A new study says that global warming has measurably worsened the ongoing California drought. While scientists largely agree that natural weather variations have caused a lack of rain, an emerging consensus says that rising temperatures may be making things worse by driving moisture from plants and soil into the air. The new study is the first to estimate how much worse: as much as a quarter. The findings suggest that within a few decades, continually increasing temperatures and resulting moisture losses will push California into even more persistent aridity. The study appears this week in the journal Geophysical Research Letters.
Dry grassland and oak landscape of central California's Coastal Mountain Range, among the parts of the state most affected by the current drought. Photo taken August 2015. (Dominick McPeake)
"A lot of people think that the amount of rain that falls out the sky is the only thing that matters," said lead author A. Park Williams, a bioclimatologist at Columbia University's Lamont-Doherty Earth Observatory. "But warming changes the baseline amount of water that's available to us, because it sends water back into the sky."
The study adds to growing evidence that climate change is already bringing extreme weather to some regions. California is the world's eighth-largest economy, ahead of most countries, but many scientists think that the nice weather it is famous for may now be in the process of going away. The record-breaking drought is now in its fourth year; it is drying up wells, affecting major produce growers and feeding wildfires now sweeping over vast areas.
The researchers analyzed multiple sets of month-by-month data from 1901 to 2014. They looked at precipitation, temperature, humidity, wind and other factors. They could find no long-term rainfall trend. But average temperatures have been creeping up—about 2.5 degrees Fahrenheit over the 114-year period, in step with building fossil-fuel emissions. Natural weather variations have made California unusually hot over the last several years; added to this was the background trend. Thus, when rainfall declined in 2012, the air sucked already scant moisture from soil, trees and crops harder than ever. The study did not look directly at snow, but in the past, gradual melting of the high-mountain winter snowpack has helped water the lowlands in warm months. Now, melting has accelerated, or the snowpack has not formed at all, helping make warm months even dryer according to other researchers.
Due to the complexity of the data, the scientists could put only a range, not a single number, on the proportion of the drought caused by global warming. The paper estimates 8 to 27 percent, but Williams said that somewhere in the middle -- probably 15 to 20 percent -- is most likely.
Severity of the California drought in 2014, a record-breaking year. The darkest red areas are the worst. (Williams et al., Geophysical Research Letters, 2015)
Last year, the U.S. National Oceanic and Atmospheric Administration sponsored a study that blamed the rain deficit on a persistent ridge of high-pressure air over the northeast Pacific, which has been blocking moisture-laden ocean air from reaching land. Lamont-Doherty climatologist Richard Seager, who led that study (and coauthored the new one), said the blockage probably has nothing to do with global warming; normal weather patterns will eventually push away the obstacle, and rainfall will return. In fact, most projections say that warming will eventually increase California's rainfall a bit. But the new study says that evaporation will overpower any increase in rain, and then some. This means that by around the 2060s, more or less permanent drought will set in, interrupted only by the rainiest years. More intense rainfall is expected to come in short bursts, then disappear.
Many researchers believe that rain will resume as early as this winter. "When this happens, the danger is that it will lull people into thinking that everything is now OK, back to normal," said Williams. "But as time goes on, precipitation will be less able to make up for the intensified warmth. People will have to adapt to a new normal."
This study is not the first to make such assertions, but it is the most specific. A paper by scientists from Lamont-Doherty and Cornell University, published this February, warned that climate change will push much of the central and western United States into the driest period for at least 1,000 years. A March study out of Stanford University said that California droughts have been intensified by higher temperatures, and gives similar warnings for the future.
Abnormally low lake level at Horseshoe Lake in the high-elevation Mammoth Lakes Basin of the Sierra Nevada Mountains. Taken June 2015. (Jennifer Bernstein)
A further twist was introduced in a 2010 study by researchers at the NASA Goddard Institute for Space Studies. They showed that massive irrigation from underground aquifers has been offsetting global warming in some areas, because the water cools the air. The effect has been especially sharp in California's heavily irrigated Central Valley—possibly up to 3.5 degrees Fahrenheit during some seasons. Now, aquifers are dropping fast, sending irrigation on a downward trajectory. If irrigation's cooling effect declines, this will boost air temperatures even higher, which will dry aquifers further, and so on. Scientists call this process "positive feedback."
Climatologist Noah Diffenbaugh, who led the earlier Stanford research, said the new study is an important step forward. It has "brought together the most comprehensive set of data for the current drought," he said. "It supports the previous work showing that temperature makes it harder for drought to break, and increases the long-term risk."
Jonathan Overpeck, co-director of the Institute of the Environment at the University of Arizona, said, "It's important to have quantitative estimates of how much human-caused warming is already making droughts more severe." But, he said, "it's troubling to know that human influence will continue to make droughts more severe until greenhouse gas emissions are cut back in a big way."
The study's other authors are Richard Seager, Jason Smerdon, Benjamin Cook and Edward Cook, all of Lamont-Doherty Earth Observatory; and geographer John Abatzoglou of the University of Idaho.
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Solomon led efforts to identify the cause of the ozone hole over Antarctica
An American atmospheric chemist who led efforts to identify the cause of the Antarctic ozone hole and a French geochemist who extracted the longest-yet climate record from polar ice cores have won the prestigious 2012 Vetlesen Prize. Susan Solomon and Jean Jouzel will share the $250,000 award, considered to be the earth sciences’ equivalent of a Nobel.
“Earth Science is a collective enterprise, and transformational advances are the product of many authors,” says the Vetlesen Prize committee’s citation. “Both nominees have made leading and fundamental contributions to climate science.” The prize is funded by the G. Unger Vetlesen Foundation in New York and administered by Columbia University’s Lamont-Doherty Earth Observatory.
Solomon’s work in identifying the cause of Antarctica’s springtime ozone losses helped bring about a global ban on manmade ozone-depleting chemicals. Working most of her career at the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colo., Solomon proposed in a 1986 study that refrigerants and other industrial chemicals were responsible for the Antarctic ozone hole discovered a year earlier. She led two expeditions to Antarctica, in 1986 and 1987, bringing back key measurements that proved her hypothesis. Recognizing that ozone protects the planet from harmful ultraviolet rays that can cause skin cancer, policymakers around the world responded with rare speed: in 1987, they agreed on a Montreal Protocol to phase out the use of chlorofluorocarbons (CFCs) and related chemicals.
Solomon and her colleagues also explained why ozone destruction was greatest during southern spring. Throughout the dark and extremely cold winter, CFC byproducts react with icy stratospheric clouds to produce ozone-depleting chlorine compounds. When sunlight returns to the South Pole, it reacts with the compounds to break ozone apart. In a 2002 study, Solomon and a colleague also linked Antarctica’s ozone hole to a strengthening of winds that circle the continent in summer, causing net cooling over most of Antarctica but extreme warming over the West Antarctic Peninsula. She has also written several books. In The Coldest March, published in 2002, she reconstructed the weather during British explorer Robert Scott’s 1911 expedition to Antarctica to argue that abnormally cold conditions and not incompetence led to the deaths of Scott and his crew. She has also worked as a member of the Intergovernmental Panel on Climate Change (IPCC), which in 2007 received the Nobel Peace Prize. Solomon, 56, is the first woman to win the Vetlesen since it was first offered in 1960.
Jouzel, 65, has been involved in collecting ice-core records from both poles since the 1970s, and has advanced isotopic techniques for extracting past climate information from them. In the longest climate reconstruction yet from ice cores, Jouzel in a 2007 study in the journal Science charted temperatures in Antarctica for the last 800,000 years, over eight consecutive ice ages. The record was long enough to highlight Antarctica’s climate response to slowly varying seasonal distribution of sunlight caused by changes in earth’s orbit. It was also detailed enough to reveal climate variations within each ice age cycle due to earth’s complex internal climate system.
He has also been a leader in bringing human-caused climate change to the public’s attention.
Jouzel has collected the longest-yet climate record from polar ice cores.
For the last 20 years, he has worked on the IPCC, and is currently vice president of the climate science working panel, whose findings will be released in the Fifth Assessment Report next September. Jouzel has co-written several books, including Climate: A Dangerous Game in 2007 and White Planet, whose English translation is due out in 2013.
Neither scientist came from a scientific family. Solomon was born and raised in Chicago; her father sold insurance and her mother taught fourth grade. She says she was inspired to become a scientist by watching oceanographer Jacques Cousteau on TV as a child, and to go into chemistry by the “exactness” of the experiments she did in high school. “You have a clear solution and you add an exact amount of base and wow, it changes to a bright color just like it’s supposed to,” she said. She received her Ph.D. from the University of California at Berkeley in 1981, and worked at NOAA until 2011, when she became a professor at the Massachusetts Institute of Technology. She has won many previous honors: the American Geophysical Union (AGU)’s Macelewane Medal in 1985 for early-career achievement; a U.S. National Medal of Science in 1999, and the Blue Planet Prize in 2004.
Jouzel grew up in a farming family in Brittany, and during his graduate studies in chemistry became interested in how large hailstones form in the atmosphere at a time when chemical seeding of clouds to produce rain was a popular research topic. He received his Ph.D. in chemistry from the Faculté d’Orsay in Paris in 1974 and eventually applied his hail-investigation techniques to polar ice cores, advancing methods for pulling climate information from air bubbles trapped in the ice and minute variations in water molecules layered in the cores.
He has spent his research career at the French nuclear agency, CEA, or Commissariat à l'Energie Atomique. In 1995, he became research director of CEA’s climate and environment lab (LSCE), and from 2001 to 2008, headed the Institut Pierre-Simon Laplace, a national climate research lab near Paris that includes LSCE. In 2002, Jouzel and his mentor Claude Lorius received France’s highest scientific honor, the CNRS gold medal, for their work on polar ice cores, including reconstruction of past greenhouse gas levels from air trapped in the ice. In 1997, he was awarded the European Geophysical Society’s Milankovitch Medal for outstanding research on long-term climate change and modeling; and in 2003, AGU’s Roger Revelle medal for outstanding contributions to atmospheric science.
“If we want to avoid large climate change we need to act now on greenhouse gases,” he said. “Global warming is not yet damaging, but if we do nothing in the coming years we will have more extreme events, droughts, storms and so on.”
The Vetlesen Prize is given “for scientific achievement resulting in a clearer understanding of the Earth, its history, or its relations to the universe.” It was established in 1959 by the trustees of the estate of G. Unger Vetlesen, a Norwegian immigrant to the United States who became a leading shipbuilder, World War II military leader and pioneer in transatlantic air travel. Vetlesen passed away in 1955. Designed to recognize sweeping achievements on par with the Nobel, it is given every several years by a selection committee appointed by the president of Columbia University. The most recent award was in 2008 to maverick geologist Walter Alvarez, who convinced the world that an asteroid strike wiped out the dinosaurs. Previous winners include British climate scientist Sir Nicholas Shackleton, and several scientists at Lamont-Doherty itself, including climate scientist Wallace Broecker, marine geologist Walter Pitman, seismologist Lynn Sykes and Lamont’s founding director Maurice “Doc” Ewing.
Solomon and Jouzel will receive the award and accompanying medal from Columbia president Lee Bollinger at the university’s Low Library on Feb. 21.
The deep-sea benthic foram Aragonia velascoensis went extinct about 56 million years ago as the oceans rapidly acidified. (Ellen Thomas/Yale University)
Some 56 million years ago, a massive pulse of carbon dioxide into the atmosphere sent global temperatures soaring. In the oceans, carbonate sediments dissolved, some organisms went extinct and others evolved.
Scientists have long suspected that ocean acidification played a part in the crisis—similar to today, as manmade CO2 combines with seawater to change its chemistry. Now, for the first time, scientists have quantified the extent of surface acidification from those ancient days, and the news is not good: the oceans are on track to acidify at least as much as they did then, only at a much faster rate.
In a study published in the latest issue of Paleoceanography, the scientists estimate that surface ocean acidity increased by about 100 percent in a few thousand years or more, and stayed that way for the next 70,000 years. In this radically changed environment, some creatures died out while others adapted and evolved. The study is the first to use the chemical composition of fossils to reconstruct surface ocean acidity at the Paleocene-Eocene Thermal Maximum (PETM), a period of intense warming on land and throughout the oceans due to high CO2.
“This could be the closest geological analog to modern ocean acidification,” said study coauthor Bärbel Hönisch, a paleoceanographer at Columbia University’s Lamont-Doherty Earth Observatory. “As massive as it was, it still happened about 10 times more slowly than what we are doing today.”
The oceans have absorbed about a third of the carbon humans have pumped into the air since industrialization, helping to keep temperatures lower than they would be otherwise. But that uptake of carbon has come at a price. Chemical reactions caused by that excess CO2 have made seawater grow more acidic, depleting it of the carbonate ions that corals, mollusks and calcifying plankton need to build their shells and skeletons.
In the last 150 years or so, the pH of the oceans has dropped substantially, from 8.2 to 8.1--equivalent to a 25 percent increase in acidity. By the end of the century, ocean pH is projected to fall another 0.3 pH units, to 7.8. While the researchers found a comparable pH drop during the PETM--0.3 units--the shift happened over a few thousand years.
“We are dumping carbon in the atmosphere and ocean at a much higher rate today—within centuries,” said study coauthor Richard Zeebe, a paleoceanographer at the University of Hawaii. “If we continue on the emissions path we are on right now, acidification of the surface ocean will be way more dramatic than during the PETM.”
Ocean acidification in the modern ocean is already affecting some marine life, as shown by the partly dissolved shell of this planktic snail, or pteropod, caught off the Pacific Northwest. (Nina Bednaršek/NOAA)
The study confirms that the acidified conditions lasted for 70,000 years or more, consistent with previous model-based estimates. “It didn’t bounce back right away,” said Timothy Bralower, a researcher at Penn State who was not involved in the study. “It took tens of thousands of years to recover.”
From seafloor sediments drilled off Japan, the researchers analyzed the shells of plankton that lived at the surface of the ocean during the PETM. Two different methods for measuring ocean chemistry at the time—the ratio of boron isotopes in their shells, and the amount of boron --arrived at similar estimates of acidification. “It’s really showing us clear evidence of a change in pH for the first time,” said Bralower.
What caused the burst of carbon at the PETM is still unclear. One popular explanation is that an overall warming trend may have sent a pulse of methane from the seafloor into the air, setting off events that released more earth-warming gases into the air and oceans. Up to half of the tiny animals that live in mud on the seafloor—benthic foraminifera—died out during the PETM, possibly along with life further up the food chain.
Other species thrived in this changed environment and new ones evolved. In the oceans, dinoflagellates extended their range from the tropics to the Arctic, while on land, hoofed animals and primates appeared for the first time. Eventually, the oceans and atmosphere recovered as elements from eroded rocks washed into the sea and neutralized the acid.
Today, signs are already emerging that some marine life may be in trouble. In a recent study led by Nina Bednaršek at the U.S. National Oceanic and Atmospheric Administration, more than half of the tiny planktic snails, or pteropods, that she and her team studied off the coast of Washington, Oregon and California showed badly dissolved shells. Ocean acidification has been linked to the widespread death of baby oysters off Washington and Oregon since 2005, and may also pose a threat to coral reefs, which are under additional pressure from pollution and warming ocean temperatures.
“Seawater carbonate chemistry is complex but the mechanism underlying ocean acidification is very simple,” said study lead author Donald Penman, a graduate student at University of California at Santa Cruz. “We can make accurate predictions about how carbonate chemistry will respond to increasing carbon dioxide levels. The real unknown is how individual organisms will respond and how that cascades through ecosystems.”
Other authors of the study, which was funded by the U.S. National Science Foundation: Ellen Thomas, Yale University; and James Zachos, UC Santa Cruz.
A new study projects that much of Ethiopia, Somalia and Djibouti in the Horn of Africa will grow drier as global warming progresses. Here in northern Ethiopia, herders often already struggle to make a living. (Brian Kahn/IRI)
A new study finds that the Horn of Africa has become progressively drier over the past century and that it is drying at a rate that is both unusual in the context of the past 2,000 years and in step with human-influenced warming. The study also projects that the drying will continue as the region gets warmer. If the researchers are right, the trend could exacerbate tensions in one of the most unstable regions in the world.
“Right now, aid groups are expecting a wetter, greener future for the Horn of Africa, but our findings show that the exact opposite is occurring. The region is drying and will continue to do so with rising carbon emissions,” said study coauthor Peter deMenocal, who heads the Center for Climate and Life at Columbia University’s Lamont-Doherty Earth Observatory.
The study, appearing in this week’s Science Advances, used a sediment core that deMenocal and his colleagues extracted from the pirate-ridden Gulf of Aden. They used the core to infer past changes in temperature and aridity. By pairing the paleoclimate record from the core with 20th century observations, the researchers determined that drying will probably continue across Somalia, Djibouti and Ethiopia. That contradicts more optimistic models that have suggested future warming might bring rainier weather patterns that could benefit the region.
“What we see in the paleoclimate record from the last 2,000 years is evidence that the Horn of Africa is drier when there are warm conditions on Earth, and wetter when it is colder,” said lead author Jessica Tierney, an associate professor at the University of Arizona and former postdoctoral associate at Lamont-Doherty.
Global-scale models used to predict future changes under global warming suggest that the region should become wetter, primarily during the “short rains” season from September to November. But the new study suggests that those gains may be offset by declining rainfall during the “long rains” season from March to May, on which the region’s rain-fed agriculture relies.
Study coauthor Peter deMenocal (left) took this deep-sea core in the pirate-infested Gulf of Aden. Lead author Jessica Tierney (right) performed an analysis linking greater heat in the past to drier conditions. (Courtesy Lamont-Doherty Earth Observatory)
The outcome has serious implications for a region that has been racked with political instability and violence as it has dried. The Horn of Africa has suffered deadly droughts every few years in recent decades, and with them humanitarian crises as famine and violence spread. It has also become one of the most unstable regions in the world. In Somalia, as the political situation deteriorated amid droughts of the 1980s and `90s, hundreds of thousands of refugees fled the country, and pirates began raiding ships off the coast.
To extract the sediment core used in the study, scientists had to evade those pirates in the Gulf of Aden. Lamont-Doherty’s R/V Maurice Ewing had been attacked with rocket-propelled grenades during a previous trip.On this run, the captain of the Dutch research ship Pelagia turned off the ship’s interior and navigation lights to slip through these waters. Around them, other vessels were reporting pirate attacks.
That sediment core, which dates back about 40,000 years, has already provided new insights into Africa’s climate. In a 2013 study analyzing parts of the core, Tierney and deMenocal showed that the Sahara, which once bloomed with regular rainfall, suddenly dried out over the span of a century or two, during a warm period some 5,000 years ago—not more gradually, as many researchers had assumed. It provided evidence that climate shifts can happen quite suddenly, even if the forces driving them are gradual.
The new study uses isotopes from leaf waxes found in the sediment sample to compare rates of drying over the past 2,000 years. Plants reflect the environment that sustains them. When the climate is drier, leaf waxes are more enriched with deuterium, or heavy hydrogen isotopes; leaf waxes from wetter climates reflect the more abundant rainfall through the presence of the normal hydrogen isotopes. The researchers found an increasing shift toward heavy hydrogen in the last century as the climate, which had experienced a wet period during the Little Ice Age (1450—1850 AD), dried out.
The findings suggest that climate modeling, frequently done at a global scale, would benefit from region-specific studies with higher resolution results in high-impact areas such as the Horn of Africa, Tierney said.
“If we can simulate rainfall in these arid tropical and subtropical regions better, we can understand the future impact of climate change,” she said.
The study’s other coauthor is Caroline Ummenhofer of Woods Hole Oceanographic Institution.