Well, it appears that this is a statistic that is being cited. Thus, the 2013 has nothing to do with the sentence. A decade is 10 years, so that would mean that either the decline was from 2002-2012 with 2012 being the end of the decade, or since a decade is usually used like 2000-2010, then the decline was from 2010-2012 and the coal reserves have began increasing once more in 2012. I hope I helped ^^

No, "decade" here surely means a period of exactly ten years. I am ALMOST sure it would mean the ten years 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, and 2012, which I would call "2003-2012 inclusive." It could just possibly mean the decade ending before 2012: 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, and 2011, which I would call "2002-2011, inclusive." "Inclusive" means that the period of time includes both of the years mentioned. I don't think this was very clear writing, because it is so common for a "decade" to mean a group of ten years starting with a year that is a round multiple of 10. I would personally have written "the ten years 2003-2012 inclusive" or perhaps "the decade 2003-2012 inclusive."

Do you have a site I can look at? The sentence itself is poorly wording and the surrounding sentences may help me answer you. ~Bhakti

Well, a decade is 10 years. So if its the decade ending in 2012, then it would be from 2002 to 2012.