E10 External Forces to the System

Topic

External forcings are perturbations that act upon the climate system from outside its own coupled internal dynamics, rather than arising from interactions between the atmosphere, ocean, and cryosphere. There are four main external forcings: solar irradiance, which varies both over an eleven-year cycle and through slower, secular changes; Milankovitch orbital parameters, which modulate the distribution of insolation received at various latitudes on timescales ranging from ten thousand to one hundred thousand years; volcanic eruptions, which inject sulfur dioxide into the stratosphere—forming sulfate aerosols that exert a cooling effect; and anthropogenic forcing, which encompasses carbon dioxide, aerosols, land use, and ozone.

Isolating the signal of these external forcings from the climate system's internal variability is the central challenge in climate detection and attribution. The "fingerprinting" method searches climate system observations for the characteristic spatial pattern—or "fingerprint"—associated with each external forcing. It then weights this pattern against the expected internal variability for that specific region; consequently, a match between the characteristic pattern and observations in a region of low expected internal variability constitutes stronger evidence for that forcing than a similar match in a region of high expected internal variability.

Among the four external forcings, volcanic eruptions produce a transient forcing lasting between one and three years, and the climate system's response to this transient volcanic forcing decays exponentially once the forcing ceases. Because this transient volcanic forcing is brief and its magnitude is well-understood, the way the climate system responds to and subsequently recovers from it allows for the estimation of the real climate system's transient climate sensitivity, without the need to wait for the system to reach a new, full equilibrium. Forcing by Milankovitch orbital parameters—the other very long-timescale external forcing mentioned at the outset—explains the rhythm of the glacial and interglacial cycles the climate system has repeatedly undergone throughout its history. However, forcing by Milankovitch orbital parameters alone is too weak to account for the full amplitude of these observed glacial and interglacial cycles; the climate system must amplify this slight orbital forcing through additional feedbacks—specifically, carbon dioxide and ice feedbacks—so that the temperature range the system actually experiences between glacial and interglacial periods matches the range observed in the paleoclimate record.

The four external forcings described—solar irradiance, Milankovitch orbital parameters, volcanic eruptions, and anthropogenic forcing—operate on vastly different timescales, ranging from the eleven-year solar irradiance cycle and the one-to-three-year duration of a volcanic eruption to the tens of thousands of years associated with Milankovitch forcing. Yet, they all share a common trait that distinguishes them from the climate system's internal variability: they are perturbations originating outside the system's coupled internal dynamics. Consequently, they can be disentangled from internal variability using the previously described method of characteristic spatial fingerprints—a necessary step for determining how much of the observed climate system change is attributable to each of these four external forcings versus the system's own internal variability.

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