"We need to show the world how serious the threat is," Mexican President Felipe Calderon said as he opened an international climate change conference in Germany on Sunday.

German Chancellor Angela Merkel, who also spoke at the opening of the conference co-hosted by both countries and aimed at laying the groundwork for the next U.N. conference on climate change, asked nations around the world for more ambition in their efforts to cut greenhouse gases.

While scientists believe global temperatures must not rise by more than 2 degrees Celsius (3.6 degrees Fahrenheit) compared to pre-industrial times, the world is now headed for a 3 to 4 degree increase, Merkel said.

"We have to realize that we have quite a long way to go to reach the 2-degree-goal," Merkel said. "Therefore we have to ascertain how we can reach our goals nonetheless."

Mexico will host the next U.N. conference on climate change in Cancun in December, the first such high-level summit after the troubled U.N. conference in Copenhagen five months ago.

Germany has long presented itself as a driving force in the international efforts to curb global warming and came up with the idea of a "mid-term" meeting.

Both countries invited ministers and representatives from around 45 countries for informal talks on the Petersberg up above Koenigswinter.

The three-day conference called the Petersberg Dialogue hopes to make some progress on details, but most of all build trust between poor and rich nations, Calderon said.

He said the conference could produce a "clear message, this will be the signal whether it will be possible to reach a uniform agreement."

Nations around the world agreed in 2007 to negotiate a new international treaty to fight global warming which scientists say has already started to cause some alarming changes such as droughts, flooding or heavier storms.

A treaty was originally hoped for in Copenhagen, but that meeting produced less than expected.

President Barack Obama and a few dozen other major players drafted the so-called Copenhagen Accord, which includes the 2-degree-goal and an immediate $30 billion three-year aid package for poorer nations.

However, the accord failed to gain full support at the summit, as some smaller countries felt left out in the process and were unhappy with the results of closed-door negotiations.

German Environment Minister Norbert Roettgen said the Petersberg meeting is designed to work intensely on some sticking points and to build trust among those who eventually have to work with each other on the U.N. level.

To have something to show for even while the negotiating is going on, nations should agree on concrete projects to curb greenhouse gas emissions or to adapt to climate change, he said.

Calderon and Merkel said one of the areas that could see some progress in Cancun was the fight against deforestation.

Mexico's president stressed that saving forests could help fight poverty at the same time as it would give residents an income.

Since Copenhagen, momentum in the drive to control global warming has slowed in some countries. The U.S. has not tackled its domestic energy bill; and Australia — one of the world's biggest per capita polluters — put off for as long as two years legislation setting up carbon trading.

Roettgen has said his country and others have not given up on striking a deal at the U.N. climate summit in Cancun Nov. 29-Dec. 10.