R QUIRK claims that "Global warming is due to cyclical variations of the Earth's elliptical orbit" (Letters, February 20, 'How can Greta be so precise?').

Is this the variation in the obliquity (the tilt of the Earth on its axis - a 41,000-year cycle), the eccentricity (how close the orbit is to a circle - a 100,000-year cycle) or the precession (the time of year when the Earth is closest to the sun - a 23,000-year cycle)?

All three are collectively called the Milankovitch cycles and are described in IPCC [Intergovernmental Panel on Climate Change] reports. They do not explain the rapid rise in temperatures in recent years, which is due to the increasing levels of greenhouse gases in the atmosphere.

Vince Devlin