Greta Thunberg told activists at UN climate talks in Madrid that the global school strikes have "achieved nothing".

Thunberg pointed out that despite many thousands of protests from schoolchildren across the globe, greenhouse gas emissions have continued to rise.

In the four years since the signing of the Paris accord, greenhouse gas emissions have increased by 4 per cent.

The young activist says the global strikes, sparked by her solo protest in 2017, have "not translated into action" by governments.

"I'm just an activist and we need more activists," she said. "Some people are afraid to change – they try so desperately to silence us."

She told protesters at the UN climate talks in Madrid that she doubts governments got the message and warned that the world cannot afford more inaction.

"I sincerely hope COP25 will reach something concrete and increase awareness among people, and that world leaders and people in power grasp the urgency of the climate crisis, because right now it does not seem that they are," she said.

"We can't go on like this; it is not sustainable that children skip school and we don't want to continue – we would love some action from the people in power. People are suffering and dying today. We can't wait any longer," she said.

Thunberg wasn't the only climate activist speaking in Madrid for the UN climate conference.

Brianna Fruean, from Samoa, speaking for the Pacific Climate Warriors, told the conference that "world leaders need to know that people like me are watching them".

"The text we put down today on paper at COP is what our future will look like."

"I've had typhoid. I've had malaria. My grandmother died from cholera. I know what I'm talking about," said Jimmy Fénelon, the national coordinator of the Caribbean Youth Environmental Network in Haiti.

"We need to raise awareness among young people. We can get them to work together and send a strong message."

Thunberg arrived in Madrid on Friday to join thousands of other young people in a march to demand world leaders take real action against climate change.