JPMorgan Chase & Co. CEO Jamie Dimon said a new accounting standard would be "pro-cyclical," meaning it would amplify economic growth periods but deepen recessions.

The standard, known as the Current Expected Credit Loss model, will force banks to set aside reserves for the expected loss over the entire life of a loan. Currently, banks set aside reserves only after a loan loss is considered probable. While JPMorgan has not yet conducted the analysis needed to estimate the impact of CECL, Dimon said it seemed clear the change would increase economic cyclicality.

"It seems to me that every single time there's a chance to make things more pro-cyclical or less, we make it more pro-cyclical," Dimon said Oct. 12 during the bank's third-quarter earnings call.

CFO Marianne Lake said the bank is not sure yet how much it will have to increase reserves to meet the standard, but that a significant increase seems possible. She called CECL a "big area of uncertainty" and one of the most open questions with which the bank currently wrestles.

"The methodologies are complicated, so operationally we are working through that across all of our businesses. We continue to expect to be running in parallel through some parts of 2019," Lake said. While she said the bank did not have any estimates, she said the bank's card portfolio would likely have the largest impact due to its size and current reserve practices for the portfolio.

"[Research reports] have suggested, not just for us but across others, that the reserve increase could be 20% to 30%," Lake said. "And while I don't have a number for you, that's not implausible."