Bloomberg News reports that banks' subprime write-downs have hit $500 billion. The last time I checked, that figure was $400 billion. Bloomberg reports that New York University economist Nouriel Roubini forecasts such losses will ultimately total $2 trillion. I wonder if he would revise his estimate upwards.

Recently banks have been taking write-downs for their Auction Rate Securities (ARS). Bloomberg reports about $1.9 billion has been set aside so far to cover ARS losses. It notes that UBS AG (NYSE: UBS) set aside $900 million to cover potential losses and Citigroup, Inc. (NYSE: C) and Wachovia (NYSE: WB) each estimate that their ARS buybacks will cost $500 million.

Write-downs have been going hand in hand with capital raising. But banks and brokers have not been able to raise enough capital to offset the losses. Bloomberg calculates that they've raised "$353 billion of capital to cope with the write-downs. The gap between the losses and capital infusions, which stands at $148 billion, has regularly narrowed to about $80 billion as capital raising follows write-down announcements."

Can banks and brokerages raise another $1.7 trillion to keep up with the write-downs that Roubini forecasts? I sincerely doubt it.

Peter Cohan is President of Peter S. Cohan & Associates. He also teaches management at Babson College and edits The Cohan Letter. He owns Citigroup stock and has no financial interest in the other securities mentioned.