CICERO - Center for International Climate Research


The summer of 2018: Hot, but how hot?

A long hot summer has brought media attention to climate change. Hundreds of news articles about drought and record temperatures in Northern and later Southern Europe were eventually topped by the news of a study by Will Steffen and co-authors warning that a ‘hot house’ may be coming. A ‘hot house’ is a state of Earth’s climate where internal feedbacks in the climate system force higher temperatures even though human drivers of greenhouse gas emissions have ceased. It helped, of course, that Johan Rockstrøm of Planetary Boundaries fame and several other famous scientists were among the authors. Before coming to this study however, let’s take a look at what data tells us about the summer of 2018. Of course, the summer is not quite over yet, but we have data for the early summer months.

The figure to the left (Figure 1) shows monthly global mean temperatures as published by NASA. July is clocked in as the second highest July temperature ever. Other institutions (e.g. NOAA) have July in third place.

The next figure (Figure 2) shows the regional distribution in July. As we see, it was pretty warm in Europe with several local temperature records, in particular in the north, while the Arctic and the Southern hemisphere are considerably colder relative to expectations. Overall, 2018 is therefore not on track to become a record hot year, but will

certainly take place among the top five, all of which have taken place after 2010. That the El Niño event, that semi-regularly takes place in the Pacific and that tends to raise the global temperature, is absent in 2018, makes the high temperature noteworthy.

Hot house

The study by Will Stephen and co-authors have also got a lot of media attention, although strictly speaking, the study does not really represent any new science. Rather it is an opinion piece, or ‘perspective’ as they call it, from scientists warning that a number of feedback mechanisms associated with for instance loss of ice and snow, and loss of carbon from forests and soils, may drive the global temperature higher even in the absence of human emissions of greenhouse gases. The feedback mechanisms are well known, but difficult to quantify, so the warning is against a potentiality, not something we know will happen for certain. The high media attention even in the absence of new science can perhaps best be understood on the background of the hot summer in the Northern hemisphere.

Hot also off-shore


It’s not only getting hotter on land. It is well known that most of the surplus energy that the greenhouse effect generates goes into the oceans. The next figure (Figure 3) shows this as well as the distribution across different depths. The yellow line is an estimate of the energy imbalance at the top of the atmosphere. As we see, most of this surplus is dumped in the ocean – year after year.

Figure 4 Source: NOAA

The consequence of this is an increased frequency and intensity of so-called Marine Heatwaves (MHWs). These are periods of extreme warm sea surface temperature that persist for days to months and can extend up to thousands of kilometres. The final figure (Figure 4) gives a snap-shot of the distribution of

warm waters on August 18 this year. Studies of earlier events reveal high vulnerability of marine ecosystems and fisheries to such extreme climate events.

A recent study by Frölicher et al. in the prestigious science journal Nature states that today, 87 per cent of Marine heatwaves are attributable to human-induced warming, with this ratio increasing to nearly 100 per cent under any global warming scenario exceeding 2 degrees Celsius. The study results suggest that MHWs will become very frequent and extreme under global warming, probably pushing marine organisms and ecosystems to the limits of their resilience and even beyond, which could cause irreversible changes.

Food for thought

Climate change is projected to negatively impact all aspects of food security according to IPCC. Now a new study by Kasegawa and co-authors published in Nature Climate Change, states that strict policies to curb the effects of climate change could leave millions more people hungry than would a warming climate itself. In a short news notice in Nature, it is noted that in one scenario, climate change alone would result in an extra 24 million people going hungry in 2050, compared with he number expected if today’s climate prevailed. When rigid policies were added, the number of hungry people jumped by a further 78 million, most of them in Africa and South Asia. The authors say officials should consider the consequences for food availability when setting climate policies. It seems that regardless what we do, food production is going to be more challenging in the future, so expect higher food prices.


Lijing Cheng, Kevin E. Trenberth, John Fasullo, Tim Boyer, John Abraham and Jiang Zhu (2017): Improved estimates of ocean heat content from 1960 to 2015, Science Advances 10 Mar 2017: Vol. 3, no. 3, DOI: 10.1126/sciadv.1601545

Frölicher, T. L., Fischer, E. M. & Gruber, N. (2018): Marine heatwaves under global warming, Nature  (2018).

Will Steffen, Johan Rockström, Katherine Richardson, Timothy M. Lenton, Carl Folke, Diana Liverman, Colin P. Summerhayes, Anthony D. Barnosky, Sarah E. Cornell, Michel Crucifix, Jonathan F. Donges, Ingo Fetzer, Steven J. Lade, Marten Scheffer, Ricarda Winkelmann, and Hans Joachim Schellnhuber (2018): Trajectories of the Earth System in the Anthropocene, Proceedings of the National Academy of Science (PNAS),

Tomoko Hasegawa, Shinichiro Fujimori, Petr Havlík, Hugo Valin, Benjamin Leon Bodirsky, Jonathan C. Doelman, Thomas Fellmann, Page Kyle, Jason F. L. Koopman, Hermann Lotze-Campen, Daniel Mason-D’Croz, Yuki Ochi, Ignacio Pérez Domínguez, Elke Stehfest, Timothy B. Sulser, Andrzej Tabeau, Kiyoshi Takahashi, Jun’ya Takakura, Hans van Meijl, Willem-Jan van Zeist, Keith Wiebe & Peter Witzke (2018): Risk of increased food insecurity under stringent global climate change mitigation policy, Nature Climate Change, volume 8, pages 699–703.