The main architect of the Paris climate agreement has said COP26 needs to be a hybrid event with some negotiations happening virtually.
Christiana Figueres told BBC Scotland that some form of in-person conference would still be needed if the talks were to be a success.
But she said it was unlikely 25,000 would attend as originally planned.
Organisers needed to find the "sweet spot" that would allow for safe and efficient negotiations, she added.
Speaking from Chicago, she admitted she did not know whether the historic Paris Agreement could have been achieved if the negotiations had been carried out virtually.
Ms Figueres, who used to head the UN's climate change arm, told BBC Scotland: "Could we have done it differently? I honestly do not know.
"Over the past 18 months we have actually shifted our mindset and realised that much can be done without our physical presence. So, therein lies the answer to your question.
"Somewhere that is only virtual I think is going to be extremely difficult. It will probably not be possible to have 25,000 people descend on Glasgow as was originally planned. And so the big question is going to be, what is the sweet spot in between that will allow for successful and efficient negotiations?"
As executive secretary of the United Nations Framework Convention on Climate Change, Christiana Figueres was the woman who steered the world - against all the odds - towards an international agreement on limiting global warming.
After five years at the helm, that happened in Paris in 2015 when the world's nations committed to limiting global warming to between 1.5-2C.
Now an observer, and with three months to go until COP26, she told me she wanted world leaders to continue that success when they come together again in Glasgow.
I asked how her nerves were doing.
With her characteristic hearty laugh, she replied: "My nerves are doing pretty well actually. Remember, I'm pretty well tested on nervy issues here.
"To be quite honest, the leadership that the UK, the EU and the United States have been showing is really taking the world forward."
She has praised the Scottish government's efforts on climate change in the past and said that needed to continue.
"It has been for many, many years, a leading country on climate change and decarbonisation," she said.
"And just because it's a small country doesn't mean that the impact is not small. The fact is that every country counts, every role modelling counts, every leadership counts. Everything counts here."
After years of criticising former American President Donald Trump for his views on climate change, Christiana Figueres says countries now needed to look to the US for their climate leadership.
She said: "What is needed here is, ironically, what the United States is doing now which is an all-of-government approach.
"President Biden has actually asked every single government agency, every single department in the United States, to ensure that the sectoral targets of that particular department are being delivered but to do so through climate change eyes."
Glasgow should be an 'immersive experience'
After serving as a vaccination centre, this Scottish Event Campus in Glasgow is now being turned back into a conference facility ready to host the UN conference in November.
And the former UN executive secretary says Glasgow now needs to show the world - and in particular the COP26 delegates - what it is made of.
She added: "They should be treated to an experience of what a city that is in the process of decarbonising. What does it feel like? What does it look like? How does it operate?
"Actually Glasgow should be aiming at giving those who will be visiting for the COP an immersive experience."
Ms Figueres confirmed that she intended to visit Glasgow in November for part of the conference.
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