First eyewitness accounts of mystery volcanic eruption

This eruption occurred just before the 1815 Tambora volcanic eruption which is famous for its impact on climate worldwide, with 1816 given memorable names such as ‘Eighteen-Hundred-and-Froze-to-Death’, the ‘Year of the Beggar’ and the ‘Year Without a Summer’ because of unseasonal frosts, crop failure and famine across Europe and North America. The extraordinary conditions are considered to have inspired literary works such as Byron’s ‘Darkness’ and Mary Shelley’s Frankenstein.

However, the global deterioration of the 1810s into the coldest decade in the last 500 years started six years earlier, with another large eruption. In contrast to Tambora, this so-called ‘Unknown’ eruption seemingly occurred unnoticed, with both its location and date a mystery. In fact the ‘Unknown’ eruption was only recognised in the 1990s, from tell-tale markers in Greenland and Antarctic ice that record the rare events when volcanic aerosols are so violently erupted that they reach the Earth’s stratosphere.

Working in collaboration with colleagues from the School of Earth Sciences and PhD student Alvaro Guevara-Murua, Dr Caroline Williams, from the Department of Hispanic, Portuguese and Latin American Studies, began searching historical archives for references to the event.

Dr Williams said: “I spent months combing through the vast Spanish colonial archive, but it was a fruitless search – clearly the volcano wasn’t in Latin America. I then turned to the writings of Colombian scientist Francisco José de Caldas, who served as Director of the Astronomical Observatory of Bogotá between 1805 and 1810. Finding his precise description of the effects of an eruption was a ‘Eureka’ moment.”

In February 1809 Caldas wrote about a “mystery” that included a constant, stratospheric “transparent cloud that obstructs the sun’s brilliance” over Bogotá, starting on the 11 December 1808 and seen across Colombia. He gave detailed observations, for example that the “natural fiery colour [of the sun] has changed to that of silver, so much so that many have mistaken it for the moon”; and that the weather was unusually cold, the fields covered with ice and the crops damaged by frost.

Unearthing a short account written by physician José Hipólito Unanue in Lima, Peru, describing sunset after-glows (a common atmospheric effect caused by volcanic aerosols in the stratosphere) at the same time as Caldas’ “vapours above the horizon”, enabled the researchers to verify that the atmospheric effects of the eruption were seen at the same time on both sides of the equator.

These two 19th century Latin American scientists provide the first direct observations that can be linked to the ‘Unknown’ eruption. More importantly, the accounts date the eruption to within a fortnight of 4 December 1808.

Dr Erica Hendy said: “There have to be more observations hidden away, for example in ship logs. Having a date for the eruption will now make it much easier to track these down, and maybe even pinpoint the volcano. Climate modelling of this fascinating decade will also now be more accurate because the season of the eruption determines how the aerosols disperse around the globe and where climatic effects are felt.”

Alvaro Guevara-Murua added: “This study has meant delving into many fields of research – obviously paleoclimatology and volcanology, but also 19th century meteorology and Spanish colonial history – and has also needed rigorous precision to correctly translate the words of two scientists writing 200 years ago. Giving them a voice in modern science has been a big responsibility.”

One further question remains: why are there so few historical accounts of what was clearly a significant event with wide-reaching consequences? Perhaps, Dr Williams suggests, the political environment on both sides of the Atlantic at the beginning of the nineteenth century played a part.

“The eruption coincided with the Napoleonic Wars in Europe, the Peninsular War in Spain, and with political developments in Latin America that would soon lead to the independence of almost all of Spain’s American colonies. It’s possible that, in Europe and Latin America at least, the attention of individuals who might otherwise have provided us with a record of unusual meteorological or atmospheric effects simply turned to military and political matters instead,” she said.

Sea-level spikes, volcanic risk, volcanos cause drought

Unforeseen, short-term increases in sea level caused by strong winds, pressure changes and fluctuating ocean currents can cause more damage to beaches on the East Coast over the course of a year than a powerful hurricane making landfall, according to a new study. The new research suggests that these sea-level anomalies could be more of a threat to coastal homes and businesses than previously thought, and could become higher and more frequent as a result of climate change, according to a new study accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.

From this week’s Eos: Assessing Volcanic Risk in Saudi Arabia: An Integrated Approach


The Kingdom of Saudi Arabia has numerous large volcanic fields, known locally as “harrats.” The largest of these, Harrat Rahat, produced a basaltic fissure eruption in 1256 A.D. with lava flows traveling within 20 kilometers of the city Al-Madinah, which has a current population of 1.5 million plus an additional 3 million pilgrims annually. With more than 950 visible vents and periodic seismic swarms, an understanding of the risk of future eruptions in this volcanic field is vital. The Volcanic Risk in Saudi Arabia (VORISA) project was developed as a multidisciplinary international research collaboration that integrates geological, geophysical, hazard, and risk studies in this important area.

From AGU’s journals: Large volcanic eruptions cause drought in eastern China


In most cases, the annual East Asian Monsoon brings heavy rains and widespread flooding to southeast China and drought conditions to the northeast. At various points throughout history, however, large volcanic eruptions have upset the regular behavior of the monsoon.

Sulfate aerosols injected high into the atmosphere by powerful eruptions can lower the land-sea temperature contrast that powers the monsoon circulation. How this altered aerosol forcing affects precipitation is not entirely clear, however, as climate models do not always agree with observations of the nature and scale of the effect.

Using two independent records of historical volcanic activity along with two different measures of rainfall, including one 3,000-year long record derived from local flood and drought observations, Zhuo et al. analyzes how large volcanic eruptions changed the conditions on the ground for the period 1368 to 1911. Understanding the effect of sulfate aerosols on monsoon behavior is particularly important now, as researchers explore aerosol seeding as a means of climate engineering.

The authors find that large Northern Hemispheric volcanic eruptions cause strong droughts in much of eastern China. The drought begins in the north in the second or third summer following an eruption and slowly moves southward over the next 2 to 3 years. They find that the severity of the drought scales with the amount of aerosol injected into the atmosphere, and that it takes 4 to 5 years for precipitation to recover. The drying pattern agrees with observations from three large modern eruptions.

China’s northeast is the country’s major grain-producing region. The results suggest that any geoengineering schemes meant to mimic the effect of a large volcanic eruption could potentially trigger devastating consequences for China’s food supply.

Rewriting the history of volcanic forcing during the past 2,000 years

Locations of Antarctic ice core sites used for volcanic sulfate aerosol deposition reconstruction (right); a  DRI scientist examines a freshly drilled ice core in the field before ice cores are analyzed in DRI's ultra-trace ice core analytical laboratory. -  M. Sigl
Locations of Antarctic ice core sites used for volcanic sulfate aerosol deposition reconstruction (right); a DRI scientist examines a freshly drilled ice core in the field before ice cores are analyzed in DRI’s ultra-trace ice core analytical laboratory. – M. Sigl

A team of scientists led by Michael Sigl and Joe McConnell of Nevada’s Desert Research Institute (DRI) has completed the most accurate and precise reconstruction to date of historic volcanic sulfate emissions in the Southern Hemisphere.

The new record, described in a manuscript published today in the online edition of Nature Climate Change, is derived from a large number of individual ice cores collected at various locations across Antarctica and is the first annually resolved record extending through the Common Era (the last 2,000 years of human history).

“This record provides the basis for a dramatic improvement in existing reconstructions of volcanic emissions during recent centuries and millennia,” said the report’s lead author Michael Sigl, a postdoctoral fellow and specialist in DRI’s unique ultra-trace ice core analytical laboratory, located on the Institute’s campus in Reno, Nevada.

These reconstructions are critical to accurate model simulations used to assess past natural and anthropogenic climate forcing. Such model simulations underpin environmental policy decisions including those aimed at regulating greenhouse gas and aerosol emissions to mitigate projected global warming.

Powerful volcanic eruptions are one of the most significant causes of climate variability in the past because of the large amounts of sulfur dioxide they emit, leading to formation of microscopic particles known as volcanic sulfate aerosols. These aerosols reflect more of the sun’s radiation back to space, cooling the Earth. Past volcanic events are measured through sulfate deposition records found in ice cores and have been linked to short-term global and regional cooling.

This effort brought together the most extensive array of ice core sulfate data in the world, including the West Antarctic Ice Sheet (WAIS) Divide ice core – arguably the most detailed record of volcanic sulfate in the Southern Hemisphere. In total, the study incorporated 26 precisely synchronized ice core records collected in an array of 19 sites from across Antarctica.

“This work is the culmination of more than a decade of collaborative ice core collection and analysis in our lab here at DRI,” said Joe McConnell, a DRI research professor who developed the continuous-flow analysis system used to analyze the ice cores.

McConnell, a member of several research teams that collected the cores (including the 2007-2009 Norwegian-American Scientific Traverse of East Antarctica and the WAIS Divide project that reached a depth of 3,405 meters in 2011), added, “The new record identifies 116 individual volcanic events during the last 2000 years.”

“Our new record completes the period from years 1 to 500 AD, for which there were no reconstructions previously, and significantly improves the record for years 500 to 1500 AD,” Sigl added. This new record also builds on DRI’s previous work as part of the international Past Global Changes (PAGES) effort to help reconstruct an accurate 2,000-year-long global temperature for individual continents.

This study involved collaborating researchers from the United States, Japan, Germany, Norway, Australia, and Italy. International collaborators contributed ice core samples for analysis at DRI as well as ice core measurements and climate modeling.

According to Yuko Motizuki from RIKEN (Japan’s largest comprehensive research institution), “The collaboration between DRI, National Institute of Polar Research (NIPR), and RIKEN just started in the last year, and we were very happy to be able to use the two newly obtained ice core records taken from Dome Fuji, where the volcanic signals are clearly visible. This is because precipitation on the site mainly contains stratospheric components.” Dr. Motizuki analyzed the samples collected by the Japanese Antarctic Research Expedition.

Simulations of volcanic sulfate transport performed with a coupled aerosol-climate model were compared to the ice core observations and used to investigate spatial patterns of sulfate deposition to Antarctica.

“Both observations and model results show that not all eruptions lead to the same spatial pattern of sulfate deposition,” said Matthew Toohey from the German institute GEOMAR Helmholtz Centre for Ocean Research Kiel. He added, “Spatial variability in sulfate deposition means that the accuracy of volcanic sulfate reconstructions depends strongly on having a sufficient number of ice core records from as many different regions of Antarctica as possible.”

With such an accurately synchronized and robust array, Sigl and his colleagues were able to revise reconstructions of past volcanic aerosol loading that are widely used today in climate model simulations. Most notably, the research found that the two largest volcanic eruptions in recent Earth history (Samalas in 1257 and Kuwae in 1458) deposited 30 to 35 percent less sulfate in Antarctica, suggesting that these events had a weaker cooling effect on global climate than previously thought.

Researchers warn against abrupt stop to geoengineering method

As a range of climate change mitigation scenarios are discussed, University of Washington researchers have found that the injection of sulfate particles into the atmosphere to reflect sunlight and curb the effects of global warming could pose a severe threat if not maintained indefinitely and supported by strict reductions in greenhouse gas (GHG) emissions.

The new study, published today, 18 February, in IOP Publishing’s journal Environmental Research Letters, has highlighted the risks of large and spatially expansive temperature increases if solar radiation management (SRM) is abruptly stopped once it has been implemented.

SRM is a proposed method of geoengineering whereby tiny sulfate-based aerosols are released into the upper atmosphere to reflect sunlight and cool the planet. The technique has been shown to be economically and technically feasible; however, its efficacy depends on its continued maintenance, without interruption from technical faults, global cooperation breakdown or funding running dry.

According to the study, global temperature increases could more than double if SRM is implemented for a multi-decadal period of time and then suddenly stopped, in relation to the temperature increases expected if SRM was not implemented at all.

The researchers used a global climate model to show that if an extreme emissions pathway-RCP8.5-is followed up until 2035, allowing temperatures to rise 1°C above the 1970-1999 mean, and then SRM is implemented for 25 years and suddenly stopped, global temperatures could increase by 4°C in the following decades.

This rate of increase, caused by the build-up of background greenhouse gas emissions, would be well beyond the bounds experienced in the last century and more than double the 2°C temperature increase that would occur in the same timeframe if SRM had not been implemented.

On a regional and seasonal scale, the temperature changes would be largest in an absolute sense in winter over high latitude land, but compared to historical fluctuations, temperature changes would be largest in the tropics in summertime, where there is usually very little variation.

Lead author of the research, Kelly McCusker, from the University of Washington, said: “According to our simulations, tropical regions like South Asia and Sub-Saharan Africa are hit particularly hard, the very same regions that are home to many of the world’s most food insecure populations. The potential temperature changes also pose a severe threat to biodiversity.”

Furthermore, the researchers used a simple climate model to study a variety of plausible greenhouse gas scenarios and SRM termination years over the 21st century. They showed that climate sensitivity-a measure of how much the climate will warm in response to the greenhouse effect-had a lesser impact on the rate of temperature changes.

Instead, they found that the rates of temperature change were determined by the amount of GHG emissions and the duration of time that SRM is deployed.

“The primary control over the magnitude of the large temperature increases after an SRM shutoff is the background greenhouse gas concentrations. Thus, the greater the future emissions of greenhouse gases, the larger the temperature increases would be, and, similarly, the later the termination occurs while GHG emissions continue, the larger the temperature increases,” continued McCusker.

“The only way to avoid creating the risk of substantial temperature increases through SRM, therefore, is concurrent strong reductions of GHG emissions.”

Forest emissions, wildfires explain why ancient Earth was so hot

This photo shows Nadine Unger with Yale University's omega supercomputer. -  Photo by Matthew Garrett/Yale School of Forestry & Environmental Studies
This photo shows Nadine Unger with Yale University’s omega supercomputer. – Photo by Matthew Garrett/Yale School of Forestry & Environmental Studies

The release of volatile organic compounds from Earth’s forests and smoke from wildfires 3 million years ago had a far greater impact on global warming than ancient atmospheric levels of carbon dioxide, a new Yale study finds.

The research provides evidence that dynamic atmospheric chemistry played an important role in past warm climates, underscoring the complexity of climate change and the relevance of natural components, according to the authors. They do not address or dispute the significant role in climate change of human-generated CO2 emissions.

Using sophisticated Earth system modeling, a team led by Nadine Unger of the Yale School of Forestry & Environmental Studies (F&ES) calculated that concentrations of tropospheric ozone, aerosol particles, and methane during the mid-Pliocene epoch were twice the levels observed in the pre-industrial era – largely because so much more of the planet was covered in forest.

Those reactive compounds altered Earth’s radiation balance, contributing a net global warming as much as two to three times greater than the effect of carbon dioxide, according to the study, published in the journal Geophysical Research Letters.

These findings help explain why the Pliocene was two to three degrees C warmer than the pre-industrial era despite atmospheric levels of carbon dioxide that were approximately the same as today, Unger said.

“The discovery is important for better understanding climate change throughout Earth’s history, and has enormous implications for the impacts of deforestation and the role of forests in climate protection strategies,” said Unger, an assistant professor of atmospheric chemistry at F&ES.

“The traditional view,” she said, “is that forests affect climate through carbon storage and by altering the color of the planet’s surface, thus influencing the albedo effect. But as we are learning, there are other ways that forest ecosystems can impact the climate.”

The albedo effect refers to the amount of radiation reflected by the surface of the planet. Light-colored snowy surfaces, for instance, reflect more light and heat back into space than darker forests.

Climate scientists have suggested that the Pliocene epoch might provide a glimpse of the planet’s future if humankind is unable to curb carbon dioxide emissions. During the Pliocene, the two main factors believed to influence the climate – atmospheric CO2 concentrations and the geographic position of the continents – were nearly identical to modern times. But scientists have long wondered why the Pliocene’s global surface air temperatures were so much warmer than Earth’s pre-industrial climate.

The answer might be found in highly reactive compounds that existed long before humans lived on the planet, Unger says. Terrestrial vegetation naturally emits vast quantities of volatile organic compounds, for instance. These are critical precursors for organic aerosols and ozone, a potent greenhouse gas. Wildfires, meanwhile, are a major source of black carbon and primary organic carbon.

Forest cover was vastly greater during the Pliocene, a period marked not just by warmer temperatures but also by greater precipitation. At the time, most of the arid and semi-arid regions of Africa, Australia, and the Arabian peninsula were covered with savanna and grassland. Even the Arctic had extensive forests. Notably, Unger says, there were no humans to cut the forests down.

Using the NASA Goddard Institute for Space Studies Model-E2 global Earth system model, the researchers were able to simulate the terrestrial ecosystem emissions and atmospheric chemical composition of the Pliocene and the pre-industrial era.

According to their findings, the increase in global vegetation was the dominant driver of emissions during the Pliocene – and the subsequent effects on climate.

Previous studies have dismissed such feedbacks, suggesting that these compounds would have had limited impact since they would have been washed from the atmosphere by frequent rainfall in the warmer climate. The new study argues otherwise, saying that the particles lingered about the same length of time – one to two weeks – in the Pliocene atmosphere compared to the pre-industrial.

Unger says her findings imply a higher climate sensitivity than if the system was simply affected by CO2 levels and the albedo effect.

“We might do a lot of work to reduce air pollution from road vehicle and industrial emissions, but in a warmer future world the natural ecosystems are just going to bring the ozone and aerosol particles right back,” she said. “Reducing and preventing the accumulation of fossil-fuel CO2 is the only way to ensure a safe climate future now.”

The modeling calculations were performed on Yale University’s omega supercomputer, a 704-node cluster capable of processing more than 52 trillion calculations per second.

What do we know — and not know — about fracking?

Fracking is in the headlines a lot these days, and everyone has an opinion about it. But how much do we really know for certain about the oil and gas extraction technique and its health effects? And how do we find out the truth among all the shouted opinions? To help cut through the static, several scientists have put together a multidisciplinary session on fracking and health at the meeting of The Geological Society of America (GSA) in Denver on Sunday.

“There is so much perceived information on fracking in the media, with so little of it based on real science and actual data,” says Thomas Darrah, a medical geologist at Ohio State University and one of the conveners of the GSA Pardee Keynote Session, “Energy and Health: The Emergence of Medical Geology in Response to the Shale Gas Boom.”

“Fracking has moved so quickly, and the research community is playing catch up on water, air, and health issues,” said Robert Jackson, an environmental scientist at Duke University who will present his research this Sunday. “The goal is to present a state of the science for researchers and the public.”

The afternoon keynote session is designed to cover a lot of ground. It will start with the geologists, hydrologists, and air-quality experts who are studying the chemistry and the physical properties of fracking in the ground, water, and air. Then the session veers into territory not often covered at a geological meeting, with talks by toxicologists, researchers in occupational medicine, and epidemiologists.

“This session includes people who would normally not be anywhere near a GSA conference,” said Darrah. “The idea is that we end the session by having the geoscience community interact with a group of people who are looking at health data sets: epidemiologists. That way we can put people working on the other end of the equation in the same room.” Included in the eleven scheduled presentations, and at the medical end of the equation, is a talk titled “Public Health Implications of Hydraulic Fracturing,” by David O. Carpenter of the University of Albany’s School of Public Health, and another, “Energy and Health: The Emergence of Medical Geology in Response to the Shale Gas Boom: An Occupational and Environmental Medicine Perspective,” to be delivered by Theodore F. Them of Guthrie Clinic Ltd.

For his part, Darrah will be presenting a talk about his work, “Understanding In-House Exposures to Natural Gas and Metal-Rich Aerosols from Groundwater within an Unconventional Energy Basin.”

There are two additional presentations on the air-quality issues of fracking, which is perhaps the topic the public knows the least about. Gabrielle Petron of the University of Colorado and NOAA will be talking about outdoor air emissions from hydraulic fracturing activities, and public health researcher Lisa M. Mackenzie of the University of Colorado will talk about work evaluating specific health risks from exposure to natural gas drilling in Garfield County, Colorado.

Improved interpretation of volcanic traces in ice

Storms, cold, poor harvests — the year 1816 was a “year without a summer” in European history. The reason was the eruption of the Indonesian volcano Tambora a year earlier. It had thrown huge amounts of sulfur compounds into the stratosphere (at altitudes of 15-50 km) where they spread around the entire globe and significantly weakened solar radiation for several years afterwards. Such intense volcanic eruptions are quite common in Earth’s history. To better understand their impact on the climate and the atmosphere, scientists try to reconstruct those eruptions accurately. Important archives of information are ice cores from Greenland and Antarctica because the sulfur particles ejected from the volcano fall back to the surface. A portion of that fallout is trapped in the ice of the polar regions and can be analyzed even thousands of years afterwards. The former aerosol contamination of the atmosphere is derived from it using a simple ratio calculation.

But this method has its limitations. “Volcanic aerosols in the stratosphere absorb infrared radiation, thereby heating up the stratosphere, and changing the wind conditions subsequently,” said Dr. Matthew Toohey, atmospheric scientist at GEOMAR Helmholtz Centre for Ocean Research Kiel. Using an atmospheric model, he has now tested the effects of this phenomenon. “We have found that the deposition of sulfur compounds in the Antarctic after very large volcanic eruptions in the tropics may be lower than previously thought,” the atmospheric researcher summarizes the findings of the study which has just been published in the current issue of the international Journal of Geophysical Research – Atmosphere.

For the study, Dr. Toohey and his colleagues from GEOMAR and the Max Planck Institute for Meteorology in Hamburg have used an aerosol-climate model to track 70 different eruption scenarios while analyzing the distribution of the sulfur particles. It was based on real volcanic eruptions during the past 200,000 years in Central America, which had been investigated in the framework of the Collaborative Research Project 574. “In our calculations, we could clearly see the differences in distribution and deposition between the northern and southern hemispheres,” explains co-author and director of the working group, Dr. Kirstin Krüger. The spatial deposition of sulfur particles in the bipolar ice cores, as calculated in the model, agrees well with the actually measured deposits of large volcanic eruptions, such as Pinatubo in 1991 or even of Tambora of 1815.

“If we know how volcanic sulfur particles affect the atmospheric winds, we can have a much improved interpretation of the traces of volcanic activities in the ice cores,” says Dr. Toohey. For one, there are better estimates of the strength of an outbreak. And secondly, the previously undetermined traces of volcanic eruptions that could not be assigned to any particular event or volcano eruption, can now be clearly traced to their origin.

“In any case, the results of our model study give a clear indication that the bipolar variability of sulfate deposits must be taken into consideration if the traces of large volcanic eruptions are to be deduced from ice cores,” says Dr. Krüger, “Several research groups that deal with this issue have already contacted us to verify their data through our model results.”

Volcanoes cause climate gas concentrations to vary

MIPAS data confirm the correlation between high sulfur dioxide concentrations (yellow-red) and high-reaching volcano eruptions (triangles). -  (Figure: KIT/M. Höpfner)
MIPAS data confirm the correlation between high sulfur dioxide concentrations (yellow-red) and high-reaching volcano eruptions (triangles). – (Figure: KIT/M. Höpfner)

Trace gases and aerosols are major factors influencing the climate. With the help of highly complex installations, such as MIPAS on board of the ENVISAT satellite, researchers try to better understand the processes in the upper atmosphere. Now, Karlsruhe Institute of Technology presents the most comprehensive overview of sulfur dioxide measurements in the journal of Atmospheric Chemistry and Physics (doi:10.5194/acpd-13-12389-2013).

“Sulfur compounds up to 30 km altitude may have a cooling effect,” Michael Höpfner, the KIT scientist responsible for the study, says. For example, sulfur dioxide (SO2) and water vapor react to sulfuric acid that forms small droplets, called aerosols, that reflect solar radiation back into universe. “To estimate such effects with computer models, however, the required measurement data have been lacking so far.” MIPAS infrared spectrometer measurements, however, produced a rather comprehensive set of data on the distribution and development of sulfur dioxide over a period of ten years.

Based on these results, major contributions of the sulfur budget in the stratosphere can be analyzed directly. Among others, carbonyl sulfide (COS) gas produced by organisms ascends from the oceans, disintegrates at altitudes higher than 25 km, and provides for a basic concentration of sulfur dioxide. The increase in the stratospheric aerosol concentration observed in the past years is caused mainly by sulfur dioxide from a number of volcano eruptions. “Variation of the concentration is mainly due to volcanoes,” Höpfner explains. Devastating volcano eruptions, such as those of the Pinatubo in 1991 and Tambora in 1815, had big a big effect on the climate. The present study also shows that smaller eruptions in the past ten years produced a measurable effect on sulfur dioxide concentration at altitudes between 20 and 30 km. “We can now exclude that anthropogenic sources, e.g. power plants in Asia, make a relevant contribution at this height,” Höpfner says.

“The new measurement data help improve consideration of sulfur-containing substances in atmosphere models,” Höpfner explains. “This is also important for discussing the risks and opportunities of climate engineering in a scientifically serious manner.”

MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) was one of the main instruments on board of the European environmental satellite ENVISAT that supplied data from 2002 to 2012. MIPAS was designed by the KIT Institute of Meteorology and Climate Research. All around the clock, the instrument measured temperature and more than 30 atmospheric trace gases. It recorded more than 75 million infrared spectra. KIT researchers, together with colleagues from Forschungszentrum Jülich, have now developed the MIPAS successor GLORIA that may be the basis of a future satellite instrument for climate research.

Do dust particles curb climate change?

Every cloud is different from the next. It is therefore important to study the types of cloud systems in which aerosols have the greatest influence. -  Max Planck Institute for Meteorology / Stevens
Every cloud is different from the next. It is therefore important to study the types of cloud systems in which aerosols have the greatest influence. – Max Planck Institute for Meteorology / Stevens

A knowledge gap exists in the area of climate research: for decades, scientists have been asking themselves whether, and to what extent man-made aerosols, that is, dust particles suspended in the atmosphere, enlarge the cloud cover and thus curb climate warming. Research has made little or no progress on this issue. Two scientists from the Max Planck Institute for Meteorology in Hamburg (MPI-M) and the American National Oceanic and Atmospheric Administration (NOAA) report in the journal Nature that the interaction between aerosols, clouds and precipitation is strongly dependent on factors that have not been adequately researched up to now. They urge the adoption of a research concept that will close this gap in the knowledge. (Nature, October 1st, 2009)

Greenhouse gases that heat up the earth’s atmosphere have their adversaries: dust particles suspended in the atmosphere which are known as aerosols. They arise naturally, for example when wind blows up desert dust, and through human activities. A large proportion of the man-made aerosols arise from sulfur dioxides that are generated, in turn, by the combustion of fossil fuels.

The aerosols are viewed as climate coolers, which compensate in part for the heating up of the earth by greenhouse gases. Climate researchers imagine the workings of this cooling mechanism in very simple terms: when aerosols penetrate clouds, they attract water molecules and therefore act as condensation seeds for drops of water. The more aerosol particles suspended in the cloud, the more drops of water are formed. When man-made dust particles join the natural ones, the number of drops increases. As a result, the average size of the drops decreases. Because smaller drops do not fall to the ground, the aerosols prevent the cloud from raining out and extend its lifetime. Consequently, the cloud cover over the earth’s surface increases. Because clouds reflect the solar radiation and throw it back into space, less heat collects in the atmosphere than when the sky is clear. Climate researchers refer to this mechanism as the “cloud lifetime effect”.

To date, however, it has not been possible to quantify the influence of the cloud lifetime effect on climate. The estimates vary hugely and range from no influence whatsoever to a cooling effect that is sufficient to more than compensate for the heating effect of carbon dioxide.

According to Bjorn Stevens from the MPI-M and Graham Feingold from the Earth System Research Laboratory at NOAA in Washington D.C the enormous uncertainty surrounding this phenomenon is indicative of the fact that the explanation of the cooling mechanism generated by aerosols is oversimplified. The two cloud researchers have analyzed the specialist literature published on this topic since the 1970s. In their survey of the literature they encountered observations that disagree with the cloud lifetime effect: for example, a field study carried out a few years ago found that clouds in the Trade Wind region rain out more quickly rather than more slowly in the presence of virtually opaque aerosols.

On the completion of their analysis of the literature, Stevens and Feingold came to the following conclusion: “Clouds react to aerosols in a very complex way and the reaction is strongly dependent on the type and state of the cloud,” says Stevens. Therefore the aerosol problem is a cloud problem. “We climate researchers must focus more on cloud systems and understand them better,” he stresses.

As the researchers write, processes in the clouds that counteract or even negate the influence of the aerosol particles have not been taken into account up to now. One example: when a cumulus cloud comes into contact with aerosols, it does not rain out. However, this has certain consequences: the fluid rises and evaporates above the cloud. The air that lies above the cloud cools down and becomes susceptible to the upward extension of the cumulus cloud. Higher cumulus clouds rain out more easily than lower ones. This is what causes precipitation. Therefore, in such situations the aerosol does not prevent the cloud from raining out.

Stevens and Feingold believe that due to such buffer mechanisms the cooling effect of the aerosols is likely to be minimal. They admit, however, that the cloud lifetime effect is not unsuitable per se as a way of explaining the processes triggered by aerosols in the clouds. “All cloud types and states cannot, however, be lumped together,” says Stevens. He calls for rethinking aerosol research and makes a comparison with cancer research: “People used to think that cancer was based on a single mechanism. Today, it is known that each type of cancer must be researched individually,” says the scientist.

According to Stevens and Feingold, research must first identify the cloud systems on which aerosols have the greatest influence. They suggest starting with particularly common types of cloud, for example flat cumulus clouds over the oceans (Trade Wind cumuli), which cover 40 percent of the global seas.

A research project to be undertaken jointly by the Max Planck Institute for Meteorology and the Caribbean Institute for Meteorology and Hydrology in Miami will make a start on this. The two-year empirical field study will commence on the Caribbean island of Barbados, which is located in the Trade Wind region, in 2010. The researchers will install remote sensing instruments on the island’s windward side that will focus on the clouds coming from the open ocean. The land measurements will be complemented by measurements taken in the clouds themselves by HALO, the German research aircraft. The data from this measurement campaign should help the scientists to reach a better understanding of the relationships between cloud cover, precipitation, local meteorological conditions and aerosols.