After years of delays, the 141-nation Kyoto Protocol took effect at 0500 GMT on Wednesday morning with celebrations occurring in the ancient Japanese city of Kyoto, where it was signed in 1997.
Kenyan environmentalist Wangari Maathai, recipient of the 2004 Nobel Peace Prize, gave the keynote address, cementing the signing ceremony.
The pact is the first legally binding plan to tackle climate change, building on a scheme launched at an Earth Summit to stabilise greenhouse gas emissions at 1990 levels by 2000, a goal that was not met.
But it excludes until at least 2012 major developing nations India, China and Brazil, which make up more than a third of the world's population.
"This is a great stride forward in our struggle to confront one of the biggest challenges we face in the 21st century: Climate change," UN Secretary-General Kofi Annan said.
"Climate change is a global problem. It requires a concerted global response," Annan said. "I call on the world community to be bold, to adhere to the Kyoto Protocol, and to act quickly in taking the next steps. There is no time to lose."
Rising temperatures are melting
ice sheets, increasing sea levels
Kyoto aims to put brakes on rising temperatures widely blamed on human emissions of heat-trapping gases that may spur more hurricanes, floods and droughts and could drive thousands of species of animals and plants to extinction by 2100.
Sea levels are also expected to rise, threatening low-lying islands and coastal cities.
Under the deal, developed nations have to cut emissions of greenhouse gases, mainly carbon dioxide from burning fossil fuels in power plants, factories and cars by 5.2% below 1990 levels by 2008-2012.
But Kyoto has been weakened by a 2001 pullout by the United States, the world's top polluter and source of almost a quarter of human emissions of carbon dioxide. The US objected to the pact because it excluded developing nations, it said.
Earth observation meet
Separately, the third earth observation summit in Brussels on Wednesday was considering strengthening international cooperation in earth observation.
About 60 nations plan to set up
a global observation network
About 60 countries are planning linking their sensor platforms to form a global network of earth observation systems.
The European Union is contributing to this system with its own global monitoring system GMES, a joint initiative of the EU and European space agency.
The data provided by the future network of systems is predicted to "revolutionise" the understanding of how the world works like the ecological processes and climate changes, according to the organisers of the summit.
The aim is to reduce losses due to natural disasters and enhance the capacity to provide emergency relief.
The summit is expected to announce the creation of an observatory for environment and sustainable development for Africa.