Negotiators are gearing for twin meetings next week that may determine whether the vaunted 2015 Paris climate accord will be a big bang or a fizzle.
A 10-day conference under the UN flag starts in Bonn on Monday, followed by the Group of Seven (G7) summit in Bavaria on June 7 and 8.
The two gatherings come at the crucial halfway point in the year's crowded climate agenda.
Everything is supposed to culminate in Paris on December 11 with a pact committing 195 countries to roll back greenhouse gases and help poor people exposed to climate change.
Desperate to avoid a blowup in this combustible process, the chairmen have solicited national viewpoints and stuffed them all into the document.
The bloated 80-page text will have to be trimmed into a manageable script for a post-2020 deal, and cosy consensus will be the victim.
"People are now going to have negotiate, and things will get tense," Tubiana told journalists on Thursday.
Contrasting ideas include an overall target for global carbon emissions, or zero emissions by 2100, or for emissions to peak "as soon as possible" or a pathway towards 2050.
Another question mark in Bonn hangs over a so-called ratchet mechanism.
On current trends, according to the UN's Intergovernmental Panel on Climate Change (IPCC), the global mean temperature could rise by up to 4.8 C this century alone, a recipe for worse drought, flood and rising seas.
What is needed is a mechanism by which countries agree to meet regularly beyond 2015 and strengthen -- ratchet up -- pledges so that the planet is aligned with a 2 C trajectory.
But when countries should meet, what the reviews should cover and whether they should be formally linked to a long-term goal have to be hammered out.
"We (have to) get the right architecture and the right regime so that we get this very durable, consistent, regular way of coming back to the negotiations and keeping on track to two degrees."
As the wrangle unfolds in Bonn, many delegates will cast a worried eye at what happens 450 kilometres (280 miles) to their southeast.
The G7 summit offers a stage for Japan -- now the laggard among the richest economies -- to spell out its plans for national curbs in carbon emissions.
Voluntary pledges are intended to be the beating heart of the Paris agreement.
But so far only 38 parties, amounting to just a third of global emissions, have made their submissions, according to the UN Framework Convention on Climate Change (UNFCCC) website.
Pledgers include the United States, the European Union, Russia and Canada, but not yet Japan, nor Australia, Brazil, India or China, the world's No 1 emitter.