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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.

The current strong El Niño brewing in the Pacific Ocean shows no signs of waning, as seen in the latest satellite image from the U.S./European Ocean Surface Topography Mission (OSTM)/Jason-2 mission. 

El Niño 2015 has already created weather chaos around the world. Over the next few months, forecasters expect the United States to feel its impacts as well. 

The latest Jason-2 image bears a striking resemblance to one from December 1997, by Jason-2's predecessor, the NASA/Centre National d'Etudes Spatiales (CNES) Topex/Poseidon mission, during the last large El Niño event. Both reflect the classic pattern of a fully developed El Niño.

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.

Mercury pollution is a global problem with local consequences: Emissions from coal-fired power plants and other sources travel around the world through the atmosphere, eventually settling in oceans and waterways, where the pollutant gradually accumulates in fish. Consumption of mercury-contaminated seafood leads to increased risk for cardiovascular disease and cognitive impairments.

In the past several years, a global treaty and a domestic policy have been put in place to curb mercury emissions. But how will such policies directly benefit the U.S.? 

The human gut harbors a teeming menagerie of over 100 trillion microorganisms, and researchers at the University of Colorado Boulder have discovered that exercising early in life can alter that microbial community for the better, promoting healthier brain and metabolic activity over the course of a lifetime.