The USA's climate envoy did not mince his words. "Key countries," said John KerryBarack Obama's secretary of state from 2013 to 2017, he was the Democrats' unsuccessful presidential candidate in 2004., "are pursuing policies that border on being very dangerous for everybody." China's promise to start reducing carbon emissions by 2030 was not good enough: it has to act now. Cop26 was the greatest test of global citizenship - the "last best hope for the world to get its act together".

Cop26Cop stands for  Conference of the Parties (COP) to the United Nations Framework Convention on Climate Change brings together nearly 200 countries to discuss the climate-change crisis. The objective is to reach an agreement on what is to be done.

In 2015, the summit resulted in the Paris Agreement to limit global warming to 2C above pre-industrial levels, and if possible only 1.5C. But governments have been slow in moving towards the agreed targets. At present, these are rising enough to reach 2.7C by 2100. Countries are due to update their targets.

The good news, according to the International Energy AgencyAn organisation representing 29 industrialised nations, set up in 1974 after an international oil crisis.   (IEA), is that we have most of the technical know-how to reduce emissions affordably, and achieve net zeroWhen the gases going into the atmosphere are balanced by those being taken out of it. by 2050. The key lies in switching to renewable sources - though hydrogen and bioenergyEnergy from organic materials such as wood and sugarcane. would also play a part, and possible nuclear power too.

Time is of the essence. In the next 10 years there must be an increase in clean electricity, and in energy efficiency - and a huge reduction in methaneA particularly strong greenhouse gas produced by fossil fuels and cattle. emissions.

New technologies need to be developed. Ways must be found of managing the new systems and securing them against attacks.

All this needs to happen worldwide. According to Martin Wolf of the Financial Times: "The battle will ultimately be won or lost in emerging and developing countries, which have the fastest growth in population and in demand for energy."

At present, nearly 770 million people in these countries live without electricity. A crucial question is how to meet their needs while phasing out coal-fuelled power stations.

Money is another factor. The IEA believes that achieving net zero emissions will need an investment of £2.9tn per year by 2030.

Many countries fear the expense of switching to clean energy will damage economies. Because of this, richer nations promised in 2009 to give $100bn a year by 2021 to help poorer ones. But the latest figure, for 2019, was under $80bn.

Individual expectations matter: experts fear a backlash if the public has to cope with higher bills or power cuts.

In the words of Martin Wolf, unless Cop26 works "the shift will probably not happen with the needed scale and speed. It is an immense responsibility, and no less immense task. Governments have no decent choice but to rise to the occasion."

Yes. We know what needs to be done, and we have the technology to do it. In the past, progress was impeded by a lack of political will and people denying the existence of climate change. Extreme weather has brought home the danger we face. People realise that whatever it costs to fix the problem, not fixing it would cost even more.

No. Lots of promises are made at events like Cop26, but few are kept. The failure of many countries to meet the targets is a case in point. Net zero cannot be achieved unless China pulls its weight - and at the moment that seems unlikely.