STOCKHOLM — Thirty more countries are expected to formally join the Paris Agreement on climate change this week, greatly improving the pact’s chances of coming into force just a year after it was negotiated in the French capital, the U.N. said Tuesday. More than 170 world leaders have signed the deal, but it won’t take effect until 55 countries accounting for at least 55 percent of global emissions have ratified or accepted it through their domestic procedures. That was initially expected to take several years, but 28 countries accounting for 39 percent of emissions including the world’s two biggest emitters, the United States and China, have already ratified the deal. The 30 ratifications expected to be handed to U.N.