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A mixture of plant oils, bacterial spores and ozone is responsible for the powerful scent of fresh rain. Image via Wikimedia Commons/Juni

Step outside after the first storm after a dry spell and it invariably hits you: the sweet, fresh, powerfully evocative smell of fresh rain.

If you’ve ever noticed this mysterious scent and wondered what’s responsible for it, you’re not alone.

Back in 1964, a pair of Australian scientists (Isabel Joy Bear and R. G. Thomas) began the scientific study of rain’s aroma in earnest with an article in Nature titled “Nature of Agrillaceous Odor.” In it, they coined the term petrichor to help explain the phenomenon, combining a pair of Greek roots: petra (stone) and ichor (the blood of gods in ancient myth).

In that study and subsequent research, they determined that one of the main causes of this distinctive smell is a blend of oils secreted by some plants during arid periods. When a rainstorm comes after a drought, compounds from the oils—which accumulate over time in dry rocks and soil—are mixed and released into the air. The duo also observed that the oils inhibit seed germination, and speculated that plants produce them to limit competition for scarce water supplies during dry times.

These airborne oils combine with other compounds to produce the smell. In moist, forested areas in particular, a common substance is geosmin, a chemical produced by a soil-dwelling bacteria known as actinomycetes. The bacteria secrete the compound when they produce spores, then the force of rain landing on the ground sends these spores up into the air, and the moist air conveys the chemical into our noses.

“It’s a very pleasant aroma, sort of a musky smell,” soil specialist Bill Ypsilantis told NPR during an interview on the topic. “You’ll also smell that when you are in your garden and you’re turning over your soil.”

Because these bacteria thrive in wet conditions and produce spores during dry spells, the smell of geosmin is often most pronounced when it rains for the first time in a while, because the largest supply of spores has collected in the soil. Studies have revealed that the human nose is extremely sensitive to geosmin in particular—some people can detect it at concentrations as low as 5 parts per trillion. (Coincidentally, it’s also responsible for the distinctively earthy taste in beets.)

Ozone—O_3, the molecule made up of three oxygen atoms bonded together—also plays a role in the smell, especially after thunderstorms. A lightning bolt’s electrical charge can split oxygen and nitrogen molecules in the atmosphere, and they often recombine into nitric oxide (NO), which then interacts with other chemicals in the atmosphere to produce ozone. Sometimes, you can even smell ozone in the air (it has a sharp scent reminiscent of chlorine) before a storm arrives because it can be carried over long distances from high altitudes.

But apart from the specific chemicals responsible, there’s also the deeper question of why we find the smell of rain pleasant in the first place. Some scientists have speculated that it’s a product of evolution.

Anthropologist Diana Young of the University of Queensland in Australia, for example, who studied the culture of Western Australia’s Pitjantjatjara people, has observed that they associate the smell of rain with the color green, hinting at the deep-seated link between a season’s first rain and the expectation of growth and associated game animals, both crucial for their diet. She calls this “cultural synesthesia”—the blending of different sensory experiences on a society-wide scale due to evolutionary history.

It’s not a major leap to imagine how other cultures might similarly have positive associations of rain embedded in their collective consciousness—humans around the world, after all, require either plants or animals to eat, and both are more plentiful in rainy times than during drought. If this hypothesis is correct, then the next time you relish the scent of fresh rain, think of it as a cultural imprint, derived from your ancestors.

As the Arctic warms, more of it will be covered by shrubs (like the Arctic National Wildlife Refuge, above) and even by forest. Image via ANWR

You probably think of the Arctic as a cold, frozen tundra—home to lichen, polar bears and scattered herds of reindeer. In many places, this view would be accurate, but in a few relatively southern areas in Canada, Alaska and Russia, warming temperatures over the past few decades have allowed new types of plants, such as shrubs, to take root.

And by 2050—if current warming trends continue—we’ll see a dramatically different ecosystem across the Arctic, starting with something that’s largely unknown in the area currently: trees. According to research published today in Nature Climate Change, tree cover in the Arctic could increase by more than 50 percent over the next few decades.

The research team, which included scientists from a number of universities and was led by Richard Pearson of the American Museum of Natural History, made the calculation based off of current projections of how the Arctic’s climate will change by 2050. So far, temperatures in the region have risen about twice as fast as those for the planet as a whole.

They created a model that predicts which class of plants (various grasses, mosses, shrubs or trees) will grow given a particular temperature and precipitation range expected for the future; for each spot on a map of the Arctic, they fed in the 2050 projections. Doing this kind of vegetative modeling for the Arctic, they say, is relatively straightforward compared to doing it for somewhere like the tropics, because there are hard limits on the temperature and growing season length that given plant types can tolerate.

They found that tree cover will expand drastically, covering up to 52 percent more land area than currently, rising far north of the current tree line in Alaska and Canada. This new tree cover will mostly come at the expense of areas currently covered by shrubs, but shrubs will take over places now dominated by tundra plants (lichens and mosses), and some areas presently under ice will convert into tundra.

In effect, the area’s warming climate and lengthening growing season will shift all current vegetation zones to more northerly and colder regions. Already, these vegetation zones have shifted an average of five degrees of latitude over the past 30 years–in other words, the vegetation in one spot resembles how a location five degrees south looked 30 years ago.

But by 2050, this shift will be even more dramatic—perhaps equaling 20 degrees of latitude—and a projected 48 to 69 percent of the Arctic’s vegetated areas will switch to a different class of plants. Some rare plant species could be at risk of extinction if they’re not able to migrate as quickly as the vegetation zones move.

Presently (left), vegetated areas of Alaska are mostly covered by small shrubs and tundra mosses (represented by the pea green color). By 2050 (right), much of this area will be dominated forests (bright green). Image via Nature Climate Change/Pearson et. al.

In Canada, areas currently covered by tundra shrubs (purple at left) will be taken over by forest (bright green at right). Image via Nature Climate Change/Pearson et. al.

Because plants are the base of any food chain, this conversion will have wide-ranging effects, both locally and elsewhere. “These impacts would extend far beyond the Arctic region,” Pearson said in a press statement. “For example, some species of birds seasonally migrate from lower latitudes and rely on finding particular polar habitats, such as open space for ground-nesting.” Their migrations patterns would presumably be altered by the growth of forests on what had been open tundra.

Most troubling, the conversion of white, snow-covered land to dark vegetation will further affect the warming of the planet. Because darker colors absorb more radiation than the white of ice and snow, shifting large masses of land to a darker color is projected to further accelerate warming, creating a positive feedback loop: more warming leads to a greener Arctic, which leads to more warming.

Given all the other problems that the area is rapidly encountering as the climate changes—melting glaciers, increasing oil exploration and hybridizing bear species—it’s clear that the Arctic will be one of the most environmentally fragile regions of the planet over the coming century.

Rapidly melting sea ice will open up shipping lanes across the Arctic, potentially making the Northwest Passage (left) and North Pole (center) navigable during the summer. Image via PNAS/Smith and Stephenson

Rapidly melting ice has already remade shipping possibilities in the Arctic. Over the past decade, commercial use of the Northern Sea Route (the blue shipping lane along the northern coast of Russia in the map above) during late summer has become commonplace, dramatically shortening the journey from Europe to the Far East.

If present trends continue, though, the options for shipping goods across the Arctic will expand even more. According to a paper published today in the Proceedings of the National Academy of Sciences, by 2040, the legendary Northwest Passage (the shipping lane on the left side of the map, along the cost of Canada and Alaska) could be accessible during some summers to normal oceangoing ships without specially reinforced ice-breaking hulls. Most surprisingly, at times, reinforced polar icebreakers might even be able to plow straight across the North Pole, making the shortest possible journey across the Arctic.

All this is due to the fact that, over the past two decades, temperatures have risen even faster in the Arctic than the planet as a whole. Although the polar ice pack grows each winter and shrinks each summer, the overall trend has been a decrease in total ice cover, as seen in the video below. In the future, this will open up a window for reinforced ships to break through weaker ice, and for normal ships to cruise through ice-free corridors.

The new study, by Laurence Smith and Scott Stephenson of UCLA, uses existing climate models to examine how this trend will change Arctic shipping for the years 2040 to 2059. They looked at theoretical ice conditions under a pair of climate scenarios from the UN’s Intergovernmental Panel on Climate Change’s most recent report, one that assumed a medium-low level of greenhouse gas emissions going forward, and another that assumed a high level. They also explored the navigational possibilities for two different types of ships: Polar Class 6 ice-breaking ships and normal oceangoing vessels.

Their analysis found that in both scenarios, the Northern Sea Route—already navigable for reinforced vessels in late summer most years—will become wider, opening up for a greater number of months each summer and allowing for a greater geographical diversity in routes. The wider lane will enable ships to take routes further away from the Russian coast and closer to the North Pole, shortening the journey over the top of our planet, and will allow unreinforced ships to travel through without an ice-breaking escort.

Currently, the Northwest Passage is inaccessible for normal vessels, and has only been transited a handful of times by reinforced ice-breaking ships. Under both of the scenarios in the model, though, it becomes navigable to Polar Class 6 ships every summer. At times, it could even be open to unreinforced vessels as well—the study shows that, when multiple simulations were run in both medium-low and high levels of greenhouse gas emissions, open sailing was possible for 50 to 60 percent of the years studied.

Finally, the straight shot over the North Pole—a route that would currently take would-be captains through a sheet of ice as much as 65 feet thick in areas—could also become possible for Polar Class 6 ships in both scenarios, at least in warmer years. “Nobody’s ever talked about shipping over the top of the North Pole,” Smith said in a press statement. “This is an entirely unexpected possibility.”

The most striking part of the study might be that these dramatic changes occurred in simulations assuming both medium-low and high levels of emissions, and that the time period studied isn’t all that far away, beginning just 27 years from the present. “No matter which carbon emission scenario is considered, by mid-century we will have passed a crucial tipping point—sufficiently thin sea ice—enabling moderately capable icebreakers to go where they please,” Smith said.

The unprecedented levels of fine particulates that pollute Beijing’s air can cause lung cancer, heart attacks and other health problems. Image via Flickr user jaaron

Beijing’s terrible air quality is currently in the news, and for good reason: The level of pollution present in the air there is unprecedented for a heavily populated area, and several times worse than what any U.S. resident has likely ever experienced.

The New York Times recently reported on the air quality problems of Salt Lake City, Utah, and how the area’s geographical features and weather systems occasionally trap pollution in the city’s bowl-shaped basin. But the highest reading on the EPA’s Air Quality Index (AQI) scale ever recorded in Salt Lake City was 69 micrograms of soot and other particles per cubic meter.

In Beijing, that number frequently rises above 300—sometimes going much higher. Yesterday, a sandstorm blew into the city, mixing sand and dust with smog and pushing the AQI to 516. The scale was only designed to go up to 500, but on January 12, a measurement from the U.S. Embassy in Beijing read 755. For reference, the EPA recommends that for any number above 200, ”People with heart or lung disease, older adults, and children should avoid all physical activity outdoors. Everyone else should avoid prolonged or heavy exertion.”

Beijing’s air pollution is literally off the charts, at least according to the EPA’s Air Quality Index. Image via EPA

What exactly makes physical activity in this sort of environment so dangerous? First, it’s important to understand exactly what AQI measures in the chart above: the weight of solid particles smaller than 2.5 micrometers wide (commonly known as fine particulates) that are suspended in an average cubic meter of air. In a heavily populated place like Beijing, most of the fine particulates are a result of industrial activity, the burning of diesel and gasoline for transport, or the burning of coal for energy or heat.

When we breathe in larger particles than those measured by the AQI (those typically bigger than 10 micrometers in size), they’re typically filtered out by cilia or mucus in our nose and throat. But those smaller than 10 micrometers can slide past these protections and settle in our bronchi and lungs. And the fine particulates commonly measured by the AQI can penetrate even further—entering the tiny air sacs known as alveoli where our bodies exchange carbon dioxide for oxygen—where they can cause some serious damage over time.

Researchers have linked many health problems to high levels of these tiny particulates in the air, but the most obvious effect has been lung cancer. One study spanning 16 years found that, over the course of an individual’s lifetime, an average increase of 10 on the AQI was associated with a 8 percent higher chance of developing the disease. When multiplied out over a broad area with a large population, the effect can be massive. A World Health Organization report estimated that fine particulates are responsible for 5% of the deaths resulting from lung cancer worldwide—800,000 deaths annually.

Fine particulates have also been linked with many other sorts of health issues, both long- and short-term. There’s evidence that, in individuals already predisposed to heart problems, they can trigger heart attacks. They can also exacerbate asthma, cause coughing or difficulty breathing in healthy people, and reduce the lungs’ ability to take in oxygen for people with COPD (chronic obstructive pulmonary disease).

Additionally, there are risks associated with even smaller particulates, known as nanoparticles, that are smaller than 100 nanometers in size. Only preliminary research on nanoparticles’ effect on the human body has been completed, but scientists believe that nanoparticles may be capable of penetrating even further into an organism, burrowing through cell membranes and potentially causing a range of problems, including damage to the lungs and circulatory system.

There has been limited research so far on the direct health impacts of air pollution in China, but one study found that, when air pollution was curtailed due to restrictions during the 2008 Olympics, several chemical biomarkers associated with cardiovascular disease in the blood of Beijing residents dropped off dramatically. Another study estimated that, if these same restrictions were extended permanently, lifetime risk of lung cancer for the city’s residents would be cut in half (a risk that has increased by 56 percent in the last 10 years, even as smoking has declined).

All told, there are very good reasons why many Beijing residents don’t venture out without a breathing mask—and why many Chinese are calling upon leaders to finally address the country’s air pollution problems in the coming political year, potentially by introducing rules that restrict industry and coal burning when air quality dips below acceptable levels.

Dust lofted up from the Sahara can be blown across the Pacific and seed clouds over California. NASA image courtesy MODIS Rapid Response Team, Goddard Space Flight Center

The fascinating idea that a butterfly flapping its wings in Asia can change the path of a hurricane over the Pacific is, alas, probably not accurate. But slight changes in one part of the atmosphere can indeed have disproportionate effects elsewhere, a concept known as the butterfly effect.

Just how slight one of these factors can be—and how incredibly far away their effects can reach—is vividly illustrated by a new finding by an international team of atmospheric scientists and chemists from the U.S. and Israel. As they document in a study published today in Science, dust blown from as far away as the Sahara desert of Africa can seed rain and snow clouds in the Sierra Nevada mountains of California.

The research team, led by Kimberly Prather of the University of California, San Diego, came to the finding after using aircraft to collect atmospheric data over the Sierra Nevada mountains, as well as analyzing precipitation that fell at the Sugar Pine Dam in Northern California. They also retroactively tracked storm masses backward across the Pacific and Asia to pinpoint the origin of the dust they found in the clouds.

Cloud formation depends upon tiny particles such as dust that serve as cloud condensation nuclei or ice nuclei—flecks that act as a surface on which water can condense. Previous studies have found that dust from as far away as the Taklimakan desert in China can be blown around the globe. But temperate deserts such as the Taklimakan and the Gobi are frozen much of the year, while the Sahara never freezes, the researchers noted. Could the Sahara and deserts in the Middle East serve as a significant source of year-round dust which, when lofted high into the atmosphere, seeded storms across the planet?

The answer is yes. Of the six storms the researchers sampled, all showed at least some trace of dust. Then, working backward to determine the origin of each of these air masses and using existing data from previous studies on wind currents across the Pacific, they found strong evidence that the majority of the dust had originated in Africa, the Middle East or Asia and traveled around the globe. Additionally, the observed height of various drafts of dust (as collected by a U.S. Navy program) on the days when the air masses would have moved past the African and Asian regions matched the altitude necessary for the particles to get lifted up into the air currents.

The dust particles carried across the Pacific from Africa, the Middle East and China are laregly responsible for cloud formation in the Western U.S. Image via Science/Creamean et. al.

Satellite analysis of the storm masses as they moved across the Pacific also confirmed that they carried dust all the way. As shown in the map above, most came from Northeast China or the Taklimakan, but a sizable amount came from as far as the Middle East or even the Sahara.

Although the butterfly’s role in all this seems to be nonexistent, the study did find that one type of living creature does play a part in cloud formation: bacteria. In recent years, scientists have discovered that bacteria, along with dust, can be lofted up high in the atmosphere and serve as nuclei for cloud formation.  In this study, the researchers found that small amounts of bacteria were mixed in with the dust, and likely originated in Asia and Africa as well.

So if you live on the West Coast, the next time you get caught in a rainstorm think of this: Each drop that hits you might contain dust and bacteria that’s traveled halfway around the planet. A close look at something as mundane as our daily weather, it turns out, can open a new window to the complex interconnectedness of our world.