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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.

Part 2 examines how the freewheeling modernism that had shocked audiences in the first two decades of the century came under state control. Initially, many practitioners thought the totalitarian regimes would be good for music and the arts. What followed in Germany was a ban on music written by Jews, African-Americans and communists, while in the Soviet Union there was a prohibition on music the workers were unable to hum. After the cataclysm of the 1940s, a new generation of composers - Boulez, Stockhausen, Xenakis, Nono, Ligeti - turned their back on what they saw as the discredited music of the past and tried to reinvent it from scratch. Or, at least, from serialism, which became as much of a straitjacket as totalitarianism's strictures had been. But from this period of avant-garde experimentation, which many listeners found baffling and terrifying, came some of the most influential and radical musical innovations of the century.

East Africa is a land which is constantly changing. To survive here, creatures must be able to deal with unpredictable twists and turns - wet turning to dry, feast to famine, cold to hot - no matter how hostile it becomes. From dense forests to snow capped peaks, steamy swamps and endless savannah, this unique and varied land is also a haven for life, supporting large animals in numbers found nowhere else on Earth. But away from the familiar, forever-travelling herds, there are a huge cast of other characters - lizards that steal flies from the faces of lions, vast dinosaur-like birds who stalk catfish through huge wetlands, and an eagle who risks everything on the arrival of ten million bats from a far off rainforest.

We can survive for weeks without food, but only days without water: it is the essential element of life. Yet many millions of us live in parched deserts around the world. In the second episode of Human Planet, we discover how the eternal quest for water brings huge challenges - and ingenious solutions - in the driest places on Earth. Battling through a sand storm in Mali, Mamadou must get his cows to a remote lake but desert elephants have arrived first. Can he find a safe way through the elephant blockade? Alone for weeks on end, Tubu women and children navigate the endless dunes of the Sahara. How does young Shede know where to find the last oasis, three days walk across the sea of sand? At the height of the drought we witness a spectacular frenzy: two thousand men rushing into Antogo Lake to catch the fish trapped by the evaporating water. When the rain finally arrives in the desert it's a time for flowering and jubilation - and love. The Wodaabe men of Niger put on make-up for an intoxicating courtship dance and beauty contest.

The Arctic is the harshest environment on Earth: little food grows, it's dark for months on end, and temperatures stay well below freezing for much of the year. Yet four million people manage to survive here. Human Planet tells remarkable stories of extraordinary people who make their homes in nature's deep freeze. In springtime, Amos and Karl-Frederik set out across the sea ice with their dogs to catch a real-life sea monster: a Greenland shark! Inuit mussel-gatherers venture underneath the sea ice at low tide for a perilous race against time as they gather their food. And the children of Churchill, Manitoba, set out on the most dangerous trick or treating Halloween in the world: they risk coming face-to-face with deadly polar bears on the streets of their town. Who'll get the tastiest snack?

Grasslands feed the world. Over thousands of years, we humans have learned to grow grains on the grasslands and domesticate the creatures that live there. Our success has propelled our population to almost seven billion people. But this episode reveals that, even today, life in the 'Garden of Eden' isn't always rosy. We walk with the Dorobo people of Kenya as they bravely attempt to scare off a pride of hungry lions from their freshly caught kill. We gallop across the Steppe with extraordinary Mongolian horsemen who were 'born in the saddle'. And in a perfect partnership with nature built up over generations Maasai children must literally talk to the birds! The honeyguide leads them to find sweet treats, but they'll have to repay the favour.