Using a massive NASA dataset, Yale researchers have created a framework that helps explain just how sensitive local temperatures are to aerosols

For many, the word "aerosol" might conjure thoughts of hairspray or spray paint. More accurately, though, aerosols are simply particles found in the atmosphere. They can be human-made, like from car exhaust or biomass burning, or naturally occurring, from sources such as volcanic eruptions or sea spray.

Aerosols account for one of the greater uncertainties in understanding the Earth's climate and, through a cooling effect, mask a significant portion of the warming caused by the increase in greenhouse gas concentrations.

One unresolved issue in understanding aerosol-climate interactions is why, for a unit change in the energy imbalance at the top of the atmosphere, the surface temperature change is higher for aerosols than for greenhouse gases. This is known as climate sensitivity. The conventional understanding is that the higher climate sensitivity to aerosols is due to their higher concentrations over land surfaces, which heat up and cool down faster than oceans. {module In-article}

In a recently published paper in the American Geophysical Union's journal Geophysical Research Letters, Yale researchers demonstrate that it is not only the geographic distribution of aerosols that explains the higher climate sensitivity but also the specific local-scale interactions with the land surface.

Using a theoretical framework to separate surface temperature response to external forcing, the study also provides mechanistic insight into spatial patterns of the local temperature change due to aerosols.

"With traditional climate models, there are huge uncertainties in how aerosols affect surface temperature," said T.C. Chakraborty, a Ph.D. student at F&ES who co-authored the paper with Xuhui Lee, the Sara Shallenberger Brown Professor of Meteorology. "This framework helps explain why and how some of these uncertainties are coming into play."

Aerosols are known to increase radiation in the longer wavelengths (longwave) and decrease radiation in the shorter wavelengths (shortwave). The strength of these effects depends on the size and chemical nature of the aerosol particles. Using the framework to analyze a massive dataset developed by NASA, Chakraborty found that although the longwave effect of aerosols has generally been considered by the scientific community to be less important, the climate is more sensitive to it than to the shortwave effect.

This is because of the absence of the shortwave effect at night, a time when the atmosphere is more stable -- and thus more sensitive to radiation. It is also the result of the high climate sensitivity in arid regions, where the longwave effect is prevalent due to the presence of aerosols from coarse mineral dust. Combined, the longwave and shortwave effects reduce the terrestrial diurnal temperature range by almost one degree Fahrenheit. Aggregating the eight major regions of interest used in the study, about half of this reduction is due to human-made aerosols.

There are also long-term trends, Chakraborty said, that show an intensification of the local climate sensitivity in the tropics due to deforestation between 1980 and 2018, demonstrating the importance of vegetation in regulating interactions between aerosols and the climate.

In separate observation, David Jewitt describes 20 previously unidentified moons around Saturn

David Jewitt, a UCLA professor of planetary science and astronomy, has captured the best and sharpest look at a comet from outside of our solar system that recently barged into our own. It is the first interstellar comet astronomers have observed.

Comet 2I/Borisov (the "I" stands for interstellar) is following a path around the sun at a blazing speed of approximately 110,000 miles per hour, or about as fast as Earth travels around the sun. Jewitt studied it on Oct. 12 using NASA's Hubble Space Telescope, which captured images of the object when it was about 260 million miles away. He observed a central concentration of dust around the comet's solid icy nucleus -- the nucleus itself is too small to be seen by Hubble -- with a 100,000-mile-long dust tail streaming behind. {module In-article}

Jewitt said it's very different from another interstellar object, dubbed 'Oumuamua, that a University of Hawaii astronomer observed in 2017 before it raced out of our solar system.

"'Oumuamua looked like bare rock, but Borisov is really active -- more like a normal comet," said Jewitt, who leads the Hubble team. "It's a puzzle why these two are so different. There is so much dust on this thing we'll have to work hard to dig out the nucleus."

That work will involve sophisticated image processing to separate the light scattered from the nucleus from light scattered by dust.

2I/Borisov and 'Oumuamua are the first two objects that have traveled from outside of our solar system into ours that astronomers have observed, but that's because scientists' knowledge and equipment are much better now than they ever have been and because they know how to find them. One study indicates there are thousands of such comets in our solar system at any given time, although most are too faint to be detected with current telescopes.

Until 2I/Borisov, every comet that astronomers have observed originated from one of two places. One is the Kuiper belt, a region at the periphery of our solar system, beyond Neptune, that Jewitt co-discovered in 1992. The other is the Oort Cloud, a very large spherical region approximately a light-year from the sun, which astronomers think contains hundreds of billions of comets.

2I/Borisov was initially detected on Aug. 30 by Gennady Borisov at the Crimean Astrophysical Observatory, when it was 300 million miles from the sun. Jewitt said its unusually fast speed -- too fast for the sun's gravity to keep it bound in an orbit -- indicates that it came from another solar system and that it is on a long path en route back to its home solar system.

Because the comet was presumably forged in a distant solar system, the comet provides valuable clues about the chemical composition and structure of the system where it originated.

2I/Borisov will be visible in the southern sky for several months. It will make its closest approach to the sun on Dec. 7, when it will be twice as far from the sun as Earth is. By the middle of 2020, it will pass Jupiter on its way back into interstellar space, where it will drift for billions of years, Jewitt said.

Comets are icy bodies thought to be fragments left behind when planets form in the outer parts of planetary systems.

20 new moons for Saturn

In separate research that has not yet been published, Jewitt is part of a team that has identified 20 previously undiscovered moons of Saturn, for a new total of 82 moons. The revised figure gives Saturn more moons than Jupiter, which has 79.

The new objects are all small, typically a few miles in diameter, and were discovered using the Subaru telescope on Maunakea in Hawaii. They can be seen only using the world's largest telescopes, Jewitt said.

The moons might have formed in the Kuiper belt, said Jewitt, a member of the National Academy of Sciences and a fellow of the American Association for the Advancement of Science and of the American Academy of Arts and Sciences.