Japan ? World leaders embraced for the first time on Tuesday an ambitious but nonbinding goal of slashing greenhouse-gas emissions in half by midcentury to stave off global warming. Unimpressed environmentalists called the effort too slow and too uncertain.

Leaders of some of the world’s richest nations praised the agreement, which endorsed President Bush’s insistence that fast-developing countries like China and India join in the effort.

At a meeting today, major developed and developing nations agreed that climate change was “one of the great global challenges of our time” and pledged to support a United Nations effort to conclude a new climate pact by 2009. The major economies said they supported long-term and midterm goals for greenhouse-gas reductions but endorsed no targets.

But the developing nations invited to the gathering were not ready to go as far as supporting the 50 percent reduction by 2050.