SONG of the Stars

In the region of Canada that is now Quebec and Ontario, a Native American tribe known as the Algonquin developed a detailed myth about the annual path of the Big Dipper around the north celestial pole, which they chanted in their Song of the Stars: "We are the stars which sing, / we sing with our light; / We are the birds of fire, / we fly over the sky". Next week an international team of astronomers known as the SONG collaboration will gather in Denmark to discuss future plans for studying "stars which sing with their light".

The SONG collaboration is a Danish-led initiative to establish a global network of small telescopes to observe the subtle signatures of star-quakes that probe the interiors of distant Suns. The idea for SONG -- an acronym for Stellar Oscillations Network Group -- is based on the successful implementation of a similar concept deployed in the 1990's for observations of the Sun. The continuous oscillations that can be detected on the surfaces of the Sun and other stars require uninterrupted observations for long periods of time. The rotation of the Earth prevents a single telescope from being able to observe for such long periods without interruption. This limitation can be overcome by coordinating the observations of telescopes that are strategically located around the planet -- the star under investigation is always visible from somewhere, so a suitable network of telescopes can monitor any given star 24/7.

The Danish astronomers are among the world's leading experts on the observation and interpretation of star-quakes to deduce the internal structure and composition of stars like the Sun. Their motivation for establishing a global network of dedicated telescopes was driven by their frustration trying to conduct such observations with existing facilities. Obtaining time to use the telescopes at most observatories is a competitive process. Proposals are normally submitted 6 months to a year ahead of time, including a detailed scientific justification, and a typical allocation usually does not exceed 1 or 2 weeks. Since the proposals at different observatories are evaluated by independent time allocation committees, coordinating the observations from many telescopes is difficult to say the least. In the end, these logistical constraints were compromising the quality of the resulting observations -- so the Danes started looking for a better way to proceed.

With an initial grant from the Carlsberg Foundation, funded by the Danish beer company, they formulated a conceptual design that could accomplish their scientific objectives. Subsequent grants from public and private sources in Denmark will fund the construction of the prototype, which is scheduled to begin operating at an observatory in the Canary Islands by 2012. The purpose of the meeting next week is to coordinate with international partners who will seek their own funding to replicate the prototype and operate one of the telescopes in the network at their local observatory.

The Danes can't do it alone, but they have made it considerably easier to establish the network by doing much of the difficult work up front. This will certainly make it an easier sell to funding agencies and private foundations around the planet.

Launching Kepler

Next Friday NASA is scheduled to launch a satellite known as "Kepler", on a several year mission to find planets like the Earth around distant stars like the Sun. As a member of the committee that leads an international collaboration of astronomers who will be analyzing some of the data from this mission, I received an invitation from NASA to attend the launch in person.

Kepler is an enormous science-grade digital camera with nearly 95 megapixels (compare that to your digital camera at home). The giant camera is attached to a telescope with a mirror more than 55 inches across, which will stare at a large patch of the sky just above the Milky Way for at least the next few years. The diameter of each image on the sky will be as wide as 24 full moons lined up side by side, and the camera will normally take 30-minute long exposures. The images will be processed on board the spacecraft to measure the brightness of more than 100,000 stars, and these measurements will be saved and transmitted to the Earth every few months.

Some fraction of the stars that Kepler monitors will have planets, and some fraction of those planetary systems will be oriented in such a way that the alien worlds pass directly in front of their host star from the vantage point of the telescope. It's sort of like an eclipse, except the planet will only cover a small fraction of the surface of the star for up to a few hours, causing a small dip in the amount of light recorded by Kepler's camera. Astronomers are particularly interested in finding planets similar to the Earth, which are far enough from their stars that water exists in liquid form, but not so far that the water becomes mostly frozen. Like the Earth, such planets will go around their stars only about once a year -- so scientists will be looking for stars that show several tiny dips in light, each separated by about a year. By looking at more than 100,000 stars, Kepler should be able to find at least a few planets like the Earth if they are at all common. If not, that will also be a very interesting thing to know!

The satellite will be launched on a Delta II rocket next Friday evening from Cape Canaveral, Florida. The Delta II is the same vehicle used by NASA to put GPS satellites into orbit, and night launches are particularly spectacular since you can easily see the rocket ascend all the way to the edge of the Earth's atmosphere. Kepler will be placed in orbit around the Sun, just behind the Earth, and will slowly drift further away over the next few years. By then its mission should be finished, and we will know exactly how special the Earth really is -- or how common it is to find planets just like ours in the neighborhood.

Although I'm not part of the official science team, I've been preparing for the Kepler mission for the past several years. I just got a grant from NASA to continue this work for the next few years. Cross your fingers that everything goes well!

Penguins in Peril

Emperor penguins -- the stars of the 2005 documentary March of the Penguins -- may be headed for a population collapse brought on by global warming, according to a study published this week in the Proceedings of the National Academy of Sciences. The impact of climate change on the extent of sea ice in Antarctica, where the penguins live and breed, is projected to reduce the penguin population by nearly 95% by the end of this century.

The authors of the report developed a sophisticated computer model to predict the population size over time for a specific colony of emperor penguins that currently contains about 6000 breeding pairs. The model was tuned to fit actual observations of a smaller population collapse that occurred during a warm period in the 1970's, and then it was coupled to the predictions of future climate from the Nobel prize-winning report of the Intergovernmental Panel on Climate Change (IPCC). The key factor that determines whether the penguin population will decline over time is how often such warm periods occur -- if they happen too often, the sea ice does not have a chance to recover between events and the penguins suffer a loss of habitat that reduces their chances of survival and corrodes their ability to reproduce.

The population model was coupled to a wide variety of individual climate predictions from the IPCC report, but all of them assumed a "business as usual" scenario in which humans continued to burn fossil fuels sufficient to double the atmospheric concentration of carbon dioxide (CO2) from pre-industrial levels by the end of the century (from 360 to 720 ppm). While this does not represent the worst case scenario for the future climate of our planet, humanity can certainly choose to avoid such a path. For example, most scientists agree that we can avoid the worst consequences of global warming by stabilizing CO2 concentrations no higher than 450 ppm, and the successor to the Kyoto climate treaty is likely to specify this target.

The results of the study suggest that the emperor penguin population could decline by 95% to only 400 breeding pairs by 2100 -- an event that would qualify as a "quasi-extinction". The chances of this happening varied from near certainty (84%) to about 1-in-3 (36%) depending on which specific climate projection was used, but all of them predicted a population in steep decline after 2080. Unlike other Antarctic species, emperor penguin behavior -- such as the timing of their migration and egg laying -- does not appear to have changed much in response to the warming that has already occurred. The authors speculate that the long lives of the penguins may prevent them from adapting to these relatively fast environmental changes, so it is unlikely that they will be able to save themselves.

The good news is that our planet is not yet committed to the amount of warming represented by these projections. We can choose to alter our behavior now, and reduce the future increases in greenhouse gas concentrations below those assumed in this report. If we do, perhaps our grandchildren will one day be able to enjoy March of the Penguins 2.

Blast from the Past

More than 400 years ago, a nearby star in our Galaxy exploded with a flash of light so bright that we can still see echoes of the event from the surrounding dust clouds. The event is known as "Tycho's supernova", after the Danish astronomer who published detailed observations of it in 1572. In today's issue of the journal Nature, astronomers announced new observations that have for the first time established the exact cause of the explosion.

When a new star appeared in the sky in 1572 -- bright enough to be visible in full daylight -- it instantly attracted public attention. At the time the "sphere of the stars" was believed to be unchanging and eternal, so the event challenged the prevailing view of the universe. Tycho Brahe was one of the first astronomers to make precise observations of the positions of stars and planets, before he famously died of a urinary infection after straining his bladder at a royal banquet as a courtesy to the host. Tycho's observations of the 1572 event were published in a book titled "Stella Nova" (Latin for "new star"), which demonstrated that the object was far beyond the orbit of the moon.

Modern analysis of the historical record suggests that the event was actually an exploding star, now commonly referred to as a "supernova". There are several types of supernova explosions, arising either from the detonation of a white dwarf star (the ember that is left at the end of the life of a regular star like our Sun), or from the gravitational collapse of a very massive star at the end of its life. The available evidence suggested that Tycho's supernova was probably the first type, but it could not distinguish between two possible scenarios that can trigger such an event. In one scenario the white dwarf is in a close orbit with a regular star -- so close that its gravity slowly consumes the mass from its companion until the white dwarf exceeds a critical size and collapses under its own weight. In the other scenario the companion is also a white dwarf, and the two white dwarfs slowly draw closer together until they merge and similarly collapse in a spectacular explosion.

The usual way of deciding what triggered the supernova explosion requires evidence that was not available in 1572. The light from the event is passed through a prism to separate it into its constituent colors (a spectrum), where dark lines appear at specific colors that reveal what the object is made of and how it is moving. For the observations that were announced today, astronomers used the interstellar dust that surrounds Tycho's supernova as a sort of time machine to record the spectrum of the event. Here's how it works: the explosion created a sudden flash that moved out in all directions at the speed of light and reached the Earth in 1572. Now imagine that there was a cloud of dust 200 light years behind the supernova that reflects some of the light of the original flash back toward the Earth. This dust cloud would not be lit up until 200 years after the explosion, and the reflected light would then have to travel another 200 years just to get back to the supernova itself -- so the flash from the dust cloud would arrive 400 years after the original explosion. Using this basic idea, astronomers recorded the spectrum of an illuminated dust cloud near Tycho's supernova and discovered that it looked exactly like an explosion caused by the first scenario -- from a white dwarf consuming a regular star.

It's a wonderful piece of detective work, and by repeating the experiment on dust clouds in every direction around the remnant of Tycho's supernova, scientists hope to eventually measure the three dimensional shape of a 400 year old explosion. Tycho would surely be impressed by our ability to study this blast from the past.

Science under Obama

The decisive presidential victory of Barack Obama last night has given millions of ordinary people hope for the future, but it also represents a symbolic end to the "war on science" that has been waged by the current administration. Scientists across the country now share the hope that the newly-elected administration will not only reverse the politicization of science that has taken place over the past eight years, but also demonstrate a renewed commitment to maintaining the competitiveness of the United States in science and technology by expanding our investment in basic research.

The abuses of the Bush administration -- distorting and suppressing scientific research for political purposes -- has been well documented within several federal agencies. Much of this information has come from a series of surveys conducted by the Union of Concerned Scientists over the past few years, and their efforts to restore scientific integrity now represent one of the top priorities of the organization. Scientists are consistently ranked by the public as more credible than politicians or even journalists, so it's important that their research is free from government interference and that their conclusions are released to the taxpayer-sponsors without political censorship. One indication that Barack Obama intends to use science to craft policy -- rather than the other way around -- can be seen in his proposal for addressing climate change, which aims to "reduce greenhouse gas emissions 80 percent by 2050". This target is not arbitrary, but was taken directly from the recommendations of the UN Intergovernmental Panel on Climate Change, representing the consensus of thousands of scientists from around the world.

With the rampant international aggression and reckless fiscal policies of the Bush administration, funding for non-military programs has stagnated even as deficit spending has soared and the national debt has more than doubled. The Bush strategy of using irresponsible tax cuts for the ultra-wealthy coupled with massive increases in military spending to justify a shrinking domestic budget has been disastrous for the basic government services that millions of people depend on. But it has also had a corrosive effect on our nation's scientific research facilities -- leaving some of the most promising technologies undeveloped, and stunting the bold new discoveries that attract the brightest young people to pursue careers in science. The newly-elected administration understands that a federal commitment to basic research provides the raw material for future economic growth and prosperity, and ensures our nation’s competitive position in the world. That's why Barack Obama co-sponsored legislation in the Senate in 2007 that tried to boost support for NASA and double the funding for basic research through the National Science Foundation by 2015. Though it sounds ambitious, the annual research budget of the NSF could be doubled for the current cost of about one week of military operations in Iraq.

The global financial crisis has created some uncertainty about what sort of economy the next administration will inherit from George W. Bush, and this has understandably led to a reflection on the nation's priorities. With the right leadership -- one that uses sound science to craft effective policy, and that understands the importance of an investment in basic research -- scientists can help turn this economy around. With Barack Obama at the helm, it just might happen.

Climate Conservatives

Over the weekend, I attended a high school reunion with my wife. One of the great things about reunions is that you get to talk with people from all walks of life, who typically have a wider variety of opinions than your co-workers or neighbors. One of my wife's classmates, after hearing that I work at a federal laboratory that does climate change research, asked whether or not I believed in global warming. The question really took me by surprise because, to me, global warming is like gravity: it doesn't really matter whether you "believe in" it -- it just is.

I responded that her question made me wonder whether or not she believed in climate change. She explained that the issue made her very upset because of how political the debate had been, and that she didn't believe in global warming at all. She said she didn't trust Al Gore, and she thought he was a hypocrite for making so much money from his film and for flying his private jet between stops on his "climate crisis" tour. If the facts in the film had been presented by a scientist, she said, it probably would have done more to persuade her about the problem. The central issue seemed to be about credibility, not about facts. I tried to imagine my own response to claims about a crisis coming from a source that I didn't trust.

In situations like these, it's important to try to communicate with people using arguments that will not instantly be rejected by their conservative viewpoint. I tried to explain that when our parents were our age there were still significant uncertainties about climate change, so we couldn't really blame them for not acting. While scientists were establishing the root causes of global warming, the greenhouse gas emissions of humanity had exceeded the capacity of the natural world to absorb them -- and there was so much surplus carbon-dioxide in the atmosphere that even if we stopped the emissions now, the planet would still continue to warm for nearly the rest of our lives. So the debate about climate change was really about what (if anything) we should do to influence the kind of world our children and grandchildren would live in.

I also pointed out that politicians were no longer arguing about the reality of climate change. The real debate was about how much we should do to mitigate the problem. Look at the proposals of the two major presidential candidates in this election year: the Democratic candidate is proposing to cut greenhouse gas emissions 80% by 2050 (a target drawn directly from the scientific recommendations of the UN Intergovernmental Panel on Climate Change) while the Republican candidate is proposing a 60% cut. Furthermore, only the Republican candidate for vice-president has expressed skepticism that climate change is primarily caused by human activity. Everyone else agrees with the scientific consensus.

I may not have convinced my wife's classmate to reconsider her position on global warming, but I'm fairly sure that I presented the issue in a manner that she could identify with. If conservatives continue to resist efforts to mitigate climate change, they are the ones who will have to explain to their grandchildren why they failed to take action even after the science was clear.

Grassroots Astronomy

A catalog of stars that a NASA satellite will search for distant solar systems over the next few years is scheduled to be released today through the Space Telescope Science Institute. In an unconventional approach to funding their research on the project, a team of astronomers is working with a non-profit organization to put the stars up for adoption on the Internet.

"NASA's budget is heavily earmarked, and research funding has been stagnant in recent years," said astronomer Travis Metcalfe, who leads the fundraising project. "We decided to engage the public directly for some grassroots financial support of this crucial element of the program."

The scientists are preparing for the launch of NASA's Kepler satellite, which will monitor more than 100,000 stars beginning in April of next year. To bring the catalog to life, the likely target stars were marked in Google Sky, where potential sponsors can browse them through the project website before selecting a star to adopt for a modest $10 donation. All contributions are used to support the analysis that will determine the physical sizes of any planets discovered by the mission.

The adopt-a-star service, named the "Pale Blue Dot" project after a phrase coined by popular astronomer Carl Sagan, has already attracted donations from hundreds of people who signed up early to get their first choice of stars. Donors receive a certificate of adoption by email, and updates when any planets are discovered around their adopted stars. Each star is tagged with the name of the sponsor, both in Google Sky and in a text version of the catalog, so no two people can adopt the same star. The final targets will not be decided until later this fall, but if an adopted star does not end up on the list, the associated donor will make a new selection.

"This is a creative way of involving the community," said Danish professor Joergen Christensen-Dalsgaard, who organized the international team that is conducting the research. "The present list of adopters is already fairly impressive, and of course the funding that it brings in is useful."

The outreach and fundraising effort is being coordinated by White Dwarf Research Corporation, a non-profit located in Boulder, Colorado. Dedicated to scientific research and public education, the organization hosts the project web pages and accepts donations on behalf of the research team free of charge. If most of the Kepler target stars are ultimately adopted, the resulting endowment is expected to provide significant support to the research project throughout the lifetime of the mission.

"This program educates the public about the Kepler satellite, but it also makes them stakeholders in its success by allowing anyone to adopt a star that NASA will search for planets," said Metcalfe. "How cool is that?"

Arctic Meltdown

This week, scientists at the National Snow and Ice Data Center (NSIDC) announced that the amount of unmelted sea ice in the Arctic has reached the second lowest level since satellite monitoring began 30 years ago, and it may be on a trajectory in the coming weeks to break the record melting that occurred just last year. The revelation comes on the heels of reports earlier this month of polar bears swimming in the open waters off the northwest coast of Alaska. It has long been known that the effects of global warming appear first near the north and south poles, so this news provides a glimpse of what's in store for the rest of the world if we don't begin to change course soon.

On the summer solstice this year, I was on an airplane returning from a two week visit to Denmark -- I purchased carbon offsets for the trip, but the irony of the situation is still tangible. In the middle of the day, I peered out the window to discover that we were flying over the east coast of Greenland. I was encouraged to see that there still seemed to be plenty of sea ice floating around, but the land was littered with cobalt blue melt ponds and the dramatic retreat of the surrounding glaciers was readily apparent from above. Little did I know at the time that only about a third of that sea ice would still be around by the annual low point in mid-September.

Through most of the melting season this year, the total area covered by sea ice in the Arctic has followed roughly the same pattern as observed in 2005, which was previously the second greatest melt in recorded history. But in early August this year, the melting seemed to accelerate -- maybe from all of those flights to China for the Olympics. "We could very well be in that quick slide downwards in terms of passing a tipping point," said Mark Serreze, a senior scientist at the Colorado-based NSIDC. Within the next few weeks as the melting bottoms out, we'll know whether this year's melt breaks the all time record from last year.

Nothing seems to bring this crisis into clearer focus for most people than the plight of the polar bears. Last week an aerial survey of wildlife off the northwest coast of Alaska reported observing 9 polar bears swimming in open waters between 15 and 65 miles offshore. Ironically, the survey was being conducted to prepare for future offshore oil development. Satellite data from the time of the survey showed that the nearest pack ice was about 100 miles from the coast. Polar bears are good swimmers, but they typically only swim distances of 10 to 15 miles. "We have some observations of bears swimming into shore when the sea ice was not visible on the horizon. In some of these cases, the bears arrive so spent energetically, that they literally don't move for a couple days after hitting shore" said Steven Amstrup, senior polar bear scientist for the U.S. Geological Survey in Anchorage. If the changes in Arctic melting were happening more gradually, the polar bears might have time to adapt over many generations. But the unprecedented pace of Arctic warming makes the outlook fairly grim.

The inconvenient truth of the matter is that these events give us a sneak preview of the types of changes we are likely to see at lower latitudes in the coming decades. Only it won't be the polar bears who are struggling to adapt to the new circumstances -- it will be us.

Secrets of the Aurora

Anyone who has ever witnessed the northern or southern lights would certainly attest to their beauty, but few would imagine that the mysteries of the aurora are still being uncovered. Last week, scientists announced a fundamental discovery about the physical mechanism that converts a stream of charged particles from the Sun into a stunning display of color and light in the Earth's upper atmosphere. The work represents a significant step forward in efforts to predict such events, which can damage satellites in orbit and disrupt power grids on the ground.

The basic mechanism that drives the aurora has been known for a long time. Our Sun is constantly shedding a "wind" of charged particles, primarily electrons and protons, which slams into the Earth's magnetic field. Since charged particles in motion create their own magnetism (the underlying concept of an electromagnet), the solar wind spirals towards the Earth's north and south magnetic poles. There, it slams into the upper atmosphere and interacts with the oxygen and nitrogen atoms to produce shimmering curtains of green and red light in the sky.

The researchers used observations of the aurora from detectors on the ground, as well as measurements from a network of five NASA satellites that orbit at different heights within the Earth's magnetic field. By combining the data from these different sources, the scientists determined the detailed series of electro-magnetic events that precede and follow an auroral display. During times of relatively mild solar wind, the Earth's magnetic field works a bit like a capacitor -- accumulating charge for several hours and then quickly releasing it. The process is accompanied by a sudden disruption of the current in the upper atmosphere (something like blowing an electric fuse), as well as a snapping of the Earth's magnetic field lines far above (a process known as magnetic reconnection). The standard model for aurora suggested that the magnetic reconnection caused a disruption in the current, which then produced the aurora. The new observations show that the aurora are produced directly by the snapping field lines, and the disruption of the current occurs later.

Although the discovery is unlikely to change the way most people view the northern and southern lights, it may improve scientist's ability to predict the potentially harmful consequences of the storms. This could ultimately allow us to avoid regional electric blackouts and service outages from satellites that provide navigation services and communications -- the dark side of the auroral lights.

Communicating with Mars

In a landmark discovery this week, NASA's Phoenix lander dug a trench in the Martian soil and watched as small chunks of water ice slowly evaporated. Scientists have known about the existence of water ice in the polar cap of Mars for a long time, but it was only in 2002 that the Odyssey spacecraft found evidence of much more ice just below the surface. This is what motivated the experiment on Phoenix that directly confirmed this vast reservoir of frozen sub-surface water. It's not easy to control an experiment from a hundred million miles away. Have you ever wondered how NASA actually does it?

Even when the Earth and Mars are as close as they can come to each other in their orbits they are separated by more than 30 million miles, and sometimes they are nearly 250 million miles apart. How do we communicate across such enormous distances? It's similar to how we do it on Earth: we encode our signals into radio waves and send them from one antenna to another. Radio waves travel at the speed of light, more than 186000 miles or eight times around the Earth in a single second. This is very practical for communicating across the planet, but it's fairly slow to communicate between planets. When it is closest, a radio signal to the Mars lander takes several minutes to get there -- and its response takes several minutes to return to Earth. More typically, the trip takes 10 minutes each way. Imagine playing a video game where any move you make with the controller doesn't show up on the screen for 20 minutes!

With such slow communication, scientists do not control the Phoenix lander the same way you might play a video game. Many of the functions of the experiment need to be automated and programmed ahead of time. So they simply tell the lander to "dig a trench" then "take a picture" and wait for the image to be sent back. This led to the discovery earlier this week of some "white stuff" beneath the soil. To test whether it might be water ice they waited for a day, asked the lander to take another picture, and compared the second picture to the first one. This showed that some of the crumbs from the digging actually disappeared in the second picture -- suggesting that they were made from ice and had evaporated.

If you think playing a video game with a 20 minute lag time might be frustrating, imagine what it would be like with people on both ends of the line. This is one of the many challenges of sending astronauts to Mars, but it's something that most people don't think about.

WorldWide Worthless

This morning I received an email from my mother-in-law with the subject line: "Travel to the Stars!' Naturally, I was intrigued. She wanted to know whether I had heard about the new free program from Microsoft called WorldWide Telescope, just released yesterday. I had actually seen some of the headlines and my impression was: oh, Microsoft is trying to compete with Google Sky, which came out last August. But to be fair, I decided to download the new software and try it out.

My first complaint is that WorldWide Telescope only runs under Windows (surprise!). Google Sky also runs on Mac and Linux machines. I decided to install it on a laptop I use for presentations, where I had successfully installed Google Sky several months ago. I went to the website at WorldWideTelescope.org, which looked very nice, and downloaded the 20 MB install application. No problem. When I ran this application, it downloaded an additional 90 MB of Microsoft software that WorldWide Telescope depends on. Everything seemed to install just fine, so I clicked the new shortcut that had been placed on my desktop. The program loaded and a dialog box appeared with instructions about what to do with my mouse to control the interface. Looks a lot like Google Sky. So I started to pan around the sky -- and within a few seconds the program crashed. I rebooted the machine for a fresh start, launched the program and tried to pan again -- another crash. And a third. And a fourth.

At this point, I decided to uninstall the software and everything that came with it. I also decided to check that my copy of Google Sky was still working -- and thankfully, it was. In the process I also discovered that Google now has a version of Sky that runs in the maps interface through a web browser. If you go to the WorldWide Telescope website you can watch flash movies of children's reactions as they explore the sky through the software. Microsoft is great at public relations, but not so great at writing software that actually works. As for me, I'm sticking with Google Sky.

Ozone and Climate

Many people confuse the issue of climate change (or global warming) with the other major influence of humanity on the Earth's atmosphere: the hole in the ozone layer. These phenomena are actually caused by two very different aspects of atmospheric chemistry, and they have distinct consequences for the environment. But this week, climate scientists announced a surprising connection between the ozone layer and climate change: if we adopt one approach to cooling the planet, it could make the ozone problem worse.

Ozone is just another form of oxygen. The oxygen that we breath is really a molecule consisting of two oxygen atoms bound together. Ozone is similar, but it has three oxygen atoms. High in the Earth's atmosphere there is a relatively thin layer of ozone, and because of its specific structure it acts like sunblock -- filtering out much of the harmful ultraviolet radiation coming from the Sun before it ever reaches the ground. Back in the 1980's, scientists discovered that large holes were developing in the ozone layer, particularly over the polar regions of the planet. Further study linked this depletion to widespread use of chlorofluorocarbons (CFCs), particularly as refrigerants and as propellants in spray cans for consumer products. Alternative gases were quickly identified for these uses, and CFCs were phased out in most industrialized countries. The results were dramatic -- further monitoring of the ozone holes showed them begin to recover as the widespread use of CFCs was discontinued. It was a landmark achievement for climate science and its influence on public policy.

More recently it has become clear that the global climate is getting warmer and that, like ozone depletion, a substantial fraction of the effect can be attributed to human activities. In this case the underlying cause is the release of so-called greenhouse gases -- molecules like carbon dioxide (CO2) and methane (CH4) that trap the Sun's infrared radiation or heat from being released back into outer space. Large volumes of these gases are produced from burning fossil fuels like coal in power plants or gasoline in automobiles -- and as humanity releases more greenhouse gases than the environment can naturally absorb, it builds up in the atmosphere, traps more heat, and gradually warms the planet. This has been happening dramatically since the industrial revolution in the late 1800's, and unless we find suitable alternatives for our energy needs we risk a major disruption of our climate system later in this century.

Since this is a much bigger problem than finding substitute refrigerants and propellants -- and because the consequences are potentially much more severe -- some climate scientists are considering how to cool the planet without reducing greenhouse gas emissions. In the long term climate record that has been reconstructed from ice cores drilled in Antarctica, scientists noticed that historical periods of global cooling often follow major volcanic eruptions. The cooling was traced to large quantities of sulfates that were hurled into the Earth's upper atmosphere during the eruptions. The sulfates block some of the incoming sunlight from ever reaching the surface, which can counteract the warming effects of increased greenhouse gas concentrations (though other problems like ocean acidification still remain). So if global warming gets really out of control, the scientists reasoned that we could manually inject sulfates into the upper atmosphere like an artificial volcanic eruption to cool the planet.

Working from this premise, the study released this week examined the other possible consequences of engineering the climate with massive injections of sulfates -- and this is where ozone returns to the picture. It turns out that sulfates also deplete the ozone layer, and if enough of them were injected into the upper atmosphere to offset the expected warming in the coming decades it would destroy between 1/4 and 3/4 of the Arctic ozone layer and delay the expected recovery of the ozone hole over the Antarctic by between 30 and 70 years. So if we want a reasonable climate and an ozone layer, sulfates might not be the best answer.