As 2015 comes to a close, Mongabay is looking back at the year that was. This year saw President Obama reject the Keystone pipeline as historic droughts and a vicious wildfire season wracked the western US and Canada. The world committed to climate action in Paris as Southeast Asia was choking on the worst Indonesian haze in years, Shell aborted its plans to drill in the Arctic for the “foreseeable” future, and ExxonMobil is being investigated for lying to the public about climate risks. Here, in no certain order, are the top 15 environmental stories of 2015.

Though it seems to be oddly absent from the mainstream media headlines, a massive methane gas leak in Southern California’s Aliso Canyon has prompted thousands of evacuations and, since Oct. 23, has pumped over 150 million pounds of the greenhouse gas into the atmosphere. Even though the owners of the methane well, Southern California Gas Co., say this week that they have finally discovered the pipe associated with the leak, it will likely be months before a seal is entirely in place.

To truly underscore the seriousness of the Porter Ranch gas leak, the Environmental Defense Fund released an infrared aerial video this week showing the tremendous, continuous release of methane:

Methane gas, in its natural state, is also odorless. However, for safety reasons, foul-smelling additives are artificially included with the gas. In Porter Ranch, a Los Angeles neighborhood and the site of the leak, the additive smell’s intensity has affected the heath of many of residents — nose bleeds, nausea, headaches and dizzy spells have been reported.

The U.S. wind power industry is celebrating after reaching a new milestone in November: 70 gigawatts (GW) of generating capacity.

"That's enough to power about 19 million homes," says Michael Goggin, senior director of research at the American Wind Energy Association (AWEA).

Fuel economy is at record highs and carmakers have surpassed strict greenhouse gas emissions standards for the third straight year, according to the Environmental Protection Agency, which released a pair of annual reports about the U.S. fleet of cars and trucks Wednesday.

Overall, fuel economy for vehicles in the U.S. did not budge from last year's record high of 24.3 miles per gallon, the EPA says. The figure includes a new high of 20.4 mpg for trucks, vans and SUVs from model year 2014.

The U.S. has a chance to be a leader in renewable energy deployment given its sheer size and resources. And some states are leading the way. Olivet Nazarene University’s engineering department ranked the top 10 green states in terms of renewable energy. How does your state stack up? Did it make the list? Read on to find out.

Lawrence Livermore researchers and collaborators have found that most climate models overestimate the increase in global precipitation due to climate change.

Specifically, the team looked at 25 models and found they underestimate the increase in absorption of sunlight by water vapor as the atmosphere becomes moister, and therefore overestimate increases in global precipitation.

The team found global precipitation increase per degree of global warming at the end of the 21st century may be about 40 percent smaller than what the models, on average, currently predict.

Is Paris the beginning of the end for coal? Coal burning is declining fast in both of the world's two largest carbon dioxide emitters, China and the the United States, with resulting declines in the emissions of both countries. The fuel looks incompatible with a world that warms by no more than two degrees Celsius, bringing calls for its rapid phaseout as the world is "decarbonized." 

But, with or without a deal here in Paris later this week, will the calls be heeded? Has the demise of King Coal been greatly exaggerated?  

In the U.S., "coal has gone from boom to bust," says Michael Brune, executive director of the Sierra Club. The black stuff's share of electricity generation has sunk from 53 percent to 35 percent in just five years.
 

Industrial-scale batteries, known as flow batteries, could one day usher in widespread use of renewable energy—but only if the devices can store large amounts of energy cheaply and feed it to the grid when the sun isn’t shining and the winds are calm. That’s something conventional flow batteries can’t do. Now, researchers report that they’ve created a novel type of flow battery that uses lithium ion technology—the sort used to power laptops—to store about 10 times as much energy as the most common flow batteries on the market. With a few improvements, the new batteries could make a major impact on the way we store and deliver energy.

Joe Davis is an artist who works not only with paints or pastels, but also with genes and bacteria. In 1986, he collaborated with geneticist Dan Boyd to encode a symbol for life and femininity into an E. coli bacterium. The piece, called Microvenus, was the first artwork to use the tools and techniques of molecular biology. Since then, bioart has become one of several contemporary art forms (including reclamation art and nanoart) that apply scientific methods and technology to explore living systems as artistic subjects. A review of the field, published November 23, can be found in Trends in Biotechnology.

Bioart ranges from bacterial manipulation to glowing rabbits, cellular sculptures, and--in the case of Australian-British artist Nina Sellars--documentation of an ear prosthetic that was implanted onto fellow artist Stelarc's arm. In the pursuit of creating art, practitioners have generated tools and techniques that have aided researchers, while sometimes crossing into controversy, such as by releasing invasive species into the environment, blurring the lines between art and modern biology, raising philosophical, societal, and environmental issues that challenge scientific thinking.

Sooner than it takes to build a nuclear power station, lithium-air batteries could be helping wind and solar to make coal, oil and nuclear obsolete, say Cambridge scientists. Five times lighter and five times cheaper than current lithium batteries, Li-air would open the way to our 100% renewable future.