This year's U.N. climate negotiations in Madrid, the longest in 25 nearly annual such gatherings, ended Sunday with major polluters resisting calls to ramp up efforts to keep global warming at bay.
Laurence Tubiana of the European Climate Foundation, and an architect of the 2015 Paris agreement, told the BBC that the result was "really a mixed bag, and a far cry from what science tells us is needed."
"Major players who needed to deliver in Madrid did not live up to expectations, but thanks to a progressive alliance of small island states, European, African and Latin American countries, we obtained the best possible outcome, against the will of big polluters," she said.
Faced with the tough task of reconciling the demands of scientists, protesters on the streets and governments back home, the negotiators ended up disappointing many and leaving for next year's talks in Glasgow, Scotland, key issues such as the regulation of global carbon markets.
Here is a look at the main issues resolved, and the sticking points for future negotiations.
ARE BOLDER TARGETS NEEDED?
While it wasn't officially on the agenda, most participants and observers agreed that the U.N. talks needed to send a strong signal that countries were willing to set bolder targets for cutting greenhouse gas emissions.
That was far more ambitious than the goal ultimately set by the so-called "Chile-Madrid Time for Action" declaration, which merely calls for the "urgent need" to cut planet-heating greenhouse gases in line with the goals of the landmark 2015 Paris climate accord.
Scientists say global emissions of carbon dioxide and other pollutants have to start falling rapidly as soon as possible to meet the Paris goal of keeping global warming by the end of the century well bellow 2 degrees Celsius (3.6 Fahrenheit), and ideally 1.5 degrees Celsius (2.7 F).
That accord allowed countries to set their own emissions-reduction targets — known as nationally determined contributions, or NDCs — which would be regularly reviewed and increased if necessary. With current emissions targets putting the world on course for a 3- to 4-degree Celsius temperature rise by 2100, scientists say sharper cuts are necessary and should be announced in advance of next year's climate conference in Glasgow.
"The global emissions' curve needs to bend in 2020, emissions need to be cut in half by 2030, and net zero emissions need to be a reality by 2050," said Johan Rockstrom, head of the Postdam Institute for Climate Impact Research.
"Achieving this is possible — with existing technologies and within our current economy," said the revered climate scientist. "The window of opportunity is open, but barely."
HOW TO REGULATE GLOBAL CARBON MARKETS?
Economists say market mechanisms can speed up the shift from fossil fuels to renewable energy sources. One way to do this is by putting a price on carbon dioxide, the most abundant man-made greenhouse gas, and gradually reducing the amount countries and companies are allowed to emit.
The European Union and some other jurisdictions around the world already have limited emissions trading systems for buying and selling carbon credits.
The Paris accord was meant to establish the rules for carbon trading on a global scale. But setting the rules for a robust and environmentally sound market and linking up existing systems is difficult. So, too, is the question of allocating a percentage of the revenue to help countries adapt to the effects of warming temperatures.
The main point of resistance lies in the existence of old carbon credits left over from a now-discredited system established under the 1997 Kyoto Protocol. Developing countries such as Brazil insisted during the past two weeks in Madrid on keeping those emissions credits, while also resisting strict accounting of future trades.
The argument that carbon markets that are not transparent enough and leave loopholes for double counting can undermine efforts to reduce emissions won at the end, postponing the decision on the issue for Glasgow.
AID FOR THE POOR
In summit lingo the issue is known as "loss and damage." In essence, it was recognized several years ago that developing nations are much more vulnerable to the negative effects of climate change, even though they contribute least to the problem.
A tentative agreement was reached in 2013 that rich countries would help them foot the bill. But attributing specific weather disasters such as hurricanes and floods, or slow but irreversible changes like sea level rise and desertification, to climate change remains a delicate issue given the potential costs involved.
The United States, in particular, had opposed any references to possible liability in the summit's conclusions, and scored a victory when a decision on it was also postponed for another year.
Developing countries also demand that compensation be kept a separate issue from funds to help the countries adapt and mitigate the effects of a warming planet. The Green Climate Fund, which was established to that end, is currently far from reaching the target of $100 billion a year in contributions.
HEAT ON THE STREET
Chile, which chaired the conference, chose to give it the slogan "Time for Action."
That echoed the blunt demands from protesters, who have been staging mass rallies around the world for the past year demanding leaders take what they call the "climate emergency" seriously.
European Union countries responded to public pressure this week by agreeing to a long-term goal of cutting the bloc's greenhouse gas emissions to net zero, meaning any that remain will be offset with carbon reduction measures.
Some observers and EU ministers had hoped this signal from Brussels would boost the talks in Madrid. If anything, it revealed the vast gap between what countries can agree at the regional level and what the U.N. process is capable of.