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 he could think of - 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 - so called because it is the 26^th summit of its kind - brings together nearly 200 countries to discuss progress in dealing with the climate-change crisis. The objective is to reach a solid agreement on what is to be done next.

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 for reducing greenhouse gas emissions. At present, in fact, these are rising enough to reach 2.7C by 2100. At Cop26, countries are due to update and strengthen 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 electricity from 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 a vast increase in the supply of 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, including better batteries and methods for capturing CO2. Ways must be found of managing the new systems and securing them against attacks.

All this needs to happen on a worldwide basis. 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 vital factor. The IEA believes that achieving net zero emissions will need an investment of £2.9tn per year by 2030 - four times the present level.

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

Individual expectations matter too: experts fear a political 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."

Some say, 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 man-made climate change. But the extreme weather which has ravaged the world has brought home the danger we all face. People realise that whatever it costs to fix the problem, not fixing it would cost even more.

Others argue that lots of promises are made at events like Cop26, but few of them are kept: in the words of Greta Thunberg, all they produce is "Blah blah blah." The failure of many countries to meet the targets they have agreed to is a case in point. Whatever the rest of the world does, net zero cannot be achieved unless China pulls its weight - and at the moment that seems unlikely.

John Kerry - Barack Obama's secretary of state from 2013 to 2017, he was the Democrats' unsuccessful presidential candidate in 2004.

Cop26 - Cop stands for  Conference of the Parties (COP) to the United Nations Framework Convention on Climate Change

International Energy Agency - An organisation representing 29 industrialised nations, set up in 1974 after an international oil crisis.  

Net zero - When the gases going into the atmosphere are balanced by those being taken out of it.

Bioenergy - Energy from organic materials such as wood and sugarcane.

Methane - A particularly strong greenhouse gas produced by fossil fuels and cattle.