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THE EVOLUTION OF MARINE HEATWAVES UNDER GLOBAL WARMING.

The ocean stores more than 90% of the extra heat that has accumulated in the Earth system as a result of the increase in greenhouse gas concentrations. Superimposed onto this long-term ocean warming trend are short-term extremes in ocean surface temperatures, so-called marine heatwaves (MHWs), during which ocean temperatures are anomalously high for periods of days to months. MHWs have occurred in all ocean basins over the last few decades, with some of them having had detrimental impacts on marine ecosystems and fisheries and cascading impacts on socioeconomic systems. Each individual MHW has its own constellation of triggers. But as the oceans are taking up more heat, MHWs are expected to become more frequent and more intense.

Using satellite observations, we show that the number of MHW days anywhere in the surface ocean has doubled over the 1982-2016 period. Over these 35 years, the average maximal intensity of an MHW has increased by 0.15[degrees]C and the spatial extent by 66%. Under the assumption that current Earth system models simulate MHWs with fidelity, we show that 87% of the currently occurring MHWs are attributable to human-induced warming. If the warming continues on its current trajectory, Earth system models project that MHWs will become more than 40 times more frequent by the end of this century. In other words, a one-in-a-hundred days event at preindustrial levels is projected to become a one-in-three-days event in 2100. MHWs will also become longer lasting and spatially more extensive. A MHW in this future hot world would typically last 112 days, with maximum temperatures 2.1[degrees]C above the intensity of an MHW today. The spatial extent would increase to 94.5 x [10.sup.5] [km.sup.2]--equivalent to the total area of China. As a comparison, at preindustrial times, an MHW lasted 11 days and had a spatial extent of 4.2 x [10.sup.5] [km.sup.2], the area of Switzerland. All ocean regions will experience an increase in MHW days. Hot spots of large increases in MHW days will be the tropical Western Pacific and Arctic oceans.

In addition to MHWs, extreme events characterized by very low oxygen or high [H.sup.+] conditions can put additional stress on marine organisms and ecosystems. Of particular concern are compound events, which correspond to events with multiple concurrent or consecutive ecosystem stressors resulting in extreme consequences for marine ecosystems. Therefore, a better understanding of the drivers and consequences of such compound events is needed to assess the full risk for marine ecosystems under global warming.--THOMAS L. FROLICHER (UNIVERSITY OF BERN, SWITZERLAND), S. STRIEGEL, E. FISCHER, N. GRUBER, C. LAUFKOTTER, "The evolution of marine heatwaves under global warming: Shifting mean versus changing variability," presented at the 32nd Conference on Climate Variability and Change, 6-10 January 2019, Phoenix, Arizona.

Caption: Longer-lasting Marine Heatwaves. Simulated multimodel mean duration of MHWs under (left) 2[degrees]C global warming and (right) 3.5[degrees]C global warming.

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Title Annotation:CONFERENCE NOTEBOOK
Author:Frolicher, Thomas L.; Striegel, S.; Fischer, E.; Gruber, N.; Laufkotter, C.
Publication:Bulletin of the American Meteorological Society
Date:Mar 1, 2019
Words:494
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