The FirstService Williams report puts Manhattan's overall availability rate at 10.9%, with more than 25% of the total represented by sublease space, the highest rate in more than three years. That includes 2.4 million square feet of sublease space entering the market in the past three months, compared to 1.8 million square feet of directly available space.

"It is highly unusual for the increase in the amount of sublease space to exceed the increase in direct space," Joyce Geiger, chief economist at FirstService Williams, tells GlobeSt.com. "Also, the sublease availability rate is normally about 1.5% to 2%. It has risen already to 2.8%, with more to come." She says sublease space may eventually comprise 40% of the total amount of available space, as it did during the dot-com bust.

The sheer volume of sublease availability poses a challenge to building owners, Geiger says. "Sublease space tends to pull rents downward because sublessors are less price sensitive," she says. "As the amount of sublease space increases, so does the competition for tenants, especially since this space is finished and frequently furnished. If the sublessor is facing financial distress, the landlord could be in danger of not receiving rent. Depending on the size of the tenant, that could cause a serious disruption to the property's cash flow."