Title: Multicentury Warming Impacts on the Oceans and the Need to Shift the Focus of Climate Science
Abstract: Global warming is currently heating the surface ocean, increasing stratification and decreasing the nutrient flux from below. This is the key mechanism driving a projected decline in marine net primary production (NPP) of ~10% by the end of this century under high-end warming scenarios. However, the response of the oceans to warming on multicentury timescales is fundamentally different, as the heat slowly works its way into the ocean interior changing circulation and nutrient distributions. Multicentury warming leads to drastic environmental change and increasing production and export in the Southern Ocean around Antarctica. This traps nutrients locally that today flow northward in surface waters, eventually fueling much of the low latitude biological productivity. The nutrient trapping, in conjunction with the globally increasing stratification (both at the surface and at mid-depths) drives a net transfer of nutrients from the upper ocean to the deep ocean, initiating a multicentury, catastrophic decline in marine biological productivity. By year 2300, NPP and biological export outside the Southern Ocean, are down by 24% and 41% respectively, and the transfer of nutrients to the deep ocean is ongoing. Many of the most dire impacts of global warming will occur on multicentury timescales, including the crumbling of the great ice sheets and accompanying sea level rise, release of greenhouse gases like methane and CO2 from melting permafrost and ocean sediments, spurring even more warming, and threats to human food security in both the marine and terrestrial realms. Yet the coordinated experiments in support of the IPCC assessments still focus almost exclusively on what will happen out to the year 2100 (now mere decades away!). There may be unpleasant surprises and tipping points in the system that we are unaware of simply because we have not explored these timescales with our ESMs. The focus of climate science needs to shift to longer timescales to fully account for the impacts and costs of global warming, over the entire human-initiated climate perturbation.