in the Arctic and neighboring high-latitude regions. Global warming has a larger effect in polar areas, as the loss of snow and ice leads to more open water, which absorbs more sunlight and warmth. Snow and ice reflect sunlight; when they disappear, so too does their ability to deflect warming rays. The large Arctic warming anomaly of 2007 is consistent with observations of record-low levels of Arctic sea ice in September 2007.



“As we predicted last year, 2007 was warmer than 2006, continuing the strong warming trend of the past 30 years that has been confidently attributed to the effect of increasing human-made greenhouse gases,” said James Hansen, director of NASA GISS.


“It is unlikely that 2008 will be a year with truly exceptional global mean temperature,” said Hansen. “Barring a large volcanic eruption, a record global temperature clearly exceeding that of 2005 can be expected within the next few years, at the time of the next El Nino, because of the background warming trend attributable to continuing increases of greenhouse gases.”



The eight warmest years in the GISS record have all occurred since 1998, and the 14 warmest years in the record have all occurred since 1990.



A minor data processing error found in the GISS temperature analysis in early 2007 does not affect the present analysis. The data processing flaw was the result of a failure to apply NOAA adjustments to U. S. Historical Climatology Network stations in 2000-2006, as the records for those years were taken from a different database (Global Historical Climatology Network). This flaw affected only 1.6% of the Earth’s surface (the contiguous 48 states) and only the last several years in the 21st century. The data processing flaw did not alter the ordering of the warmest years on record and the global ranks were unaffected. In the contiguous 48 states, the statistical tie among 1934, 1998 and 2005 as the warmest year(s) was unchanged. In the current analysis, in the flawed analysis, and in the published GISS analysis, 1934 is the warmest year in the contiguous states (but not globally) by an amount (magnitude on the order of 0.01°C) that is an order of magnitude smaller than the certainty.



The NASA Goddard Institute for Space Studies, at Columbia University in New York City, is a laboratory of the Earth Sciences Division of NASA’s Goddard Space Flight Center and a unit of the Columbia University Earth Institute.

NASA Observes La Nina: This ‘Little Girl’ Makes A Big Impression





The blue area throughout the center of this image shows the cool sea surface temperature along the equator in the Pacific Ocean during this La Niña episode. (Credit: NASA/Goddard's Scientific Visualization Studio)
The blue area throughout the center of this image shows the cool sea surface temperature along the equator in the Pacific Ocean during this La Niña episode. (Credit: NASA/Goddard’s Scientific Visualization Studio)

Cool, wet conditions in the Northwest, frigid weather on the Plains, and record dry conditions in the Southeast, all signs that La Niña is in full swing.



With winter gearing up, a moderate La Niña is hitting its peak. And we are just beginning to see the full effects of this oceanographic phenomenon, as La Niña episodes are typically strongest in January.



A La Niña event occurs when cooler than normal sea surface temperatures form along the equator in the Pacific Ocean, specifically in the eastern to central Pacific. The La Niña we are experiencing now has a significant presence in the eastern part of the ocean.



The cooler water temperatures associated with La Niña are caused by an increase in easterly sea surface winds. Under normal conditions these winds force cooler water from below up to the surface of the ocean. When the winds increase in speed, more cold water from below is forced up, cooling the ocean surface.



“With this La Niña, the sea-surface temperatures are about two degrees colder than normal in the eastern Pacific and that’s a pretty significant difference,” says David Adamec of NASA’s Goddard Space Flight Center, Greenbelt, Md. “I know it doesn’t sound like much, but remember this is water that probably covers an area the size of the United States. It’s like you put this big air conditioner out there — and the atmosphere is going to feel it.”



While this “air conditioner” may be located in the equatorial Pacific Ocean, it has a great influence on the weather here in the United States and across the globe.



The cool water temperatures of a La Niña slow down cloud growth overhead, causing changes to the rainfall patterns from South American to Indonesia. These changes in rainfall affect the strength and location of the jet stream — the strong winds that guide weather patterns over the United States. Since the jet stream regulates weather patterns, any changes to it will have a great impact on the United States.



Those changes can be felt throughout the country. The Northwest generally experiences cooler, wetter weather during a La Niña. On the Great Plains, residents normally see a colder than normal winter and southeastern states traditionally experience below average rainfall.



The cooler waters of a La Niña event also increase the growth of living organisms in this part of the ocean. La Niñas amplify the normal conditions in the Pacific. These typically cool and abundant waters experience an increase in phytoplankton growth when the water temperature drops even further.


The increased circulation that brings up cold water from below also brings up with it nutrients from the deeper waters. These nutrients feed the organisms at the bottom of the food chain, starting a reaction that increases life in the ocean. NASA’s SeaWiFS satellite documented this increase in phytoplankton during the last La Niña period in 1998.



La Niña and El Niño episodes tend to occur every three to five years. La Niñas are often preceded by an El Niño, however this cycle is not guaranteed.



The lengths of La Niña events vary as well. “We need to watch to see if this La Niña diminishes, because they can last for multiple years. And if it does last for multiple years, the southern tier of the United States, especially the Southeast, can expect dryer weather. That is not a good situation. If this La Niña behaves like a normal event, we should see signs that it is beginning to weaken by February,” says Adamec.



So far this La Niña is behaving like a textbook case: following the predicted weather patterns, strengthening throughout the winter, and peaking toward January. According to NOAA’s Climate Prediction Center, this La Niña episode is expected to continue until the spring of 2008, with a gradual weakening starting in February.



NASA will continue to monitor this phenomenon with several of its key Earth observing satellites.



Instruments on NASA’s Terra and Aqua satellites measure sea surface temperature and observe changes to life in the ocean, changes of great importance to the fishing industry. The MODIS instruments on these satellites detected the temperature drop that signaled this La Niña period, and SeaWiFS continues to monitor ocean life.



Scientists also look at sea surface height to understand La Niña. The cooler ocean water associated with a La Niña contracts, lowering sea-surface heights. Over the past year, NASA’s Jason satellite has observed a lower than normal sea level along the equatorial Pacific where this current La Niña episode is taking place.



NASA also looks at changes in wind and rain patterns to study La Niña. The QuikSCAT satellite measures changes in oceanic surface winds, while the Tropical Rainfall Measuring Mission satellite observes changes in rainfall. These observations add to a fuller understanding of this phenomenon.



The current La Niña episode has far many reaching effects. What some may see as just a small change in sea surface temperature has a much greater impact on our climate here in the U.S. and across the globe, as well as implications for the fishing industry and the global economy. With the help of NASA’s earth observing fleet, scientists are becoming better equipped to observe and understand this phenomenon.