BRUSSELS, Dec. 12 — The leaders of Europe adopted what they described as a historic pact to combat global warming Friday and challenged President-elect Barack Obama to join in their commitment to drastically reduce greenhouse gases despite the global economic crisis.

The 27 European Union nations also endorsed a $260 billion economic stimulus plan, equivalent to 1.5 percent of the bloc’s gross domestic product, and urged the Obama administration to prepare similarly ambitious measures and financial reforms for coordinated action at a summit of the world’s 20 major economic powers scheduled April 2 in London.