Environmentalism
By 2009, Klein's attention had turned to environmentalism, with particular focus on climate change, the subject of her book This Changes Everything (2014). According to her website in 2016, the book and its accompanying film (released in 2015) would be about "how the climate crisis can spur economic and political transformation." She served on the board of directors of the non-profit group 350.org from 2011, through the fiscal year ending September 2018, and took part in their "Do the Math" tour in 2013, encouraging a divestment movement.
In an interview by Graeme Greene in New Internationalist, Klein rejected criticism that This Changes Everything politicized the climate issue and that the issue should be apolitical, asserting that such criticism reflected "how blind so many within the mainstream climate discussion are to the fact that they themselves are fully immersed within the confines of neoliberalism; ... It's a fantasy that you could fundamentally shift the building blocks of your economy without engaging with politics."
She encouraged the Occupy movement to join forces with the environmental movement, saying the financial crisis and the climate crisis are similarly rooted in unrestrained corporate greed. She gave a speech at Occupy Wall Street where she described the world as "upside down", where people act as if "there is no end to what is actually finite—fossil fuels and the atmospheric space to absorb their emissions", and as if there are "limits to what is actually bountiful—the financial resources to build the kind of society we need."
She has been a particularly vocal critic of the Athabasca oil sands development in Alberta, describing it in a TED Talk as a form of "terrestrial skinning." On September 2, 2011, she attended the demonstration against the Keystone Pipeline outside the White House and was arrested. Klein celebrated Obama's decision to postpone a decision on the Keystone pipeline until 2013 pending an environmental review as a victory for the environmental movement.
She attended the 2009 United Nations Climate Change Conference in Copenhagen. She put the blame for the failure of Copenhagen on President Barack Obama, and described her own country, Canada, as a "climate criminal". She presented the Angry Mermaid Award (a satirical award designed to recognize the corporations who have best sabotaged the climate negotiations) to Monsanto. Writing in the wake of Hurricane Sandy, she warned that the climate crisis constitutes a massive opportunity for disaster capitalists and corporations seeking to profit from crisis. But equally, the climate crisis "can be a historic moment to usher in the next great wave of progressive change", or a "People's Shock".
In 2016, following the election of Donald Trump as the 45th president of the United States, Klein called for an international campaign to impose economic sanctions on the United States if his administration refused to abide by the terms of the Paris Agreement. In October 2022, Klein published an article on The Intercept that addressed the 2022 United Nations Climate Change Conference and the repression of the Egyptian government; the conference took place in Egypt, a country widely seen as repressive and autocratic. She goes on to state "Sisi's Egypt is making a big show of solar panels and biodegradable straws ... but in reality, the regime imprisons activists and bans research. The climate movement should not play along", calling it greenwashing. In an interview with Democracy Now!, she says "what is not welcome would be pointing out this enormous lucrative network of deals that the military itself is engaged in that are linked to fossil fuels, that are linked to destroying remaining green space in cities like Cairo". Klein also stressed the release of prominent political prisoner and activist Alaa Abd El-Fattah, and wrote a foreword to You Have Not Yet Been Defeated (2021), his collected writings translated by an anonymous collective.