Knowing the origin of the iconic phrase "London calling" helps situate the song in the "punk revolution" milieu.
"London calling" was a phrase commonly used by the BBC in the early years of radio. Transmissions would begin "This is BBC London calling listeners in North America," for example. And later, the BBC World Service named its program-listing magazine London Calling. Considering the punk movement of the 1970s as embedded in an anti-establishment, DIY scene, the co-optation of the radio call of the BBC (the UK's state-sponsored news source!) imbues the song with a sense of uprising: the BBC has been cored and The Clash has taken the reigns, guiding a new generation of punks as the apocalypse approaches.
Could this be a reference to global warming? In '70s popular culture?? WHAT!?
It definitely could be. If Joe Strummer's lyrics tell us anything about the man, it is that he was very well informed. He was talking about the depletion of oil 30 years before "peak oil" became a big issue (the oil shocks of the 1970s had a different cause). While global climate change has only recently become a burning issue in public debate, scientists have been studying the phenomenon for decades. As early as the 1950s, scientists were making projections about the future climate of the world. In the 1970s scientists began to believe that the climate was trending towards a hotter world. But it wasn't always like this. For decades earlier, most scientists believed that we were entering a miniature ice age! Here Strummer has the "ice age" coming, but also a zooming in sun, suggesting warmth? Which is it? Either way, the sense of impending, apocalyptic doom is strong.
The near-nuclear meltdown at Three Mile Island, Pennsylvania occurred earlier in 1979, just months before Strummer penned "London Calling."
The partial meltdown of the nuclear core reactor at Three Mile Island was the worst accident at a nuclear reactor the world had yet seen. (Another one, much worse, would come at Chernobyl in the Soviet Union a few years later.) Though the incident at Three Mile Island ended short of total catastrophe, it raised apocalyptic fears about the dangers of nuclear energy among many in the West. The dark mood of "London Calling" hinges on that "nuclear error," potentially the source of the end of the world.
London, a city built along the banks of the Thames River, is extremely susceptible to flooding.
With poor conditions and a high tide, it is possible for the Thames (which runs straight through central London) to flood and "drown" the city. This was especially apparent during the 1970s, because the government was organizing construction of the massive Thames Barrier to prevent just such an occurrence from ever taking place.