The recent normalization of historical marine heat extremes

Abstract

Climate change exposes marine ecosystems to extreme conditions with increasing frequency. Capitalizing on the global reconstruction of sea surface temperature (SST) records from 1870-present, we present a centennial-scale index of extreme marine heat within a coherent and comparable statistical framework. A spatially (1° × 1°) and temporally (monthly) resolved index of the normalized historical extreme marine heat events was expressed as a fraction of a year that exceeds a locally determined, monthly varying 98th percentile of SST gradients derived from the first 50 years of climatological records (1870–1919). For the year 2019, our index reports that 57% of the global ocean surface recorded extreme heat, which was comparatively rare (approximately 2%) during the period of the second industrial revolution. Significant increases in the extent of extreme marine events over the past century resulted in many local climates to have shifted out of their historical SST bounds across many economically and ecologically important marine regions. For the global ocean, 2014 was the first year to exceed the 50% threshold of extreme heat thereby becoming “normal”, with the South Atlantic (1998) and Indian (2007) basins crossing this barrier earlier. By focusing on heat extremes, we provide an alternative framework that may help better contextualize the dramatic changes currently occurring in marine systems.

The full paper is on POS Climate; "The recent normalization of historical marine heat extremes."

In short, this means the imminent bleaching of all coral worldwide with the resultant loss of habitat and species diversity. By imminent I mean that the science indicates that when global climate warming reaches 1.5C coral bleaching will occur worldwide. I believe we are at around 1.2C already, and well on the way to 1.5C.

Note the May 2021 assessment from the World Meterological Organization and the UK Met Office;

There is about a 40% chance of the annual average global temperature temporarily reaching 1.5°C above the pre-industrial level in at least one of the next five years.

While the above isn't the permanent shift to +1.5C discussed by Van Houtan and Kisei Tanaka it is apparent that the upper thermal tolerance for many marine species will be exceeded soon. And a permanent shift in temperature envisaged in the paper will follow.

In the course of the next two or three decades the 1.5C global mean temperature rise is likely, an existential event that threatens not just marine life but also the global population of more than one billion people who depend on the sea for their survival.

Figure 1a. Earth’s global average surface temperature has risen as shown in this plot of combined land and ocean measurements from 1850 to 2019, derived from three independent analyses of the available data sets. The temperature changes are relative to the global average surface temperature of 1961−1990. Source: NOAA Climate.gov; data from UK Met Office Hadley Centre (maroon), US National Aeronautics and Space Administration Goddard Institute for Space Studies (red), and US National Oceanic and Atmospheric Administration National Centers for Environmental Information (orange).
Figure 1.. Earth’s global average surface temperature has risen as shown in this plot of combined land and ocean measurements from 1850 to 2019, derived from three independent analyses of the available data sets. The temperature changes are relative to the global average surface temperature of 1961−1990. Source: NOAA Climate.gov; data from UK Met Office Hadley Centre (maroon), US National Aeronautics and Space Administration Goddard Institute for Space Studies (red), and US National Oceanic and Atmospheric Administration National Centers for Environmental Information (orange).