KATOWICE, Poland (Reuters) – Brexit turmoil and French riots have kept many government chiefs away from the final and crucial week of U.N. climate talks, with only four national leaders present on Tuesday.

Some 190 countries are meeting in Katowice, Poland to agree a “rule book” on implementing the Paris climate deal to reduce greenhouse gas emissions, in what has been billed as the most important U.N. conference since that pact was agreed in 2015.

Back then, former U.S. President Barack Obama, Chinese President Xi Jinping and India’s Prime Minister Narendra Modi attended the talks that ended with a sense of global unity to confront climate change that scientists say will have devastating consequences if emissions are not curbed.

Out of around 134 national representatives delivering speeches over the next two days, only four are heads of government: the president of Kiribati and the prime ministers of Samoa, the Cook Islands and Tuvalu – nations among the most vulnerable to climate change.

“Kiribati is among the low-lying islands which has been brutally battered by climate change over the past two decades,” said President Taneti Maamau.

“We all need to act now and fast but I stress it requires strong political will by us leaders … climate change is everyone’s business.”

Expectations for the talks are low: political unity built in Paris has been shattered by populist governments that have placed national agendas before collective action.

Last year, at talks in Germany, Chancellor Angela Merkel and French President Emmanuel Macron urged more global action to combat global warming after U.S. President Donald Trump decided to pull out of the Paris pact.

France was initially expected to be represented in Katowice by Prime Minister Edouard Philippe but he had to cancel after a weekend of violent protests in Paris.

Germany’s Merkel was busy meeting with Prime Minister Theresa May on Tuesday about Britain’s floundering Brexit deal.

Britain’s minister for business, energy and industrial strategy, Claire Perry, is expected to arrive in Katowice on Thursday.

“What matters most is the ministers who are here this week have the necessary political mandate to make the big decisions we need,” said Jens Mattias Clausen of Greenpeace International.

“Talks here in Katowice urgently need to speed up to achieve a strong rulebook and a commitment to raise ambition. The Polish presidency has a critical role in driving this process forward.”