‘Fossil earthquakes’ abundant

Rocks formed only under the extreme heat and friction during earthquakes, called pseudotachylytes, may be more abundant than previously reported, according to new research focused on eight faults found in the Sierra Nevada. The research appears in the February issue of the Bulletin of the Seismological Society of America.

Geologists have previously debated whether these rocks are rarely produced or not based on an apparent absence in the rock record, most likely brought about by the difficulty in identifying them. Only a small fraction of the energy released in an earthquake is consumed by seismic waves, the formation of pseudotachylytes reveals the importance of the heat generated by the earthquake process.

Pseudotachylytes form by frictional melting during co-seismic faulting at significant depths in the crust. They are not easy to identify, requiring evidence that the fault rock has passed through a melt phase. They are generated by frictional heating of the slip surface, the melting of which may account for a significant proportion of energy released during an earthquake.

Past surveys of the Sierra Nevada, which reported an absence of pseudotachylytes, have focused on the geometry and mechanics of the faults rather than the geological details of the rock types and composition. However, the authors of this study report an abundance of pseudotachylytes throughout the area. The pseudotachylytes they describe range from easily identified to impossible to identify from field data alone. The authors suggest further study of pseudotachylytes will ultimately reveal more about energy partitioning during earthquakes.

Geoengineering could complement mitigation to cool the climate

The first comprehensive assessment of the climate cooling potential of different geoengineering schemes has been carried out by researchers at the University of East Anglia (UEA).

Funded by the Natural Environment Research Council and published today in the journal ‘Atmospheric Chemistry and Physics Discussions‘, the key findings include:

  • Enhancing carbon sinks could bring CO2 back to its pre-industrial level, but not before 2100 – and only when combined with strong mitigation of CO2 emissions
  • Stratospheric aerosol injections and sunshades in space have by far the greatest potential to cool the climate by 2050 – but also carry the greatest risk
  • Surprisingly, existing activities that add phosphorous to the ocean may have greater long-term carbon sequestration potential than deliberately adding iron or nitrogen
  • On land, sequestering carbon in new forests and as ‘bio-char’ (charcoal added back to the soil) have greater short-term cooling potential than ocean fertilisation
  • Increasing the reflectivity of urban areas could reduce urban heat islands but will have minimal global effect
  • Other globally ineffective schemes include ocean pipes and stimulating biologically-driven increases in cloud reflectivity
  • The beneficial effects of some geo-engineering schemes have been exaggerated in the past and significant errors made in previous calculations

“The realisation that existing efforts to mitigate the effects of human-induced climate change are proving wholly ineffectual has fuelled a resurgence of interest in geo-engineering,” said lead author Prof Tim Lenton of UEA’s School of Environmental Sciences.

“This paper provides the first extensive evaluation of their relative merits in terms of their climate cooling potential and should help inform the prioritisation of future research.”

Geo-engineering is the large-scale engineering of the environment to combat the effects of climate change – in particular to counteract the effects of increased CO2 in the atmosphere.

A number of schemes have been suggested including nutrient fertilisation of the oceans, cloud seeding, sunshades in space, stratospheric aerosol injections, and ocean pipes.

“We found that some geoengineering options could usefully complement mitigation, and together they could cool the climate, but geoengineering alone cannot solve the climate problem,” said Prof Lenton.

Injections into the stratosphere of sulphate or other manufactured particles have the greatest potential to cool the climate back to pre-industrial temperatures by 2050.

However, they also carry the most risk because they would have to be continually replenished and if deployment was suddenly stopped, extremely rapid warming could ensue.

Using biomass waste and new forestry plantations for energy, and combusting them in a way that captures carbon as charcoal, which is added back to the soil as ‘bio-char’, could have win-win benefits for soil fertility as well as the climate.

A new combined heat and power plant at UEA is pioneering this type of technology.

UEA’s School of Environmental Sciences leads the world in climate change research and is creating a new GeoEngineering Assessment & Research initiative (GEAR) to take this groundbreaking work forward.

The Dead Sea: Tectonic concurrence below ten kilometers of sediments

The Dead Sea lies in a basin structure situated below the sea level. This deep subsidence is a result of a tectonic concurrence between processes in the upper lithosphere that led to subsiding and a compensating upward flow of rocks in the deeper layers of the lithosphere.

This is a result presented by A. Petrunin and A. Sobolev from the GFZ – German Research Centre for Geosciences in the current issue of ?PHYSICS OF THE EARTH AND PLANETARY INTERIORS” (Vol. 171, S. 387 – 399). In a series of thermomechanical numerical experiments they were able to show that the brittle layer of the Earth’s crust subsides when the African and the Arabian plate pass each other along a strike-slip fault. Due to the lateral displacement, the Earth’s crust gets thinner at this point and a pull-apart basin develops that in the course of roughly 15 million years has been filled with a layer of sediments that is up to ten kilometres thick. In the upper part of the underlying Earth’s mantle, this basin development leads to a corresponding upward flow of hot and ductile rock material. These concurring tectonic processes thus determine the process of the basin development.

Petrunin and Sobolev demonstrate that the subsidence rate is controlled by four parameters: firstly the thickness of the brittle layer and the basin width, secondly the length of the strike-slip displacement, thirdly the rate of frictional softening of the crust and finally the viscosity of the ductile material in the upper mantle.

The detected mechanisms give insight into the formation and development of such pull-apart basins forming a natural rheology lab in which the history of the lithospheric deformation can be studied.

Charcoal evidence tracks climate changes in Younger Dryas

A new study reports that charcoal particles left by wildfires in sediments of 35 North American lake beds don’t readily support the theory that comets exploding over the continent 12,900 years ago sparked a cooling period known as the Younger Dryas.

The study — appearing online this week ahead of regular publication in the Proceedings of the National Academy of Sciences — however, did find clear links between abrupt climate changes and fire activity during the transition between the last Ice Age and the warm interglacial period that began 11,700 years ago. These links are also consistent with the impacts of climate-change conditions on wildfires during recent decades in North America, the researchers noted.

Charcoal particles, along with tree pollen, provide snapshots of types of vegetation and frequencies of wildfire activity in a given area, said study co-author Patrick J. Bartlein, a professor of geography at the University of Oregon. His doctoral student Jennifer R. Marlon led the collaborative study of 23 co-authors (including seven current or former UO students) at institutions in the U.S., Canada and Europe.

“The charcoal data don’t support the idea of widespread fires at the beginning of the Younger Dryas interval,” Bartlein said. “The results don’t reject the comet hypothesis, but do suggest that one element of it — widespread fires — didn’t occur. Instead, the data show that biomass burning tracked general climate changes closely. Biomass burning increased as conditions warmed during deglaciation until the beginning of the Younger Dryas cold interval at 12,900 years ago, leveled off during the cool interval, and then increased again as warming resumed after the end of the cold interval, about 11,700 years ago.”

The fires that left the charcoal records reflect the impact of climate changes independent of potential contributions of early Paleoindians who may have been living on the continent. Proponents of the comet theory suggest Clovis culture may have been dramatically disrupted across the continent.

Marlon began the National Science Foundation-sponsored study of charcoal-pollen records soon after the comet theory was proposed in PNAS by an international team of 26 researchers led by Richard B. Firestone. A co-author of that study, UO archaeologist Douglas Kennett, in the Jan. 2 issue of Science, documented the existence of possible comet-triggered nanodiamond-rich soil at six North American sites dating to 12,900 years ago in apparent support of the hypothesis. The formation of nanodiamonds requires intense pressure and heat, much higher than those of biomass wildfires but possible in comet explosions.

“We had the data to look for widespread fires if they had occurred,” Marlon said, “but what we saw instead was a general increase in biomass burning whenever the climate warmed.”

The lakes containing the charcoal are in Alaska (3 sites), British Columbia (7), U.S. Pacific Northwest (6), the Sierra Nevada (3), northern U.S. Rocky Mountains (6), Southwest (4), Midwest (2), Northeast (3 sites in Quebec), and Southeast (1). Thirty of the samples came from the Global Charcoal Database; another five were drawn from more recent research by co-authors currently studying sediments from the Younger Dryas.

The new study’s conclusion that climate is a major control of wildfires matched that of a study published last year in Nature Geosciences by the same researchers on global biomass burning over the last 2,000 years. “Together,” Bartlein said, “these studies suggest that episodes of global warming are accompanied by increases in wildfires.”

US scientific ocean drilling vessel sets sail for science sea trials

After a complete transformation to modernize and upgrade the research vessel JOIDES Resolution (JR, for short), the ship has set sail from the Singapore shipyard where the work was done, for science sea trials and transit to Honolulu.

The JOIDES Resolution is the U.S. research vessel for exploring and monitoring the sub-seafloor. It operates as part of the international Integrated Ocean Drilling Program (IODP.)

Already a 20-year workhorse on behalf of scientific ocean drilling, the JOIDES Resolution has been completely refurbished. It’s now poised to help IODP continue to push the edge of science by collecting unique sub-seafloor samples and data that would otherwise remain out of reach to researchers.

“We are extremely pleased to see the JOIDES Resolution set sail once more for science–with a vastly improved capability for performing cutting-edge research into the secrets of Earth’s past climate and ocean conditions, and our planet’s interior and deep biosphere,” said Tim Killeen, NSF assistant director for geosciences. “NSF congratulates all involved in the successful refit and modernization of this major research platform.”

“This is a long-awaited day for the world of ocean research,” said Robert Gagosian, president and CEO of the Consortium for Ocean Leadership. “The JOIDES Resolution is operational again for IODP expeditions at an extremely important time, when our need for scientific understanding of our ocean planet–and climate and energy challenges–is at its greatest. The research pursuits of thousands of scientists around the globe will be enhanced in a major way by the future work of the JR.”

Steven Bohlen, interim director of science services at IODP’s U.S. Implementing Organization–Texas A&M University, Lamont-Doherty Earth Observatory of Columbia University and the Consortium for Ocean Leadership–said that “from close ties with the community of scientists involved with IODP’s work, it’s a huge thrill for all of us to have this important vessel back in action.”

Expeditions onboard the JR help improve scientific understanding of the causes of natural hazards such as earthquakes and tsunamis; Earth’s changing climate conditions over the past 100 million years; methane hydrates–“frozen” deposits of methane gas that may be a major factor in climate change; and future rising sea levels, by examining the evidence of past sea level rise and climate change in seafloor sediments.

Comet impact theory disproved

New data, published today, disproves the recent theory that a large comet exploded over North America 12,900 years ago, causing a shock wave that travelled across North America at hundreds of kilometres per hour and triggering continent-wide wildfires.

Dr Sandy Harrison from the University of Bristol and colleagues tested the theory by examining charcoal and pollen records to assess how fire regimes in North America changed between 15 and 10,000 years ago, a time of large and rapid climate changes.

Their results provide no evidence for continental-scale fires, but support the fact that the increase in large-scale wildfires in all regions of the world during the past decade is related to an increase in global warming.

Fire is the most ubiquitous form of landscape disturbance and has important effects on climate through the global carbon cycle and changing atmospheric chemistry. This has triggered an interest in knowing how fire has changed in the past, and particularly how fire regimes respond to periods of major warming.

The end of the Younger Dryas, about 11,700 years ago, was an interval when the temperature of Greenland warmed by over 5�C in less than a few decades. The team used 35 records of charcoal accumulation in lake sediments from sites across North America to see whether fire regimes across the continent showed any response to such rapid warming.

They found clear changes in biomass burning and fire frequency whenever climate changed abruptly, and most particularly when temperatures increased at the end of the Younger Dryas cold phase. The results are published today [26 January] in the Proceedings of the National Academy of Science.

Understanding whether rapid changes in climate have caused wild fires in the past will help understand whether current changes in global temperatures will cause more frequent fires at the present time. Such fires have a major impact on the economy and health of the population, as well as feeding into the increase in global warming.

The continents as a heat blanket

Drifting of the large tectonic plates and the superimposed continents is not only powered by the heat-driven convection processes in the Earth’s mantle, but rather retroacts on this internal driving processes. In doing so, the continents function as a thermal blanket, which leads to an accumulation of heat underneath, and which in turn can cause the break-up of the super-continents.

These results of numerical modelling have been published by scientists from the GFZ German Research Centre for Geosciences in the latest volume of the journal PHYSICS OF THE EARTH AND PLANETARY INTERIORS (Vol. 171, S. 313-322).

Alfred Wegener’s theory of continental drift was turned up when the driving forces for continental drift were discovered during the 50s and 60s: The enormous heat in the Earth’s core and Earth’s mantle generates the flow of rocks within the Earth’s mantle, a process similar to the movement of warm water in a cooking pot. This heat-driven mass transport is called convection. On the Earth’s surface, this process leads not only to plate movement but also to drifting of the continents floating on the plates.

To date however, there has been no realistic mathematical-physical theory describing the interaction between the convective movement in the Earths mantle and the continental drift. V. Trubitsin, M. Kaban und M. Rothacher from the GFZ have now developed a numerical model, based on the current position of the continents, the structures of the Earth’s mantle obtained through geophysical measurements, and the current displacement rates on the surface. Hence they were able to calculate the future position of the continents in hundreds of millions of years.

It could be shown that the enormous heat in the Earth’s interior does not generally lead to a chaotic mass transport within the Earth’s mantle. On the contrary, the continents influence the heat distribution within the Earth’s mantle and the associated convective mass flow. In other words the continents act as a thermal blanket causing heat to accumulate beneath. A self-regulating system develops, beginning and ending with a super-continent. This super-continent breaks apart due to heat accumulation which in turn leads to a reorganoization of mantle convection with the pieces ultimately joining again to form a large super-continent.

Danube delta holds answers to ‘Noah’s flood’ debate

Did a catastrophic flood of biblical proportions drown the shores of the Black Sea 9,500 years ago, wiping out early Neolithic settlements around its perimeter? A geologist with the Woods Hole Oceanographic Institution (WHOI) and two Romanian colleagues report in the January issue of Quaternary Science Reviews that, if the flood occurred at all, it was much smaller than previously proposed by other researchers.

Using sediment cores from the delta of the Danube River, which empties into the Black Sea, the researchers determined sea level was approximately 30 meters below present levels-rather than the 80 meters others hypothesized.

“We don’t see evidence for a catastrophic flood as others have described,” said Liviu Giosan, a geologist in the WHOI Geology and Geophysics Department.

Ten thousand years ago, at the end of the last glacial period, the Black Sea was a lake-cut off from the Sea of Marmara and beyond it the Mediterranean by the Bosphorus sill. Debate in geological and archaeological circles has focused on whether, as glaciers melted and global sea levels began to rise, the Bosphorus sill overflowed gradually or whether a flood broke through the sill, drowning some 70,000 square kilometers and wiping out early Neolithic civilizations in the region. In addition to questions about the rate of the flood, investigators continue to debate the extent of the flood — a debate centered around what the level of the Black Sea was 9,500 years ago.

In the late 1990s, Columbia University researchers Bill Ryan and Walter Pitman examined the geological evidence and estimated the Black Sea level at the time of the flood was approximately 80 meters lower than present day levels. They suggested that the impact of a Black Sea flood could have forced the movement of early agriculturist groups to central Europe and established the story of Noah and his ark, as well as flood myths among other peoples.

The source of the uncertainty fueling the Black Sea flood debate is the difficulty of finding reliable sea level markers to date the flood. “Sea level is like the Holy Grail,” said Giosan. “You can’t really talk about a flood if you don’t know the exact levels of the sea level in both the Black Sea and outside it in the Mediterranean. And that’s what we tried to find.”

Scientists examine the geochemistry of sedimentary deposits for evidence of fresh water fauna and the morphology of features on the seafloor, trying to infer drowned beaches or wind-generated dunes, but there are pitfalls associated with these indicators. Sediments are subject to erosion by waves and currents, and sand deposits formed by underwater currents can misleadingly be interpreted as dunes or beaches. “Instead, what we use as indicators of sea level is the level of the Danube River delta plain, an immense landform that cannot be mistaken for something else,” Giosan stated.

A delta is formed when a river empties into a body of water. It dumps sediments and builds a flat plain-the delta-that is within a couple of meters of the shore and is, therefore, an indicator of sea level. In 2006, a team led by Liviu Giosan showed that contrary to Soviet-era data suggesting large oscillations of Black Sea level, the development phases of the Danube delta demonstrate that the level was more or less as today in the last 6000 years.

To extend their record back in time beyond 6000 years, in 2007, Giosan and his colleagues drilled a new core to 42 meters depth at the mouth of the Danube River, the largest river emptying into the Black Sea. Their goal was to reconstruct the history of that part of the delta-before and after the flood-through an examination of the sediments. In analyzing the delta sediment from the new core as well as others taken in the region, Giosan’s team discovered fresh water deposits of the newly forming delta dating back approximately 10,000 years, subsequently overlaid by fine marine sediments, followed by the modern delta deposits.

“It’s amazing,” said Giosan. “The early delta was forming in a fresh water lake just a couple of hundred years before the flood. And after the flood you have these marine deposits overlaying the whole delta region.”

Using sediment cores to reconstruct the delta with accurate dates is challenging. To attach a date to the layers of a core, scientists use radiocarbon dating on the fossil shells of animals found in the core-for instance, clams or snails. But in energetic areas, waves can erode sediment on the seabed and heave up older shells, depositing them in “younger” sediments. To address these concerns, Giosan and his team used an approach that had not been used before in the Black Sea. They employed high resolution dating performed at WHOI’s Accelerator Mass Spectrometer (AMS) facility and only used “articulated” bivalves – those where both sides of the shell were still attached as they are when alive. The shells are held together by an organic substance that degrades easily when they are dead, so the valves usually separate when the animal dies. When bivalves are found intact, it means they were not moved by waves and they are likely to be in situ.

Once the researchers dated and reconstructed the delta plain, they could determine sea level for the Black Sea. They found that the Black Sea level at the time of the flood was around 30 meters below present levels. Determining how much water poured over the Bosphorus sill remains problematic. There is no direct reconstruction of the sea level for the Marmara, but, according to Giosan, indirect methods put it at approximately 5 to 10 meters above the Black Sea level at the time of the flood.

“So if this is true, it means that the magnitude of the Black Sea flood was 5 or 10 meters but not 50 to 60 meters,” said Giosan. “Still, having flooded the Black Sea by 5 meters can have important effects, for example, drowning of the Danube Delta and putting an area of 2,000 square kilometers of prime agricultural land underwater. This has important implications for the archaeology and anthropology of southern Europe, as well as on our understanding of how the unique environment of the Black Sea formed.”

Sea bed provides information about present climatic change

Lately, every drought, flood or hurricane which happens in the planet is connected with climatic change, and therefore the interest of society and scientists is getting to know this phenomenon better.

Climatic change is connected at present with the phenomenon of global warming. This is characterized by the increase of carbon dioxide (CO2 gas), which produces the reduction of heat emission to the space and provokes a higher global warming.

Although gases in the atmosphere tell us about this greenhouse effect, oceans have accumulated information for million years which allow us a better understanding of this phenomenon.

The past lets us know the present

In this process, which involves a better knowledge of carbon cycle in the sea, David Gallego Torres developed the research work “Acumulación y preservación de materia orgánica en sedimentos marinos: implicaciones en los ciclos del carbono y nutrientes” (Accumulation and preservation of organic matter in marine sediments: implications in the cycles of carbon and nutrients), under the supervision of Professors Francisca Martínez Ruiz and Miguel Ortega Huertas of the University of Granada (Andalusian Institute of Earth Sciences, CEAMA and Department of Mineralogy and Petrology).

“Oceans may act as a drain of carbon, in the way of inorganic carbonates or as organic matter settled in sediments”, says Gallego Torres, who did research, among other phenomena, into the accumulation of organic matter in the geological past (Plioceno-Holoceno), in the East of the Mediterranean.

According to the researcher, “for the carrying out of this work we applied different techniques of geochemical analysis, mineralogy and isotopic analysis of organic matter for the reconstruction of the paleoceanographic conditions which induced to the accumulation of organic matter in marine sediments (sapropels), its implications in the carbon cycle and, consequently, in the climatic variation in the Mediterranean area and in the African craton, the main source of nutrients of these sediments”.

Professor Francisca Martínez Ruiz highlights that the analysis research line of the climatic variability in the geological past “provides scenes of climatic changes which help us to understand the answers of the components of the climatic system in future”.


Doctor David Gallego Torres says that one of the main conclusions of his research is that “climatic fluctuations affect the marine environment in such a way that there may be a carbon taking by organic matter, due to these changes in marine environment’s oceanography, in such a way that the organic matter would remain accumulated again in the earth’s crust of sediments and would remain there for a while”.

Other conclusions are:

  • the accumulation of organic matter in marine sediments is mainly connected with an intensification of marine productivity;

  • anoxic conditions (without oxygen) of the sea bed favour the preservation of such organic matter, but they can not produce an enrichment in the sediment by themselves;
  • anoxia causes nutrients’ recycling maintaining a high productivity, in such a way that the interaction between primary high productivity and anoxia may promote the accumulation of organic matter in sediments.
  • Such fixing of organic carbon in the litosphere is connected with the climatic system, as high concentrations of CO2 in the atmosphere facilitate a high productivity, due to an excess of carbon. Such CO2 is later fixed, in the way of organic matter, and removed of the atmospheric reservoir inducing a fall in Earth’s superficial temperature.

Satellites confirm half-century of West Antarctic warming

The Antarctic Peninsula juts into the Southern Ocean, reaching farther north than any other part of the continent. The southernmost reach of global warming was believed to be limited to this narrow strip of land, while the rest of the continent was presumed to be cooling or stable.

Not so, according to a new analysis involving NASA data. In fact, the study has confirmed a trend suspected by some climate scientists.

“Everyone knows it has been warming on the Antarctic Peninsula, where there are lots of weather stations collecting data,” said Eric Steig, a climate researcher at the University of Washington in Seattle, and lead author of the study. “Our analysis told us that it is also warming in West Antarctica.”

The finding is the result of a novel combination of historical temperature data from ground-based weather stations and more recent data from satellites. Steig and colleagues used data from each record to fill in gaps in the other and to reconstruct a 50-year history of surface temperatures across Antarctica.

Over the years, climate research in northern latitudes led researchers to believe that the Arctic is where impacts of global climate change would be seen first. Less certain is how climate is affecting Antarctica where inland temperatures are known to plunge to minus 112°F, and ground-based weather stations have been sparse.

It’s this sparse data collection-from ground-stations on the Antarctic Peninsula and previous reports that much of East Antarctica has experienced cooling since 1978-that led the International Panel on Climate Change to conclude in its most recent report that Antarctica is the one continent where we have failed to detect human-caused temperature changes.

With funding from the National Science Foundation’s Office of Polar Programs, Steig and colleagues set out to reconstruct Antarctica’s recent past. Ground-based stations have recorded temperatures since 1957, but most of those readings come from the peninsula and areas on the edges of the continent. But at the same time, scientists such as study co-author Joey Comiso of NASA’s Goddard Space Flight Center in Greenbelt, Md., have been gathering measurements from a series of Advanced Very High Resolution Radiometer (AVHRR) instruments deployed on satellites since 1981.

To construct the new 50-year temperature record, the team applied a statistical technique to estimate temperatures missing from ground-based observations. They calculated the relationship between overlapping satellite and ground-station measurements over the past 26 years. Next, they applied that correlation to ground measurements from 1957 to 1981 and calculated what the satellites would have observed.

The new analysis shows that Antarctic surface temperatures increased an average of 0.22°F (0.12°C) per decade between 1957 and 2006. That’s a rise of more than 1°F (0.5°C) in the last half century. West Antarctica warmed at a higher rate, rising 0.31°F (0.17°C) per decade. The results, published Jan. 22 in Nature, confirm earlier findings based on limited weather station data and ice cores.

While some areas of East Antarctica have been cooling in recent decades, the longer 50-year trend depicts that, on average, temperatures are rising across the continent.

West Antarctica is particularly vulnerable to climate changes because its ice sheet is grounded below sea level and surrounded by floating ice shelves. If the West Antarctic ice sheet completely melted, global sea level would rise by 16 to 20 feet (5 to 6 meters).

To identify causes of the warming, the team turned to Drew Shindell of NASA’s Goddard Institute for Space Studies in New York, who has used computer models to identify mechanisms driving Antarctica’s enigmatic temperature trends.

Previously, researchers focused on Antarctic ozone depletion, which influences large-scale atmospheric fluctuations around the continent-most notably, the Southern Annular Mode, which speeds up wind flow to isolate and cool the continent.

Shindell compared Steig’s temperature data with results from a computer model that can simulate the response of the atmospheric system to changes in land surface, ice cover, sea surface temperatures, and atmospheric composition. He found the ozone-influenced Southern Annular Mode is not necessarily the primary influence on Antarctic climate. Instead, it appears that smaller-scale, regional changes in wind circulation are bringing warmer air and more moisture-laden storms to West Antarctica.

“We still believe ozone depletion can increase wind speeds around Antarctica, further isolating the interior,” Shindell said. “But it’s clear now that it’s not such a dominant influence on temperature trends.”