How salt from the Caribbean affects our climate

The distribution of salt by ocean currents plays a crucial role in regulating the global climate. This is what researchers from Dalhousie University in Canada, GEOMAR Helmholtz Centre for Ocean Research Kiel, Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and MARUM -- Center for Marine Environmental Sciences at the University of Bremen have found in a new study. They studied natural climate anomalies, including the so-called Little Ice Age. This cold period from the 15th to the mid-19th century led to poor harvests, famine and disease in Europe. Although the Little Ice Age is one of the most studied periods in recent history, the underlying climatic mechanisms remain controversial.

"Looking at recent, natural climate anomalies helps to understand the processes and mechanisms that human-induced global warming may trigger," says Dr Anastasia Zhuravleva, lead author of the study. She was a PhD student at GEOMAR and received the Annette Barthelt Prize for her dissertation in 2019. She then worked as a post-doctoral researcher at GEOMAR and Dalhousie University, where the study was completed.

"Researchers often consider an increase in sea ice extent and desalination in the subpolar North Atlantic as possible triggers for past cold periods, but processes in the tropical Atlantic appear to be equally important," says Dr Zhuravleva. "In fact, in contrast to the northern and mid-latitudes, there is little information on these recent climate events from the subtropical-tropical Atlantic and their impact on regions in the Northern Hemisphere," adds Dr Henning Bauch, paleoclimatologist at AWI and GEOMAR, co-initiator and co-author of the study. "This is where our research comes in."

So, what happened in the tropical Atlantic during historical climate anomalies, and how might potential changes there have affected ocean circulation and climate much further north? To answer these questions, the team worked on a sediment profile from the southern Caribbean and reconstructed the salinity and temperature of the surface water over the last 1700 years. Among other things, the researchers determined the isotopic and elemental composition of the calcareous shells of plankton.

The results show a cooling of about 1°C during the Little Ice Age. "It is a significant temperature change for this region," says Dr Mahyar Mohtadi, co-author of the study and head of the Low Latitude Climate Variability group at MARUM. "Particularly noteworthy is the occurrence of another pronounced cooling for the 8th-9th centuries. Colder temperatures in the otherwise warm tropical ocean led to lower regional rainfall, which coincided with severe droughts in the Yucatan Peninsula and the decline of the Classic Maya culture."

In addition, the researchers found that the cold climate anomalies in the subpolar North Atlantic and Europe were accompanied by weaker ocean circulation and increased salinity in the Caribbean. "Advection, or the movement of tropical salt to high northern latitudes, is essential for maintaining high surface densities in the subpolar North Atlantic. This is a prerequisite for the overall stability of the large-scale ocean circulation, including the transfer of warm Gulf Stream water, which is responsible for our mild temperatures in Europe," says Dr Bauch.

The data on the historical past thus allow a reconstruction of the connection across the North Atlantic. Initial cooling can be caused by volcanic eruptions, low solar activity and feedbacks between sea ice and the ocean in the north. The new study provides evidence that a decrease in salt movement to high northern latitudes will amplify and prolong these climate events. Conversely, the slow movement of positive salinity anomalies from the tropics will eventually increase the density at the surface of the subpolar North Atlantic. This may favour the northward transport of heat by ocean currents, resulting in milder temperatures over Europe and North America.

"Such a salinity feedback is known from models and has been assumed for the Little Ice Age. However, in the absence of tropical ocean data, these assumptions have been based on less direct precipitation records," says Dr Zhuravleva.

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Insolation affected ice age climate dynamics

In past ice ages, the intensity of summer insolation affected the emergence of warm and cold periods and played an important role in triggering abrupt climate changes, a study by climate researchers, geoscientists, and environmental physicists suggests. Using stalagmites in the European Alps, they were able to demonstrate that warm phases appeared primarily when the summer insolation reached maxima in the Northern Hemisphere. Study participants included scientists from Germany, Austria, and Switzerland led by researchers from Heidelberg University and the GFZ German Research Centre for Geosciences Potsdam.

Past ice ages in the Northern Hemisphere were marked by sudden transitions between cold and warm phases, each lasting several thousand years. The reason for these fluctuations has yet to be resolved, but research does point to effects relating to the size of the continental ice sheets. Greenland ice records 25 such warm-cold cycles between 115,400 and 14,700 years ago. Investigating stalagmites in the Melchsee-Frutt cave system in the Swiss Alps, the researchers were able to investigate for the first time and with high precision 16 such fluctuations in the penultimate glacial period 185,000 to 130,000 years ago.

Stalagmites in caves are crucial archives in climate research and offer clues as to changes in temperature, precipitation, and vegetation cover. "We are able to precisely determine their age and hence analyse the chronological sequence of abrupt ice age climate fluctuations, which we identify using oxygen isotope values," explains Prof. Dr Norbert Frank of the Institute of Environmental Physics at Heidelberg University. "Our investigations targeted whether, in addition to ice volumes in the Northern Hemisphere, orbitally driven changes in the global distribution of insolation could have influenced the abrupt changes in climate," states study head Dr Jens Fohlmeister, who earned his doctorate in environmental physics at Heidelberg University and worked at the GFZ German Research Centre for Geosciences Potsdam and the Potsdam Institute for Climate Impact Research during the investigations.

The researchers studied the transitions of warm-cold cycles in the penultimate ice age by analysing the age and oxygen isotope composition of stalagmites from the Melchsee-Frutt cave system. "Based on the newly acquired data, we were able to show that warm phases occurred mainly during the peak phase of summer insolation in the Northern Hemisphere, even when the sea level, which is dependent on the volume of the continental ice sheets, remained close to its minimum during peak glacial periods," explains Dr Fohlmeister. Model simulations confirmed these findings. In accordance with the research data from the cave system, the simulations predict the frequency as well as the duration of warm phases at the corresponding sea level and existing insolation.

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Significant decline of snow cover in the Northern hemisphere over the last half century

In the face of the ongoing climate crisis, scientists from many fields are directing their expertise at understanding how different climate systems have changed and will continue to do so as climate change progresses. Robert Lund, professor and department chair of statistics at the UC Santa Cruz Baskin School of Engineering, collaborated on a new study that uses rigorous mathematical models and statistical methods and finds declining snow cover in many parts of the northern hemisphere over the last half century.

Understanding snow cover trends is important because of the role that snow plays in the global energy balance. Snow's high albedo -- the ability to reflect light -- and insulating characteristics affects surface temperatures on a regional scale and thermal stability on a continent-wide scale.

In the new study published in the Journal of Hydrometeorology, researchers analyzed snow cover data gathered from weekly satellite flyovers between 1967 (when satellites became more common) and 2021, which was divided into grid sections for analysis. Of the grids that researchers determined had reliable data, they found that snow cover is declining in nearly twice as many grids as it is advancing.

"In the Arctic regions, snow is going away more often than not -- I think climatologists sort of suspected this," Lund said. "But it's also going away at the southern boundaries of the continents."

In a study that took about four years to complete, the researchers show that snow presence in the Arctic and southern latitudes of the Northern hemisphere is generally decreasing, while some areas such as Eastern Canada are seeing an increase in snow cover. This could be due to increasing temperatures in areas that are typically very cold but still below freezing, allowing the atmosphere to hold more water, which then falls as snow.

Lund believes this is the first truly dependable analysis of snow cover trends in the Northern hemisphere due to the rigor of the researchers' statistical methods. It is often challenging for non-statisticians to extract trends from this type of satellite data, which comes as a sequence of 0s or 1s to indicate if snow was present during a certain week. The researchers also had to take correlation into account when looking at trends, as the presence of snow cover one week greatly affects the likelihood of snow cover the following week. These two factors were taken into account with a Markov chain based model. Accurate uncertainty estimates of the trends could be computed from the model. The researchers found hundreds of grids where snow cover was declining with at least 97.5% certainty.

However, they also found that some of the satellite data gathered in mountainous regions was unreliable, showing no snow in the winter and several weeks of snow in the winter. This was likely due to a flaw in the algorithm that processed the satellite data to determine if snow was present or not.

"The reason this study took a lot of work is because the satellite data is so doggone poor," Lund said. "Whatever the meteorologists did to estimate snow from the pictures in some of the mountainous regions just didn't work, so we had to take all the grids in the Northern hemisphere, and figure out whether the data was even trustworthy or not."

By determining which satellite data is unreliable, this study can serve as a resource to the scientific community who also may want to evaluate this snow cover data for their research.

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Mountains vulnerable to extreme rain from climate change

As the world warms, extreme weather events grow -- and they also change. Researchers at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) found that climate change is shifting snowfall to rainfall on mountains across the Northern Hemisphere. Those surges of liquid water bring a distinct set of dangers, including floods, landslides, and soil erosion.

"One quarter of the global population lives in or downstream from mountainous regions," said Mohammed Ombadi, first author of the paper published today in Nature. "They are going to be directly affected by this risk."

Scientists already expect climate change to increase the volume of water falling during extreme events (which typically take place over a few hours to a day), but this study is the first time researchers have looked at whether that extreme precipitation comes as rain or snow. They found that the fraction of water falling as snow decreased in mountainous regions, falling instead as rain -- making mountains particularly susceptible to extreme rain hazards. They even put a number to it: For every 1 degree Celsius increase in the global temperature, researchers expect an average of 15% more rain at high elevations.

"This increase in rainfall extremes is not only something that is going to happen from now until the end of the 21st century -- we're already seeing it," Ombadi said. "That same rate was also evident in the data from 1950 to 2019. Rainfall extremes in mountains have already been increasing, and will continue to change with that 15% rate."

While all the mountain ranges in the Northern Hemisphere are seeing the shift from snow to rain, those at greatest risk of extreme rainfall events are the North American Pacific mountain ranges (the Cascades, Sierra Nevada, and coastal ranges from Canada to Southern California), the Himalayas, and high-latitude regions. Researchers are still working to understand why those areas are at higher risk than other mountain ranges such as the Rockies or the Alps.

"We think that North American Pacific mountain ranges are more susceptible to the risk of rainfall extremes than other mountain ranges because a significant portion of snowfall in this region typically occurs at temperatures just below zero degrees Celsius," Ombadi said. "The slightest change in air temperature will shift this snowfall to rainfall. This is unlike other mountain ranges where snowfall may occur at very low temperatures below zero degrees."

Ombadi hopes that fellow climate scientists will incorporate the distinction between snowfall and rainfall to improve global climate models, and that civil engineers and planners will use the data to better prepare for intense rain events.

"We need to factor these results into how we design and build the infrastructure in these mountainous regions, so that they can withstand the negative consequences of increases in rainfall extremes," Ombadi said.

Meanwhile, countries continue efforts to meet targets established by the Paris Agreement that would limit global warming to less than 2 degrees Celsius above pre-industrial levels.

"Our findings revealed a linear relationship between the level of warming and the increase in extreme rainfall: For instance, 1 degree of warming causes 15% more rain, while 3 degrees leads to a 45% increase in rainfall," Ombadi said. "There are many technologies in progress that could help us reduce greenhouse gas emissions and how much the planet warms. To me, this study shows the need to invest in those clean solutions, and also start preparing for the consequences of warming now."

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