“If we don’t act [on climate] in the future we can be pretty sure there will be conflicts,” said Belkacem, adding that climate scientists have come up with victories of truth. “Thanks to you, no-one can seriously challenge the role of humans in climate change.”

Fellow French minister Ségolène Royal, who is responsible for ecology, sustainable development and energy, called the conference a key event in the preparation of the Paris climate summit. “I would like you to have an impact on the COP21 negotiations,” Royal told delegates. “You are free to express your ideas, you don’t have to hide behind euphemisms.”

Thomas Stocker of the University of Bern, Switzerland, who was co-chair of working group I of the IPCC fifth assessment report, said that no policymaker has an excuse to say they have no time to read the report. That’s because the IPCC team condensed its 7579 printed pages down to 31 summary pages, and then compressed these again into 21 headline statements that print on just one sheet “admittedly on two sides”.

Stocker also detailed how climate change is a threat to sustainable development. “There are 17 sustainable development goals,” he said. “Addressing climate change would put us on a good path for at least half of them. Without it, it will be impossible.”

Science challenges

Sandrine Bony of LMD-Institute Pierre Simon Laplace, France said that climate science has answered yes to three questions - is the atmospheric composition changing, is the climate warming and are human activities responsible? But we still need to know about the pace and pattern of warming over the next few decades and the longer term; the effects of climate change in specific regions on aspects such as water, extreme events, air quality; and the impact on ecological and social systems.

“We have entered uncharted territory,” said Bony. Whilst some of our knowledge is robust, other areas contain uncertainties, which are important for assessing risk and could affect our decisions.

The World Climate Research Programme (WCRP) has identified five grand challenges, Bony explained, where it hopes to bring the different scientific communities together: melting ice, sea-level change, water availability; clouds and sensitivity; and climate extremes. Two more are under consideration for adoption – carbon and chemistry, and decadal prediction.

It’s crucial that we understand more about the circulation of the atmosphere and the oceans, and how they will respond to climate change, Bony said, as well as the interaction between the large and small scales. “Many of the big questions have been answered from a thermodynamic view,” she said. “Many of the new questions need a more dynamic view.” Whilst a thermodynamic approach can show us the mean rate of warming it can’t help with climate variability or spatial patterns.

Carbon sinks, sea-level rise and rainfall are all sensitive to circulation, whilst a key uncertainty in climate sensitivity is shallow cumulus clouds, which disappear very fast as temperatures rise in some models but not in others. Recent research has indicated that this cloud response may be due to vertical wind shear. Appropriately enough for a conference in a city that was in the throes of a heatwave, Bony explained that climate extremes too are often driven by atmospheric circulation. “What causes it to make persistent ‘blockings’?” she asked.

Bony believes that climate science has never been more relevant. “The urgency and specificity of the societal demand is not incompatible with investment in basic research,” she said. “It is our best chance to prepare ourselves to explain future surprises in our climate.”