Cities across the world went dark between 8.30-9.30pm on Saturday, marking Earth Hour.
Spearheaded by the World Wide Fund for Nature (WWF), the Earth Hour campaign calls for greater awareness and more sparing use of resources, especially fossil fuels that produce carbon gases and lead to global warming.
It was first launched 15 years ago by the organisation, with major landmarks in capital cities shutting off their lights, with the general population encouraged to do the same.
Since its beginning in Sydney in 2007, Earth Hour has spread to more than 180 countries, with tens of millions of people usually joining in.
"Whether it's the decline of pollinators, the decline of fish in oceans and rivers, the disappearance of forests or the wider loss of biodiversity, the evidence is mounting that nature is in freefall," said Marco Lambertini, director general of WWF.
"Protecting nature is our moral responsibility, but losing it also increases our vulnerability to pandemics, accelerates climate change and threatens our food security," he added.
Organisers of the event put together a list of ways people could take part from home, which - aside from the obvious option of turning off your lights - included a live Earth Hour streams on the internet, a board games night, or by signing a petition.
This year organisers said they wanted to highlight the link between the destruction of the natural world and the increasing incidence of diseases - such as COVID-19 - making the leap from animals to humans.
Landmarks that switched off their lights in 2021 included the Eiffel Tower in Paris, the Brandenburg Gate in Berlin, and the Colosseum in Rome.