american tower corporation and subsidiaries notes to consolidated financial statements 2014 ( continued ) as of december 31 , 2006 , the company held a total of ten interest rate swap agreements to manage exposure to variable rate interest obligations under its amt opco and spectrasite credit facilities and four forward starting interest rate swap agreements to manage exposure to variability in cash flows relating to forecasted interest payments in connection with the securitization which the company designated as cash flow hedges . the eight american tower swaps had an aggregate notional amount of $ 450.0 million and fixed rates ranging between 4.63% ( 4.63 % ) and 4.88% ( 4.88 % ) and the two spectrasite swaps have an aggregate notional amount of $ 100.0 million and a fixed rate of 4.95% ( 4.95 % ) . the four forward starting interest rate swap agreements had an aggregate notional amount of $ 900.0 million , fixed rates ranging between 4.73% ( 4.73 % ) and 5.10% ( 5.10 % ) . as of december 31 , 2006 , the company also held three interest rate swap instruments and one interest rate cap instrument that were acquired in the spectrasite , inc . merger in august 2005 and were not designated as cash flow hedges . the three interest rate swaps , which had a fair value of $ 6.7 million at the date of acquisition , have an aggregate notional amount of $ 300.0 million , a fixed rate of 3.88% ( 3.88 % ) . the interest rate cap had a notional amount of $ 175.0 million , a fixed rate of 7.0% ( 7.0 % ) , and expired in february 2006 . as of december 31 , 2006 , other comprehensive income includes unrealized gains on short term available-for-sale securities of $ 10.4 million and unrealized gains related to the interest rate swap agreements in the table above of $ 5.7 million , net of tax . during the year ended december 31 , 2006 , the company recorded a net unrealized gain of approximately $ 6.5 million ( net of a tax provision of approximately $ 3.5 million ) in other comprehensive loss for the change in fair value of interest rate swaps designated as cash flow hedges and reclassified $ 0.7 million ( net of an income tax benefit of $ 0.2 million ) into results of operations during the year ended december 31 , 2006 . 9 . commitments and contingencies lease obligations 2014the company leases certain land , office and tower space under operating leases that expire over various terms . many of the leases contain renewal options with specified increases in lease payments upon exercise of the renewal option . escalation clauses present in operating leases , excluding those tied to cpi or other inflation-based indices , are recognized on a straight-line basis over the non-cancelable term of the lease . ( see note 1. ) future minimum rental payments under non-cancelable operating leases include payments for certain renewal periods at the company 2019s option because failure to renew could result in a loss of the applicable tower site and related revenues from tenant leases , thereby making it reasonably assured that the company will renew the lease . such payments in effect at december 31 , 2007 are as follows ( in thousands ) : year ending december 31 .
what is the percentage change in aggregate rent expense from 2006 to 2007?
aggregate rent expense ( including the effect of straight-line rent expense ) under operating leases for the years ended december 31 , 2007 , 2006 and 2005 approximated $ 246.4 million , $ 237.0 million and $ 168.7 million , respectively. .