Recent reports that the first month of the Occupy Wall Street protests cost the city $3.4 million in police overtime no doubt led to some raised eyebrows. While a substantial sum, it equals just a fraction of police overtime spending in recent years.

In fiscal year 2011, which ended on June 30, police overtime totaled $549.5 million. And it has been climbing steadily. Just looking at the prior five years, spending on police overtime grew from $412.0 million in 2006 to $538.4 million in 2010, according to numbers assembled by IBO’s Bernard O’Brien. Some portion of the increase is probably a reflection of wage growth during the five years, not just more overtime hours.

The New York Police Department categorizes part of its overtime spending in terms of planned and unplanned events. Planned events, meaning the event has occurred annually for at least three consecutive years, include goings-on such as the New York City Marathon ($2.3 million in police overtime last year), the Thanksgiving Day Parade ($192,763), and the Steinway Street Festival ($3,474).

Not surprisingly, Occupy Wall Street falls under the category of unplanned events. And there are a lot of them each year, some stemming from acts of nature, others very much manmade. The tornados that swept Brooklyn, Queens, and Staten Island in September 2010 caused $318,407 in police overtime. Last December’s blizzard dumped $1.6 million in overtime costs on the city. The snowstorms that followed in January piled on another $883,721.

Baseball games are events of our own making that can also mean police overtime, especially when the Red Sox come here to play the Yankees. The two teams squared off in the Bronx in August and September last year, generating headlines and $410,948 in police overtime spending. And that doesn’t include the playoffs against the Twins last October, which knocked in $305,045 more in police overtime. (The Mets, it seems, simply don’t ignite the same passion—or extend their season long enough—to warrant added overtime.)

Presidential visits, international events, and Mayoral initiatives also can boost the city’s police overtime bill. Last fiscal year, President Obama visited the city seven times, resulting in $2.4 million in overtime for the local police force. When Osama bin Laden was killed by Navy SEALS in Pakistan in May, it triggered $773,981 overtime for extra security in the city. Mayor Bloomberg’s Summer Streets initiative, which opens seven miles of streets for walking, biking, and playing on three consecutive Saturdays in August, cost $709,358 in 2010.

With all the events, planned and unplanned, that occur in the city, the need for police overtime might seem like a given—especially as the number of officers declines. Since 2006 the number of police officers has dropped by nearly 2,000 to 33,777 at the end of last fiscal year.

If recent history is any guide, size of the force isn’t all that matters when it comes to police overtime. For example, in the two fiscal years prior to 9/11, police staffing hit all time highs, yet police overtime spending continued to rise. Then, in the aftermath of 9/11, antiterrorism efforts multiplied while the number of officers began to decline. Yet overtime spending, excluding costs stemming from 9/11) leveled off (see IBO’s Police Overtime: Tracking the Big Growth in Spending for more details).

While the cost of police overtime seems to follow its own laws of gravity, it’s likely that we’ll see substantial overtime costs for Occupy Wall Street and other goings-on around town for some time.