A Long dry spell doomed Mexican city 1,000 years ago

Ruins of the city of Cantona in the Mexican state of Puebla, with the mountain Cerro Pizarro in the background. The city was abandoned almost 1,000 years ago, probably as a result of a prolonged dry spell. (Ines Urdaneta image courtesy of Wikimedia Commons.)

A UC Berkeley study sheds new light on this question, providing evidence that a prolonged period of below-average rainfall was partly responsible for the abandonment of one such city, Cantona, between A.D. 900 and A.D. 1050.

At its peak, Cantona, located in a dry, volcanic basin (La Cuenca Oriental) east of today’s Mexico City, was one of the largest cities in the New World, with 90,000 inhabitants. The area was a major source of obsidian, and the city may have played a military role alongside an important trade route from the Veracruz coast into the highlands.

To assess the climate in that area before and after Cantona’s collapse, UC Berkeley geographers analyzed sediment cores from a lake located 20 miles south of the former city. They found evidence of a 650-year period of frequent droughts that extended from around A.D. 500 to about A.D. 1150. This was part of a long-term drying trend in highland Mexico that started 2,200 years ago, around 200 B.C. The climate became wetter again in about A.D. 1300, just prior to the rise of the Aztec empire.

“The decline of Cantona occurred during this dry interval, and we conclude that climate change probably played a role, at least towards the end of the city’s existence,” said lead author Tripti Bhattacharya, a UC Berkeley graduate student.

Surprisingly, the population of Cantona increased during the early part of the dry period, perhaps because of political upheaval elsewhere that increased the importance of the heavily fortified city, she said. Teotihuacan, less than 100 miles to the west, was in decline at the time, also possibly because of more frequent droughts. 

Lake Aljojuca, Mexico
The maar lake Aljojuca, 20 miles south of Cantona, yielded sediments that recorded a lengthy series of droughts between A.D. 500 and 1150. (Tripti Bhattacharya photo)

“In a sense the area became important because of the increased frequency of drought,” said UC Berkeley associate professor of geography Roger Byrne. “But when the droughts continued on such a scale, the subsistence base for the whole area changed and people just had to leave. The city was abandoned.”

Bhattacharya, Byrne and their colleagues report their findings in an article appearing this week in the early edition of the journal Proceedings of the National Academy of Sciences. The UC Berkeley researchers analyzed lake cores provided by scientists at the National Autonomous University of Mexico in Juriquilla, Querétaro, Mexico and the German Research Centre for Geosciences in Potsdam, Germany.

Political upheaval and climate change

Byrne emphasized that the area’s typical monsoon weather with wet summers and dry winters did not stop, but was interrupted by frequent short-term droughts, no doubt affecting crops and water supplies. Today the area is close to the northern limit of maize production without irrigation, and would have been particularly vulnerable to drier conditions, he said.

Byrne, a member of the Berkeley Initiative on Global Change Biology (BiGCB) and curator of fossil pollen in the Museum of Paleontology, has studied sediment cores from many lakes in Mexico and California, and is particularly interested in possible links between climate change and human activities.

Nearly 20 years ago, he learned of Cantona and traveled with students to the areas three times to obtain cores from lakes near the site, most of which are maar lakes created by magma explosions. They are deep and often contain undisturbed and regularly layered sediments ideal for chronological studies.

Tripti Bhattacharya
Tripti Bhattacharya analyzed carbonates in lake sediments to explore the climate history of the Cuenca Oriental east of Mexico City. (Ellie Broadman photo)

German colleagues cored this particular lake, Aljojuca, in 2007, and Bhattacharya traveled to Potsdam to collect sediment samples. Oxygen isotope ratios in carbonate sediments are correlated with the ratio of precipitation to evaporation and thus indicate aridity. Organic material in the sediments was used for accelerator mass spectroscopy carbon-14 dating.

“We can show that both the growth and decline of the site took place during a time period of frequent drought, which forces us to think in more nuanced ways about how political and social factors interact with environmental factors to cause social and cultural change,” Bhattacharya said. “That makes the study particularly interesting.”

Bhattacharya noted that more studies are necessary to reconstruct the prehistoric climate of highland Mexico. Such studies could reveal the causes of prehistoric climatic change and whether they were similar to the factors that regulate the region’s climate today, such as the El Niño/Southern Oscillation.

Co-authors include Harald Böhnel and Kurt Wogau of UNAM, Juriquilla; Ulrike Kienel of the German Research Center for Geosciences in Potsdam; B. Lynn Ingram of UC Berkeley; and Susan Zimmerman of Lawrence Livermore National Laboratory. The work was funded by the National Science Foundation.

Source: UC Berkeley

Dolly Varden are a species of char common in southeast Alaska. The fish shown is in spawning coloration.

Not all species may suffer from climate change. A new analysis shows that Dolly Varden, a species of char common in southeast Alaska, adjust their migrations so they can keep feasting on a key food source -- salmon eggs -- even as shifts in climate altered the timing of salmon spawning.

The resiliency of species to climate change may depend on how well they adapt to climate-driven changes in their food and habitat, such as altered growth of plants they feed on. A mismatch in timing between predators and the availability of prey could cause some species to lose access to food. But others such as Dolly Varden that successfully adjust to shifts in climate and prey offer a climate change story with a happy ending, according to the study published in Freshwater Biology.

Ignoring environmental cues may help this predator

The Dolly Varden's secret appears to be that instead of taking its migration cues from environmental variables such as water temperature or streamflow, the species cues directly off the presence of salmon the Dolly Varden depend on for food, the study found.

"Despite warming temperatures and shifting salmon migrations, Dolly Varden do a great job of following their food," said lead author Chris Sergeant of the National Park Service's Inventory and Monitoring Program in southeast Alaska. "Species that can handle a high degree of variability are the ones that should be most resilient to further changes associated with climate."

Dolly Varden get most of their energy over the course of each year by gorging themselves on salmon eggs, which are abundant in summer and rich in energy thanks to the same fatty acids that make fish healthy for humans. Eggs from any single species of salmon may be available during a narrow spawning window of two to six weeks. The Dolly Varden must follow salmon migrations closely to take full advantage of this annual salmon egg bonanza.

Sticking with salmon to find food

But salmon migrations are shifting as the climate warms. Previous research by the University of Alaska and NOAA Fisheries' Alaska Fisheries Science Center in southeast Alaska's Auke Creek has shown that pink and coho salmon now migrate to their spawning grounds 10 to 17 days earlier while sockeye salmon migrate eight days earlier.

Instead of falling out of synch with salmon, though, seagoing Dolly Varden in Auke Creek have accurately adjusted their annual migrations from the ocean back to freshwater to stick with the salmon. The adjustment has maintained their access to egg meals, according to the new research that includes coauthors from the University of Wyoming and NOAA Fisheries' Northwest Fisheries Science Center.

Researchers rely on long record of fish migration data

The research depended on a wealth of fish data from a weir on Auke Creek maintained primarily since 1980 by NOAA Fisheries' Alaska Fisheries Science Center Auke Bay Labs, supported by collaborations with the University of Alaska Fairbanks and Alaska Department of Fish and Game. From 1997 to 2006 crews counted and measured Dolly Varden migrating from the ocean, past the weir and into Auke Creek, providing an unusual long-term picture of fish migration times.

"We're really indebted to the people who kept that record going for so long," said Eric Ward of the Northwest Fisheries Science Center. "It turns out to be very valuable in understanding how species are responding to the changing climate."

Researchers in the new study used the weir data to examine the relationship between migrations of salmon and Dolly Varden from year to year. They found the timing of Dolly Varden migration more closely related to the presence and timing of the salmon than on environmental variables such as temperature and precipitation that are often seen as driving animal migrations.

In short, the Dolly Varden are shifting their migration to follow their food instead of following temperatures or other environmental cues that, as the climate changes, might otherwise lead them to migrate at a different time than the salmon that provide their most important food.

The researchers cautioned that it's unclear whether other salmon predators could adjust their timing to follow salmon as effectively as Dolly Varden do, apparently by watching salmon passing by or detecting salmon eggs through smell. But the adaptability of Dolly Varden suggests that at least some species may be more resilient to climate-induced changes in migration timing than ecologists might assume.

Source: NOAA Fisheries West Coast Region

'Family' matters when predicting ecosystems' reaction to global change

This is a picture of the experimental setup in the greenhouse.
Credit: Jennifer Schweitzer, co-author and associate professor at UT

Humans are rapidly changing the look and function of earth's ecosystems, from the increase of greenhouse gases to the unintentional and harmful spread of plants and animals to new environments. A major challenge for ecologists is to understand how and why communities respond to factors that underlie global change.

A University of Tennessee, Knoxville, study is finding some clues. It shows that just as our family histories dictate what we look like and how we act, plant evolutionary history shapes community responses to interacting agents of global change.

The research, published in the open-access journal PLOS ONE, may help predict what ecosystems will look like in the future and how they will work.

"The issues of global change have already begun to jeopardize the natural functioning of ecosystems and important services that we often take for granted like clean air, clean water, food and fiber production," said Rachel Wooliver, lead author and doctoral student in ecology and evolutionary biology. "Our study is the first to experimentally show that plant communities with different evolutionary backgrounds will respond differently to human-caused physical and biological changes."

In other words, regarding the future effects of global change on ecosystem services and processes humans rely upon, it's all in the family.

Wooliver and colleagues from UT, the University of Tasmania and Villanova University used eucalypt species native to Tasmania, Australia, to compare plant growth in cultures of all the same species to that of mixtures with native species with an introduced hardwood plantation species. They analyzed plant activity in an ambient environment versus one of increased levels of carbon dioxide and soil nitrogen.

"We found that only those communities composed of native species within one evolutionary lineage responded significantly to elevated carbon dioxide and nitrogen by taking carbon from the atmosphere and sequestering it into biomass," said co-author John Senior of the University of Tasmania. "Communities from another lineage, on the other hand, showed no response, which suggests that they will play a less crucial role in offsetting the rise of carbon dioxide and global warming."

This means that evolutionary history will shape which species will effectively sequester carbon and which won't.

Further, the presence of the nonnative species in these communities influenced productivity differently depending on the evolutionary background of the interacting native species. Thus, family trees can be used to predict how the spread of nonnative species by humans will shape the look and function of ecosystems as global change continues.

"Overall, this study provides new direction for global change scientists by highlighting that evolutionary history is key to understanding outcomes of plant function and diversity with rapid ecological change," said Wooliver.

The work is promising to researchers that are trying to figure out if species interactions change how ecosystems are responding to global change, as well as conservation biologists who aim to determine which species might be at higher risk for extinction in the future.

Source: University of Tennessee

California's drought is the worst in 1,200 years, evidence suggests

The 2012-2014 California drought, unusual in the context of the last 1,200 years, greatly diminished water reserves in Lake Nacimiento of the upper Salinas Valley. Credit: Photo by Daniel Griffin

As California finally experiences the arrival of a rain-bearing Pineapple Express this week, two climate scientists from the University of Minnesota and Woods Hole Oceanographic Institution have shown that the drought of 2012-2014 has been the worst in 1,200 years.

Daniel Griffin, an assistant professor in the Department of Geography, Environment and Society at the University of Minnesota, and Kevin Anchukaitis, an assistant scientist at Woods Hole Oceanographic Institution, asked the question, "How unusual is the ongoing California drought?" Watching the severity of the California drought intensify since last autumn, they wondered how it would eventually compare to other extreme droughts throughout the state's history.

To answer those questions, Griffin and Anchukaitis collected new tree-ring samples from blue oak trees in southern and central California. "California's old blue oaks are as close to nature's rain gauges as we get," says Griffin. "They thrive in some of California's driest environments." These trees are particularly sensitive to moisture changes and their tree rings display moisture fluctuations vividly.

As soon as the National Oceanic and Atmospheric Administration (NOAA) released climate data for the summer of 2014, the two scientists sprang into action. Using their blue oak data, they reconstructed rainfall back to the 13th century. They also calculated the severity of the drought by combining NOAA's estimates of the Palmer Drought Severity Index (PDSI), an index of soil moisture variability, with the existing North American Drought Atlas, a spatial tree-ring based reconstruction of drought developed by scientists at Columbia University's Lamont-Doherty Earth Observatory. These resources together provided complementary data on rainfall and soil moisture over the past millennium. Griffin and Anchukaitis found that while the current period of low precipitation is not unusual in California's history, these rainfall deficits combined with sustained record high temperatures created the current multiyear severe water shortages. "While it is precipitation that sets the rhythm of California drought, temperature weighs in on the pitch," says Anchukaitis.

"We were genuinely surprised at the result," says Griffin, a NOAA Climate & Global Change Fellow and former WHOI postdoctoral scholar. "This is California--drought happens. Time and again, the most common result in tree-ring studies is that drought episodes in the past were more extreme than those of more recent eras. This time, however, the result was different." While there is good evidence of past sustained, multi-decadal droughts or so-called "megadroughts"' in California, the authors say those past episodes were probably punctuated by occasional wet years, even if the cumulative effect over decades was one of overall drying. The current short-term drought appears to be worse than any previous span of consecutive years of drought without reprieve.

Tree rings are a valuable data source when tracking historical climate, weather and natural disaster trends. Floods, fires, drought and other elements that can affect growing conditions are reflected in the development of tree rings, and since each ring represents one year the samples collected from centuries-old trees are a virtual timeline that extend beyond the historical record in North America.

So what are the implications? The research indicates that natural climate system variability is compounded by human-caused climate change and that "hot" droughts such as the current one are likely to occur again in the future. California is the world's 8th largest economy and the source of a substantial amount of U.S. produce. Surface water supply shortages there have impacts well beyond the state's borders.

With an exceptionally wet winter, parts of California might emerge from the drought this year. "But there is no doubt," cautions Anchukaitis, "that we are entering a new era where human-wrought changes to the climate system will become important for determining the severity of droughts and their consequences for coupled human and natural systems."

Source:  Woods Hole Oceanographic Institution

Cost of cloud brightening for cooler planet revealed

University of Manchester scientists have identified the most energy-efficient way to make clouds more reflective to the sun in a bid to combat climate change. Credit: © magann / Fotolia

Marine Cloud Brightening is a reversible geoengineering method proposed to mitigate rising global temperatures. It relies on propelling a fine mist of salt particles high into the atmosphere to increase the albedo of clouds -- the amount of sunlight they reflect back into space. This would then reduce temperatures on the surface, as less sunlight reaches Earth.
Clouds form when water droplets gather on dust or other particles in the air. Increasing the amount of salt particles in the atmosphere allows more of these water droplets to form, making the clouds denser and therefore more reflective.

A new paper, published in the journal Philosophical Transactions of the Royal Society A, has looked at four different ways of getting the particles into the sky, to compare how effective they may be. The researchers found that a technique called the 'Rayleigh Jet' proved to be best.

Named after Lord Rayleigh, who provided the theory, the technique relies on spraying a fine jet of water that breaks down into small droplets into the sky. The liquid droplets evaporate quickly, leaving behind just the salt particles.

These particles, say the paper's authors, could be generated from specially built ships that could travel the world's oceans spraying salt particles into the air where they then hang in the atmosphere for several days until they return to Earth as rain.

Previous studies have optimised the size of the salt particles needed to produce the best increase in cloud reflectance but haven't taken into account how much energy the technique would need and how much it would cost to operate. This new paper, by teams at the universities of Manchester, Washington and Edinburgh, tackled this question. The researchers tested each technique so there was an increase in reflection of 5%, a figure that would combat the predicted effects of increased carbon dioxide levels over the rest of this century. They then looked at how much energy each would consume.

The scientists say that the Rayleigh jet method could produce the desired effect using 30 megawatts of energy, about the same energy that two large ships produce.

Dr Paul Connolly, based in the School of Earth, Atmospheric and Environmental Sciences at The University of Manchester, said: "It can be incredibly energy intensive to propel water high into the atmosphere and the energy required had never really been tested before. Our paper optimises the salt particle sizes to produce the required change in cloud reflectance for the least energy cost. It is an important finding if these techniques should be needed in the future.

"I am not recommending that we use any of these techniques now, but it is important to know how best to use them should they become necessary. Should no progress be made to reduce CO2 levels, then geoengineering techniques, similar to this, might become necessary to avoid dangerous rises in global temperatures."

Source: Manchester University