Meteorologist Peter Gibbs looks back on the impact of his old boss, the British Antarctic Survey scientist Joe Farman, who spotted the hole in the ozone layer using an antiquated piece of kit at the Halley research station in Antarctica.

Caroline Lucas, the only British Green MP, explores the impact of the radical German politician Petra Kelly, who founded Die Grünen and inspired a generation of European Greens.

Naturalist and TV presenter Gillian Burke assesses the role of her Kenyan 'auntie', Wangari Maathai, in drawing attention to environmental causes in Africa.

Corin Throsby considers the impact of Australian poet and activist Judith Wright, who campaigned to create the Great Barrier Reef Marine Park.

James Lovelock’s Gaia Theory, first put forward in the mid 1970s, was a ground-breaking hypothesis of how the earth works and one of the most influential ideas on 20th century environmentalism.

It proposed that the earth is one self-regulating system in which everything on the planet, living and non-living, interacts with each other to maintain the right conditions for life to exist. It’s quasi-spiritual sentiment captured the imagination of the New-Agers of the 1980s, sci-fi writers and philosophers, as well as a growing global environmental movement.

Lovelock’s idea has been a source of controversy within the scientific community. But many of his ideas about the impact of life, and humans in particular, on the environment have made their way into the scientific status quo.

Having recently celebrated his 100th birthday, the humble Lovelock continues to inspire. Environmental activist and filmmaker Jack Harries looks back on the career of this rare breed freelance scientist, and traces Gaia’s legacy across science and culture.

“As we discover more about humanities role in tipping the fragile balance of life on earth,” he says, “Lovelock’s Gaia theory becomes incredibly compelling.”

Emma Shortis reflects on the influence of David McTaggart, founder of Greenpeace International.

A pioneer of the aqua lung and nature documentaries, Jacques Cousteau’s groundbreaking series The Undersea World of Jacques Cousteau had an enormous impact on the 1970s, gripping an entire generation of children with its kaleidoscopic beauty. The stunning films of sea-life, coupled with Cousteau's natural history lectures in romantically accented English, are credited with spawning the environmental movement.

Cousteau spent more time filming underwater than probably anyone else and, as such, was alert to the devastating impact of over-fishing and pollution, particularly in the Mediterranean. Away from the camera, he lobbied tirelessly for tighter government regulations to protect the marine environment and biodiversity.

Champion free-diver Tanya Streeter reflects on the life and work of the explorer and film-maker turned oceanographer and considers the challenges that remain for the protection of our Oceans.

“Cousteau’s inventions opened up the underwater world to exploration,” she says. “He inspired us to see the planet in an entirely new way.”

n the early 1970s, village women in the foothills of the Indian Himalayas resisted deforestation by literally hugging the trees that loggers came to chop down. These original tree-huggers became known as the Chipko movement, from the Hindi word meaning “to embrace”.

At the heart of the movement was the Gandhi-inspired activist Sunderlal Bahuguna, who spread Chipko’s message of forest conservation by undertaking an almost 5,000km foot march across the Himalayas. In 1981, Bahuguna successfully persuaded India’s Prime Minister Indira Gandhi to ban the felling of all trees in the region above an altitude of 1,000 metres.

The Chipko movement inspired similar tree-hugging movements around the world, from Switzerland to the USA. In the 1990s, Bahuguna campaigned against the construction of India’s tallest dam in the state of Uttarakhand – this time without success.

The environmental activist Vandana Shiva, herself an early volunteer with the Chipko movement, assesses Bahuguna’s legacy. She considers what his campaign against Tehri Dam teaches about what happens when environmental activism fails.

“Bahuguna was a natural politician,” she says. “He pioneered the use of non-violent tactics – including marches, fasts and roadblocks – to draw attention to environmental issues.”

Margaret Thatcher might seem to some like an unlikely pioneer of the need for climate action but, in the late 1980s, she made a series of remarkable speeches and interventions on the subject and catapulted the issue to the foreground of media and public attention.

In 1988, at a Royal Society dinner, she gave a speech warning of the dangers of what was then known as the greenhouse effect, and the need for action. Tellingly, a key paragraph setting out practical suggestions for global action was struck out by the then Chancellor of the Exchequer, Nigel Lawson.

She encouraged discussion of the subject at Cabinet level, inviting leading climate scientists into Downing Street to educate her Ministers, and described the urge to protect the environment as a key plank of Tory philosophy.

In 1989, she addressed the UN General Assembly on the subject of climate change and called for immediate and urgent action to address it.

Alice Bell is co-director at climate change charity, Possible, and is writing a book about the history of climate change. She reflects on Margaret Thatcher’s brief and vigorous engagement with the question of climate change.

Contributors include Lord Debden, Sir Crispin Tickell, Professor Sir Brian Hoskins and Jonathan Porrit.

Ken Saro-Wiwa was a writer, television producer, human rights and environmental activist.

He was hanged by the Nigerian government in 1995 after a hearing John Major described as fraudulent.

He campaigned for the rights of the Ogoni people in the Niger Delta who found their land devastated by the pollution caused by oil extraction. They saw little financial benefit from the vast oil and gas resources and suffered from oil spills, gas flaring and water contamination.

Helon Habila is a Nigerian novelist and professor of creative writing at George Mason University in Virginia. His novel Oil on Water dealt with the human cost of the oil industry in Nigeria.

With his trademark hat and clear message about climate change, Dr James Hansen has been described as somewhere between an old testament prophet and Indiana Jones.

In 1988, when he was Director of the Goddard Institute for Space Studies at NASA, he gave evidence to Congress that changed the conversation about the greenhouse effect and climate change. Since then, he’s been a powerful advocate for the importance of listening to scientists on climate change and has himself become an activist.

"Einstein said to think and not act is a crime," Hansen said in 2011. "If we understand the situation, we must try to make it clear."

In this episode, the science educator, writer and broadcaster, Dr Emily Grossman explains why James Hansen is an inspiration for the scientists who are increasingly stepping out of their labs and onto the streets to protest about inaction from governments on climate change.

Reflections on the modern pioneers of the environmental movement. Today - Joni Mitchell.

When Chico Mendes was gunned down in the Amazon in December 1988, his assassin, a rancher named Darcy Alves, hoped to kill off his campaign to prevent forest clearance. Instead, it raised the alarm on the issue in Brazil and across the world, influencing a generation of conservationists and policy makers. Mendes is now a symbol of the global environmental movement in South America.

After starting to tap rubber as a child in Acre state in Brazil’s far west, Mendes co-founded the local branch of the rural workers union and organised demonstrations to stop thousands of square kilometres of rainforest being destroyed for timber and ranching, saving hundreds of families from destitution.

“He showed what we could do against the power of these people who had money, and gunmen, and the protection of the judiciary,” says Marina Silva, a former environment minister and presidential candidate.

The Brazilian singer Monica Vasconcelos reflects on the work of the union leader who only learned to read at 18 but went on to become a powerful advocate for forest people.

“Since Jair Bolsonaro became Brazil’s president at the start of last year, his government has weakened forest protections and encouraged land grabbers to move in. As we face an increase in violence towards local communities and deforestation in the Amazon, its worth looking back and remembering Chico’s extraordinary life,” she says.

Severn Cullis-Suzuki was twelve years old when she gave a speech demanding action on the environment at the Rio Earth Summit in 1992.

Like many young people who came after her, she demanded that adults listen and act swiftly to protect her future. She had grown up with a love of nature and was scared and angry about the extinction of animal species, pollution, and the destruction of forests.

In this programme, the naturalist and broadcaster Chris Packham reflects on the impact of her speech and the power of children’s voices in the climate debate.