Human-induced factors such as greenhouse gas concentrations, other occurrences such as volcanic eruptions, solar activity, and air pollution peaks are responsible for global temperature rise, suggests a new study by Oxford University.
While this has been the consensus of the scientific community for quite a while, uncertainty remained around how natural ocean cycles may impact global temperature warming over the course of multiple decades.
For this study, scientists observed ocean and land temperature data since 1850. They found that slow-acting ocean cycles do not explain the long-term changes in global temperature, which include several decades of accelerated or slowed warming.
Study lead author Dr. Karsten Haustein said, “We can now say with confidence that human factors like greenhouse gas emissions and particulate pollution, along with year-to-year changes brought on by natural phenomena like volcanic eruptions or the El Niño, are sufficient to explain virtually all of the long-term changes in temperature.”
“The idea that oceans could have been driving the climate in a colder or warmer direction for multiple decades in the past, and therefore will do so in the future, is unlikely to be correct.”
Peter Jacobs, a co-author of the study and Ph.D. student at George Mason University in the USA, said, “Unfortunately, a number of previous studies have compared flawed observations with flawed modeling results to claim naturally occurring ocean cycles have played a large role in changes in the global temperature record.”
“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.
Co-author Dr. Friederike Otto said, “Our study showed that there are no hidden drivers of global mean temperature. 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.”
The study is published in the Journal of Climate.