xkcd.WTF!?

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Soil

You might want to bring your frost-sensitive plants in from the patio. The high-level aerosols may result in short-term cooling across the entire backyard.

Explanation

Beret Guy seems to be using volcano seeds to grow a tiny volcano to fertilize Cueball's garden. In reality, volcanoes are caused by the pressure of magma (underground lava) from below the Earth's crust pushing up through it, not seeds.[citation needed]

In the first panel, Beret Guy states that he made these seeds himself, presumably using his proven skill at exotic bioengineering. It is unclear whether his volcano is capable of self-propagating but, until his 'help' with maintaining the garden, there would probably be no particular risk of volcanic activity.

Volcanic soil is generally fertile and replenishes depleted elements in the soil, just like commercial fertilizers. However, the process of eruption is destructive to whatever plants are presently in the garden, as Cueball points out. Beret Guy's announcement in the final panel resembles the voice-over from a nature documentary.

The title text alludes to the fact that sulfate aerosols from volcanic eruptions may cause a short-term cooling effect because the aerosols in the atmosphere block out energy from the sun. This expands on the comedy from a small scale volcano by adding small scale volcano effects. Realistically a small supply of aerosols would be blown away and dispersed by wind. Not to mention the fact that, of course, the currently flowing lava is potentially harmful to all plants near the garden anyway.