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 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.

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.

A new study, published today in the International Journal of Epidemiology [1], has dismissed the concept of 'fat but fit'. In contrast, the results from the new study suggest that the protective effects of high fitness against early death are reduced in obese people. 

Although the detrimental effects of low aerobic fitness have been well documented, this research has largely been performed in older populations. Few studies have investigated the direct link between aerobic fitness and health in younger populations. This study by academics in Sweden followed 1,317,713 men for a median average of 29 years to examine the association between aerobic fitness and death later in life, as well as how obesity affected these results. The subjects' aerobic fitness was tested by asking them to cycle until they had to stop due to fatigue.

African countries have agreed to cooperate in setting limits for use of lead in paints with a view to phasing it out by 2020. This is because of its dangers to human beings, especially to children, and the environment.

Contrary to recent headlines — and a talk by actor Arnold Schwarzenegger at the United Nations Paris Climate Change Conference — eating a vegetarian diet could contribute to climate change. In fact, according to new research from Carnegie Mellon University, following the USDA recommendations to consume more fruits, vegetables, dairy and seafood is more harmful to the environment because those foods have relatively high resource uses and greenhouse gas (GHG) emissions per calorie. 

It's National Pear Month and the perfect time to enjoy juicy, sweet pears. If that isn't reason enough to fill your shopping basket, there's another reason to add this fruit to your grocery list. A new study, 'Fresh Pear Consumption is Associated with Better Nutrient Intake, Diet Quality, and Weight Parameters in Adults: National Health and Nutrition Examination Survey 2001-2010,' published in Nutrition and Food Science, revealed new information regarding the health benefits of pear consumption.1 Of particular interest given the high rates of obesity in the United States, the study found that adult pear consumers had a lower body weight than non-pear consumers and they were 35 percent less likely to be obese.

The epidemiologic study, led by Carol O'Neil of the Louisiana State University Agricultural Center, used a nationally representative analytic sample to examine the association of fresh pear consumption with nutrient intake, nutrient adequacy, diet quality, and cardiovascular risk factors in adults. 

Taking a deep breath might be a bit harder for children exposed early in life to a widely used class of pesticides in agriculture, according to a new paper by UC Berkeley researchers.

The greater the pesticide exposure, the smaller the lungs, a new study finds.

As representatives from more than 190 countries convene in France for the second week to address ways to slow global warming, an MIT-led team has published a paper outlining a set of options for incorporating equity considerations in a national Emissions Trading System (ETS) for China that could reduce carbon emissions while minimizing economic impact on poorer or less-developed regions.

The paper, “Equity and Emission Trading in China,” published in the journal Climatic Change, outlines a sophisticated menu aimed at Chinese policymakers showing how the burden of reducing carbon emissions could be shared or divided across the country’s provinces under a market-based carbon pricing system.

“Emission trading systems have been shown to be highly effective when they are allowed to work, but one of the toughest challenges involves how to distribute the cost,” said lead author Valerie Karplus, Assistant Professor of Global Economics and Management at the MIT Sloan School of Management. “We compare alternative schemes for allocating emissions rights that can effectively de-couple who pays for reductions from where the reductions actually occur.”