The UN's climate chief urged country negotiators Monday to "build bridges" at talks in Bonn towards a new, global pact that received fresh endorsement from world leaders last month.
The six-day forum must lay the foundations for the annual round of ministerial-level UN talks to be held in Lima in December, Christiana Figueres told delegates as the meeting opened.
Lima, in turn, must pave the way to a historic pact in Paris in December 2015 that will for the first time apply to 195 nations, rich and poor alike.
Figueres said a September 23 climate summit called by UN chief Ban Ki-moon had "shifted the ground on what is possible in climate change".
"Collectively, your heads of state have reassured the world that we will address climate change," she said via webcast.
"Today, dear delegates, the world's eyes turn to you. It is up to you to chart the path of that solution."
Negotiators face a tough task ironing out long-standing differences over how to share responsibility for curbing Earth-warming greenhouse gas emissions.
These cuts seek to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels, and save the planet from potentially catastrophic climate damage.
The UN's Intergovernmental Panel on Climate Change (IPCC) says that on current trends, the planet could be up to 4.8 C warmer by 2100 and sea levels up to 82 centimetres (32 inches) higher.
Conflict, hunger, floods and homelessness would be the likely result, it warns.
Figueres stressed the new climate pact, due to enter into force in 2020, "must irreversibly bend the curb of emissions", which have continued rising.
But many technicalities have to be resolved, including the very legal nature of the pact and how it will be monitored and enforced.
The talks in Germany will be the first chance for negotiators to discuss a rough 22-page outline for the deal that has been drawn up by working group leaders and distributed for scrutiny in July.
The meeting must also start narrowing down what data countries must provide when they submit their pledges for emissions curbs in the first quarter of next year.
This topic will likely see negotiators return to a sore point -- whether rich countries should have tougher targets because of their longer history of burning fossil fuels.
Developed nations reject the notion, and point the finger in turn at fast-growing emerging giants like China and India.
Attending the Bonn talks, Environment Minister Manuel Pulgar-Vidal of Peru, host of the December talks, said there were only 13 months to the deadline for signing a new, global pact.
"Lima must be successful if we want to pave the road to Paris," he said, and urged negotiators to "work constructively this week to prepare the substantive outcomes we expect from Lima."