The Taiga forest, on the edge of the Arctic, is a silent world of stunted conifers. The trees may be small but filming from the air reveals its true scale. A third of all trees on Earth grow here and during the short summer they produce enough oxygen to change the atmosphere. In California General Sherman, a giant sequoia, is the largest living thing on the planet, ten times the size of a blue whale. The oldest organisms alive are bristlecone pines. At more than 4,000 years old they pre-date the pyramids. But the baobab forests of Madagascar are perhaps the strangest of all.

This episode explores the nature of the greenhouse effect (discovered by Joseph Fourier and Svante Arrhenius), and the evidence demonstrating the existence of global warming from humanity's influence. Tyson begins by describing the long-term history of the planet Venus; based on readings from the Venera series of probes to the planet, the planet had once had an ocean and an atmosphere, but due to the release of carbon dioxide from volcanic eruptions, the runaway greenhouse effect on Venus caused the surface temperatures to increase and boiled away the oceans. Tyson then notes the delicate nature of the amount of carbon dioxide in the atmosphere can influence Earth's climate due to the greenhouse effect, and that levels of carbon dioxide have been increasing since the start of the 20th century. Evidence has shown this to be from mankind's consumption of oil, coal, and gas instead of from volcanic eruptions due to the isotopic signature of the carbon dioxide. The increase in carbon dioxide has led to an increase in temperatures, in turn leading to positive feedback loops of the melting polar ice caps and dethawing of the permafrost to increase carbon dioxide levels. Tyson then notes that humans have discovered means of harvesting solar power, such as Augustin Mouchot's solar-driven motor in the 19th century, and Frank Shuman's solar-based steam generator in the 1910's. Tyson points out that in both cases, the economics and ease of using cheap coal and oil caused these inventions to be overlooked at the time. Today, solar and wind-power systems would be able to collect enough solar energy from the sun easily. Tyson then compares the motivation for switching to these cleaner forms of energy to the efforts of the Space race and emphasizes that it is not too late for humanity to correct its course.

An immersive cinematic experience of nonspeaking autistic people across the world, The Reason I Jump is based on a book written by Naoki Higashida when he was just 13. The film follows a young Japanese boy on a journey through an epic landscape. As a maelstrom of thoughts, feelings, impulses, and memories affects his every action, he gradually discovers what his autism means to him, how his perception of the world differs from others’, and why he acts the way he does—the reason he jumps.

Morgan Freeman narrates Island of Lemurs: Madagascar, the incredible true story of nature's greatest explorers - lemurs. Scientist Patricia Wright has a passion for preserving the life of one particular creature: the lemur. These lively animals found in Madagascar face all sorts of threats, just like so many other animals in a world in which they are increasingly crowded out by humans. Her passion for the subject is compelling and plainly evident, but it's the silent testimony of the animals themselves that really makes the case. Countless hours of footage are trimmed down to create this tapestry depicting the lives of these creatures of remarkable intelligence and ingenuity. Their daily routines and extraordinary adventures are awesome to behold, as is the way that the community functions. A full immersion into the world of the lemurs means great insights on these unusual animals.

Sir David Attenborough journeys to both Polar Regions to investigate what rising temperatures will mean for the people and wildlife that live there and for the rest of the planet. David starts out at the North Pole, standing on sea ice several metres thick, but which scientists predict could be Open Ocean within the next few decades. The Arctic has been warming at twice the global average, so David heads out with a Norwegian team to see what this means for polar bears. He comes face-to-face with a tranquilised female, and discovers that mothers and cubs are going hungry as the sea ice on which they hunt disappears. In Canada, Inuit hunters have seen with their own eyes what scientists have seen from space; the Arctic Ocean has lost 30% of its summer ice cover over the last 30 years. For some, the melting sea ice will allow access to trillions of dollars worth of oil, gas and minerals. For the rest of us, it means the planet will get warmer, as sea ice is important to reflect back the sun's energy. Next David travels to see what's happening to the ice on land: in Greenland, we follow intrepid ice scientists as they study giant waterfalls of meltwater, which are accelerating iceberg calving events, and ultimately leading to a rise in global sea level. Temperatures have also risen in the Antarctic - David returns to glaciers photographed by the Shackleton expedition and reveals a dramatic retreat over the past century. It's not just the ice that is changing - ice-loving adelie penguins are disappearing, and more temperate gentoo penguins are moving in. Finally, we see the first ever images of the largest recent natural event on our planet - the break up of the Wilkins Ice Shelf, an ice sheet the size of Jamaica, which shattered into hundreds of icebergs in 2009.

At 4.6 billion years old, the Solar System is our solid, secure home in the Universe. But how did it come to be? In this episode we trace the system's birth from a thin cloud of dust and gas. Shocked by a nearby supernova, the pull of gravity and natural rotation spun it into a flat disc from which the Sun and planets coalesced. It all happened in the space of 700 million years, during which the planets jockeyed for position, dodging the Late Heavy Bombardment of deadly asteroids and setting into the neat, stable system that we now realize might be a rarity in the universe.