The economy has lost jobs for four months now, but it is losing them at a significantly lower rate than it did in the earlier downturn. And with sublease space rising more slowly, it is having less of an effect on office rents, not driving them down as far or as fast as it did previously, according to Bach.

"This time around, rents are softening and concessions are going up, but sublease is playing less of a role than it did the last time," Bach says. He adds, however, it's always difficult to calculate exactly how much the flattening or decline of rental rates results from sublease space versus direct space and how much of the decline results from the generally weakening of the office market.

Another factor will be how high the US office vacancy rate ultimately rises in this downturn before it starts to recover. The last time around, the country's vacancy rate rose from 8.5% at the start of the downturn to 17.9% before it started turning around in 2004. "The vacancy rate is going to rise, but possibly not to the extent that it did then," Bach says.

Bach's latest figures show that office sublease space for the entire US has increased by 12% from the three quarters since its low in second quarter 2007, ending first quarter 2008 at 81.9 million sf. In the 2001 downturn, sublease space had more than doubled after three quarters of softening.

As in all statistical studies that track the entire US, however, the nationwide sublease figures include some wide variations on a market-by-market basis. And looking at sublease space on the basis of a percentage increase can be misleading because some markets with some of the biggest percentage leaps have some of the smallest increases in terms of actual square footage of space.

For example, San Antonio and Austin, TX have posted some of the highest percentage increases lately, with both more than doubling on a percentage basis during the past three quarters. But the actual amount of increase for San Antonio was very low, rising only from 75,000 sf to 178,000 sf on a base inventory of 24 million sf. Austin's gain was larger--326,000 sf to 858,000 sf on a base inventory of 41 million sf. By contrast, the percentage of increase in Los Angeles was less than 2%, but that works out to more than one million sf of the 185-million-sf base in Los Angeles.

Job losses thus far have been shallow and sublease space is rising gradually are both encouraging signs, according to Bach. He says how high vacancy and sublease rates go will depend primarily on employment trends, which are reflected in monthly statistics from the federal government. "The future of the office market and the future of the sublease inventory really depend on numbers like that," he says.