Modern astronomical telescopes are extraordinarly powerful. And we keep making them more powerful. With telescopes like the Extremely Large Telescope and the Giant Magellan Telescope seeing first light in the coming years, our astronomical observing power will be greater than ever.
But a new commentary says that climate change could limit the power of our astronomical observatories.
The commentary is titled “The impact of climate change on astronomical observations.” The lead author is Faustine Cantalloube, a post-doctoral researcher at Max Planck Institute for Astronomy. It’s published in the journal Nature Astronomy.
Much of the observing power of modern astronomical observatories is a result of advances in technology and engineering, especially the development of segmented mirrors and adaptive optics. But there’s more to it than that. Their locations are chosen to give them the best environmental conditions.
“Each telescope site is likely to have its own microclimate that calls for individual study…”
From the Paper “The impact of climate change on astronomical observations.” Cantalloube et al, 2020.
Altitude, atmospheric turbulence, and atmospheric water content are all key considerations when deciding where to build one. The world’s most powerful telescopes are mostly clustered in Mauna Kea in Hawaii, and at Atacama in the high altitude Chilean desert. Both locations provide excellent seeing conditions.
But will they always?
In the introduction to their commentary, the authors write that they “have investigated the role some key weather parameters play in the quality of astronomical observations and analysed their long-term (longer than 30 years) trends in order to grasp the impact of climate change on future observations.”
The researchers focused on a single telescope to make their case: the European Southern Observatory’s (ESO) Very Large Telescope (VLT) at Cerro Paranal, in the Atacama Desert in Chile. The VLT is actually four telescopes in one facility, allowing it to perform interferometry. Each one has an 8.2 meter primary mirror, and the facility began operations in 1998. It’s one of the most powerful telescopes in the world, and it contains a number of ground-breaking secondary instruments like SPHERE, and MUSE.
In their work, the team of researchers used data from atmospheric sensors on the VLT, and other climate data going back to 1980. In their paper, they outline four ways that climate change affects observing.
Surface Temperature
On a powerful instrument like the VLT, temperature is critical. The primary mirrors are housed inside gigantic domes that open at dusk to begin observing. The temperature inside the dome is actively controlled, so that the temperature inside the dome during the day matches the coming temperature at dusk when the dome opens.
At the end of each night of observing, observatory personnel try to predict what the temperature will be for the following sunset, and they set the system to maintain the telescope at that temperature. The aim is to minimize the temperature difference between the telescope and the ambient temperature. If there’s too much difference between the two temperatures, it creates localized turbulence which degrades astronomical observations.
The current temperature control system at the VLT can’t go above 16 Celsius (61 F). But the temperature of the planet has risen in the past decades, and now there are more and more occurrences of the temperature exceeding the limitations of the temperature control system.
The problem is that the inner dome can’t be brought up to the ambient temperature when the ambient temperature exceeds 16 C.
When there’s a temperature differential between the primary mirror and the surface temperature, it creates turbulence inside the dome. Astronomers call it “dome seeing.” Dome seeing is a serious problem because it degrades the quality of the images. When the instrument is observing something that’s tens, hundreds, even thousands of light years away or more, dome seeing is detrimental.
The temperature will keep climbing. It’s expected that the temperature at Paranal will rise another 4 Celsius by the end of the current century, according to some models.
This is cause for deep concern, and for action. The European Extremely Large Telescope is currently under construction only 20 km (12.4 mi) from the VLT. With its enormous 39.5 meter primary mirror, it’s scheduled to see first light in 2025. Its instruments are still under development, so there’s still time to take climate change into consideration.
Surface Turbulence
Adaptive Optics (AO) is a powerful technological advance in modern astronomical observatories. Typically, an AO system will shoot a laser into the sky, creating a laser guide star, and then read the light for atmospheric distortions in the wavefront. The system then adapts to those distortions by changing the shape of its deformable mirror, or, more often, by micro-adjusting the segments in its mirror. For example, the ELT will have 798 separately adjustable segments in its 39.5 meter primary mirror.
But surface turbulence might outfox even AO. The surface layer is a thin layer of air that makes up the first few tens of meters of air above the ground. It’s altitude and depth can vary greatly over time. It’s caused by the “inefficient heat exchange between the ground and the airflow,” as the authors write.
The four unit telescopes (UTs) of the VLT are sensitive to the surface turbulence from about 10m to 30m (32.8 to 98.4 ft) and upwards. Luckily, the surface turbulence isn’t affecting image quality. Well, not yet, anyway.
But the warming temperature will increase temperature gradients, and is expected to cause an increase in surface turbulence. Every technology has its limits, and AO systems might have to be redesigned, or even reinvented, to deal with increasing surface turbulence.
Exoplanet Imaging
The next generation of astronomical observatories will have the power to capture images of some exoplanets. That’s a very difficult thing to achieve, and to accomplish it, lots of things have to work out right.
Like other astronomical targets, exoplanet images require a telescope with a high angular resolution. Basically, that means the ability to see small details. The AO system on the VLT can correct for atmospheric turbulence, and will allow the power of the VLT’s angular resolution to be exploited. But there’s another problem when it comes to exoplanets.
Exoplanets orbit other suns, and to see them, there needs to be a way to block out the light from that star. Otherwise, the starlight overwhelms the puny light from the exoplanet. For that, telescopes use coronagraphs.
Coronagraphs are used on solar observatories to block out the light from the Sun, and to let astronomers see more detail.
But there’s a problem, and the problem’s name is “wind-driven halo.”
The wind-driven halo is caused by the lag time between the “analysis of the atmospheric turbulence and its correction by a deformable mirror,” as the authors explain. It’s largely because of the speed of the jet-stream.
The sub-tropical jet stream near the VLT is up at about 12 km (7.5 mi). What happens is that by the time the telescope’s AO system has adapted to the conditions in the overlying sub-tropical jet stream, conditions have changed again. Hence the name wind-driven halo.
According to the study, this condition arises about 30% to 40% of the time. And it causes a significant reduction in the telescopes’s angular resolution.
The occurrence rate of the wind-driven halo changes by season, by El Nino, and by the Pacific Decadal Oscillation. But the frequency of the wind-driven halo is expected to intensify as climate change continues. This is because increasing climate change will mean stronger and longer El Nino’s.
Scheduling and Availability
There’s high demand for observing time on the world’s astronomical observatories. But even with all their power, and all the technological advances like AO, there are limits on their availability. Sometimes nature doesn’t cooperate.
The authors point out that there are “three critical parameters affecting the scheduling and availability of astronomical instruments at large observatories…” They are integrated water vapour (IWV), relative humidity, and cloud coverage.
A dry atmosphere is critical to the peak performance of telescopes like the VLT. The driest place on Earth is Antarctica, and the Atacama region is a close second. It’s a desert high in the mountains.
But even in the dry, high-altitude environment of the Atacama Desert, there are fluctuations in humidity. Yearly altiplanic winters, also called South American summer monsoons, are high humidity events that negatively impact observing. They occur in January and February as episodic atmospheric rivers.
These altiplanic winters are also associated with El Niño. In their paper the authors write that “Recent climate studies give a hint that an increased CO2 concentration in the atmosphere will give rise to a global increase in humidity and to more violent El Niño events.” More violent El Niño events mean flooding, which is a problem not specific just to astronomical observing. But it’s the potential increase in himidity and atmospheric rivers that could put the squeeze on observing opportunities.
However, in the case of altiplanic winters, the authors point out that the pedictions of climate change and El Niño may not actually make humidity worse. The evidence isn’t clear, and some evidence suggests that it could be drier. “By exploring different projection models, there are hints that the area will become drier,” the authors write, “but this needs to be more thoroughly checked due to the coarse resolution of the current models.”
Part of the problem is that the Paranal Observatory is at a very specific place: at the interface between the Andes and the Chilean coastal range.
To understand how climate change will affect the altiplanic winters and the atmospheric rivers there will require more study specific to that region. “In the future, we will refine this analysis to better apprehend the potential variation of IWV in the context of large near-infrared, submillimetre and radio wavelength observatories,” the authors write.
“There is No Planet B”
It’s clear that the changing climate will affect the world’s premier astronomical observatories. The authors wanted to draw attention to this issue so that the astronomical community can adapt to it.
“As astronomers, we are privileged to work in such a fascinating field, studying objects beyond Earth for the sole purpose of increasing humanity’s knowledge.”
From “The impact of climate change on astronomical observations.” – Cantalloube et al, 2020.
“The purpose of this work is to raise awareness among the astronomy community about the effect and immediate consequences of climate change on astronomical data,” they say in their conclusion. “Each telescope site is likely to have its own microclimate that calls for individual study, but we have highlighted three different areas — dome seeing, surface layer turbulence and the wind-driven halo effect — that will have an increasingly detrimental impact on astronomical observations at Paranal as climate change worsens.”
The paper is a call to action, from one team of astronomers to the rest of the astronomical community. “As astronomers, we are privileged to work in such a fascinating field, studying objects beyond Earth for the sole purpose of increasing humanity’s knowledge.” They point out that this initial research, and the voice of the astronomical community, could help lead to “concrete and sustainable actions against climate crises.”
Their goal is to optimize the energy and resources that the community uses to do its work, and to reshape global astronomy with the goal of reducing the environmental footprint of the entire endeavour.
“To do so, a massive cultural shift is needed, and it is of prime importance that astronomy uses its unique perspective to claim this simple fact: there is no planet B.”