Global warming, also called climate change, was environmental damage that resulted in planets becoming hotter.
On Earth[]
Earth was a notable victim of global warming.
During his Exile on Earth, the Third Doctor attempted to warn Alistair Gordon Lethbridge-Stewart of the global warming Earth would experience in the 21st century but his warnings fell on deaf ears. (AUDIO: Terror of the Master)
According to Rose Tyler, global warming was partly caused by cows farting methane into the atmosphere. In her searching the multiverse for N-Space, Rose found a parallel Earth that had fallen victim to global warming, suffering a continuous rain. To try and combat this, the people of this Earth banned further technological innovation in an attempt to reduce greenhouse gases. These methods had no effect with it being suspected that the planet's state was actually the result of alien activity. (AUDIO: The Flood)
The process of global warming was accelerated by a group of Zygons who sought to transform the Earth's atmosphere, and make it more amenable to Zygon life. They invested in industrial projects around the world, with the express goal of damaging the ozone layer, encouraging the introduction of modified CFCs into the atmosphere. They released products, like hairsprays and gassed-up CDs, that would allow humans to poison the world for them. (AUDIO: The Zygon Who Fell to Earth) The Sea Devils who ran Ravenscaur School had a similar agenda, aiming to restore Earth to what it had been during their reign. (COMIC: Clara Oswald and the School of Death)
Sam Jones, who was from the 1990s, thought of global warming as things that were regular features of her world but not Fitz Kreiner's, who was from the 1960s. (PROSE: The Taint)
Compared to the 21st century, Britain was on average three degrees colder in the Middle Ages. (TV: The Last Oak Tree)
Upon taking Rose Tyler from 2005 to see the Earth's destruction in the year 5,000,000,000, the Ninth Doctor mentioned global warming as one of the things humanity thought would cause their extinction. (TV: The End of the World)
In 2007, Professor Mallingan and his team of scientists began draining the power of the icy planet of Isqar hoping that it would put an end to global warming on Earth. The Tenth Doctor came across Mallingan's plans and enlisted the help of one of his scientists to destroy his lab, saving Isqar. (COMIC: Cold War)
Climate change was a great worry around this time. (TV: The Lost Boy) Santiago Jones' father, an environmental activist, was arrested twice at a climate change conference. (TV: Death of the Doctor)
Global warming existed on Pete's World on 2010, which was exacerbated by the Cybermen coming through the breach leading to N-Space over the course of three years. Pete Tyler noted that the temperature increasing by two degrees over 6 months and the melting of the ice caps wasn't caused "just [by] global warming", but by the breach. (TV: Doomsday)
In 2029, Constance Clarke was appalled that good men and women died in World War II just for their heirs to ruin the planet. (AUDIO: Shield of the Jötunn)
When she visited the time of the Second Great and Bountiful Human Empire with the Tenth Doctor, 21st century native Donna Noble was fascinated that Earth was still populated, despite all the news articles stating the near end of the world, including global warming and the disappearance of bees. (TV: Planet of the Ood)
Agriculture was harmed by severe climate change, starting in 2040 with horrific storm conditions. New ways of growing food were developed. (TV: The Waters of Mars)
Sometime before the 2050, a group of scientists created a device to cool the Earth, but it was misused, (TV: The Korven) resulting in the Great Cataclysm. (TV: Aeolian)
Rising sea levels caused by global warming flooded the Netherlands, forcing the Dutch people to become refugees. Later in the century, they founded a new homeland on an island in the Atlantic Ocean, which they named the New Dutch Republic. (PROSE: St Anthony's Fire)
As a stop-gap solution to global warming, a device called the Great Solar Shield was designed in the 21st century to stall the effects of global warming by partially blocking off the sun. When the Shield broke apart because of the Tenth Doctor's efforts to stop the Silhouettes from attacking Earth, the Doctor told Martha Jones that humanity would come up with a more permanent solution a few years afterwards. (COMIC: Sunscreen)
By 2099, the governments of Earth, in answer to global warming, had taken vast sections of the Arctic and Antarctic and placed them inside huge domes called Snowglobes. (PROSE: Snowglobe 7)
In the 23rd century, three-quarters of the planet's landmass was drowned by water. Humans built islands and colonised parts of the oceans for research and other uses. London was totally flooded. (GAME: TARDIS) Venice prepared to finally submerge into the waters of the Laguna. (AUDIO: The Stones of Venice)
By the 52nd century, winters never occurred due to global warming. (PROSE: The Frozen)
In one possible timeline, global warming and nuclear warfare had turned Earth into a wasteland inhabited by Dregs, a mutated form of humanity, and the planet was known by the name Orphan 55. (TV: Orphan 55)
On other planets[]
Global warming on Flydon Maxima led to an increase in water levels, rising above the previous 88% water surface coverage. This led to the renaming of the planet as "Despair" and an evacuation, mainly to Flydon Minima. Others adapted to living in floating cities, surviving on fish, seaweed and rainwater. (PROSE: The Depths of Despair)
New Oceana had been entirely flooded due to catastropic global warming, wiping out the colonists who had lived there. When the Twelfth Doctor visited the planet he met a nomadic amphibian race who had settled under the water, and claimed that the entire planet had been recycled. (COMIC: Beneath the Waves)