tina krause is in london with the details. >> reporter: good morning. it's not easy to getetearly 200 countries to agree on anything, especially something with such an economic impact. the summit opens this morning and it's expected to last for two weeks. negotiators will try to come to terms on a deal that will slow and reduce greenhouse gas emissions. and at least slow down the effect of global warming. president obama will be in paris two days. he arrived last night and went directly to the concert hall, the bataclan, where the most people were killed during the terrorist attacks two weeks ago. also last night, protesters clashed with police. french forces used tear gas to stop demonstrators who defied a government ban on protests. french authorities did not want large crowds gathering after the terrorist attacks. this is the kind of pollution smog in shanghai reached hazardous levets this morning and two previous summits did nothing on change the way developing economies like china and india operate. that is the biggest stumbling blocks. developing