A5 Extreme External Forcing Disturbances

Topic

Extreme external forcing perturbations are climate system anomalies originating from processes external to the climate system's own internal dynamics. The three primary examples of such perturbations are explosive volcanic eruptions, which inject sulfur dioxide into the stratosphere, forming sulfate aerosols with a residence time of one to three years; large asteroid or meteorite impacts, which deposit dust and soot into the atmosphere; and extreme variations in solar irradiance, such as the Maunder Minimum—a period between 1645 and 1715 marked by sustained low solar activity, associated with the Little Ice Age.

Of these three types of extreme external forcing, volcanic forcing is the best documented. A volcano like Pinatubo—which erupted in 1991—injects approximately twenty teragrams of sulfur dioxide into the stratosphere; this injected sulfur dioxide is converted into roughly thirty teragrams of sulfate aerosols (specifically, sulfuric acid aerosols) within the stratosphere. These sulfuric acid aerosols exert a negative forcing of three to four watts per square meter for one or two years, cooling the global climate system by three to five-tenths of a degree. The climate system's response to this negative forcing extends beyond direct cooling: the forcing also reduces global precipitation, alters stratospheric circulation, and warms the stratosphere itself, as the sulfate aerosols absorb infrared radiation. Once the negative forcing dissipates, the climate system's recovery follows an exponential curve determined by the residence time of sulfate aerosols in the stratosphere: the longer these aerosols remain aloft, the slower the climate system recovers to its pre-eruption temperature.

For explosive volcanic eruptions far larger than Pinatubo—such as the Toba-scale eruption seventy-four thousand years ago, which injected nearly three thousand teragrams of sulfur dioxide—or for large asteroid or meteorite impacts, the climate system's response is no longer proportional to the negative forcing in the way it was for a Pinatubo-like event. The negative forcing from such extreme external perturbations could trigger global cooling of five to fifteen degrees—enough to halt photosynthesis and cause an "impact winter." Unlike the moderate volcanic forcing described earlier, where the system responds and then recovers along a fixed exponential curve, negative forcing of this magnitude can cause the climate system to continue responding long after the initial forcing has ceased; this is because each brief perturbation leaves a cumulative, lingering effect rather than dissipating immediately.

Taken together, these three types of extreme external forcing—explosive volcanic eruptions, large asteroid or meteorite impacts, and extreme variations in solar irradiance—demonstrate that the climate system can undergo abrupt cooling driven by factors entirely external to its own internal dynamics. The magnitude of this cooling ranges from tenths of a degree lasting a year or two (as seen in moderate explosive eruptions) to drops of several degrees capable of halting photosynthesis (as seen in extreme explosive eruptions or large asteroid/meteorite impacts). Understanding these extreme external forcing perturbations makes it possible to anticipate both the magnitude and the duration of the climate system's response to a forcing that—unlike gradual warming caused by other factors—strikes abruptly and originates from outside the climate system itself.

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