Venus. The differences between the atmospheres of Earth and Venus that give Venus a "runaway" greenhouse effect are down to the amount of CO2, and the results are that Venus has clouds of sulphuric acid, insanely high atmospheric pressure, and is hot enough to melt lead.
On Earth, millions of years after this happened on Venus, we have seen a 40% rise in the CO2 level since the industrial revolution. Tyson directly addresses the opponents of climate change by asking the sort of questions they ask (most likely because they think their questions raise unanswerable objections to the scientific point of view), beginning with: Could climate change be down to volcanic eruptions instead of human actions?
Each year the volume of CO2 produced by all volcanic eruptions is equivalent to only 2% of that produced by humans. To give a sense of the staggering scale of the amount of Carbon Dioxide our civilisation creates each year, Tyson explains that it is equal in volume to the white cliffs of Dover, illustrated with an effect showing those cliffs rising out of the ground and doubling in height.
In reality, of course, CO2 is invisible, and Tyson suggests this may be part of the reason we find it so difficult to take seriously as a threat. This is followed by a neat effects sequence showing the CO2 being emitted by cars and planes as a sinister purple smoke.
Linking the scenes on Venus and Earth, Carl Sagan's PhD thesis was on the Venusian greenhouse effect, and in the original Cosmos series he spoke about the greenhouse effect on Earth, as things stood in 1980.
The differences between weather (with chaotic, short term patterns) and climate (showing a predictable pattern in the long run) are illustrated by Tyson taking a dog for a walk and looking at the dog's path - a fun way of making a serious point.
The animated sequences for this episode concern historical attempts to introduce alternatives to humans burning fossil fuels for energy, which go back further than you - or I, before I saw this programme - might have thought. At the great exhibition at Paris in 1878, Augustin Mouchot demonstrated solar energy, but at that time coal was just too cheap for anyone to be interested so it never caught on.
A similar fate befell Frank Shuman in 1913. He went out to Egypt and planned to irrigate the desert using solar power. The British and German governments were interested, but the arrival of cheap coal, and the First World War, got in the way.
We would only need to harness a fraction of the available solar and wind energy to power our civilisation, it seems disheartening that we have not yet made that transition. But, as with previous episodes, we end on a note of optimism and hope, with Tyson giving two examples of where humanity has successfully made difficult transitions before: the passing of the Cold War (which was, of course, still ongoing at the time of the first Cosmos series) and the ending of the threat of instant nuclear armageddon. And, much further back, the dawn of civilisation as humans made the change from nomadic to agricultural societies, made possible by climate change of different sort - a global warming coming out of an ice age.
This says that we can avert global warming on Earth and prevent it from becoming like Venus. The programme finishes with a few lines of John F Kennedy's inspirational, uplifting speech "We choose to go to the moon."
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