Nereus Alumni Thomas Fröelicher (ETH Zurich) has been invited to give a joint seminar at the Max Planck Institute for Meteorology in Hamburg, Hungary, on June 8th.
Heat and CO2 exchange between the atmosphere and the ocean is a major control on Earth’s climate. Since preindustrial times, the global ocean has taken up more than a quarter of the carbon emitted from human activities and more than 90% of the excess heat that has accumulated in the Earth System as a result of these emissions. Thus, the ocean greatly moderates the rate of climate change. I will show that the Southern Ocean is especially important, accounting for about half of the oceanic uptake of carbon and more than three quarters of its heat uptake. The Southern Ocean’s dominant influence on the global heat and carbon balance stems from the fact that it is the primary gateway through which cold, centuries old and nutrient rich deep waters interact with the atmosphere.
Redistribution of the ocean’s heat content generated by internal variability also has the potential to alter – for decades or so – the longer-term trends that are associated with anthropogenic-induced changes. An example of this is the slowdown of global warming from 1998 to 2014. So far, the importance of the Equatorial Pacific was highlighted, with periods of frequent and/or intense La Niña phases having been identified as periods of intensified ocean heat uptake, keeping the rate of global mean warming low. I will challenge this popular view of the Equatorial Pacific being the single most important region by examining changes in oceanic heat content in a 30-member ensemble simulation with a fully coupled carbon-climate Earth System model. I will show that Hiati periods may occur without negative sea surface temperature anomalies in the Equatorial Pacific indicating that the surface warming pattern may be very different during future Hiati decades.
During the last part of my talk, I will argue that the understanding of the geographic structure of ocean heat uptake is crucial to understand the global warming response to cumulative carbon emissions. I will show that ocean heat uptake, which occurs preferentially at subpolar latitudes, has a larger temperature impact per watt per square meters than the CO2 radiative forcing. In other words, the cooling effect of a high-latitude heat sink is larger than that of an equivalent tropical heat sink. As a result, surface temperature may actually increase on multi-century timescales after zero carbon emissions, despite of a decline in radiative forcing that exceeds the decline in ocean heat uptake – a circumstance that would otherwise be expected to lead to a decline in global temperature.