Earth's rivers are increasingly dammed, disrupted by development and fragmented — all of which are threatening food and clean water sources that hundreds of millions of people depend on, a new study finds.

What's new: A first-of-its-kind study published Wednesday in Nature provides a global census of the world's rivers, and seeks to answer the question of how many are still free-flowing.

By the numbers:

• 2.8 million: The estimated number of dams constructed worldwide.
• More than 3,700: Hydropower dams currently planned or under construction worldwide, particularly in Asia.
• 15 gigawatts: The amount of hydropower capacity added in Asia during 2016 alone. The study highlights the Balkans, Amazon, China, and the Himalayas as hotspots of hydropower construction.
• 37%: Share of rivers longer than 1,000 km (about 620 miles) in length that remain free-flowing.
• 41%: Share of global river volume that still flows freely into the ocean.
• 77%: Share of rivers greater than 1,000 km in length that have seen the connection from their source region to the sea severed.

The study finds large contiguous river networks with intact natural connectivity are limited to places where few humans live, including the Arctic, Amazon Basin, and Congo Basin in Africa.

• Dams and reservoirs are the top contributors to major connectivity loss of sections of rivers.
• Consumptive water use and infrastructure development also rank high on the list of river disturbances.

"The Mekong River is a prime example of a river that has been severed by several hydropower dams," study co-author Michele Thieme tells Axios via email. "Two more dams planned on the lower Mekong river, the Sambor and Stung Treng hydropower projects, could be the final straw..." This could deprive communities downstream of fish, imperiling livelihoods.

What they did: Researchers used satellite imagery and global hydrological data to compile an atlas of rivers around the world. They examined rivers' longitudinal connectivity, hydrological alterations of river flow, and the exchange of water between the atmosphere and groundwater.

They then used this data to rank rivers' intact natural connectivity.

• The study finds the last 2 remaining very long free-flowing rivers in Asia — the Irrawaddy and Salween rivers — provide more than 1.2 million metric tons of fish catch per year, and help nurture coastal agriculture where more than 30 million people live.

What's next: The study's authors recommend river connectivity be considered in future development decisions, and note that the study illustrates how fragile and important free-flowing rivers are in providing benefits to humans and natural systems.

What they're saying:

"Rivers provide abundant fisheries that feed millions, nutrients to downstream floodplains and agriculture, sediments that help stop deltas from sinking, refuges for biodiversity in a rapidly changing world, and healthy floodplains and wetlands that can act as a buffer against extreme weather events."

Go deeper: Map of the world's free-flowing rivers.

The U.S. scientific community is slowly chipping away at the lack of diversity seen in medical research, but it still has a long way to go, writes Eileen Drage O'Reilly.

Why it matters: "We are not a nation of white men," LA BioMed's David Meyer says.

Because race and ethnicity sometimes play a role in how people develop diseases or react to medications, it's important to expand research to include people of color, older people, poorer communities, and those in the LGBTQ community.

Driving the news: Earlier this week, the National Institutes of Health discussed progress made in the first year of its All of Us program, which aims to enroll at least 1 million people from all types of U.S. communities over a 5-year-period.

• The reason behind the program is "we're making a lot of scientific conclusions based on a small slice of the population," Stephanie Devaney, the program's deputy director, tells Axios.
• "We're excited" that roughly 80% of program participants self-describe as being in at least 1 of the 9 categories of underrepresented populations, Devaney says.

But, there's much progress still to be made, experts say.

• For example, the Centers for Disease Control and Prevention announced this week that maternity mortality rates remain high, with American Indian/Alaska Native and black women 3 times more likely to die than white women, particularly in the year period after giving birth.
• While research shows the serious problem of bias in health care professionals and lack of health care access for underrepresented groups, this doesn't always explain the whole problem, notes Jane Delgado, president and CEO of the National Alliance for Hispanic Health.
• The Hispanic community also faces problems of bias, but their maternity mortality rate is actually lower than white women (11.4 deaths per 100,000 live births compared with 13 deaths of white mothers.)

What they're saying: Experts urge increased focus on research to develop precision medicine.

"Diversity in medical research is becoming increasingly important as the population becomes increasingly diverse. Consequently, investment in precision medicine is only as good as the genetic knowledge bank we are able to accumulate. By conducting clinical trials and research that reflect the citizens they aim to help, the industry is better able to accurately identify specific disease markers and new rare diseases that are unique to various races and ethnicities."

The bottom line: As the U.S. population continues to change, medical research needs to actively seek participants from all communities to understand the role that genetics, socioeconomic, and lifestyle factors play in a person developing disease or chronic conditions.

A rare supernova is helping scientists unlock the mysteries of how these bright, exploding stars come to be, Axios' Miriam Kramer reports.

Why it matters: Supernovas — the violent explosions of some dead stars at the end of their lives — are thought to be responsible for seeding our universe with many of the heavy elements we see around us today. If researchers can figure out exactly how these stellar explosions are created, it could help explain some of the inner-workings of our universe.

Background: On its surface, the supernova in question — named ASASSN-18tb — looked like other Type Ia supernovas, which are typically used to measure distances in our galaxy thanks to their predictable brightness.

• These types of supernovas are thought to come from the explosions of white dwarf stars — the dead remnants of a sun-like star — in a binary system with another star.
• Some astronomers think these types of explosions are triggered when a white dwarf eats up a large amount of the material from its companion in the binary, but another hypothesis suggests the explosion occurs when 2 white dwarfs slam into each other.

The big question: The chemistry of ASASSN-18tb is unlike others spotted before.

• Most Ia supernovas have no hydrogen signature at all, but ASASSN-18tb appears to have ejected some hydrogen when it exploded, causing scientists to question exactly what made the star explode.
• While some other Type Ia supernovas have been found enveloped in a large amount of hydrogen, ASASSN-18tb didn’t fit the usual model for those kinds of stellar explosions, since those supernovas usually occur in young galaxies that are still forming stars.
• In contrast, ASASSN-18tb was found in a galaxy with older stars.

What they found: Scientists now think it’s possible the hydrogen ejected by ASASSN-18tb was actually from the white dwarf’s other star in the binary system.

• “One exciting possibility is that we are seeing material being stripped from the exploding white dwarf’s companion star as the supernova collides with it,” Anthony Piro, one of the authors of the new supernova study in the journal Monthly Notices of the Royal Astronomical Society, said in a statement.
• “If this is the case, it would be the first-ever observation of such an occurrence.”

More symptoms of Arctic climate change have emerged in recent weeks, including a near complete lack of multi-year sea ice in the Arctic Ocean. This means the sea ice is especially fragile as melt season begins.

The big picture:

• The Greenland melt season started about a month early, according to NASA research aircraft as well as scientists on the ground.
• Utqiagvik (formerly Barrow), Alaska — the northernmost U.S. city —has had its warmest January through April stretch on record, NOAA found.
• Sea ice extent is also hovering near record lows for this time of year, a precarious position as the midnight sun nears.

Driving the news: The U.S. threw an unexpected curve ball into efforts to cooperate with other Arctic nations to adapt to such changes and avert more severe impacts at a key regional summit this week.

Instead of portraying climate change as a threat to the U.S. and the 4 million who call the Arctic home, Secretary of State Mike Pompeo told an audience in Rovaniemi, Finland, at the edge of the Arctic Circle, that the increasingly accessible Arctic should be viewed as an emerging zone of economic opportunity and great power competition.

• The U.S. then objected to including references to climate change in a ministerial declaration, scuttling such a document for the first time in the 23-year history of the organization.
• “It makes no sense in light of everything that the Arctic is experiencing and what it means for the rest of the world,” Gosia Smieszek of the University of Lapland, tells Axios of Pompeo's speech, which she attended.

Why it matters: While a seasonally ice-free Arctic Ocean will present some economic pluses, such as tourism, the warming climate is already upending lives in northern communities and favoring extreme weather events in the Lower 48 states and Europe.

• In addition, Greenland ice melt directly threatens U.S. cities with sea level rise, and thawing permafrost alone could cost the globe trillions if it leads to a large release of long-stored greenhouse gases, such as methane.

The bottom line: Timo Koivurova, director of the Arctic Center at the University of Lapland, observes that America's Arctic policies are divorced from climate science. "Pompeo just said that ice is melting, but did not mention why," he told Axios via email.

"[T]he Trump presidency is trying to stir the calm Arctic atmosphere by creating these narratives of conflict, tension and resource possibilities. They are not much based on facts, but they may influence how people think about the region."

Earth hurtles toward extinction crisis — 1 million species at risk

Most U.S. deaths from pregnancy complications are preventable (Eileen Drage O'Reilly)

Private satellite data increases marijuana citations in California county (Miriam Kramer)

We wouldn't know it if we found another Earth (Miriam Kramer)

Black, hot ice may be nature’s most common form of water (Joshua Sokol, Quanta)

Stunning video of a ‘particularly dangerous’ tornadic storm from 22,236 miles in space (Matthew Cappucci, Washington Post)

Rural areas drive increases in global obesity (Barry M. Popkin, Nature News)

Gas That Makes a Mountain Breathe Fire Is Turning Up Around the World (JoAnna Klein, New York Times)

India’s water crisis is already here. Climate change will compound it. (James Temple, MIT Technology Review)

Scientists have found new evidence that bat-like dinosaurs once fluttered through the skies.

What's new: A tiny dinosaur with membranous, bat-like wings lived during the Upper Jurassic period, according to a study in Nature on Wednesday.

• A fossil of the species — named Ambopteryx longibrachium — was found in northeastern China's Liaoning Province and dated to around 163 million years ago, bolstering the conclusions of a 2015 study detailing a less well-preserved but similar fossil finding called Yi qi.
• The specimen in the new study, by Min Wang and colleagues from the Chinese Academy of Sciences, is superbly preserved — researchers were even able to glean information from its stomach contents.

The details: The new species likely weighed only about 200 grams and could have fit in your hand, paleontologist and study author Jingmai O’Connor, told Science. “It would have been this tiny, bizarre-looking, buck-toothed thing like nothing alive today.”

• It also had feathers, along with bat-like, membrane wings supported by a long, pointed wrist bone, a type of structure that has been found in non-dinosaur flying vertebrates such as bats and pterosaurs.
• Scientists aren't yet sure exactly how this winged dinosaur flew.
• Ambopteryx longibrachium likely points to changes in wing structure around the time that scansoriopterygids, a family of climbing and gliding dinosaurs, split with bird lineages.
• In other words, it may represent an evolutionary dead-end that consisted of bat-like dinosaurs, and there may be other flying dinos that we don't yet know about.

The bottom line: The study, along with other research, puts forward the idea that membranous wings and elongated forelimbs in scansoriopterygids were likely short-lived evolutionary experiments with flight, since feathered wings and the birds we know of today came to predominate later.