Prince Harry has interviewed The Prince of Wales for his guest editorship of BBC Radio 4's Today programme.
Climate change is causing "untold horrors" across the world, The Prince of Wales said.
Prince Harry joined the presenters of the current affairs programme in the studio and said the issues he had chosen, which included mental health, the charitable sector and the Armed Forces, were "incredibly important topics we all need to think about and need to be discussed."
The Prince interviewed former US president Barack Obama for the programme and the statesman warns that the irresponsible use of social media is distorting the public's understanding of complex issues and leading to the spread of misinformation.
Prince Harry interviewed his father, His Royal Highness The Prince of Wales, at Clarence House. When asked by his son to pick one issue to focus on next year, The Prince of Wales said over the years there have been a "whole lot" of things he has tried to focus on that he thought needed attention.
"Not everyone else did, but maybe now some years later they are beginning to realise that what I was trying to say may not have been as dotty as they thought" he said.
"The issue really which has to go on being focused on, big time I think, is this one around the whole issue of climate change.
"Which is now, whether we like it or not, is the biggest threat multiplier we face because what is happening now is what I was dreading.
"Which is we are having to deal all the time with the symptoms that are springing up all round the world, and they are diverting us off down all these different channels to try and deal with ghastly conflicts and humanitarian and natural disasters, and goodness knows what else.
"But at the root of it all, much of it, is climate change which is causing untold horrors in different parts of the world."