Maine will participate in the nation's first auction of greenhouse gas emission allowances Thursday in an effort to slow global warming.

The state's six largest power plants, along with other companies throughout the northeastern United States, will need to purchase allowances in order to release carbon dioxide, which scientists say is a major contributor to climate change. The overall number of allowances will be capped and companies also will be allowed to buy and sell the allowances among themselves.

The first auction will take place Thursday, although others will follow until all 10 participating states have distributed their allowances.

The so-called Regional Greenhouse Gas Initiative is similar to European efforts and could become a model for a national system in the United States. The effort is not expected to have any immediate impact on global warming pollution or electricity costs because the allowances will be relatively plentiful and inexpensive at first. The number of allowances sold at auction will shrink in future years, forcing companies to reduce emissions or bid for the declining number of allowances.

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