WASHINGTON (AP) — A Senate climate bill will require a 20 percent cut in greenhouse gases by 2020, deeper than the reductions mandated by the House, but also include stronger measures to try to avoid energy price spikes, according to a draft of the bill.

The draft obtained by The Associated Press, remains subject to change, but the overall carbon reduction requirements are expected to stand. The Democratic bill is to be released Wednesday by the Senate Environment and Public Works Committee with a vote by the panel likely in late October.

The draft includes an economy-wide cap and trade system that would require power plants, industrial facilities and refineries to cut carbon dioxide and other climate changing pollution.

While there would be an overall emission cap, polluters would be able to purchase emissions allowances to limit reductions. But the Senate bill does not lay out how emission allowances would be distributed, leaving that for later.

The bill is viewed widely as an early focus of Senate negotiations over climate in the coming months, but just a starting point as Democratic leaders seek to work out compromises — adding new provisions and dropping others — to garner the 60 votes that will be needed for passage.

The 684-page draft calls for a ceiling on greenhouse gas emissions beginning in three years, and then to be tightened annually so that emissions would be 20 percent lower in 2020 than they were in 2005. Emissions would have to be 83 percent lower by 2050.

While the long-term cuts are the same as required by the House in June, the Senate bill has a faster early ramp-up, something many in industry wanted to avoid.

The House called for a 17 percent emission cut by 2020 and President Barack Obama originally had sought only a 14 percent cut.

The bill also includes provisions that its sponsors argue will help avoid severe energy price spikes as the economy shifts more and more away from cheaper fossil fuels, especially coal for electricity production.