Mary Robinson has warned that the world "needs to wake up" to the threat posed by the climate crisis and nuclear weapons.
The former Irish president was speaking as scientists moved the so-called 'doomsday clock' to '100 seconds to midnight'.
The clock was established in 1947 and serves as a metaphor for global apocalypse.
Every year, the Bulletin of the Atomic Scientists reveal their latest calculation of the danger to humanity - and this year the clock is the closest it has ever been to 'midnight'.
Announcing their decision to move it from two minutes to midnight to 100 seconds, the scientists pointed to the "existential dangers" of nuclear war and climate change.
Mrs Robinson - who attended the event as the chair of the Elders group - described it as "a solemn occasion".
She observed: "I must say as I listened to the excellent but devastating presentation of the scientists, I felt more and more a strong personal reaction - the reaction of an angry granny. This is not acceptable.
"This is no mere analogy - we are now 100 seconds to midnight, and the world needs to wake up.
"Our planet faces two simultaneous existential threats - the climate crisis, and nuclear weapons.
"The science of the climate crisis makes it imperative that we take urgent action in 2020. We need a change of mindset in politics, finance, business and civil society."
She claimed failure to acknowledge the threat from climate change means a "death sentence for humanity" - but said world leaders continue to "ignore the science".
However, she added: "Despite this stark picture of failing leadership and narrow self-interest, we can draw hope and inspiration from the young school children and activists who are striking under the banner Fridays for Future."
She also highlighted the continuing risk from nuclear war, suggesting: "As long as nuclear weapons remain in existence, it is inevitable that they will one day be used - whether by accident, miscalculation or design."
She called on Donald Trump and Vladimir Putin to "get down to business" and agree to new nuclear limitation deals.