Roy Halladay and the rest of the starting rotation are a big reason why the Phillies have the best record in baseball. (David Banks/GETTY IMAGES)

One trend in baseball now dominates all others: Better pitching, plus swifter defense, equals lower scoring. Not just this year or last year. We’re watching a sea change, not a brief cycle. The sport is now in its sixth straight season with fewer runs scored. And the lines on this graph are still dropping — fast.

Soon, baseball may have a problem if offensive shrinkage continues at its recent rate: 9.72 total runs per game, 9.60, 9.30, 9.22, 8.76 and so far in 2011 just 8.38. That’s a plummet from the very top of the range of aesthetically acceptable offense to somewhere within sight of “arrrgghhh, too low!” And the big drops the past two seasons have been dramatic to traumatic.

For now, no migraines yet. This year’s scoring is about like 1952, ’73, ’83 or — in the last season when runs were near this level — ’92. But get used to it. Through history, when scoring drops to this level, it stays there for years — unless it goes even lower.

Washington Nationals fans certainly know what’s up. The Nats have had 21 games of 1-0, 2-0 or 2-1! Their games average just 7.85 runs. But they aren’t even among the six teams with the lowest-scoring games. Pity fans in San Diego or Seattle with fewer than seven runs per game.

Who defines how many runs are enough? We do. And history does. For 136 years, the average is 9.07 per game. When scoring gets above 10 runs (’96, ’99, ’00) or below eight (7.54 in ’67 and 6.84 in ’68), everybody goes nuts. Rules change. The ball is (secretly) tweaked.

For the moment, we’ve returned to a palatable balance familiar to fans from ’73 through ’92 (8.54 runs). But different eras favor different styles of play. The current trend is so pervasive that the teams that don’t heed it will probably be on the wrong side of history.

“Just look at the players,” Nats General Manager Mike Rizzo said when he took charge in ’09. “They are built more like they used to be. I’m not saying why. But we have to adapt to it. The game will be played more like it was in the past. Not home run derby. So, adjust.”

Several teams already have. Last year’s world champion San Francisco Giants exemplify it with their four-deep postseason rotation, super closer Brian Wilson and just enough offense to win tense games. The Phillies, this season’s best team, are built on pitching with Roy Halladay, Cole Hamels and Cliff Lee curing an offense that ranks 14th in runs.

If you want archetypes for teams that dominated in periods of comparable offense, look back to the 1972-73-74 champion Athletics, the ’86-88 Mets powerhouses of Davey Johnson or the ’91-93 Braves. All were defined by rotations with famous aces: Catfish Hunter, Dwight Gooden or Tom Glavine. But those behind them, always to the third and sometimes to the fourth starter, were almost as dominant. Pitching was the core.

Just as pertinent, all three teams blended power and speed in their three peak years: A’s (average of 138 homers, 126 steals), Mets (164, 139) and Braves (149, 139). That athleticism meant “plus” range at most defensive spots.

The reasons for the current drop in offense make a nice but inconclusive debate. However, one practical implication is clear: In hindsight, steroids and human growth hormone helped hitters more than pitchers. Why? More strength instantly results in more bat speed. (The bat literally feels lighter in your hands.) And that quickness, like swinging a broomstick instead of a club, means that it’s easier to hit fastballs. It just is.

So, as overall bat speeds decline, what pitch immediately becomes more valuable? The fastball. In recent years, every power arm in the minors who lacked the repertoire, command or temperament to be in a big league rotation got sent to the bullpen. Last season, the 90-win Padres had five relievers who averaged almost 95 mph on fastballs.

Of this season’s young promising teams — such as the Indians, Pirates and Nationals — perhaps none is as consciously modeled on the “traditional” teams of the pre-steroid era as Rizzo’s Nats. He’s looking for the same kind of players whom his dad, the lifelong scout, searched for a generation ago and whom he himself sought until muscle mass ruled everything.

The GM practically has his tenets nailed to his office door. Get me more power arms. Find more athletic, less bulky players with some power and enough speed. That syncs with better defense up the middle. Get catchers to stop the running game, because it will soon return to fashion. Goodbye, Adam Dunn. Hello, Wilson Ramos and Ivan Rodriguez.

Rizzo wanted pitchers who “miss the bat.” So far he’s only found Stephen Strasburg and Jordan Zimmermann as starters. But he’s packed the bullpen with four high-K men.

Players with several of the five classic tools — not just “hit for power” — are central to this slightly sub-normal offensive era. So, Danny Espinosa, Ian Desmond and Roger Bernadina are gold, if they pan out. Maybe 20-20 is the new (honest) 30-30. As for balance, the Nats are on pace for 147 homers and 135 steals.

In similar fashion, big bucks go to Carl Crawford, Jayson Werth or Adrian Beltre, who seem multitalented. But pick the wrong $100-million free agent, and you’ve bought only an anchor.

The early results: the Nats are on pace to add 22 wins in two years, before they ever get Strasburg back from surgery or glimpse the power-speed of Bryce Harper.

The Nats probably have the right blueprint. But so do others. It’s not just the Phils with their four aces and the second-fewest errors in baseball. Right inside the National League East, the Braves have been rebuilt along the current proper lines. If you want to score, avoid Atlanta.

Welcome to baseball as it is today. The game stopped being home run derby several years ago. But, now, as scoring has dropped to levels not seen in 20 years, we must adapt again. Last October, San Francisco introduced to its new motto. Giants baseball: Torture.

Embrace the pain. The low-scoring nail-biter is the wave of the present.