Global warming, already on the hook for declining polar bear populations, disappearing glaciers and rising sea levels, may also increase your sewer bill.

A U.S. Environmental Protection Agency climate change expert says warming temperatures over the next several decades will be accompanied by an increase in the number and severity of storms. The combination will reduce the effectiveness of scores of federally mandated sewer improvements and water treatment upgrades designed to stop almost all of the sewage pollution flowing into rivers and creeks when it rains.

Such a projection could have significantly expensive implications for southwestern Pennsylvania. Sewer authorities and municipalities are already looking at more than $10 billion worth of sewer system work, and the Allegheny County Sanitary Authority and its 82 member municipalities and the city of Pittsburgh must by 2026 eliminate almost all of the 22 billion gallons of untreated sewage annually discharged into area waterways.

"They're going to redesign the [sewer] systems, but because of the storm activity they're not going to get the outcomes they desire," said Joel Scheraga, national program director for the EPA's Global Change and Mercury research programs, speaking at Stanford University last month. "So they do need to consider climate change."

If sewer authorities and municipalities design systems to handle those bigger, more frequent storms, the increased costs will be reflected in ratepayers' bills.

In March, a little-publicized EPA draft report predicted that because of surging storm severity in the Great Lakes region, the design and construction of sewer systems could cost at least 10 percent more. If that held true in southwestern Pennsylvania, it would add more than $1 billion to sewer and treatment plant reconstruction costs.

The climate change projections should interest the more than 300 municipal and sewer authority officials expected to attend the 3 Rivers Wet Weather 9th annual Sewer Conference today and tomorrow in Cranberry, said John Schombert, executive director of 3 Rivers Wet Weather, a nonprofit formed to work on a cooperative approach to the region's sewer problems.

"We seem to be getting 100-year storms with the frequency of every couple of years if you look anecdotally at what's happened in Pitcairn and Etna, or with Hurricane Ivan, but there's nothing I see in any of the consent decrees that mentions climate change," Mr. Schombert said. "It's not something EPA informed Alcosan or any of us about."

In the 11-county region there are 595 municipal jurisdictions with 492 separate water and sewer providers. There are also a total of 755 combined sewer outfalls into rivers and streams -- more than the number found in all but two states.

Angela McFadden, chief of EPA's regional water discharge permit review and enforcement branch, said the agency's climate change report on sewer system projects was listed for public comment in the Federal Register in March, but said the agency hasn't publicized the report to local sewer authorities or municipalities and doesn't plan to.

She said EPA's 1994 combined sewer overflow policy encourages sewer system operators to design facilities that can be easily modified or expanded.

"If we have concerns that what's proposed in the redesign won't be adequate," Ms. McFadden said, "we will work with Alcosan or whoever to ensure that it will be adequate."

Art Tamilia, Alcosan's director of environmental compliance, said its consulting engineering firms are aware of the issue and the authority believes it should be part of upcoming system design considerations.

"We have to design for bigger storms and we've seen that the last couple of years. The rainfall amounts go up and down but now we need to design for more intense storms," Mr. Tamilia said. "The treatment plant has already been expanded to the limit of the site. But climate change considerations will affect where and what and how much we build out in the system."

He said the $3 billion price tag on the improvements Alcosan and its member municipalities are required to make under the federal, state and county consent orders is 10 years old and has already been pushed upward by inflation. Sizing the projects to handle the bigger storms, he said, will also drive up costs.

Even though the EPA is a dozen years into a crackdown on sewage contamination of the nation's waterways, Mr. Tamilia and Mr. Schombert said there's still plenty of time to add climate change variables to the planning for new sewer systems, collection basins and treatment facilities.

Under terms of the consent agreements, the first draft plans for system reconstruction are due in 2012, with final wet weather plans for the region due in 2015. Construction of the new facilities must be completed by 2026, although some construction work to fix the worst of the problem areas has been done and will continue as the process goes along.

Mr. Schombert said a yearlong project to measure flow throughout the Alcosan system will begin in February.

Pittsburgh is one of 1,100 cities, most of them in the Northeast and Great Lakes region, that has combined sewer systems that regularly overflow during rainstorms. Many of those sewer systems, including the one operated by Alcosan, were designed to overflow during storms to prevent damage to treatment facilities. Subsequent changes in federal law have made those overflows illegal.

EPA studies have found that such overflows have damaged the quality of water, resulting in beach closings and fish kills, and have the potential to harm human health.

About 90 percent of the water provided by public water suppliers is drawn from the region's rivers and streams.