Remember when 2016 was the new Year of the Homer, featuring the second-highest home run rate in MLB history and supplanting 1987 in terms of unexpectedness? Well, 2017 laughed at that notion and bumped the homer rate by another 10 percent, setting a new record of 1.26 home runs per team per game. So now the question is whether 2018 will surpass even last year's "Year of the Homer 2: Electric Boogaloo."
The most maddening aspect of guessing where offense is going in baseball is the why. A 25 percent increase in homers over a two-year period is stunning. A similar change occurred from 1992 to 1994, and even a quarter of a century later, that shift is largely unexplained. League expansion isn't enough to account for that change, and one of the pop-science explanations -- performance-enhancing drugs -- would necessitate everybody discovering the benefits of PEDs in an 18-month period, because the home run rate stayed flat for most of the next decade. With no expansion teams, as well as drug testing since 2004, even those hole-filled theories aren't available to explain the latest home run boost.
One possible theory is that the baseballs are constructed differently, something commissioner Rob Manfred and Major League Baseball have denied, though without actually providing any rebuttal to what researchers have found. One thing will be different this year: MLB has announced that all baseballs will be stored in air-conditioned rooms in 2018, to help determine if they should subsequently be stored in humidors in 2019 to standardize the temperature and humidity they're kept in across the game. In theory, this change could ultimately result in lower exit velocities for a hit baseball; harder-hit baseballs are more likely to be home runs.
So one question that brings up is what effect this would have on the results, for both players and teams. Projections are made with certain assumptions for levels of offense around the league, and organizations are aware of those assumptions as they construct their teams. But what happens if we turn back the clock and the level of offense is more like 2015 than 2016-2017? To answer this question, I went back and ran my 2018 projections at 2015's level of offense and looked for the largest differences. I also used playing time generated from estimated playing time based on current rosters, rather than the straight-up ZiPS projections (ZiPS is agnostic on which minor leaguers will play).