Global Temperature Change Attributable to External Factors Says New Study

Credit: Ed Hawkins / http://www.climate-lab-book.ac.uk/spirals/  Data from the HadCRUT4 dataset http://www.metoffice.gov.uk/hadobs/hadcrut4/

‘Unfortunately a number of previous studies have compared flawed observations with flawed modelling results to claim naturally occurring ocean cycles have played a large role in changes in the global temperature record,’ says Peter Jacobs, co-author on the study and PhD student at George Mason University in the USA.  ‘We show here that in fact there’s little role for such cycles in explaining temperature changes when more accurate representations of both the temperature record and factors like volcanic eruptions, solar energy, and of course human activities are used. The climate system is endlessly interesting and no doubt has many mysteries left to explore, but this is really not one of them. Being sure that we’re comparing like-with-like before jumping to the conclusion that there are discrepancies between our understanding of the climate and how it is behaving in the real world is a lesson we seem to have to relearn over and over again.’

The study showed that global warming that occurred during the ‘early warming’ period (1915 – 1945) was in fact caused by external factors as well. Formerly, it had been largely attributed to natural ocean temperature changes, which is why there has been uncertainty over how much of global warming is influenced by unpredictable natural factors.

‘Our study showed that there are no hidden drivers of global mean temperature,’ says co-author Dr Friederike Otto. ‘The temperature change we observe is due to the drivers we know. This sounds boring, but sometimes boring results are really important. In this case, it means we will not see any surprises when these drivers – such as gas emissions – change. In good news, this means when greenhouse gas concentrations go down, temperatures will do so as predicted; the bad news is there is nothing that saves us from temperatures going up as forecasted if we fail to drastically cut greenhouse gas emissions.’

Contacts and sources:
University of Oxford

Citation: A limited role for unforced internal variability in 20th century warming.
Karsten Haustein, Friederike E.L. Otto, Victor Venema, Peter Jacobs, Kevin Cowtan, Zeke Hausfather, Robert G. Way, Bethan White, Aneesh Subramanian, Andrew P. Schurer. . Journal of Climate, 2019; DOI: 10.1175/JCLI-D-18-0555.1