Methane is a greenhouse gas with dual personalities. It heats Earth’s atmosphere 28 times as potently as carbon dioxide, gram for gram. But its absorption of the sun’s radiation high in the atmosphere also alters cloud patterns — casting a bit of shadow on its warming effect.

So rather than adding even more thermal energy to the atmosphere, as previously thought, methane’s solar absorption sets off a cascade of events that reduces its overall warming effect by about 30 percent, researchers report March 16 in Nature Geoscience.
“These are really interesting and important results,” says Rachael Byrom, a climate scientist at the CICERO Center for International Climate Research in Oslo who wasn’t involved in the new study. Nonetheless, she says, “methane still remains a really key gas that we need to target in emissions reductions.”

Humans are responsible for most of the methane entering the atmosphere, where it worsens global warming. Concentrations of the potent greenhouse gas have risen about 162 percent since preindustrial times, according to the U.S. National Oceanic and Atmospheric Administration.

The largest sources of anthropogenic methane include fossil fuel use, livestock, rice farming, landfills and biomass burning (SN: 9/29/22; SN: 7/14/20). Scientists fear that as warming triggers thaw of permafrost in the Arctic regions, this could also lead to increased methane emissions, as microbes in the soil consume dead plant material and release the gas (SN: 9/25/19).
Greenhouse gases like methane exert their strongest effects by absorbing infrared “longwave” radiation emitted from the planet’s surface. Earth emits this longwave radiation when it is struck by “shortwave” radiation coming directly from the sun. Most studies of greenhouse gases focus on longwave absorption.

But scientists are learning that greenhouse gases, including methane, also absorb some of the sun’s shortwave radiation. Recent estimates suggested that methane might contribute up to 15 percent more thermal energy to the atmosphere than previously thought, due to this additional shortwave absorption.

However, the new study reveals that methane’s shortwave absorption has the opposite effect. This finding is based on a detailed analysis of the gas’s absorption at various wavelengths.

The result is “counterintuitive,” says climate scientist Robert Allen of the University of California, Riverside. It happens because of the way that methane’s shortwave absorbance affects clouds in different layers of the atmosphere, Allen and colleagues’ simulations suggest.

When methane absorbs shortwave radiation in the middle and upper troposphere, above about three kilometers, it further warms the air — leading to fewer clouds in that upper layer. And because methane absorbs shortwave radiation high up, less of that radiation penetrates down to the lower troposphere. This actually cools the lower troposphere, leading to more clouds in that layer.

These thicker low-level clouds reflect more of the sun’s shortwave radiation back out to space — meaning that less of this solar radiation reaches Earth’s surface, to be converted into longwave radiation.

Meanwhile, upper-level clouds, in addition to greenhouse gases, are known to absorb longwave radiation. So fewer of these clouds means that less of the longwave radiation emitted by Earth is captured in the atmosphere — and more of it escapes to space without contributing to climate change.

With methane’s shortwave absorption, “you expect warming of the climate system,” Allen says. “But these cloud adjustments actually overwhelm the heating due to absorption, leading to a cooling effect.”

Allen and his colleagues conducted the study using a computational model of Earth’s climate. When they took the traditional approach — considering only methane’s longwave absorbance — they estimated that the gas has caused 0.2 degrees Celsius of warming since preindustrial times, out of 1.06 degrees C total warming. But when they also included shortwave absorbance, methane’s contribution to warming fell to about 0.16 degrees C.

In addition to warming the planet, methane is also thought to increase global precipitation, due to greater evaporation of water with higher temperatures. But the researchers found that inclusion of shortwave absorbance also reduced methane’s precipitation effect, from a predicted 0.3 percent increase in precipitation (based on longwave absorbance alone), down to an increase of about 0.18 percent.
It will be important to include methane’s shortwave effects in future climate projections, says Daniel Feldman, an atmospheric scientist at the Lawrence Berkeley National Laboratory in California, who was not involved in the study. But he thinks that more work needs to be done to clarify those effects.

The new study analyzed methane’s shortwave impact using only one comprehensive model that included both the atmosphere and ocean, he says. “I would just like to see that sort of analysis done across multiple models,” increasing confidence in the results.

People have different tastes. It turns out that octopuses, squid and cuttlefish do too.

These soft-bodied cephalopods have proteins on suckers along their tentacles that allow them to “taste” by touching objects. But the species have evolved to detect different compounds, researchers report in two studies published in the April 13 Nature. And the differing tastes may be tied to the species’ hunting styles.

All the species have modified versions of proteins called neurotransmitter receptors, which detect brain chemicals. Evolution morphed the brain proteins to take on new roles as taste-sensing proteins. But octopus evolution led them to develop a taste for greasy things, while squid and cuttlefish evolution tweaked the brain proteins to detect bitter compounds, the researchers discovered.
“This is an entirely new sensory system,” says Maude Baldwin, an evolutionary biologist at the Max Planck Institute for Biological Intelligence in Seewiesen, Germany, who was not involved in the work. “Together these papers offer unprecedented insight into how sensory systems evolve.”

Studying cephalopod receptors might also shed some light on how human taste-sensing proteins evolved. “It greatly enhances our understanding of how proteins evolve in general,” Baldwin says, as well as how proteins and even entire organisms acquire new functions.

Octopuses can taste many “greasy, sticky” molecules
In a previous study, Harvard physiologist Nicholas Bellono and colleagues discovered that barrel-shaped proteins known as chemotactile receptors in the suckers of California two-spot octopuses (Octopus bimaculoides) allow the animals to taste terpenes — “greasy,” insoluble molecules — with their arms (SN: 10/29/20).

To get a detailed look at these proteins, Bellono teamed up with structural biologist Ryan Hibbs of the University of Texas Southwestern Medical Center at Dallas. Hibbs and colleagues used cryoelectron microscopy to examine the three-dimensional structure of the protein.

When looking at the structure of the octopus protein, the researchers found an unexpectedly large molecule stuck in a special pocket used to detect certain chemicals. Finding a molecule stuck in one of these pockets can give clues to the protein’s function.
The mystery molecule turned out to be part of the detergent the researchers used to prepare the protein for the microscope. That’s very different from the types of molecules that bind to the neurotransmitter receptors from which chemotactile receptors evolved, says Hibbs, now at the University of California, San Diego. “Neurotransmitters are small and soluble. This thing is bulky and greasy.”

By testing a variety of molecules collected from neighboring labs, Bellono’s team determined that the octopus receptors can detect a variety of “greasy, sticky molecules” that don’t dissolve in water. Because octopuses feel around for their prey, it makes sense that their taste receptors evolved to detect molecules that remain stuck to underwater surfaces such as crab shells or their own eggs, rather than small chemicals that easily diffuse in water, Hibbs says.
But octopuses don’t seem to find all greasy molecules tasty. In one experiment, the researchers tested the response of a severed tentacle to one such chemical. The arm crawled off the measuring apparatus and right out of the bath.

Squid and cuttlefish can discern bitter compounds
To see if other cephalopods share octopuses’ tastes, the researchers turned to genetic analyses. Octopuses have 26 genes that each encode a slightly different chemotactile receptor protein. Those proteins can come together in combinations of five to detect a wide variety of molecules, the team found.

Examining genes from squid and cuttlefish, the researchers discovered that these cephalopod species also have modified neurotransmitter receptors in their suckers. But some of the squid and cuttlefish receptors detect bitter compounds which can diffuse in water, not the greasy ones octopuses taste. (Squid could also taste some terpenes, but not all of the greasy molecules octopuses detect.)

Bitter taste is often a signal that something is spoiled or poisonous, so animals usually avoid bitter compounds, says Harold Zakon, a neuroscientist and evolutionary biologist at the University of Texas at Austin who was not involved in the work.

Bitter compounds also caused squid to turn up their noses — or in this case, tentacles — at prey. Squid given shrimp soaked in a bitter compound handled the food longer before eating it than they did with undoctored prey. Or the squid rejected the bitter shrimp, something researchers never saw the animals do with regular prey.

The type of receptors the species have reflect their hunting strategies. Octopuses ”explore everything with their arms,” Bellono says, and likely use chemotactile receptors to guide their explorations. While octopuses use sight to catch prey out in the light of day, chemotactile receptors help them hunt in the dark and to find prey hidden in cracks and crevices, Bellono says. Squid and cuttlefish are ambush predators that rely on eyesight alone. The bitter receptors help squid decide whether to eat their prey only after they have it in their grasp.

The octopus and squid receptors evolved about 300 million years ago, early in the species’ histories. But it’s impossible to tell whether hunting style or receptor type came first or if the traits evolved together.

Octopuses also have another type of chemotactile receptor, the researchers found, but they don’t yet know what sorts of molecules those receptors sense.

It will take years to work out the details of what all the cephalopods’ receptors detect and how they influence animals’ behavior, Zakon says. “This is really a first announcement that these receptors have changed in fundamentally important ways.”

The rings that make Saturn such a spectacle are probably heating its atmosphere and making it glow at ultraviolet wavelengths.

Researchers detected an excess of ultraviolet emission in Saturn’s northern hemisphere that comes from hydrogen atoms. The emission, known as Lyman-alpha radiation, is probably the result of water ice, which contains hydrogen, falling into the atmosphere from the planet’s rings, the researchers propose March 30 in the Planetary Science Journal.

The detection of similar emission from a distant world could someday lead to the discovery of a Saturn-like planet orbiting another star.
The key to the discovery came after two spacecraft — the Hubble Space Telescope and Cassini — observed Saturn simultaneously in 2017, right before Cassini plunged into the planet’s atmosphere, says Lotfi Ben-Jaffel, an astrophysicist at the Institut d’Astrophysique de Paris.

This allowed Ben-Jaffel and colleagues to calibrate the ultraviolet detectors on those spacecraft as well as detectors on Voyager 1 and 2, which flew past Saturn in 1980 and 1981, and the International Ultraviolet Explorer, an Earth-orbiting satellite that also observed Saturn. Comparing these ultraviolet observations revealed a band of excess Lyman-alpha radiation spanning 5° to 35° N latitude on Saturn.

The researchers’ explanation for the extra ultraviolet glow is plausible, says Paul Estrada, a planetary scientist at NASA’s Ames Research Center in Moffett Field, Calif., who was not involved with the new work.

“We know material is falling in from the rings,” he says, because Cassini detected it during the spacecraft’s spiral into Saturn (SN: 12/14/17). “The rings are predominantly water ice. It may be the source of the atomic hydrogen” emitting the Lyman-alpha radiation that the researchers have detected, he says.

When icy ring particles fall into Saturn’s atmosphere, they carry kinetic energy with them. “They have to release that energy to the surrounding gas,” Ben-Jaffel says, and that energy heats up the atmosphere. When the icy particles vaporize, they release additional energy, further heating the atmosphere and making it glow at UV wavelengths. The researchers suspect that the emission also appears in the planet’s southern hemisphere.

All the giant planets of our solar system have rings, but only Saturn’s are so bright and beautiful. Astronomers don’t yet know whether any of the thousands of worlds found orbiting other stars have rings that are equally magnificent.

The new discovery may help astronomers identify those spectacularly ringed worlds, if they exist. Future planet hunters could look for the telltale ultraviolet glow of the Lyman-alpha radiation, Ben-Jaffel says, and then further observations could confirm the rings’ existence.