OSWIECIM, Poland — A decade ago, 1,500 Holocaust survivors traveled to Auschwitz to mark the 60th anniversary of the death camp’s liberation. On Tuesday, for the 70th anniversary, organizers are expecting 300, the youngest in their 70s.“In 10 years there might be just one,” said Zygmunt Shipper, an 85-year-old survivor who will attend the event in southern Poland to pay homage to the millions killed by the Third Reich. In recent years, Shipper has been traveling around Britain to share his story with school groups, hoping to reach as many people as he can while he has the strength. “The children cry, and I tell them to talk to their parents and brothers and sisters and ask them ‘why do we do it and why do we hate?”‘ he said. “We mustn’t forget what happened.” But as the world moves inevitably closer to a post-survivor era, some Jewish leaders fear that people are already starting to forget. And they warn that the anti-Semitic hatred and violence that are on the rise, particularly in Europe, could partly be linked to fading memories of the Holocaust.Ronald Lauder, president of the World Jewish Congress, says that the recent massacres in Paris, which targeted Jews and newspaper satirists, are proof of growing hatred and extremism. It’s a message he plans to stress in a speech Tuesday at the former site of Auschwitz-Birkenau, where the Nazis killed more than 1.1 million people, most of them Jews.