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A limited role for unforced internal variability in 20th century warming

New Research published in the Journal of Climate sheds new light on the long-standing question of how much of the observed changes in Earth’s temperature are due to natural ocean cycles. The short answer is, very little.

While the scientific community overwhelmingly agrees that human activities are responsible for the observed increase in temperatures for the last half-century, the relative influences of natural drivers of climate change like volcanic eruptions, ocean cycles, and the sun on warmer and cooler phases superimposed on the long-term warming trend is still an area of active research. The new study, led by Oxford University’s Karsten Haustein and colleagues from around the world, concludes that so-called internal variability due to slow-acting ocean cycles is not necessary to explain the changes in the historical temperature record.

Rather, the team concludes that human factors like greenhouse gas emissions and particulate pollution, volcanic eruptions, and changes in solar activity (collectively known as external forcings), along with year-to-year ups and downs related to the El Niño-Southern Oscillation phenomenon, are sufficient to explain virtually all of the long-term change in the temperature record. In the course of this work the team also found that the near-term sensitivity of the Earth’s climate to influences like greenhouse gas emissions is consistent with previous estimates from the Intergovernmental Panel on Climate Change, and have provided evidence for unresolved biases both in parts of the ocean temperature record and the procedure used to compare climate models to the instrumental record.

The team also provides compelling evidence that the warming during so-called Early Warming period between 1915 and 1945 might in fact be caused entirely by external factors. Currently, half of the observed warming during that time is attributed to internal ocean variability, which is a key reason why the estimate of the human-induced warming fraction has been very uncertain in the past. Together with a revised index to describe North Atlantic Ocean variability more realistically, this study helps to resolve some of the most puzzling questions in climate science until now.

In detail

A new study published online in the Journal of Climate is providing strong evidence that virtually all of the observed changes in global mean temperatures over the last 170 are caused by external drivers, leaving only little room for an (unforced) internal ocean contribution. The team of researchers found that alleged ocean cycles on timescales of 60-70 years are unlikely to be a factor in the observed evolution of the Global Mean Surface Temperature (GMST) since 1850. Instead, external factors such as periods of strong volcanic activity and the release of anthropogenic aerosol particles (air pollution) have caused the temperature (including the ocean surface temperature) to fluctuate. While not an entirely novel suggestion (e.g. Mann et al. 2014), the new study provides strong evidence that virtually all of the observed variability is externally forced.

Most prominently, the so-called Early Warming period between 1915 and 1945 can now be explained by external influences. Currently, only half of the observed warming during this period was attributable to external factors, with the remaining half attributed to unforced natural variability (e.g. Hegerl et al. 2018). Since this very period fits right into the traditional concept of the existence of a quasi-oscillatory 60-70 year ocean cycle (with colder periods during 1880-1910 and 1950-1980, followed by warmer periods in between), the new findings robustly challenge this prevailing view. This is particularly true as the study’s results reveal that the conclusions hold when compared with paleo-climate data during the pre-instrumental period from 1500-1850.

Figure caption: Global (upper panel) and Northern Hemisphere (lower panel) response model result (green bold line) plotted against three observational datasets for the 1875-2017 period: Cowtan/Way land temperature data combined with HadISST2 sea surface temperature data over ocean (grey), standard Cowtan/Way which uses HadSST3 sea surface temperature data over ocean (yellow) and Berkeley Earth global temperature data (black). The anomalies are expressed relative to 1850-1879. El Nino Southern Oscillation index variability is added onto the response model time series.
Figure caption: Global (upper panel) and Northern Hemisphere (lower panel) response model result (green bold line) plotted against three observational datasets for the 1875-2017 period: Cowtan/Way land temperature data combined with HadISST2 sea surface temperature data over ocean (grey), standard Cowtan/Way which uses HadSST3 sea surface temperature data over ocean (yellow) and Berkeley Earth global temperature data (black). The anomalies are expressed relative to 1850-1879. El Nino Southern Oscillation index variability is added onto the response model time series.

Importantly, the team does not dispute short-term variability associated with El Niño Southern Oscillation (ENSO), a well understood climate mode known to cause strong interannual changes in GMST. In fact, the team demonstrates that the incorporation of ENSO variability into their model leads to an explained variability of the observed temperature of 93%. The simple model used in their study takes fast and slow climate feedback processes from natural and anthropogenic radiative forcing perturbations into account, thus allowing the researchers to estimate the resulting temperature response with high accuracy. By virtue of a more precise description of the anthropogenic aerosol feedback processes as well as the removal of known biases in the instrumentally observed sea surface temperature (SSTs) record such as a warm bias during World War II, mismatches between model and observational data disappear.

Asked whether the findings will have notable repercussions for our views of climate change, Karsten Haustein, the lead author of the new study, said that “the current estimate of the human contribution to warming (~100% human-induced) is all but confirmed, yet the confidence in this estimate is considerably enhanced.” He added: “we now have to worry less about the large uncertainty associated with unforced natural variability, as there never was a substantial contribution on sub-centennial timescales to start with”. The same is valid for our estimate of the climate sensitivity. It remains unchanged, but it is now underpinned with more robust evidence. As far as the alleged ocean cycles in general and multidecadal Atlantic Ocean variability (also known as AMV index) in particular is concerned, the results published by the team suggest that the ocean responds to external changes rather than the other way around. The North Atlantic may amplify this signal, it cannot, however, drive hemispheric temperatures. To describe the North Atlantic Ocean variability more realistically, a revised AMV index – called North Atlantic Variability Index – has been introduced.

To conclude: while the climate system continues to be influenced by interannual and presumably multi-annual internal variability, the idea that oceans have magically been driving the climate in a colder or warmer direction for multiple decades in the past, and hitherto do so in the future, is unlikely to be correct. Most of the complex global climate models strongly support the hypothesis that oceans have only limited ability to alter global temperatures on multidecadal timescales. Hence this study provides a very useful constraint on the simulated internal variability in climate models.

References

Mann, M.E., B.A. Steinman, and S.K. Miller (2014). On forced temperature changes, internal variability, and the AMO. Geophysical Research Letters, 41: 3211-3219. doi: 10.1002/2014GL059233

Hegerl, G.C., S. Brönnimann, A. Schurer, and T. Cowan (2018). The early 20th century warming: Anomalies, causes, and consequences. WIREs Climate Change, 2918:9:e522. doi: 10.1002/wcc.522