"This is significant because climate models currently do not simulate that much warming of the global ocean during the last interglaciation," Jeremy Hoffman, formerly of Oregon State University, US, and now at the Science Museum of Virginia, told environmentalresearchweb. "This ultimately implies that important climate feedback mechanisms, or processes within the climate system that may have intensified the warming of the last interglaciation, may be missing in the current generation of climate models."
If the impacts of this feedback are underestimated by climate models for times in Earth's past, Hoffman says, what might they underestimate in projections of future climate under human influence?
"To better understand how our climate will continue to change in response to human activity, we need to maintain or increase our investment in understanding both how the Earth worked naturally and how it's working today," he said. "Investing in this understanding of the natural world will ultimately make our cities, towns, farms, main streets, schools, coastlines and infrastructure more resilient to future climate-related disruptions."
Hoffman and colleagues from the US and Ireland compiled publically available ocean-sediment core data collected over decades. They set up a near-global database of 104 sea-surface temperature records from 83 marine-sediment core sites, robustly assessing the uncertainties in the data.
"Combining techniques used in previous research with a new analysis of ocean-sediment core age ranges for this time period, we refined our understanding of how ocean temperatures changed at the global and regional scale during the last interglaciation," Hoffman said.
The global data showed that the ocean-surface temperature was already similar to the 1870–1889 mean at the onset of the interglacial period around 129 thousand years ago. The ocean warmed until 125 thousand years ago before cooling back to the 1870–1889 mean by 120 thousand years ago.
On a regional level, the picture was a little different. Tropical sea-surface temperatures exhibited a similar trend but remained slightly below the 1870–1889 mean, which is lower than in previous reconstructions. Outside the tropics, sea-surface temperatures behaved differently in the northern and southern hemispheres.
In the southern hemisphere, sea-surface temperatures were already around 1.1° greater than the 1870–1889 mean by 129 thousand years ago. They remained at this level until 120 thousand years ago, when cooling began. Northern hemisphere seas warmed by roughly 1.3° but not until around 125 thousand years ago. The difference between hemispheres was particularly pronounced for the Atlantic basin.
The relative warmth of the southern hemisphere during the early part of the last interglacial may be due to the bipolar seesaw mechanism, with freshwater from remnant ice sheets in the north disrupting the Atlantic meridional overturning circulation, keeping sea temperatures cold despite the additional solar radiation whilst at the same time reducing ocean heat transport and so warming the southern oceans.
According to Hoffman, the results further refine the uncertainty around global and regional sea-surface temperature estimates that are used as targets for comparison to climate modeling experiments. "For example, our results support the interpretation that near the beginning of the last interglaciation there were sea-surface temperature patterns in the North Atlantic ocean that identify the lingering influence of a collapsed Atlantic meridional overturning circulation," he added. "This hypothesis has previously been supported by climate modelling experiments and other regional data-compilation studies."
Now the researchers will continue to investigate "how information from natural thermometers and climate modelling experiments can be compared to help guide advancements in our understanding of global and regional climate change".
Hoffman and colleagues reported their findings in Science.