The Caspian Sea, the world's largest inland body of water, is in major trouble. Since the mid-1990s, this saline lake, located between Europe and Asia, has lost an area nearly the size of Sicily. But until now, it wasn't clear why.

Researchers finally provided answers in a study published June 5 in the journal Earth's Future. Using ground- and satellite-based observations, they connected falling water levels to a drastic drop in river inputs from Iran, Turkmenistan, Azerbaijan, Kazakhstan and Russia, all of which border the Caspian Sea.

The team particularly highlighted losses from Russia's Volga River, the longest river in Europe, which supplies roughly 80% of the Caspian's inflow.

To find out why the Volga is discharging less water into the Caspian Sea now than it did 30 years ago, we spoke with Amir AghaKouchak, a co-author of the study and professor of civil and environmental engineering at the University of California, Irvine.

AghaKouchak, who directs UC Irvine's Center for Hydrometeorology and Remote Sensing, explained why river inputs have dropped and how decades of water withdrawals have put the Caspian Sea on a path to collapse. He also touched on a solution to this crisis — and revealed why Russia's hunger for ammunition to attack Ukraine is making it harder to save the Caspian Sea.

Sascha Pare: Falling water levels in the Caspian Sea have been attributed to natural variability, and more recently, there's been a focus on climate change and increased evaporation due to rising temperatures. But what did the results of your study show?

Amir AghaKouchak: People quickly jump to conclusions about climate change. Of course, that is important, but it cannot explain the change that we have observed. Over the last 30 years, water levels in the Caspian Sea have shrunk by around 2 meters [6.5 feet] or so. Just to put things in perspective, this translates to about 24,000 square kilometers [9,300 square miles] of sea area loss — about 5%. When we look at drivers, we notice that rainfall hasn't changed significantly over the lake and upstream, so the decline cannot be explained simply by changes in rain.

Evaporation has increased, so it explains maybe about one-third of the change. There is less water in the lake, and that indicates there's probably some water lost along the way. We don't have a lot of human-related data ‪—‬ for example, water withdrawal for irrigation or groundwater wells. That area is very difficult; the countries are not open to sharing data. A lot of our results are based on looking at proxies. Our interpretation is that temperature caused about 30% of the change and the rest is probably because of human activities.

SP: Which human activities does your research suggest are responsible for the Caspian Sea's decline?

AA: Several of the countries on the Caspian Sea have built large dams — Russia, for example. They are all desperate for hydropower. All of the countries around the lake [have also built infrastructure to supply agriculture], and one reason is that they want to create jobs and improve the local economy. Water demand is increasing in all those countries around the Caspian Sea. It's not surprising to see lakes drying or groundwater levels falling.

It's hard to find data about how much water countries are using; they keep data very private, especially places like Russia. We don't have detailed information, but the Volga-Don Canal in Russia is one of the projects that is contributing to this crisis. [Editor's note: The Volga-Don Canal is a ship canal that connects the Volga and Don rivers at their closest points.]

But we cannot blame this whole thing on that. We have to be careful with our language; we can't say one project is responsible. Most likely, this is the collective impact of many different small withdrawals from here and there, overall contributing to a significant decline in the Volga River outflow into the lake. In the paper, we have different rivers, but the Volga dominates because it's the largest river. The Volga has a higher impact than all the other rivers combined. There are lots of demands along that river basin.

SP: Did you expect human activities to play such a big role, or were you surprised?

AA: We did not have any expectations at the beginning. In fact, when we started this project, I reached out to a lot of colleagues from neighboring countries around the Caspian Sea. We didn't want to do it on our own, sitting in California, looking at another area of the world, without having local people involved. If you look at the co-authors, you see people from Azerbaijan, Iran, Kazakhstan — so, we have local experts. We thought this was really important to have those perspectives. I didn't want to reflect the opinions of only one research group. What we have here is the collective opinion of people who have not met in person; we only exchanged emails to look at the results, and then we came up with those conclusions.

We do have a Russian involved, but not from an official Russian institution. I thought this was really important, because there's more back-and-forth [with a large group of people]. If two people have to convince themselves of an idea, it's much easier than with 20. Also, we brought in people from other countries who had projects in the area; for example, there is one group from Finland and another group from Germany. We tried to come up with something that this large group of people could agree on. It was not easy, but it worked out.

SP: What is the worst-case future scenario for the Caspian Sea if shoreline countries fail to decrease their water demands?

AA: My concern is that, in the same general area, we have the Aral Sea, which is now about 5% to 10% of what it used to be, and Lake Urmia, in northwest Iran, that has also shrunk really fast and is now very small, with low levels of ecological activity. So we have two smaller lakes in a very similar climate that, from an ecological perspective, are dead. Because we have seen those degrade, we have to consider that this is the worst-case scenario for the Caspian Sea, too.

The Caspian Sea is much, much bigger. I hope it doesn't happen, but it's on the same path. Again, it's much larger, so it would probably take 100 years [to shrink as much as the Aral Sea and Lake Urmia], even with the current trend. But because we have seen those lakes shrink really fast, it deserves attention. Due to the size of this lake, the potential impacts will be huge. Even if 20% of the Caspian Sea dries out, the consequences will be very significant. It would be a disaster.

SP: You mentioned that drying lakes can become ecologically "dead." What other impacts could the Caspian Sea's falling water levels have on the surrounding landscape and people?

AA: The lake's retreat will expose the lake bed, and because it's saline, you have all kinds of minerals that will be exposed. With the next strong wind, they will go into the air. The health impacts include respiratory diseases, and even eye diseases, that are linked to salt minerals in the atmosphere. Those are the immediate consequences. Additionally, those minerals can travel long distances. In general, salt deposition on agricultural land reduces productivity. It takes a while, but if those lands are exposed for a long time, this will have an impact on the agricultural sector in those areas, which means potential food security issues. We don't exactly know what those materials are on the lake bed, so it's hard to know precisely what the consequences will be.

There is also serious shipping happening in this region. Right now, most of the shrinking is happening in the Caspian Sea's northern part, which is shallower [than the rest of the lake]. The drop in water levels is about 2 meters, and you can imagine that if you have a port and water levels drop more, you won't be able to operate. You'll need to extend your shipping routes southward; you'll need to invest and spend a lot of money to use other ports and docking sites. We have evidence of that around the Aral Sea and also Lake Urmia. There used to be these docking sites and ports, but those are now far away from the water.

SP: What must bordering countries do to save the Caspian Sea and prevent these negative effects?

AA: Demand management is absolutely critical. Demand management doesn't get a lot of attention, because unlike big projects that make the news, management means you have to cut things and do things that people don't notice. Some people argue that politicians don't show a lot of interest in demand management, but it's something that we have control over. The technological opportunities are there.

But when you look at that area, there is a big war ‪—‬ so probably it's not the priority to invest in management. There are a lot of things that we don't talk about in the paper that are relevant. I recently learned through discussions with people from the region — this is something that came to us from co-authors and collaborators — that many countries in that area grow cotton, which uses a lot of water. It has a lot of applications, but apparently, one of them is military. Cotton is used to produce ammunition. Russia needs a lot of ammunition, so not only are they growing it themselves, but they also bought neighboring reserves from Uzbekistan and Kazakhstan. So now, even those countries have an incentive to grow more cotton. This is driving more irrigation, more groundwater extraction, more surface water depletion, and probably more impacts on the Caspian Sea.

This interview has been condensed and edited lightly for clarity.

Each year, tons of soil, silt and rocks flow into reservoirs in the U.S. and get stuck there, creating a "ticking time bomb" that threatens lives and livelihoods. According to a new study, around a fifth of the world's reservoirs are currently at high risk of filling with sediment, and by 2060, half could be inoperable as a result.

The sediment takes up space meant for water, putting additional pressure on structures that were not built to contain large quantities of sand and sludge. As the reservoir fills with the material instead of water, sediment-filled reservoirs pose a serious threat to water security and the safety of people living near or downstream of them — and as the climate changes, scientists expect the problem to worsen.

"We are losing storage space in our reservoirs faster from sediment accumulation than we gain space from new dam construction," Amy East, a geologist at the U.S. Geological Survey, told Live Science.

What is dam sedimentation?

Dam sedimentation is not a uniquely U.S. problem. Rain pulls sediment down slopes into waterways. Normally, this sediment would travel through the river system, shaping the water flow, creating habitats, and fortifying coastal wetlands and estuaries against erosion. In many parts of the world, this sediment is also a vital natural fertilizer for crops. But after a river is dammed, this sediment can collect in the reservoir.

A 2023 study published in the journal Science found that sediment buildup was the greatest threat to global water storage in existing reservoirs — even more than drought.

New research, which includes smaller reservoirs, has offered a more nuanced and concerning picture. In the study, published June 5 in the journal Nature Sustainability, researchers assessed the state of reservoir sedimentation globally. They looked at more than 550,000 reservoirs, most of which spanned less than 0.4 square miles (1 square kilometre). They found that almost 1 in 5 of reservoirs globally are at high risk of filling up with sediment.

The researchers found that sedimentation is eating into global reservoir storage capacity at about 7.3% every decade. There are 16 global hotspots that are at high risk of reservoir sedimentation, threatening water supplies to more than 2 billion people and impacting more than a quarter of global irrigated land. One of these hotspots includes the western U.S.

The authors found that sedimentation risk was especially pronounced in regions with dense small-reservoir networks supporting key crop systems.

"Without intervention, we project that, by 2060, more than half of all reservoirs could become functionally inoperable," the authors wrote. This includes more than half of all small reservoirs and more than a third of large reservoirs globally.

Where is it a problem in the U.S.?

North America has the second largest number of reservoirs, after Asia, according to the global survey. And at 7.8%, its sedimentation rate is higher than the global average, ranking it third after Oceania (9.9%) and South America (8.1%),

The U.S. is home to many small dams, said Matthias Kondolf, a geomorphologist at the University of California, Berkeley who wasn't involved in the study. "All the focus tends to be on the big dams, and these many, many small ones have been neglected," he told Live Science.

From the 1940s to 1970s, there was a big push by the U.S. government to build "farm ponds," he said. In western Oklahoma, for example, "all these dams are now filling up with sediment, and there's nothing in place to assess the safety of these features, which have now become threats."

Some of these dams have already failed, following large storms as the ageing dams were not able to cope with the volume of water. A 2018 study found that since 1980, there have been on average 24 dam failures a year in the U.S.

Reservoirs in some regions are more at risk than others. The western U.S. is a natural hotspot because it has a steeper topography, meaning more sediment naturally ends up in watersheds, East, who wasn't involved in the new research, said.

And in areas where human activities like farming or logging disrupt the landscape, rainfall is likely to drag even more sediment into reservoirs, East said. "Those types of land use can create sediment-producing hotspots almost anywhere."

In California, for example, more than half of small reservoirs in maize-growing areas are filling up with sediment at a rate of more than 10% a year, which threatens local maize production.

The threat to water security will be particularly acute in places struggling with wildfires, such as the western U.S. After fires, rain washes the debris and sediment into watersheds. A 2024 study in California found that most of this post-fire sediment erosion occurred upstream of reservoirs, rather than in places where sediment would move downstream directly to the ocean.

The dam-sedimentation situation is likely to get worse as the climate changes, researchers said. As the climate changes, there will be more extreme weather events, such as droughts and wildfires, adding more sediment to watersheds. Dams are designed to cope with the pressure of water, not sediment which exerts more force on dam walls. Moreover, in seismically active areas, the sediment vibrates at a different frequency than the concrete of the dam, which creates further impact against the structure. Additionally, the dams are just getting older.

"What do we do 50 or 100 years from now when we have all these reservoirs across the landscape filled with sediment, sitting there as ticking time bombs?" Kondolf said.

What can be done about it?

For a reservoir to be sustainable long-term, the amount of sediment entering it and leaving it should be the same.

"A big part of preventing reservoir sedimentation problems is upstream soil conservation in the watershed," East said. Interventions include ensuring that land use, such as agriculture or timber harvesting, follow best practice to reduce the amount of sediment washing off the landscapes.

If sediment is already in the dam, it can be difficult to manage and expensive to remediate, Kondolf said. Some dams, such as Fall Creek Dam in Oregon, have mechanisms that enable engineers to release sediment. But in the past, reservoirs were not designed to manage sediment and little thought was given to how to decommission them.

"This is one of those issues that falls in the cracks between different agencies and responsibilities," Kondolf said. Some larger facilities fall under the remit of the federal government. Other dams fall under state governments. California, for example, recently launched a state-sponsored program to future-proof the state's dams.

But smaller dams often get overlooked, Kondolf said.

One option is to remove the dams entirely. For example, sediment-related concerns contributed to the decision to remove the Elwha and Glines Canyon dams in Washington. The Elwha River later carried most of the sediment to its river mouth.

For smaller dams with a relatively small amount of sediment, "you just blow up the dam and let the sediment work its way downstream", Kondolf said.

There are other technological solutions, but they are expensive, such as retrofitting dams to enable them to pass sediment; creating a bypass to route sediment-heavy water around the dam; and mechanical dredging.

Another option is to empty the reservoir of water and let the river cut a channel through the sediment, Kondolf said.

However, released dam sediment brings its own risks. If not managed carefully, sediment can flood the river system and disrupt downstream ecosystems.

"The economics of managing reservoir sediment long-term were not generally incorporated into dam design and planning in the last century when many western-U.S. water projects began," East said. This has "pushed the cost burden on future generations."

But the reality is that reservoirs around the country are filling with sediment. It is reducing their ability to store water and potentially endangering the communities living downstream of them — and unless addressed, the problem will only get worse.

Science word of the day: Seamount

Pronunciation: SEE'-mount

What it means: A seamount is an undersea mountain that rises at least 3,300 feet (1,000 meters) above the seafloor but does not come close to breaking the surface. According to the National Oceanic and Atmospheric Administration, there are at least 100,000 seamounts in the oceans, and less than 0.1% of them have been explored. Seamounts typically form from volcanic activity, and many host dramatic communities of sea life ‪—‬ from coral, to sharks, to an assortment of weird jellyfish-looking things.

How to use it in a sentence: Every time marine biologists explore a seamount with their remotely operated vehicles, they seem to discover a new universe of strange and blobby deep-sea creatures.

Can you crack our science word of the day puzzle, Chain Word?

Runoff from Greenland's rapidly melting ice sheet could severely disrupt currents in the Atlantic Ocean that regulate the global climate, researchers say. But the fresh water won't trigger the complete and irreversible collapse of the circulation that some research predicts, a new study suggests.

By 2100, Greenland meltwater could cause a reduction in the strength of the Atlantic Meridional Overturning Circulation (AMOC) equivalent to an additional 10% to 20% on top of the weakening that many scientists already expect for this huge system of currents, researchers found.

Previous research suggests the AMOC, which moves heat from the tropics to the Northern Hemisphere and cold water back south toward Antarctica, is at its weakest in more than 1,000 years due to Arctic ice melt and soaring ocean temperatures. Models indicate that the circulation will continue to weaken for decades to come, potentially unleashing freezing weather in Northwest Europe, additional sea-level rise along the U.S. East Coast and droughts around the equator.

The new study revealed that by 2300, Greenland's ice sheet could cause a whopping 40% of AMOC weakening on top of the declines projected by some climate models if we continue to cook the planet. The results highlight the effects on the AMOC of a growing freshwater pulse from Greenland that most climate models currently don't take into account, the authors said.

"By the year 2100, yes, we have something, but the greatest fraction is what we add later," study co-author Jost von Hardenberg, a professor in the Department of Environment, Land and Infrastructure Engineering at the Polytechnic University of Turin in Italy, told Live Science.

Meltwater from the Arctic and Greenland slows the AMOC by preventing the formation of deep currents in the North Atlantic that drive the circulation south toward Antarctica. Deep currents form when masses of cold, salty water sink to the seabed, but inputs of fresh water and rising water temperatures are blocking this step by diluting and warming surface waters.

While Arctic meltwater is commonly integrated into climate simulations, researchers have been slower to include runoff from the Greenland ice sheet in models, partly because this runoff may not be as disruptive as Arctic meltwater on shorter timescales, von Hardenberg said.

"In order to represent correctly the Greenland melt, you need a specific, dynamic model for the Greenland ice sheet. It's nice to have, but it requires a lot of resources to develop," von Hardenberg said

The Community Ice Sheet Model version 2 (CESM2) is one of the few available simulations of the Greenland ice sheet. For the study, von Hardenberg and his colleagues used a global climate model called EC-Earth3, which does not include the Greenland ice sheet. The team added the ice sheet from CESM2 to explore how it might affect the AMOC over the next few centuries. Adding the ice sheet separately enabled them to compare AMOC weakening with and without Greenland, clearly revealing the impact of the ice sheet on the AMOC's strength.

The results, published June 19 in the journal Science Advances, suggest the Greenland ice sheet will have a big influence on Atlantic ocean currents, especially after 2100.

"Greenland starts really melting strongly only after that date," von Hardenberg said. "If you go farther in our simulation — we go up to the year 2300 — that is also when this meltwater really becomes extremely strong."

Yet unlike other studies that show the AMOC collapsing irreversibly under climate change, in this model, the circulation recovered once the team switched off the meltwater or dialed down greenhouse gas emissions. Both the simulations that didn't include the Greenland ice sheet and those that did had similar results, recovering partially when the meltwater was turned off and fully when greenhouse gas emissions were stopped.

The recovery in the model suggests that the AMOC is more stable than previously thought and doesn't suddenly flip from an active state into a collapsed one, von Hardenberg said. "It's consistent with a strong weakening, but not a shutdown," he said.

Overall, the findings show that it is important to include the Greenland ice sheet in climate models — something von Hardenberg and his colleagues are planning to do in the next version of the EC-Earth3 model.

"It's really something which is worthwhile doing to better constrain the uncertainty of these future simulations," von Hardenberg said. "If we have a larger set of models which include this component, we will be in a better position to try to answer the question, 'How much do we expect AMOC to decrease in different scenarios?'"

Other experts agreed that integrating the Greenland ice sheet in climate models will increase the accuracy of AMOC predictions.

"What makes the study particularly interesting is that it directly tests a process that has often been highlighted as an important source of uncertainty in future AMOC projections," said Jonathan Baker, a senior climate scientist at the Met Office in the U.K. who was not involved in the study.

The results are consistent with previous research showing that Greenland meltwater causes additional AMOC weakening, but whether the system could collapse is still uncertain, given that the team used only one model, Baker told Live Science in an email. "Repeating these experiments across a wider range of models will be important to assess how robust the findings are," he said.

Nicholas Foukal, an assistant professor at the University of Georgia's Skidaway Institute of Oceanography who was not involved in the study, said he doesn't think the use of a single model limits the analysis.

"I think this result is robust and I am glad to see the authors come to this conclusion in such a high-resolution and realistic coupled climate model," Foukal told Live Science in an email. "This finding also agrees with other modeling results that require immense amounts of fresh water, much more fresh water than is available on Greenland, to cause irreversible change in the AMOC."

However, a third expert said the model used in the study is unrealistic, because its parameters make the AMOC more resilient than those of other models. Therefore, the study should be replicated in other models to draw unbiased conclusions.

"While the experiment reported is interesting, its outcomes are neither surprising nor should be interpreted as exemplary for the whole model ensemble," Sybren Drijfhout, a professor of physical oceanography at the University of Southampton in the U.K., told Live Science in an email.

Researchers should investigate the impact of the Greenland ice sheet on the AMOC further, von Hardenberg said. "It would be interesting in the future to check it [the AMOC's recovery with Greenland meltwater] with other models, but of course it's encouraging to see it in one case," he added.

This ethereal satellite snap shows beautiful, "painting-like" folds within an ancient underwater structure in the Bahamas.

The Great Bahama Bank (GBB) is a massive submerged platform stretching around 330 miles (530 kilometers) across a shallow ocean channel between Andros Island and the Exuma islands, according to Britannica. The crescent-shaped bank surrounds a darkly colored ocean drop-off known as "The Tongue of the Ocean," which plunges to depths of 6,500 feet (2,000 meters), according to NASA's Earth Observatory.

The photographed section of the GBB is around 23 miles (37 km) across and features a series of submerged sandbanks — most of which are covered with dense seagrass beds — that lie between 7 and 40 feet (2 to 12 m) below the ocean's surface.

The varying water depth and seagrass concentration cause multiple hues of green and blue to shine across the sandbanks, which have been carved into smooth, folding ribbons by ocean currents over thousands of years.

"The varying colors and curves remind us of graceful strokes on a painting," Earth Observatory representatives wrote.

This section of the GBB was first photographed in 2001 by the Landsat 7 satellite and has often appeared on lists of the most iconic aerial images of our planet.

"There are many nice seagrass and sand patterns worldwide, but none like this anywhere on Earth," Serge Andréfouet, an oceanographer at the French National Research Institute for Sustainable Development who first shared the 2001 image, told the Earth Observatory. "I am not surprised it is still a favorite, especially for people who see it for the first time."

The sandbanks are like giant underwater dunes carved out by ocean currents.

"Tides and ocean currents in the Bahamas sculpted the sand and seaweed beds into these multicolored, fluted patterns in much the same way that winds sculpted the vast sand dunes in the Sahara Desert," Earth Observatory representatives wrote in 2001.

The GBB lies atop a roughly 3-mile-thick (5 km) bed of limestone dating to the age of the dinosaurs, more than 65 million years ago. This hefty mass, made up of the remains of long-dead coral reefs, is so large that Earth's crust directly below the GBB has "sagged under the weight," according to the University of Texas at Austin's Marine Science Institute.

Before the end of the last ice age, around 12,000 years ago, this limestone slab had risen to more than 400 feet (120 m) above sea level and only resubmerged as global sea levels rose due to the melting of glaciers, according to the Marine Science Institute.

The Bahamas has more than 3,000 islands and smaller cays and is home to several other intriguing oceanographic features, including deep tidal channels off the coast of Great Exuma Island (which once harbored famous pirate ships) and a series of aurora-like sandbanks that appear to shine along the nation's northernmost islands.

See more Earth from space

A giant blob of abnormally cold water in the North Atlantic Ocean is shifting the Indian summer monsoon, threatening the livelihoods of more than 1 billion people, new research suggests.

The link between these two systems highlights a previously unrecognized connection that could inform weather forecasts in South Asia and shed light on climatic events elsewhere, scientists say.

The Indian summer monsoon is a rainfall pattern that lasts from June to September and is driven by temperature differences between the warm northern Indian Ocean and cooler seawater below the equator. Historically, the monsoon triggered heavy rainfall along the west coast of India and a huge region of northern India called the Indo-Gangetic Plain. But since 1999, this pattern has changed dramatically, the researchers reported in a new study.

Northwest India now receives about 25% more rain during the monsoon season than it did before 1999, while the Indo-Gangetic Plain gets roughly 4% less, the team found. This is disastrous for farmers, in particular, because their soils and crops are adapted to the old rainfall pattern, said study first author Mahendra Nimmakanti, a climate scientist at the Indian Institute of Science's Centre for Atmospheric and Oceanic Sciences.

"India is largely dependent on agriculture," Nimmakanti told Live Science in a joint interview with study co-author Matthew Huber, a professor in the Department of Earth, Atmospheric, and Planetary Sciences at Purdue University in Indiana. Higher-than-normal precipitation in northwest India is causing flash floods and crop losses, because agriculture in this region is adapted to dry conditions. Meanwhile, the Indo-Gangetic Plain has seen periods of drought that also caused crop declines and impacted farmers' livelihoods, Nimmakanti said.

Previous studies have linked shifts in the Indian summer monsoon to changes in the Atlantic Meridional Overturning Circulation (AMOC), a giant web of ocean currents in the Atlantic that regulates the global climate and carries heat to the Northern Hemisphere. Data suggests the AMOC is slowing due to climate change and releasing less heat in the North Atlantic Ocean than it did before. This may be causing a southward shift of the Intertropical Convergence Zone, a band of low-pressure atmospheric conditions around the equator that drives tropical monsoons, including the Indian summer monsoon.

But these studies didn't specify how the Indian monsoon might shift or explain the underlying mechanism in detail, Nimmakanti said. "They generally explain that if there is a weakening of AMOC, that suppresses the Indian summer monsoon," he said.

One part of the problem is that current climate models do not show the changes in the Indian monsoon that have happened in real life, possibly because they also don't fully capture shifts in North Atlantic sea surface temperatures. This is especially true for a region southeast of Greenland known as the "cold blob," where the water was colder between 1901 and 2021 than it was in the late 1800s, even as the ocean around it heated up.

The cold blob suggests the AMOC is weakening because it points to a reduction in the amount of heat reaching the North Atlantic.

To pinpoint how and why the Indian monsoon has changed, the researchers fed precipitation data, sea surface temperature records and other real-life observations into dozens of climate models. This reproduced the shifts observed over the past 27 years. However, the results only implied a correlation between shifts in the North Atlantic and changes in the monsoon, not that the former directly caused the latter.

To figure out if North Atlantic sea surface temperatures caused the Indian monsoon to behave strangely, the team added and removed the cold blob in a simulation. The results showed that the cold blob has shifted the Indian monsoon by creating a strong temperature gradient over the North Atlantic, which, in turn, affects jet stream winds and pressure systems in the atmosphere above Eurasia.

Specifically, the jet stream above the North Atlantic has intensified, and a "blocking" system over the Ural Mountains in western Russia has strengthened, Huber said. As a result, weather systems in India have changed, sucking moist air toward the country's northwest and away from other regions.

"It's a shifting of the high- and low-pressure systems," Huber said. "Realizing that these two systems were directly linked through this wave train coming off the North Atlantic, that was novel."

More than 1 billion people in India and other parts of South Asia depend on the monsoon for food security and economic stability. The new results, published April 27 in the journal AGU Advances, could help forecasters predict extreme rainfall and drought events in India and neighboring countries like Pakistan during the monsoon season.

Some researchers think the Indian monsoon is a key tipping point within the global climate, and the findings suggest the system crossed a threshold in 1999. Since then, the cold blob has caused a "persistent jet stream reorganization," leading to abrupt shifts in the monsoon, according to the study.

It's unclear how the Indian monsoon will evolve under intensifying climate change, because climate models produce different predictions of what will happen in the North Atlantic, and other drivers may come into play as the world changes, Huber said.

"What we do now know is that this is one of the key building blocks of formulating a theory for what will happen in the future," he said.