New study challenges previous assumptions about the surface-warming ‘hiatus’

Many climate scientists claim the ocean caused the 1998-2012 surface-warming 'hiatus' by drawing heat downward, away from the Earth's surface. In a new study, scientists from the Max Planck Institute for Meteorology (MPI-M) challenge this view: the heat could also have been radiated upward from the surface into space, and the uncertainty in observations means that we will probably never know what really caused the slowdown in surface warming.

The surface temperature of the Earth warmed more slowly over the period 1998-2012 than could be expected by examining either most model projections or the long-term warming trend. Climate scientists usually blame the surface-warming slowdown or 'hiatus' on the uptake of heat into the subsurface ocean, and they have found observational evidence to back their claim. The problem is, they have found the decisive heat uptake not once, but many times over. Each time, the smoking gun is in a different region: the Atlantic, the Pacific, the Indian Ocean, the Southern Ocean, or a combination of these.

'Not so fast', says Christopher Hedemann, lead author of the new study. He and his co-authors Thorsten Mauritsen, Johann Jungclaus and Jochem Marotzke find that the heat uptake in just one ocean region - and even in all the world's oceans put together - cannot explain a surface-warming hiatus.

The team at MPI-M wanted to explore what causes the surface temperature to vary against the backdrop of long-term global warming, sometimes warming faster than expected, sometimes slower. To extract these quasi-random variations ('internal variability') from the global warming signal, they used an ensemble of climate simulations that was unprecedented in size, requiring massive computing power. An opportunity had presented itself: the Swiss National Computing Centre (CSCS) at ETH Zurich offered use of their new supercomputer during its empty start-up phase.

In the large ensemble that was generated, the authors find around 360 events where the surface temperature behaves in a similar way to the 1998-2012 surface-warming hiatus, and they make some unexpected discoveries.

First, the heat needed to cause a hiatus in the ensemble is smaller than they thought. Previous studies had suggested deviations as large as 0.5 Wm-2 would be necessary, whereas the authors of the new study find that the value is closer to 0.1 Wm-2. They explain that the surface layer of the ocean, which regulates the decadal variability in surface temperature, is a lot thinner than assumed in previous hiatus studies. Thus, less variation in the surface's energy budget is needed to slow down surface warming.

Second, the authors find that a hiatus does not only occur because the surface loses heat to the ocean below. A slowdown can also be caused by a temporary increase in the energy radiating upward from the surface and into space. In most cases though, the authors require the full surface energy budget to explain a hiatus - the global ocean heat uptake and the total outgoing radiation. Studies that neglect to consider the full budget or that focus on individual ocean basins will likely misdiagnose the origin of a hiatus, finding evidence where there is none.

Hedemann and colleagues further compare the results in their study to different observations of the Earth's energy budget over the 2000s. Pioneering efforts to monitor the world's oceans with robotic buoys and to monitor radiation into space with improved satellites have been underway for a decade or so. But because the variation in energy needed to cause a hiatus is smaller than previously thought, determining the origin of such an event requires that the energy budget is known with pinpoint accuracy. The MPI-M team find that the observations currently available to the scientific community are just not up to the job. Therefore, the origin of the recent hiatus might never be identified.

Original publication:
Hedemann, C., T. Mauritsen, J. Jungclaus and J. Marotzke (2017) The subtle origins of surface warming hiatuses. Nature Climate Change, Opens external link in current windowdoi: 10.1038/NCLIMATE3274.


Christopher Hedemann
Max Planck Institute for Meteorology
Phone: +49 40 41173 193
Email: Opens window for sending emailchristopher.hedemann@we dont want

Dr Thorsten Mauritsen
Max Planck Institute for Meteorology
Phone: +49 40 41173 182
Email: Opens window for sending emailthorsten.mauritsen@we dont want

Dr Johann Jungclaus
Max Planck Institute for Meteorology
Phone: +49 40 41173 109
Email: Opens window for sending emailjohann.jungclaus@we dont want

Prof Dr Jochem Marotzke
Max Planck Institute for Meteorology
Phone: +49 40 41173 311 (Assistant Kornelia Müller)
Email: Opens window for sending emailjochem.marotzke@we dont want