Leaders including French President Emmanuel Macron, German Chancellor Angela Merkel and British Prime Minister Boris Johnson are expected to contribute by video link to the Climate Adaptation Summit, being hosted by the Netherlands.
The aim of these talks is to make the planet more resilient to the effects of climate change.
The meeting, also expected to involve current UN chief Antonio Guterres and former head Ban Ki-moon, will produce an "adaptation action agenda" for dealing with effects such as rising sea levels, extreme weather and crop failure.
Former US secretary of state and Joe Biden's newly appointed climate envoy John Kerry will speak at the summit, as will Deputy Chinese Prime Minister Han Zheng.
The summit is being held online because of the coronavirus pandemic.
Johnson to announce new climate action coalition
At the summit, Boris Johnson will announce the launch of an Adaptation Action Coalition, in partnership with Egypt, Bangladesh, Malawi, the Netherlands, Saint Lucia and the United Nations.
The new grouping "will work to turn international political commitments made through the United Nations Call for Action on Adaptation and Resilience into on-the-ground support for vulnerable communities," his office said in a statement.
"It is undeniable that climate change is already upon us and is already devastating lives and economies," Johnson is expected to say.
"We must adapt to our changing climate, and we must do so now."
A summit on adaptation to inevitable change
While previous summits have focused on tackling the causes of climate change, including reducing emissions, this is the first to concentrate on dealing with its effects, say organisers.
These include reducing the vulnerability of countries to rising sea levels, extreme weather and food shortages.
Plans could include shoring up sea defences but also taking advantage of opportunities including longer growing seasons for crops, and new areas for cultivation, organisers said.