The climatic effects of the direct injection of water vapour into the stratosphere by large volcanic eruptions
We describe a novel mechanism that can signifi- cantly lower the amplitude of the climatic response to cer- tain large volcanic eruptions and examine its impact with a coupled ocean-atmosphere climate model. If sufficiently large amounts of water vapour enter the stratosphere, a cli- matically significant amount of water vapour can be left over in the lower stratosphere after the eruption, even after sul- phate aerosol formation. This excess stratospheric humidity warms the tropospheric climate, and acts to balance the cli- matic cooling induced by the volcanic aerosol, especially be- cause the humidity anomaly lasts for a period that is longer than the residence time of aerosol in the stratosphere. In par- ticular, northern hemisphere high latitude cooling is reduced in magnitude. We discuss this mechanism in the context of the discrepancy between the observed and modelled cooling following the Krakatau eruption in 1883. We hypothesize that moist coignimbrite plumes caused by pyroclastic flows travelling over ocean rather than land, resulting from an erup- tion close enough to the ocean, might provide the additional source of stratospheric water vapour.