Question

Why did the Mt. Pinatubo eruption result in global temperatures dropping almost two degrees?

Answer 1

Short answer: the eruption injected huge amounts of sulfur gases into the stratosphere that formed a global layer of fine sulfate aerosols. Those aerosols reflected and scattered incoming sunlight back to space, so less solar energy reached the surface — producing a net cooling of the lower atmosphere for a year or two.

A bit more detail and context:
- SO2 injection and aerosol formation: Mt. Pinatubo (June 1991) put roughly 15–20 million tonnes (Tg) of SO2 into the stratosphere. Oxidation of that SO2 made sulfuric acid droplets (sulfate aerosols).
- Why the stratosphere matters: because the aerosols were in the stratosphere (above the weather), they were not quickly washed out by rain and persisted for 1–3 years, spread globally by winds.
- How they cool the surface: the sulfate aerosols increase Earth’s albedo — they scatter and reflect incoming shortwave (solar) radiation back to space and absorb some sunlight in the stratosphere — so less energy reaches the surface and troposphere, producing surface cooling.
- Size of the effect: the peak negative radiative forcing from Pinatubo was a few watts per square meter (a large instantaneous forcing), and the global-mean surface temperature fell by roughly 0.4–0.6 °C in the year or two after the eruption. Some regions and seasons experienced larger short-term drops (locally up to about 1–2 °C), which is probably why you’ve seen “almost two degrees” cited for particular places/times.
- Other consequences: the stratosphere actually warmed (because the aerosols absorbed sunlight there), while surface/tropospheric temperatures fell. The effect faded as the aerosols slowly settled out of the stratosphere.

So the mechanism is straightforward: sulfate aerosols from the eruption increased planetary reflectivity and reduced incoming solar energy, causing a temporary global cooling that lasted a couple of years.