Three of the most prominent ad-forecasting firms kicked off the UBS Annual Global Media and Communications Conference this morning — as they typically do at this event — by unveiling their updated forecasts for 2012. And they pretty much agree: 2012 will be better than 2011. MagnaGlobal is on the conservative side, projecting 3.2% growth in North America, and GroupM is more bullish at 4%. MagnaGlobal EVP Vincent Letang says that the U.S. ad market will benefit from the strongest-ever quadrennial effect with $2.4B hitting the market from political campaigns, and $600M related to the Summer Olympics in London. That’s why GroupM Futures Director Adam Smith warns that the ad-growth number in 2013 — without the quadrennial effect — “might be harder to look at.” The forecasters agree that television and digital will benefit most from the growth in ad spending. But they warn that pay TV providers need to watch out: Subscriptions “will go down by 500,000 a year for next five years” partly due to competition from Web video providers such as Netflix, says ZenithOptimedia CEO Steve King, whose company released its forecast last night. He also says that there’s a reason to take the forecasts with a big grain of salt. “The factor that none of us have incorporated is a default” by European country.