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.

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

Of that 1.05ºC temperature departure from pre-industrial, roughly 1.0ºC is due to the anthropogenic forcing, about 0.05ºC (0.09ºF) to 0.1ºC (0.18ºF) is due to El Niño and about 0.02ºC (0.04ºF) is due to higher solar activity. The remainder is well within the range of variations due to random weather, especially winter weather in Siberia and Canada. Volcanoes contribute very little at this time.

Methodology

This analysis uses well-established techniques from the peer-reviewed literature (Foster and Rahmstorf, 2011; Suckling et al, 2015).
First, the team extended the NCEI global mean surface temperature time series through the end of the year by assuming that November and December temperatures are similar to September and August. (October temperatures were unusually high, in part due to random weather fluctuations that are unlikely to persist into the next months.) This extrapolation gives an expected value for the global mean surface temperature of about 0.87ºC above the 1951-1980 average, which translates to 1.05ºC above the 1850–1900 average that the IPCC takes to be “pre-industrial.”

This temperature time series was fitted to the logarithm of the equivalent CO2 distribution (this includes CO2, other greenhouse gases, and aerosols) from IIASA, volcanic aerosols and solar radiation. As can be seen in Figure 1, this gives a very good fit, with 2014 almost on the fitted trend and 2015 clearly above it. The forced trend is for just about 1.0ºC due to anthropogenic forcing, and 0.02ºC due to the solar cycle, which was above average in 2015. The remainder, about 0.05ºC, is mainly due to the developing El Niño, which tends to heat the globe with a delay of around five months. Various ways to take this delay into account give different estimates of the contribution of the current very strong El Niño, from 0.05 to 0.1ºC. Assuming a shorter delay for this event gives higher values. Due to the delayed response of the global temperature, the influence of El Niño on global mean temperature will be greater in 2016 than in 2015, as it was in 1998 in the year after the peak El Niño ocean temperatures in December.
In addition the observational data was compared with simulated global mean temperature rise from 1850–1900 in the CMIP5 models (Figure 2). The CMIP5 ensemble was normalized against observations over a long period without volcanic eruptions, 1911-1960. This is driven by the observation that the global mean cooling due to volcanoes is much larger in CMIP5 simulations than in the observations and hence taking a period with large volcanoes, such as 1986-2005, gives an offset between the two curves. The recent observations are back in the middle of the plume after a few years on the low side. This comparison is influenced by many factors: the reference periods, the speed at which air quality has improved worldwide, the effects of the relative weak solar cycle and volcanic eruptions. It is therefore impossible to deduce the effect of only CO2 from the warming trend up to now.

References

Foster, G. and Rahmstorf, S. (2011) Global temperature evolution 1979–2010. Environmental Research Letters, 6: 044022, doi: 10.1088/1748-9326/6/4/044022

Krivova, N.A., Vieira, L.E.A. and Solanki, S.K. (2010) Reconstruction of solar spectral irradiance since the Maunder minimum. Journal of Geophysical Research: Space Physics, 115(A12) CiteID A12112. doi: 10.1029/2010JA015431

Suckling, E., Hawkins, E., van Oldenborgh, G.J. and Eden, J. (2015) An empirical model for probabilistic decadal prediction: A global analysis. Climate Dynamics, 48(9–10): 3115–3138. doi: 10.1007/s00382-016-3255-8

The KNMI and Melbourne teams found that the odds of average temperatures across Europe reaching this year’s record-setting levels were increased by at least 35 to 80 times due to human influence on our climate. The team at the University of Oxford found that, even on a more local scale where variability is often greater, global warming had increased the odds of a year as hot as the one just experienced in most of continental Europe by at least a factor of 10.

Hot years are getting hotter

Climate change makes hot years hotter and more common. Nine of the 10 hottest years ever recorded globally have occurred since 2000. According to NOAA’s National Climatic Data Center (NCDC) there hasn’t been an annual cold record globally since 1911.

In Europe, nine of the 10 hottest years ever recorded have also all occurred since 2000. There hasn’t been an annual cold record across Europe since 1956.

Using early instrumental records and proxies like tree rings to extend the temperature record for Europe back to 1500, climate scientists found that the average January-through-December temperature in Europe during 2014 was very likely warmer than at any time during the past 500 years. Initial estimates from KNMI forecast the January-December annual mean temperature for Europe to be 10.5ºC, or 0.3ºC above the previous record set in 2007.

“In the early 1900s, before global warming played a significant role in our climate, the chances of getting a year as warm as 2014 were less than 1-in-10,000. In fact, the number is so low that we could not compute it with confidence,” Geert Jan van Oldenborgh, a climate scientist at KNMI.

The analysis by van Oldenborgh concluded that global warming has made a temperature anomaly like the one observed in 2014 in Europe at least 80 times more likely. For many individual countries the probability has increased by at least a factor of 30 (e.g., the UK, many countries in Central Europe).

Using a large computing network (weather@home), Oxford scientists simulated possible European weather based on the observed global ocean temperatures. At the same time, they also simulated a 2014 where there is no human-influenced climate change. Comparing those two “worlds” they found that the 2014 European temperatures were much more likely in the world with climate change than the one without.

“It is important to highlight that Oxford’s result crucially depends on the 2014 global ocean temperatures. The same study using 2000-2011 conditions gives a different result although the anthropogenic warming is the roughly same in these years,” said Fredi Otto, a climate scientist at the University of Oxford. “When looking at smaller regions in Europe, we notice that there is a higher variability of temperatures,” Karsten Haustein, a colleague of Otto who conducted the analysis, said. “For example, in central Europe we found that the probability of reaching the observed 2014 temperatures is about 40 times higher. In an even smaller region such as the UK, we found that the probability has increased by a factor of about 10.”

The analysis from climate scientists at the University of Melbourne and the Australian National University showed similar results. Their approach utilised the Coupled Model Intercomparison Project phase 5 (CMIP5) experiments.” By comparing climate model simulations representing the world as it is with simulations of a world without humans, we show that the risk of warm years like 2014 occurring has very likely increased by at least 35-fold,” said Andrew King, a climate scientist from the University of Melbourne who conducted the analysis. “This means that human-induced climate change has very likely played a significant role in 2014 being a record hot year for Europe.”

Nineteen European countries are very likely to see their hottest year on record:

• Austria
• Belgium
• Croatia
• Czech Republic
• Denmark
• France
• Germany
• Hungary
• Iceland
• Italy
• Luxembourg
• The Netherlands
• Norway
• Poland
• Serbia
• Slovakia
• Slovenia
• Sweden (equal to 1953)
• United Kingdom

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