Human influence on climate is set to make otherwise unusually hot summers in the Northern Hemisphere more frequent, even if the current warming slowdown continues. That finding, from a new study by Youichi Kamae from the National Institute for Environmental Studies in Tsukuba, Japan, and his colleagues, could now heat up climate talks. “The recent hot summers over land regions and the climate hiatus have opposite effects on ongoing global negotiations for climate policies,” Youichi underlined. “The findings of this study can have significant implications for policy makers.”
Over the past 15 years, growing ‘anthropogenic’ or human-emitted CO2 hasn’t turned into significant average temperature rises on the Earth’s surface. The top levels of the oceans haven’t warmed significantly either, even though heat is still building up deeper down. However in that time sometimes deadly hot summers have become more common in Earth’s northern half. It’s not clear how that’s happening without average temperatures increasing faster. One possible part of the explanation could be a fast response to greenhouse gas emissions that Youichi and other scientists had previously found. “The fast response over can largely be interpreted as direct land surface warming due to CO2,” Youichi told me.
The Japanese team’s search for a better explanation had a big question at the centre: How much of this climate change is natural, and how much is man-made? Not able to easily experiment on the planet to investigate, they did what climate scientists usually do for such ‘attribution studies’, and turned to computer models. Simulating the world with and without human greenhouse gas emissions and comparing the results, scientists are increasingly trying to pinpoint whether climate change directly caused particular extreme weather events. They’re trying to build up lots of evidence about a single event to be sure that their result isn’t random, and that takes lots of computer time and power.
Sea of knowledge
However, Youichi’s team weren’t interested in hot summers in a specific place – they wanted to look at how often they were happening across half the world. Covering so many locations automatically gave the scientists lots of data to help rule out a random finding. They therefore focussed on three sets of ten climate simulations for 1949–2011. Called MIROC5, the model they used had been developed by a team including Youichi’s coauthors Masahiro Watanabe and Masahide Kimoto from the University of Tokyo. They turned of the part of MIROC5 that simulates Earth’s oceans and used it as an atmospheric general circulation model (AGCM). That lets the scientists feed in previously constructed datasets for sea surface temperature and ice.
The Japanese team exploited this capability in an unusual way, breaking their attribution question down even further than just human versus natural. They could now compare the direct impact of human emissions, against human influence exerted via our effect on the oceans, against natural factors. “This type of attribution study, using atmosphere-only model long-term integrations with and without anthropogenic ocean warming and radiative forcing, is a new idea,” Youichi noted.
Therefore, in one of the three sets of simulations, they reproduced 60 years of climate using existing records of sea temperatures and ice and other key driving forces. That included measured changes in natural factors, like volcanoes and energy from the sun, and human influences like greenhouse gases, dust and ozone emissions and land use change. The second set reflected ‘natural warming’ by holding human influences as they had been in 1850, before industrialisation started, and removing estimated human impact on sea temperatures and ice. The final set also held human influences at levels from 1850, but used the real-world record of sea temperature and ice changes that included human impact.
Compared to a reference set of other climate models MIROC5 was much better at simulating year-to-year measured real-world temperature changes. Comparing the three MIROC5 sets showed direct effects from human influences, the warming sea, and natural factors were each responsible for around a third of average summer temperature rises since 1981. Similarly, direct effects were responsible for around a third of the increase in hot summers, with sea-driven and natural influence together contributing around two-thirds on average. However, the role of the oceans and natural factors are more variable, while the direct effect is more consistent.
Looking in detail at climate patterns, Youichi and his team found two key components in their simulations explaining why hot summers are getting more common. One was a natural effect related to two long-duration cyclical patterns: the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). The other was the direct effects from human emissions in more northerly regions, which is set to carry on. “A significant part of the recent increase in the frequency of hot summers can be attributed to the direct influence of anthropogenic forcing,” Youichi emphasised. “It can continuously increase their frequency in the future even if the climate hiatus persists for the coming decades.”
Kamae, Y., Shiogama, H., Watanabe, M., & Kimoto, M. (2014). Attributing the increase in Northern Hemisphere hot summers since the late 20th century Geophysical Research Letters DOI: 10.1002/2014GL061062