Clouds: Lighter than air but laden with lead

By sampling clouds — and making their own — researchers have shown for the first time a direct relation between lead in the sky and the formation of ice crystals that foster clouds. The results suggest that lead generated by human activities causes clouds to form at warmer temperatures and with less water. This could alter the pattern of both rain and snow in a warmer world.

The lead-laden clouds come with a silver lining, however. Under some conditions, these clouds let more of the earth’s heat waft back into space, cooling the world slightly. Atmospheric lead primarily comes from human sources such as coal.

The international team of researchers reported their results in the May issue of Nature Geoscience. The collaboration included researchers from institutions in the United States, Switzerland and Germany.

“We know that the vast majority of lead in the atmosphere comes from man-made sources,” said atmospheric chemist Dan Cziczo of the Department of Energy’s Pacific Northwest National Laboratory and study author. “And now we show that the lead is changing the properties of clouds and therefore the balance of the sun’s energy that affects our atmosphere.”

Globe Trotting for Lead


Scientists first attempted to goad rain from the sky with silver and lead iodide in the 1940s. Since then, researchers have known that lead can pump up the ice crystals in clouds. But daily human activities also add lead to the atmosphere. The top sources include coal burning, small airplanes flying at the altitude where clouds form, and construction or wind freeing lead from the ground. Cziczo and colleagues wanted to know how lead from these sources affects clouds.

To find out, the researchers collected air from high atop a mountain peak on the Colorado-Wyoming border. In their high altitude lab, they created artificial clouds from the air in a cloud chamber about the size of a small refrigerator. Half of the ice crystals they plucked from the synthetic clouds, they found, contained lead.

The team then collected a dollop of real cloud atop a mountain in Switzerland. About half of those ice crystals also contained lead. But finding lead in an incriminating position doesn’t mean it causes ice crystals.

To determine whether lead causes ice crystals and clouds to form, the team turned to a lab in Germany that houses a cloud chamber three stories tall, as well as a smaller chamber in Switzerland. They created dust particles that were either lead-free or contained one percent lead by weight, which is about what scientists find in the atmosphere. They put these dust particles into the chambers and measured the temperature and humidity at which point ice nucleated around the dust.

They found that lead changed the conditions under which clouds appeared. The air didn’t have to be as cold or as heavy with water vapor if lead was present.

“Most of what nucleates clouds are dust particles,” said Cziczo. “Half of the ones we looked at had lead supercharging them.”

Leaden Clouds, Cooler Climes


To investigate what this might mean for the earth’s climate, the researchers simulated the global climate with either lead-free dust particles floating around, or with either 10 percent or all of them containing lead.

The computer simulation showed that the clouds they looked at — typically high, thin clouds — formed at lower altitudes and different locations in the northern hemisphere when lead was present in dust particles. This will probably affect precipitation, said Cziczo.

“In our atmosphere, lead affects the distribution and density of the kinds of clouds we looked at,” said Cziczo, “which might then affect where and when rain and snow fall.”

Clouds at lower altitudes let more of the earth’s heat, or so-called longwave radiation, escape out to space. So lead-triggered clouds could partly offset global warming due to greenhouse gases.

But that doesn’t mean lead in the atmosphere will simply cool the planet, said Cziczo, since they looked at only one type of cloud. Cloudy skies are far more complicated than their wispy image lets on.

“This work highlights how complex these interactions between lead and water vapor and temperature are,” said Cziczo. “They’re not as simple as greenhouse gases.”

Future work will look at the type of lead and how much is needed to affect clouds and precipitation, as well as the atmospheric distribution of the metal dust.

Origins of sulfur in rocks tells early oxygen story

Sedimentary rocks created more than 2.4 billion years ago sometimes have an unusual sulfur isotope composition thought to be caused by the action of ultra violet light on volcanically produced sulfur dioxide in an oxygen poor atmosphere. Now a team of geochemists can show an alternative origin for this isotopic composition that may point to an early, oxygen-rich atmosphere.

“The significance of this finding is that an abnormal isotope fractionation (of sulfur) may not be linked to the atmosphere at all,” says Yumiko Watanabe, research associate, Penn State. “The strongest evidence for an oxygen poor atmosphere 2.4 billion years ago is now brought into question.”

The researchers, who also include James Farquhar, associate professor of geology, University of Maryland and Hiroshi Ohmoto, professor of geoscience, Penn State, present the possibility that the rocks with an anomalous sulfur isotope fractionation came from locations on the ocean floor where hydrothermal fluids seeped up from submarine vents through organic carbon rich sediments and mixed with the ocean water. Watanabe used laboratory experimentation to test their theory and report on the results in today’s (Apr. 17) issue of Science.

Chemical elements often have more than one form. While the number of protons and electrons are all the same, the element may have forms with a greater or lesser number of neutrons and consequently a different atomic weight. Sulfur has four naturally occurring isotopes none of which are radioactive. Although 95 percent of sulfur has an atomic weight of 32, the other 5 percent is composed of sulfur with atomic weights of 33, 34 or 36. The relationship between the amounts of 33, 34 and 36 are predictable based on the differences in their weights, but in the early rocks examined, the relationship was often anomalous. Other scientists have previously determined that the sulfur dioxide, ultraviolet light reaction in the absence of oxygen can produce the anomalous isotope fractionation.

Watanabe looked at samples of amino acids and sodium sulfur compounds to try to recreate the anomalous sulfur isotope composition in another way. She chose amino acids as a proxy for organic material because the anomalous sulfur isotopes often come from sedimentary rock, black shale, that also contains abundant mature kerogen — a mixture of organic compounds. She chose sodium compounds because of the large amounts of sodium and sulfate in the ocean.

Initial experiments used two amino acids — alanine and glycine — and sodium sulfite, which is less oxidized compared to sulfate. When heated, these did not produce abnormal fractionation. Watanabe then tested five amino acids, adding histidine, arginine and tryptophan, and mixed them with sodium sulfate. In this case, alanine and glycine produced the anomalous isotope composition found in the rocks. In all, she ran 32 series of experiments with more than 100 individual samples.

“At high temperatures it sometimes took 24 hours for the sulfate to reduce to sulfide,” said Watanabe. “At lower temperatures it took about two months, 1,000 hours. I ran the experiments until I had enough product to test the isotopic distribution.”

Although Watanabe captured the sulfur from the experiments as hydrogen sulfide gas, she converted it to silver sulfide for analysis because it is easier to work with a solid than a gas.

“People never thought that anomalous sulfur isotope fractionation could be caused by a process other than atmospheric reactions,” said Ohmoto. “Our study significantly shifts possibilities to something different, to a biological and thermal regime. There are now at least two ways that the anomalous sulfur isotope fractionation seen in some rocks could be achieved.”

While sulfate-reducing bacteria do not produce anomalous isotope relationships, the remains of simple organisms coupled with thermal sulfate reduction does produce the anomalous isotope signature.

The researchers plan to look at dead cyanobacteria — blue green algae — next to see if their organic material will fuel the thermal reaction to produce anomalous sulfur isotope relationships.

Megadroughts in sub-Saharan Africa normal for the region

Winston Wheeler, a researcher at the University of Arizona, collecting tree cores from a partially submerged tropical tree at Lake Bosumtwi, Ghana. A submerged forest in the lake provides evidence for severe and long-lasting droughts in West Africa just a few centuries ago. This tree grew at a time of prolonged drought when the lake level was tens of meters lower than today. Tree rings in the core sample were used to date the age of the tree and provide an additional line of evidence about megadroughts in the past. -  Jonathan T. Overpeck.
Winston Wheeler, a researcher at the University of Arizona, collecting tree cores from a partially submerged tropical tree at Lake Bosumtwi, Ghana. A submerged forest in the lake provides evidence for severe and long-lasting droughts in West Africa just a few centuries ago. This tree grew at a time of prolonged drought when the lake level was tens of meters lower than today. Tree rings in the core sample were used to date the age of the tree and provide an additional line of evidence about megadroughts in the past. – Jonathan T. Overpeck.

Devastating droughts worse than the infamous Sahel drought are part of the normal climate regime for sub-Saharan West Africa, according to new research.

For the first time, researchers have developed an almost year-by-year record of the last 3,000 years of West Africa’s climate. In that period, catastrophic droughts occurred every 30 to 65 years, and the pattern can be expected to continue in the future, the team reports.

“What’s disconcerting about this record is that it suggests that the most recent drought was relatively minor in the context of the West African drought history,” said first author Timothy M. Shanahan, who conducted the research while he was a doctoral student at The University of Arizona in Tucson.

The Sahel drought, which began in the late 1960s and continued for several decades, killed at least 100,000 people and displaced many more.

“What’s really striking about droughts in this area is that they last such a long time,” he said. “You have droughts that last 30 to 60 years, and then some that last four times as long.”

The region has undergone multicentury droughts, most recently from 1400 A.D. to 1750 A.D., the researchers found.

“If we were to switch into one of these century-scale patterns of drought, it would be a lot more severe, and it would be very difficult for people to adjust to the change,” said Shanahan, now an assistant professor of geosciences at the University of Texas at Austin.

Changes in the North Atlantic sea-surface temperatures have a significant effect on the West African monsoon, which is a climate pattern of alternating wet-and-dry periods, the scientists found.

“This area switched between very long wet periods and long, very severe dry periods,” he said.

As global warming progresses, the increases in temperature may exacerbate the normal climate pattern, producing even more severe and prolonged droughts than those of the past, said co-author Jonathan T. Overpeck, a UA professor of geosciences.

“We also know that global warming will make these droughts a lot hotter. This could be devastating,” said Overpeck, co-director of UA’s Institute for Environment and Society.

The research team figured out the region’s past climate by analyzing the annual layers of sediment deposited in Ghana’s Lake Bosumtwi, geological records of the lake level and other climate indicators.

Paul Filmer, the National Science Foundation program director who funded the study, said, “This project is a good example of how work in the tropics on sediment records provides more detailed insight into climate patterns that affect millions of people in a highly vulnerable area of the world.”

Shanahan, Overpeck and their colleagues will publish their paper, “Atlantic forcing of persistent drought in West Africa,” in the April 17 issue of the journal Science. A complete list of authors is at the bottom of this release. The National Science Foundation funded the research.

Many of the Earth’s climate patterns are influenced by sea-surface temperatures.

Climate scientists have proposed that temperatures in the North Atlantic rise and fall naturally in an approximately 60-year cycle called the Atlantic Multidecadal Oscillation. Some computer models and tree-ring data from around the North Atlantic support this hypothesis.

If it exists, the oscillation should have a strong effect on the West African monsoon.

But an accurate annual record of Atlantic sea-surface temperatures exists only for the last 80 to 100 years, and the tree-ring data going farther back in time is patchy. Therefore, up until now, it has been hard to tell whether there is a long-term cyclical pattern or only a shorter-term trend.

The new reconstruction of the West African Monsoon by Shanahan and his colleagues shows a close match between the 350-year-old paleoclimate record of sea-surface temperatures and the wet-dry cycles of the monsoon.

“This paper provides a long-term context suggesting that the Atlantic Multidecadal Oscillation does actually exist,” says Shanahan.

“Our rainfall records are strongly related to these really distant sea-surface temperature reconstructions, at least on this multidecadal time scale. It suggests that the rainfall patterns are being generated by the sea-surface temperature patterns and not by some other influence.”

Some current climate models have forecast a wetter climate for West Africa, while others have forecast a drier one. Armed with this new information, scientists who create climate models will be better equipped to sort out some of the conflicting climate predictions for West Africa.

Satellites show how Earth moved during Italy quake

Studying satellite radar data from ESA’s Envisat and the Italian Space Agency’s COSMO-SkyMed, scientists have begun analyzing the movement of Earth during and after the 6.3 earthquake that shook the medieval town of L’Aquila in central Italy on 6 April 2009.

Scientists from Italy’s Istituto per il Rilevamento Elettromagnetico dell’ Ambiente (IREA-CNR) and the Istituto Nazionale di Geofisica e Vulcanologia (INGV) are studying Synthetic Aperture Radar (SAR) data from these satellites to map surface deformations after the earthquake and the numerous aftershocks that have followed.

The scientists are using a technique known as SAR Interferometry (InSAR), a sophisticated version of ‘spot the difference’. InSAR involves combining two or more radar images of the same ground location in such a way that very precise measurements – down to a scale of a few millimetres – can be made of any ground motion taking place between image acquisitions.

The InSAR technique merges data acquired before and after the earthquake to generate ‘interferogram’ images that appear as rainbow-coloured interference patterns. A complete set of coloured bands, called ‘fringes’, represents ground movement relative to the spacecraft of half a wavelength, which is 2.8 cm in the case of Envisat’s ASAR.

The first Envisat data, acquired after the earthquake on 12 April, were made immediately available to the scientists.

“We produced an interferogram just a few hours after the Envisat acquisition by combining these data with data acquired before the earthquake on 1 February. We were pleased that we were able to immediately see the pattern of the earthquake,” said Riccardo Lanari of IREA-CNR in Naples, Italy.

The Envisat interferogram, as explained by Stefano Salvi from INGV’s Earthquake Remote Sensing Group, shows nine fringes surrounding a maximum displacement area located midway between L’Aquila and Fossa, where the ground moved as much as 25 cm (along a line between the satellite’s orbital position and the earthquake area).

“By using available 3D ground displacements from five GPS location sites around the affected area, we were able to confirm the preliminary results obtained with Envisat data,” Salvi said.

The COSMO-SkyMed constellation, which is currently made up of three satellites, allows for frequent data. This means new interferograms can be calculated every few days.

The COSMO-SkyMed data together with the Envisat data and possibly SAR data from other satellites will ensure a dense sampling of the ground deformation around the L’Aquila area in the next months, which could make this earthquake one of the most covered by SAR Interferometry measurements.

To ensure all scientists are able to contribute to the analysis of the earthquake, ESA is making its Earth observation dataset collected over the L’Aquila area freely accessible with an innovative fast data download mechanism. The dataset will be continuously updated with the newest Envisat acquisitions.

Study uncovers tectonic events behind earthquake that killed 595 in Peru

Three-dimensional deformation following the 2007 Pisco, Peru earthquake. The red areas show ~1m of uplift offshore and the blues areas about 50 cm of subsidence on land. The hinge-line between uplift and subsidence closely matches the location of the coastline. -  UM Rosenstiel School
Three-dimensional deformation following the 2007 Pisco, Peru earthquake. The red areas show ~1m of uplift offshore and the blues areas about 50 cm of subsidence on land. The hinge-line between uplift and subsidence closely matches the location of the coastline. – UM Rosenstiel School

A magnitude 8.0. earthquake destroyed 90 percent of the city of Pisco, Peru on August 16, 2007. The event killed 595 people, while another 318 were missing. Tsunami waves were observed locally, off the shore of Chile, and as far away as New Zealand.

In a study published in the Geophysical Journal International, scientists from the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, in collaboration with scientists from the University of Oxford (U.K.) have analyzed data on this earthquake and its impact on regional topography. Using InSAR-based geodetic data and teleseismic data, the scientists were able to use satellite images to identify details of this major plate boundary event.

“Unfortunately, historical earthquakes in Central Peru show a complex repeat pattern making it difficult to identify which area will be affected in the future,” said Rosenstiel School Postdoctoral Fellow and Principal Investigator Dr. Juliet Biggs. “The convergence of the Nazca and the South American plates is slowly building the Andes, but the relationship between great earthquakes and mountain building processes is still unclear.”

Intriguingly, models developed as a result of this event in 2007 demonstrated no upper lifting of the region after this major earthquake. Long-term uplift of the upper plate must either occur aseismically or as ‘slow earthquakes’ during the interseismic or postseismic part of the earthquake cycle.

Support for the project came from the U.S. National Science Foundation. The earthquake confirmed a common feature for earthquakes in central Peru: maximum intensity and damage occur few tens of kilometers south of the epicenter. This is a key observation for disaster management and tsunami prediction.

“Visiting Peru immediately after the earthquake together with fellow researcher Kim Outerbridge provided us with a desolating picture of the affected region, but it was critically important for data-gathering,” Biggs said.

The collaboration with Dr. Edmundo Norabuena, a former graduate from the Rosenstiel School now at Instituto GeofĂ­sico del PerĂș, to deploy GPS equipment in the region permitted collection of essential data, which will be the subject of a new study. This will provide details on the movement generated deep inside the earth after the earthquake, which is another crucial part of the puzzle in terms of our understanding of the recurrence intervals of major earthquakes.

How do they spread?

Propagation of earthquake waves within the Earth is not uniform. Experiments indicate that the velocity of shear waves (s-waves) in Earth’s lower mantle between 660 and 2900 km depth is strongly dependent on the orientation of ferropericlase. In the latest issue of “Science” (Vol. 325, 10.04.2009), researchers from the German Research Center for Geosciences GFZ, the Karlsruhe Institute of Technology, the University of Bayreuth, and Arizona State University report unexpected properties of ferropericlase, which is presumably the second most abundant mineral of the lower mantle.

“The dependence of wave velocity on direction increases significantly at a pressure of about 50 Giga-Pascal, corresponding to approximately 1300 km depth. This is caused by a change in electronic arrangement of the iron ions in ferropericlase” explains Hauke Marquardt from GFZ. In addition, flow in the lower mantle results in a preferred mineral orientation; this causes the detectable non-uniform propagation of earthquake waves. This flow is the driving force of tectonic plate movements, formation of mountains, earthquakes, and volcanic activities and therefore, strongly affects our life on Earth’s surface

Because the deep interior of our planet is not accessible to direct observations, the researchers simulate the conditions of Earth’s interior by generating the extreme pressures in their laboratory. Diamond anvil cells are used at GFZ to perform the high-pressure experiments, which are complemented by X-ray diffraction experiments at “Diamond Light Source” in Didcot, UK.

These new findings are of practical importance: Only if we know the properties of the materials that constitute the deep Earth, we can derive information about its internal flow from the non-uniform propagation of earthquake waves. This can help to better understand plate tectonic processes.

Monitoring Yellowstone earthquake swarms

The Seismological Society of America (SSA) is an international scientific society devoted to the advancement of seismology and its applications in understanding and mitigating earthquake hazards and in imaging the structure of the earth.

The second largest earthquake swarm ever recorded in Yellowstone National Park occurred during the two weeks from 27 December 2008 and 7 January 2009 and included more than 1000 earthquakes. Analysis of the swarm suggests epicenters migrated north over the 12 day period and maximum hypocenter depths abruptly shallowed from 12 km to 3 km depth at the time of rapid cessation of activity on Jan. 7. Source properties of the swarm earthquakes suggest that the swarm may be due to the movement of hydrothermal fluids through pre-existing cracks, as suggested by recent analysis by University of Utah scientists.

The Yellowstone Volcano Observatory (YVO) was created as a partnership among the U.S. Geological Survey (USGS), Yellowstone National Park, and University of Utah to strengthen the long-term monitoring of volcanic and earthquake unrest in the Yellowstone National Park region. Yellowstone is the site of the largest and most diverse collection of natural thermal features in the world and the first National Park.

California’s central coast earthquake hazards: New information about recently identified faults

Seismologists are re-evaluating the earthquake potential of the Central Coast, a very complex tectonic region located west of the San Andreas Fault, between Monterey Bay and the Western Transverse Ranges. This area of increasing population growth ranks as one of the top 40 U.S. metropolitan areas with significant earthquake risk.

Speakers from the US Geological Survey, PG&E and academia will compare fresh data to illuminate the complexity of faulting in the central California coastal region.

Three talks will use separate datasets to focus on the California Central Ranges, Hosgri Fault Zone and nearby faults:

  • Fault structure of the California Central Coast: Jeanne Hardebeck, US Geological Survey, will present and interpret new earthquake relocations and focal mechanisms for earthquakes occurring along the central California coast, including the offshore region near San Luis Obispo. A prominent newly-observed feature is a 25 km long linear trend of seismicity running just offshore and parallel to the coast-line in the region of Point Buchon. This seismicity trend is accompanied by a linear magnetic anomaly, and both the seismicity and the magnetic anomaly are truncated where they obliquely meet the Hosgri Fault. Focal mechanisms indicate that this feature is a vertical strike-slip fault.

  • Geophysical characterization of the Hosgri Fault zone: High-resolution
    marine magnetic and seismic-reflection data collected offshore Point
    Buchon show that the Hosgri Fault represents a complex zone of steeply
    dipping faults that varies significantly in character along strike. The boundary of a
    northwest-trending linear magnetic anomaly off Point Buchon corresponds
    to a linear trend of small earthquakes, suggesting an active fault. Continued interpretation and geophysical modeling of magnetic, seismic reflection, and seismicity data will help determine whether or not the magnetic boundaries are fault boundaries, and if so, how these structures relate to the Hosgri Fault Zone.

  • Constraints on 3-dimensional structure from gravity and magnetic data: V. E. Langenheim, US Geological Survey, will present analysis based on a new physical dataset that is sensitive to magnetic properties of rock, mapping fault boundaries. Her research suggests complex, non-linear features with intersecting faults. Fault and basin geometry will be important for estimating shaking potential of scenario earthquakes.

Study compares sound from exploding volcanoes with jet engines

Scripps researchers are using infrasound recording to study Tungurahua volcano in Ecuador. -  D.Fee/University of Hawaii
Scripps researchers are using infrasound recording to study Tungurahua volcano in Ecuador. – D.Fee/University of Hawaii

New research on infrasound from volcanic eruptions shows an unexpected connection with jet engines. Researchers at Scripps Institution of Oceanography at UC San Diego speeded up the recorded sounds from two volcanoes and uncovered a noise very similar to typical jet engines. These new research findings provide scientists with a more useful probe of the inner workings of volcanic eruptions. Infrasound is sound that is lower in frequency than 20 cycles per second, below the limit of human hearing.

The study led by Robin Matoza, a graduate student at Scripps Oceanography, will be published in an upcoming issue of the journal Geophysical Research Letters, a publication of the American Geophysical Union (AGU). Matoza measured infrasonic sound from Mount St. Helens in Washington State and Tungurahua volcano in Ecuador, both of which are highly active volcanoes close to large population centers.

“We hypothesized that these very large natural volcanic jets were making very low frequency jet noise,” said Matoza, who conducts research in the Scripps Laboratory for Atmospheric Acoustics.

Using 100-meter aperture arrays of microbarometers, similar to weather barometers but sensitive to smaller changes in atmospheric pressure and low-frequency infrasonic microphones, the research team tested the hypothesis, revealing the physics of how the large-amplitude signals from eruptions are produced. Jet noise is generated by the turbulent flow of air out of a jet engine. Matoza and colleagues recorded these very large-amplitude infrasonic signals during the times when ash-laden gas was being ejected from the volcano. The study concluded that these large-scale volcanic jets are producing sound in a similar way to smaller-scale man-made jets.

“We can draw on this area of research to speed up our own study of volcanoes for both basic research interests, to provide a deeper understanding of eruptions, and for practical purposes, to determine which eruptions are likely ash-free and therefore less of a threat and which are loaded with ash,” said Michael Hedlin, director of Scripps’ Atmospheric Acoustics Lab and a co-author on the paper.

Large-amplitude infrasonic signals from volcanic eruptions are currently used in a prototype real-time warning system that informs the Volcanic Ash Advisory Center (VAAC) when large infrasonic signals have come from erupting volcanoes. Researchers hope this new information can improve hazard mitigation and inform pilots and the aviation industry.

“The more quantitative we can get about how the sound is produced the more information we can provide to the VAAC,” said Matoza. “Eventually it could be possible to provide detailed information such as the size or flow rate of the volcanic jet to put into ash-dispersal forecasting models.”

Earth under global cooling

The Late Eocene Earth -- Hothouse, Icehouse, and Impacts by
Christian Koeberl and Alessandro Montanari (editors). -  Geological Society of America
The Late Eocene Earth — Hothouse, Icehouse, and Impacts by
Christian Koeberl and Alessandro Montanari (editors). – Geological Society of America

Thirty-four-million years ago, Earth changed profoundly. What happened, and how were Earth’s animals, plants, oceans, and climate affected? Focusing on the end of the Eocene epoch and the Eocene-Oligocene transition, a critical but very brief interval in Earth’s history, GSA’s latest Special Paper provides new answers to these questions.

According to the book’s editors, Christian Koeberl of the University of Vienna and Alessandro Montanari of the Observatorio Geologico di Coldigioco in Italy, the end of the Eocene and the Eocene-Oligocene (E-O) transition mark the most profound oceanographic and climatic changes of the past 50 million years of Earth’s history.

Earth experienced global cooling beginning in the middle Eocene, with a sharp temperature drop of about two degrees Celsius in the Late Eocene. This drop was characterized by an increase in marine oxygen isotope values and significant floral and faunal turnovers. The global climate changes are commonly attributed to the expansion of the Antarctic ice cap following its gradual isolation from other continental masses. However, as examined in this volume, multiple extraterrestrial bolide impacts, possibly related to a comet shower that lasted more than two million years, may have played an important role in deteriorating the global climate.

The volume provides an excellent overview of conditions on Earth during the last few million years of the Eocene and around the time of the Eocene-Oligocene boundary. Chapters include an expanded look at Earth across time by Walter Alvarez and colleagues; an updated and enhanced understanding of the Eocene-Oligocene boundary transition using different climate proxies, improved time control, and climate models; integrated stratigraphy of the late Eocene-early Oligocene transition and reevaluation of the Global Stratotype Section and Point (GSSP); paleoecology and paleoclimate through the critical period of transition from hothouse to icehouse; and Late Eocene impact processes and impact stratigraphy.