The U.N. warned last year that we barely have a decade to avoid the worst consequences of climate change, an emergency due in no small part to major fossil fuel companies. Dutch activist Mark van Baal wanted oil giants to change their policies, so he bought shares in one of the biggest oil companies in the world, Shell, and became a so-called activist investor, pushing for change from the inside.

In bombshell testimony today on Capitol Hill, key impeachment inquiry witness Ambassador Gordon Sondland made it crystal clear he was working at the direction of President Trump as he tried to get Ukraine to investigate the Bidens and the 2016 election. To analyze, Christiane speaks with former White House counsel Bob Bauer, and Evelyn Farkas, the former Russia expert at the Department of Defense.

Imelda Marcos, the former first lady of the Philippines, is known for the ruthless dictatorship she and her late husband headed for two decades, massively enriching themselves in the process. Director Lauren Greenfield and former investigator of the Marcos family, Andy Bautista, join Alison to discuss "The Kingmaker," a new documentary about the larger-than-life Imelda.

Today, impeachment hearings in Washington entered a critical stage. Eight key witnesses from the Trump administration are defying White House orders and taking the stand this week, with White House Ukraine expert Lt. Col. Vindman and Vice President Pence's national security aide Jennifer Williams kicking off proceedings. Christiane discusses Tuesday's events with Chris Ruddy and Susan Glasser.

Daria Kaleniuk is one of Ukraine's most respected independent corruption fighters, having spent years taking on officials seeking to abuse their authority. She joins the program from Ukrainian capital Kyiv to discuss corruption, the impeachment inquiry and Ukraine-Russia relations.

Professor S. James Gates has spent his career pioneering modern physics and diversity in science. His most recent book, written with novelist Cathie Pelletier, is about how scientists proved Einstein's groundbreaking theory of relativity all those years ago. Walter sits down with Gates to find out just how they did this, and how his own upbringing played such a pivotal role in his success.

Never in 50 years has Venice seen a catastrophe like this: historic storm waters have deluged the historic site three times in a single week. Six foot tides are engulfing 85% of Venice's streets and buildings, including the iconic St. Mark's Basilica. Henk Ovink is a world renowned expert on flooding, as a special water envoy to the United Nations, and joins the program to discuss this crisis.

As a climate scientist, educator, and lead author for U.S. national climate assessments, Katharine Hayhoe is devoted to exposing diverse—sometimes suspicious—audiences to the cataclysmic events happening right before our eyes as a result of climate change. She joins the program to explain how to convince people and what changes are needed.

Extreme weather events are galvanizing a new climate conscious generation of consumers who are prepared to commit to sustainable lifestyles even if it means paying higher prices to do so. Allbirds, the company known for its seductively comfy wool shoes, helped drive this growing demand. The company's co-CEO Joey Zwillinger joins the program to discuss eco-friendly business practices.

Historian and journalist Rick Perlstein has been called the “chronicler extraordinaire of American conservativism,” and he has studied what the Nixon and Clinton scandals can tell us about today’s hearings. Perlstein sits down with Alison Stewart to provide a historical lens and some much-needed perspective to the ongoing impeachment hearings.

It took the buy-in of Democratic representatives from swing districts to prompt Speaker Pelosi to begin the impeachment investigation. One of those was Democratic Congresswoman Elissa Slotkin, who has faced challenges convincing her constituents of the need to consider impeachment. She joins the program from Michigan to assess how the week’s hearings have gone down in her part of the country.

Today, Marie Yovanovitch testified publicly in the impeachment inquiry against President Trump. James Melville, a career diplomat who knows her personally, can understand more than most what she has experienced – he himself quit as U.S. Ambassador to Estonia last year in protest over President Trump’s comments about European allies. He joins the program to assess Yovanovitch’s testimony.

Marie Yovanovitch describes vital U.S. interests as trying to help set up a stable, free and democratic Ukraine that could resist pressure from Russia. New Yorker columnist Masha Gessen has deep insight into how authoritarian leaders like Vladimir Putin function, and says she is “shocked but not surprised” by what we are learning in this impeachment inquiry. She joins the program from New York.

When Adam Frankel was 25 years old, he discovered something shocking: he wasn’t his father’s biological son, a secret his mother had kept from him his whole life. At the time, he was making waves as a talented speechwriter for President Obama. His debut book “The Survivors” captures his experience, and he sits down with Walter to discuss his powerful story.

Washington is gearing up for a second day of public impeachment hearings on Friday, when Marie Yovanovitch will testify. Nancy McEldowney was a colleague of Yovanovitch’s for many years, having served in the foreign service under both Republican and Democratic administrations; she joins the program with former FBI general counsel James Baker, who oversaw the launch of the Mueller investigation.

A new film starring Mark Ruffalo is a true story about cover-ups and conspiracies at the highest levels of the corporate world. “Dark Waters” follows Rob Bilott, played by Ruffalo, who took on the chemical company DuPont after one of its West Virginia plants leaked the chemical PFOA into the water supply. Ruffalo and Bilott join Christiane to discuss this true and timely investigation.

At the heart of the impeachment inquiry is Ukraine, and today’s testimony brought into sharp relief the challenges that country faces both militarily and over corruption. To understand what’s at stake for Ukraine and why it is vital to US foreign policy interests, Anthony Gardner joins the program in London.

Today, Congress began hearing public testimony in the impeachment inquiry into President Trump. To digest this first day of public impeachment testimony, Christiane speaks with former representatives from both sides of the aisle: former chair of the House Intelligence Committee Mike Rogers and former member of the House Judiciary Committee Elizabeth Holtzman.

In today’s hearings, there was talk of President Trump trying to publicly box in President Zelensky, involving a planned TV interview where the Ukrainian leader was to publicly announce investigations into the Bidens and alleged Ukrainian meddling in 2016. To provide perspective on these tactics, Christiane speaks with veteran journalist Jeff Greenfield and legendary reporter Carl Bernstein.

Garry Kasparov became the youngest world chess champion in history at the age of 22, while his famous matches against the IBM supercomputer Deep Blue brought chess and artificial intelligence into the mainstream. Now, he’s focusing on the quiet war Russia is waging against U.S. democracy. Kasparov sits down with Miles O’Brien to discuss everything from troll farms to election interference.