New study unveils 16,000 years of climate history in the tropical Andes

A new study that explores ancient temperatures and rainfall patterns in the tropical Andes of South America has revealed how 16,000 years of climate history in this part of the world was driven by carbon dioxide levels and ocean currents from global climate events.

Led by Brown University researchers, the study marks the first high-resolution temperature record covering the past 16,000 years in the tropical Andes and could help scientists predict and mitigate future climate impacts in tropical regions of the planet. The work is described in the Proceedings of the National Academy of Science.

"Usually when we study climate change in the past, we emphasize the Northern Hemisphere or Southern Hemisphere because of the outsized role they play in affecting climates all over the globe," said Boyang Zhao, a scientist in Brown's Department of Earth, Environmental and Planetary Sciences and the study's first author. "One of the biggest questions we are getting at is what are the driving factors behind temperature history in this part of the tropics, so that we can begin to potentially apply that data and knowledge to other tropical regions."

Along with future implications, the new study provides a unique look at the way distinct and distant parts of the world influence temperature and weather elsewhere, emphasizing how regional climates are connected to global climate changes.

"Our evidence here suggests that temperatures in this region of the world are more influenced by the Southern Hemisphere -- so places like Antarctica," Zhao said.

The study focused on an analysis of sediment samples from Laguna Llaviucu, a lake located in Ecuador's Cajas National Park. The measurements from the sediment samples showed that temperature variations in the tropical Andes closely aligned with climate events that saw the planet's temperatures rise and fall during the past 16,000 years.

Overall, the evidence showed that the main driver in these temperature fluctuations was the concentration of CO2. Researchers saw evidence that the tropical Andean temperatures track with Antarctic temperatures, which are mainly controlled by carbon dioxide concentrations. The findings showed that about 17,000 to 14,000 years ago, tropical Andean temperatures rose when carbon dioxide increased and that the temperatures remained relatively stable about 12,000 years ago when carbon dioxide levels remained relatively stable.

Zhao said that the findings support previous research identifying carbon dioxide as a key driver of global temperature changes since the last Ice Age.

The study also highlighted the role of ocean currents that move warm water from the tropics to the North Atlantic. During a cooling period known as the Antarctic Cold Reversal 14,500 years ago, the northward current was strengthened, causing it to transport more heat northward and cool sea surface temperatures in the south. The researchers found that this cooling effect extended to the tropical Andes.

To reconstruct past climate conditions, the researchers analyzed lipid biomarkers and hydrogen isotopes collected in 2009 from Laguna Llaviucu by scientist Mark Bush from the Florida Institute of Technology. Lipid biomarkers are chemical compounds that provide clues about past temperatures and rainfall patterns. The high-resolution data, combined with accurate dating techniques, allowed the team to create a detailed timeline of climate changes over the past 16,000 years.

According to the researchers, this is the first time organic biomarkers have been used to put together a quantitative climate history for tropical South America. It is traditionally difficult to reconstruct temperatures from the tropics.

The study also points out a number of regional differences in temperature patterns, like how the tropical Andes and Southeast Asia cooled during certain historical periods while other regions like Africa did not.The research also shows how local factors may help to counteract the global effects of rising CO2 levels by looking at differences between past temperature models and what the sediment data show.

The researchers from Brown plan to continue to explore past temperature patterns in this part of the world that historically lacks complete climate records.

"Mountain environments are some of the most sensitive regions on Earth to climate change," said James Russell, a professor of Earth, environmental, and planetary sciences at Brown who oversaw the new research. "In fact, in the near future, high tropical mountains, such as the Andes, are predicted to experience a rate of warming second only to the Arctic, which is the fastest warming part of the planet."

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Study offers most detailed glimpse yet of planet's last 11,000 summers and winters

By analyzing Antarctic ice cores, CU Boulder scientists and an international team of collaborators have revealed the most detailed look yet at the planet's recent climactic history, including summer and winter temperatures dating back 11,000 years to the beginning of what is known as the Holocene.

Published today in Nature, the study is the very first seasonal temperature record of its kind, from anywhere in the world.

"The goal of the research team was to push the boundaries of what is possible with past climate interpretations, and for us that meant trying to understand climate at the shortest timescales, in this case seasonally, from summer to winter, year-by-year, for many thousands of years," said Tyler Jones, lead author on the study, and assistant research professor and fellow at the Institute of Arctic and Alpine Research (INSTAAR).

The study also validates one aspect of a long-standing theory about Earth's climate that has not been previously proven: how seasonal temperatures in polar regions respond to Milankovitch cycles. Serbian scientist Milutin Milankovitch hypothesized a century ago that the collective effects of changes in Earth's position relative to the sun -- due to slow variations of its orbit and axis -- are a strong driver of Earth's long-term climate, including the start and end of ice ages (prior to any significant human influence on the climate).

"I am particularly excited that our result confirms a fundamental prediction of the theory used to explain Earth's ice-age climate cycles: that the intensity of sunlight controls summertime temperatures in the polar regions, and thus melt of ice, too," said Kurt Cuffey, a co-author on the study and professor at the University of California Berkeley.

These more highly detailed data on long-term climate patterns of the past also provide an important baseline for other scientists, who study the impacts of human-caused greenhouse gas emissions on our present and future climate. By knowing which planetary cycles occur naturally and why, researchers can better identify the human influence on climate change and its impacts on global temperatures.

"This research is something that humans can really relate to because we partly experience the world through the changing seasons -- documenting how summer and winter temperature varied through time translates to how we understand climate," said Jones.

Finer definition amidst diffusion

Scientists around the world have long studied Earth's past climate using ice cores gathered from the poles. These slender, cylindrical columns of ice, drilled from ancient ice sheets (mostly in Antarctica and Greenland), provide valuable long-term data trapped in time about everything from past atmospheric concentrations of greenhouse gases to past temperatures of the air and oceans.

The West Antarctic Ice Sheet (WAIS) Divide ice core, the longest ice core ever drilled by U.S. researchers, measures 11,171 feet (or over 2 miles) long and 4.8-inches in diameter -- containing data from as old as 68,000 years ago. Ice cores like this one are then carefully cut into smaller sections which can be safely transported to and stored or analyzed in ice core labs around the country -- like the Stable Isotope Lab at CU Boulder.

For this study, researchers analyzed a continuous record of water-isotope ratios from the WAIS ice core. The ratios between the concentration of these isotopes (elements with the same number of protons but different numbers of neutrons) reveal data about past temperatures and atmospheric circulation, including transitions between ice ages and warm periods in Earth's past.

Measuring seasonal changes in our planet's history from ice cores is especially difficult, however, due to the fine detail required for their shorter timescales. A process within ice sheets known as diffusion, or natural smoothing, can blur this needed detail.

These water isotopes tend to not stay in one place in the upper ice sheet, but instead move around in interconnected pathways (similar to the air pockets in Styrofoam) as they change states between vapor and ice, over decades or centuries, before sufficiently solidifying. This process can "blur" the data researchers are trying to examine. But by using the high-quality ice cores from the West Antarctic Ice Sheet, extremely high-resolution measurements and advances in ice core analysis from the past 15 years, the team was able to correct for the diffusion present in the data and complete the study.

"Even beyond that, we had to develop new methods entirely to deal with this data, because no one's ever seen it before. We had to go above and beyond what anyone's done in the past," said Jones.

Studying stable isotopes

While the study details the history of Earth's climate, the work behind it has a history of its own.

For more than three decades, researchers at INSTAAR's Stable Isotope Lab have been studying a variety of stable isotopes -- nonradioactive forms of atoms with unique molecular signatures -- found everywhere from the inside ice cores and the carbon in permafrost to the air in our atmosphere. Jones joined the lab in 2007 as a master's student and has never left.

"I have this distinct memory of walking into my advisor, Jim White's office in about 2013, and showing him that we would be able to pull out summer and winter values in this record for the last 11,000 years -- which is extremely rare. In our understanding, no one had ever done this before," said Jones. "We looked at each other and said, 'Wow, this is going to be a really big deal.'"

It then took almost a decade to figure out the proper way to interpret the data, from ice cores drilled many years before that meeting.

Bruce Vaughn, co-author and a chief scientist on the project, and manager of the Stable Isotope Lab, and Bradley Markle, co-author on the study and assistant professor at INSTAAR and the Department of Geology, were there to collect the ice in West Antarctica that was shipped back and analyzed.

The team's next step is to attempt to interpret high-resolution ice cores in other places -- such as the South Pole and in northeast Greenland, where cores have already been drilled -- to better understand our planet's climate variability.

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The last 12,000 years show a more complex climate history than previously thought

We rely on climate models to predict the future, but models cannot be fully tested as climate observations rarely extend back more than 150 years. Understanding the Earth's past climate history across a longer period gives us an invaluable opportunity to test climate models on longer timescales and reduce uncertainties in climate predictions. In this context, changes in the average surface temperature of the Earth during the current interglacial Epoch, the Holocene (approximately the past 12,000 years), have been thoroughly debated over the past decades. Reconstructions of past temperature seem to indicate that global mean temperature showed a maximum around 6,000 years ago and has cooled until the onset of the current climate crisis during the industrial revolution.

Climate model simulations, on the other hand, suggest continuous warming since the start of the Holocene. In 2014, researchers named this major mismatch between models and past climate observations the "Holocene Temperature Conundrum."

In this new study, scientists used the largest available database of past temperature reconstructions extending back 12,000 years to carefully investigate the geographic pattern of temperature change during the Holocene. Olivier Cartapanis and colleagues find that, contrary to previously thought, there is no globally synchronous warm period during the Holocene. Instead, the warmest temperatures are found at different times not only in different regions but also between the ocean and on land. This questions how meaningful comparisons of the global mean temperature between reconstructions and models actually are.

According to the lead author Olivier Cartapanis, "the results challenge the paradigm of a Holocene Thermal Maximum occurring at the same time worldwide." And, while the warmest temperature was reached between 4,000 and 8,000 years ago in western Europe and northern America, the surface ocean temperature cooled since about 10,000 years ago at mid-high latitudes and remained stable in the tropics. The regional variability in the timing of maximum temperature suggests that high latitude insolation and ice extent played major roles in driving climate changes throughout the Holocene.

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News from the climate history of the Dead Sea

The lake level of the Dead Sea is currently dropping by more than one metre every year -- mainly because of the heavy water consumption in the catchment area. However, very strong lake level drops due to climate changes are also known from earlier times. At the end of the last ice age, for example, the water level dropped by almost 250 metres within a few millennia. A study published today in the journal Scientific Reports now provides new insights into the exact course of this process. Daniela Müller and Achim Brauer from the German Research Centre for Geosciences (GFZ) in Potsdam, together with colleagues from the Hebrew University of Jerusalem, studied 15,000-year-old sediments from the Dead Sea and the surrounding area using newly developed methods. With unprecedented accuracy, they show that the long period of drought was interrupted by wet periods lasting ten to a hundred years. This also offers new insights into the settlement history of this region, which is significant for human development, and enables better assessments of current and future developments driven by climate change.

The water cycle at the Dead Sea -- then and now

In highly sensitive regions such as the Eastern Mediterranean, where water availability is an important factor for socio-economic and political development, it is crucial to understand how the water cycle is changing in response to climate change. Geologists can achieve this by assessing strong hydroclimatic changes that occurred several millennia back in time. For example, during the transition from the last ice age to the Holocene, the water level of Lake Lisan dropped by about 240 metres in the period 24-11 thousand years ago, which eventually led to its transition into today's Dead Sea.

Sediments as witnesses of time

The sediments at the edge of lake Lisan near the archaeological site of Masada and from the bottom of what is now the Dead Sea are unique witnesses to this development. In their new study, researchers led by Achim Brauer, head of Section 4.3 "Climate Dynamics and Landscape Evolution" at the German Research Centre for Geosciences Potsdam, and doctoral student Daniela Müller together with colleagues from the Geological Survey Israel and the Hebrew University of Jerusalem, analysed these sediments with unprecedented precision. The investigations took place within the framework of the PALEX project 'Paleohydrology and Extreme Floods from the Dead Sea ICDP Core', which is funded by the German Research Foundation (DFG).

New high-resolution methods for sediment analysis

For this study, new high-resolution analytical methods were developed at the GFZ to gain precise information from the stratification of the sediments and their geochemical composition, even about seasonal deposition processes and thus about the type, duration and course of climatic phases.

In particular, the combination of light microscopic methods with so-called 2D element mapping using X-ray fluorescence scanners is new. This enables the precise identification and localisation of elements in the sediments. Important and challenging for this is the preparation of the sediments for this analysis: The moisture must be removed from them by freeze-drying -- not easy given the high salt content of the Dead Sea and its affinity for water. Then the sediments are impregnated in synthetic resin and thin sections are made from them. In all this, the microstructure must not be altered.

Pause in climate change: humid phases interrupted long dry periods

The researchers found out that the dramatic long-term drop in the lake level due to increasing dryness was interrupted several times by wetter phases when climate change took breaks. "In this study, we were able for the first time to precisely determine the duration of these phases with several decades and in one case up to centuries by counting annual layers in the sediment," says Daniela Müller, lead author of the study. The exact reason for these pauses in the climate change of this region still remain elusive. Possible links to North Atlantic climate are suspected.

"What was particularly surprising was that during these wetter phases, in some cases over several decades, there we even did not find any traces of extreme floods, which are typical for this region even today and during wetter times in the past," Müller explains.

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Lakes are changing worldwide: Human activities to blame

International research led by Luke Grant, Inne Vanderkelen and Prof Wim Thiery of the VUB research group BCLIMATE shows that global changes in lake temperature and ice cover are not due to natural climate variability and can only be explained by massive greenhouse gas emissions since the Industrial Revolution. The influence of human-induced climate change is evident in rising lake temperatures and in the fact that the ice cover forms later and melts sooner.

"These physical properties are fundamental to lake ecosystems," says Grant, a researcher at VUB and lead author of the study. "As impacts continue to increase in the future, we risk severely damaging lake ecosystems, including water quality and populations of native fish species. This would be disastrous for the many ways in which local communities depend on lakes, ranging from drinking water supply to fishing."

The team also predicted future development under different warming scenarios. In a low-emission scenario, the average warming of lakes in the future is estimated to stabilise at +1.5°C above pre-industrial levels and the duration of ice cover to be 14 days shorter. In a high-emission world, these changes could lead to an increase of +4.0 °C and 46 fewer days of ice.

At the beginning of the project, the authors observed changes in lakes around the world: temperatures are rising and seasonal ice cover is shorter. However, the role of climate change in these trends had not yet been demonstrated.

"In other words, we had to rule out the possibility that these changes were caused by the natural variability of the climate system," says fellow VUB researcher and study co-author Vanderkelen.

The team therefore developed multiple computer simulations with models of lakes on a global scale, on which they then ran a series of climate models. Once the team had built up this database, they applied a methodology described by the Intergovernmental Panel on Climate Change (IPCC). After determining the historical impact of climate change on lakes, they also analysed various future climate scenarios.

The results show that it is highly unlikely that the trends in lake temperatures and ice cover in recent decades can be explained solely by natural climate variability. Moreover, the researchers found clear similarities between the observed changes in lakes and model simulations of lakes in a climate influenced by greenhouse gas emissions.

"This is very convincing evidence that climate change caused by humans has already impacted lakes," says Grant. Projections of lake temperatures and ice cover loss unanimously indicate increasing trends for the future. For every 1°C increase in global air temperature, lakes are estimated to warm by 0.9°C and lose 9.7 days of ice cover. In addition, the analysis revealed significant differences in the impact on lakes at the end of the century, depending on the measures taken by humans to combat climate change.

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Climate change: Threshold for dangerous warming will likely be crossed between 2027-2042

 

Photo concept, hourglass on beach

The threshold for dangerous global warming will likely be crossed between 2027 and 2042 -- a much narrower window than the Intergovernmental Panel on Climate Change's estimate of between now and 2052. In a study published in Climate Dynamics, researchers from McGill University introduce a new and more precise way to project the Earth's temperature. Based on historical data, it considerably reduces uncertainties compared to previous approaches.

Scientists have been making projections of future global warming using climate models for decades. These models play an important role in understanding the Earth's climate and how it will likely change. But how accurate are they?

Dealing with uncertainty

Climate models are mathematical simulations of different factors that interact to affect Earth's climate, such as the atmosphere, ocean, ice, land surface and the sun. While they are based on the best understanding of the Earth's systems available, when it comes to forecasting the future, uncertainties remain.

"Climate skeptics have argued that global warming projections are unreliable because they depend on faulty supercomputer models. While these criticisms are unwarranted, they underscore the need for independent and different approaches to predicting future warming," says co-author Bruno Tremblay, a professor in the Department of Atmospheric and Oceanic Sciences at McGill University.

Until now, wide ranges in overall temperature projections have made it difficult to pinpoint outcomes in different mitigation scenarios. For instance, if atmospheric CO2 concentrations are doubled, the General Circulation Models (GCMs) used by the Intergovernmental Panel on Climate Change (IPCC), predict a very likely global average temperature increase between 1.9 and 4.5C -- a vast range covering moderate climate changes on the lower end, and catastrophic ones on the other.

A new approach

"Our new approach to projecting the Earth's temperature is based on historical climate data, rather than the theoretical relationships that are imperfectly captured by the GCMs. Our approach allows climate sensitivity and its uncertainty to be estimated from direct observations with few assumptions," says co-author Raphael Hebert, a former graduate researcher at McGill University, now working at the Alfred-Wegener-Institut in Potsdam, Germany.

In a study for Climate Dynamics, the researchers introduced the new Scaling Climate Response Function (SCRF) model to project the Earth's temperature to 2100. Grounded on historical data, it reduces prediction uncertainties by about half, compared to the approach currently used by the IPCC. In analyzing the results, the researchers found that the threshold for dangerous warming (+1.5C) will likely be crossed between 2027 and 2042. This is a much narrower window than GCMs estimates of between now and 2052. On average, the researchers also found that expected warming was a little lower, by about 10 to 15 percent. They also found, however, that the "very likely warming ranges" of the SCRF were within those of the GCMs, giving the latter support.

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High-fidelity record of Earth's climate history puts current changes in context

 

View of Planet Earth

For the first time, climate scientists have compiled a continuous, high-fidelity record of variations in Earth's climate extending 66 million years into the past. The record reveals four distinctive climate states, which the researchers dubbed Hothouse, Warmhouse, Coolhouse, and Icehouse.

These major climate states persisted for millions and sometimes tens of millions of years, and within each one the climate shows rhythmic variations corresponding to changes in Earth's orbit around the sun. But each climate state has a distinctive response to orbital variations, which drive relatively small changes in global temperatures compared with the dramatic shifts between different climate states.

The new findings, published September 10 in Science, are the result of decades of work and a large international collaboration. The challenge was to determine past climate variations on a time scale fine enough to see the variability attributable to orbital variations (in the eccentricity of Earth's orbit around the sun and the precession and tilt of its rotational axis).

"We've known for a long time that the glacial-interglacial cycles are paced by changes in Earth's orbit, which alter the amount of solar energy reaching Earth's surface, and astronomers have been computing these orbital variations back in time," explained coauthor James Zachos, distinguished professor of Earth and planetary sciences and Ida Benson Lynn Professor of Ocean Health at UC Santa Cruz.

"As we reconstructed past climates, we could see long-term coarse changes quite well. We also knew there should be finer-scale rhythmic variability due to orbital variations, but for a long time it was considered impossible to recover that signal," Zachos said. "Now that we have succeeded in capturing the natural climate variability, we can see that the projected anthropogenic warming will be much greater than that."

For the past 3 million years, Earth's climate has been in an Icehouse state characterized by alternating glacial and interglacial periods. Modern humans evolved during this time, but greenhouse gas emissions and other human activities are now driving the planet toward the Warmhouse and Hothouse climate states not seen since the Eocene epoch, which ended about 34 million years ago. During the early Eocene, there were no polar ice caps, and average global temperatures were 9 to 14 degrees Celsius higher than today.

"The IPCC projections for 2300 in the 'business-as-usual' scenario will potentially bring global temperature to a level the planet has not seen in 50 million years," Zachos said.

Critical to compiling the new climate record was getting high-quality sediment cores from deep ocean basins through the international Ocean Drilling Program (ODP, later the Integrated Ocean Drilling Program, IODP, succeeded in 2013 by the International Ocean Discovery Program). Signatures of past climates are recorded in the shells of microscopic plankton (called foraminifera) preserved in the seafloor sediments. After analyzing the sediment cores, researchers then had to develop an "astrochronology" by matching the climate variations recorded in sediment layers with variations in Earth's orbit (known as Milankovitch cycles).

"The community figured out how to extend this strategy to older time intervals in the mid-1990s," said Zachos, who led a study published in 2001 in Science that showed the climate response to orbital variations for a 5-million-year period covering the transition from the Oligocene epoch to the Miocene, about 25 million years ago.

"That changed everything, because if we could do that, we knew we could go all the way back to maybe 66 million years ago and put these transient events and major transitions in Earth's climate in the context of orbital-scale variations," he said.

Zachos has collaborated for years with lead author Thomas Westerhold at the University of Bremen Center for Marine Environmental Sciences (MARUM) in Germany, which houses a vast repository of sediment cores. The Bremen lab along with Zachos's group at UCSC generated much of the new data for the older part of the record.

Westerhold oversaw a critical step, splicing together overlapping segments of the climate record obtained from sediment cores from different parts of the world. "It's a tedious process to assemble this long megasplice of climate records, and we also wanted to replicate the records with separate sediment cores to verify the signals, so this was a big effort of the international community working together," Zachos said.

Now that they have compiled a continuous, astronomically dated climate record of the past 66 million years, the researchers can see that the climate's response to orbital variations depends on factors such as greenhouse gas levels and the extent of polar ice sheets.

"In an extreme greenhouse world with no ice, there won't be any feedbacks involving the ice sheets, and that changes the dynamics of the climate," Zachos explained.

Most of the major climate transitions in the past 66 million years have been associated with changes in greenhouse gas levels. Zachos has done extensive research on the Paleocene-Eocene Thermal Maximum (PETM), for example, showing that this episode of rapid global warming, which drove the climate into a Hothouse state, was associated with a massive release of carbon into the atmosphere. Similarly, in the late Eocene, as atmospheric carbon dioxide levels were dropping, ice sheets began to form in Antarctica and the climate transitioned to a Coolhouse state.

"The climate can become unstable when it's nearing one of these transitions, and we see more deterministic responses to orbital forcing, so that's something we would like to better understand," Zachos said.

The new climate record provides a valuable framework for many areas of research, he added. It is not only useful for testing climate models, but also for geophysicists studying different aspects of Earth dynamics and paleontologists studying how changing environments drive the evolution of species.

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