Mr. Roubini’s numbers may be the highest, but many others share his rising sense of alarm. Simon Johnson, a former chief economist at the International Monetary Fund, estimates that the United States banks have a capital shortage of $500 billion. “In a more severe recession, it will take $1 trillion or so to properly capitalize the banks,” said Mr. Johnson, an economist at the Massachusetts Institute of Technology.

At the end of January, the I.M.F. raised its estimate of the potential losses from loans and other credit securities originated in the United States to $2.2 trillion, up from $1.4 trillion last October. Over the next two years, the I.M.F. estimated, United States and European banks would need at least $500 billion in new capital, a figure more conservative than those of many economists.

Still, these numbers are all based on estimates of the value of complex mortgage-backed securities in a very uncertain economy. “At this moment, the liabilities they have far exceed their assets,” said Mr. Posen of the Peterson institute. “They are insolvent.”

Yet, as Mr. Posen and other economists note, there are crucial issues of timing and market psychology that surround the discussion of bank solvency. If one assumes that current conditions reflect a temporary panic, then the value of the banks’ distressed assets could well recover over time. If not, many banks may be permanently impaired.

“We won’t know what the losses are on these mortgage-backed securities, and we won’t until the housing market stabilizes,” said Richard Portes, an economist at the London Business School.

Raghuram G. Rajan, a professor of finance and an economist at the University of Chicago graduate business school, draws the distinction between “liquidation values” and those of calmer times, or “going concern values.” In a troubled time for banks, Mr. Rajan said, analysts are constantly scrutinizing current and potential losses at the banks, but that is not the norm.

“If they had to sell these securities today, the losses would be far beyond their capital at this point,” he said. “But if the prices of these assets will recover over the next year or so, if they don’t have to sell at distress prices, the banks could have a new lease on life by giving them some time.”