Global leaders, development experts, bankers, academics and activists are gathering here this week to celebrate the anniversary of the landmark Earth Summit of 1992 and to try to address the linked problems of poverty, hunger, energy shortages and environmental degradation.

But the conference — expected to draw as many as 50,000 participants — is in many ways overshadowed by economic and political crises around the world. While more than 100 heads of state and government are planning to attend the formal talks starting Wednesday, President Obama, Prime Minister David Cameron of Britain and Chancellor Angela Merkel of Germany are staying away, preoccupied by domestic politics and the financial turmoil in Europe.

As a result, while diplomats feverishly negotiate a complex 80-page document spelling out the meeting’s goals, there are few expectations for concrete actions or pledges of new aid to developing countries.