Sky & Telescope - March 2025 - 17

but many with relative humidity higher than 50%. High
humidity complicates some observations, particularly those
in the mid- and near-infrared, because water absorbs light at
these wavelengths.
Their study, however, did not confirm the overall deterioration
in seeing - turbulence's blurring effect - that Cantalloube
and Milli's work had found.
Closed Domes and Lost Photons
Other observatories face different challenges. In 2022,
astronomer Maaike van Kooten (now at the Herzberg Astronomy
and Astrophysics Research Centre, Canada) collaborated
with atmospheric scientist Jonathan Izett (University of California,
Santa Cruz) to investigate the impact of climate on
Mauna Kea, another landmark for ground-based astronomy.
While overall conditions at the observatory have remained
relatively stable, the study revealed that maximum wind
speeds have increased over the decades. The team also found
that the frequency of weather conditions that would require
dome closures - including freezing temperatures, high winds,
and excessive humidity - has more than doubled in the last
two decades.
The domes aren't necessarily closed every time these conditions
occur; the local operators have the final call. Also, for
telescopes like Keck's two 10-meter instruments, domes are
already closed about 40 nights per year for non-meteorological
reasons, such as planned maintenance. Still, the increase
in bad weather conditions means more nights when no observations
are done.
As in the Chilean studies, Van Kooten and Izett found that
40 years are not enough to see clear trends, due to the variable
nature of weather data. They also had problems with data
quality, for instance when instruments were replaced. No
record of these changes was made to guide future researchers
accessing the data.
Apart from weather, the atmosphere itself is also becoming
less transparent to photons coming from space, as it
heats up and its water vapor and aerosol content increase.
In a 2022 study, Eric Steinbring (Herzberg Astronomy and
Astrophysics Research Centre, Canada) showed that there's
been a nearly 0.2% decrease per decade in the amount of
light that reaches the ground. He compared 17 years of data
collected by the 8-meter Gemini North and South telescopes
in Hawai'i and Chile, respectively, with observations from the
Gaia spacecraft, which isn't affected by Earth's atmosphere.
This decrease is in line with what would be expected from the
global rise in air temperature.
The drop is the equivalent of poking a 12-centimeter hole
in the primary mirror of an 8-meter telescope every year,
Steinbring said in a presentation. While it might not seem
like a lot, " it's adding up. "
SMOKY SKIES Lightning-sparked fires and their smoke compete with
the starry vista above the Mount Kobau Star Party in August 2018. Mars
sits above the trees at left.
sk yand tele scope .o r g * MARCH 2025 17
Extreme Locations, Difficult Predictions
Anticipating climate change's effects on astronomical sites is
challenging. The best available simulations generally forecast
deteriorating conditions, but their resolution is too coarse to
accurately represent small geographical features like islands
and mountaintops, where most observatories are located.
A comprehensive study published in 2022 by Caroline
Haslebacher (University of Bern, Switzerland) and others
analyzed climate projections for eight major observing sites
located on four different continents and in Oceania: Mauna
Kea, Cerro Paranal, La Silla, Cerro Tololo, La Palma, Sutherland,
Siding Spring, and San Pedro Mártir. The only common
trends the researchers could find for all the sites were an
increase both in average temperatures and in the total water
vapor content in the atmosphere.
However, while these projections account for the observatories'
elevations, they neglect crucial topographical factors
that significantly influence local weather patterns, and they
aren't good at simulating interactions between land and
JIM FAILES
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