Atmospheric greenhouse gases have continued to rise during recent years, yet global mean surface temperature has shown no clear warming since about 2000. This slowdown in surface warming, often referred to as the global warming “hiatus,” is in sharp contrast to model simulations, which on average show strong warming since 2000.

A National Science Foundation-supported study co-authored by Shang-Ping Xie, a climate scientist at Scripps Institution of Oceanography, UC San Diego, attributes nearly the entire difference between observations and simulations to a climate cycle known as the Inter-decadal Pacific Oscillation (IPO). The cycle is similar to the more commonly known El Niño Southern Oscillation but significantly longer in duration, reversing phases over the course of decades. The study found that the behavior of the IPO explains essentially all the difference between observed and model-simulated global warming rates on decadal time scales since 1920, and in particular the warming hiatus since year 2000.

The research supports earlier work by Xie, Scripps’s inaugural Roger Revelle Chair in Environmental Science, and colleagues that implicates long-term climate cycles as the source of the hiatus.

When climate deviates from historical norms, it is usually in response to external influences such as the recent increase of carbon dioxide in the atmosphere caused by human activities. It can also be in response to natural variability within the climate system such as short El Niño and longer IPO cycles. Climate models simulate both the response to outside influences and internal variability but cannot control random fluctuations of the internal variability. This leads simulations to deviate from observations.

In a 2013 study published in the journal Nature, Xie’s team developed a method to force the IPO in climate models to follow its observed evolution. The simulation with the realistic IPO evolution successfully captured the ongoing global warming hiatus, implicating the tropical Pacific cooling—the negative phase of IPO—as the major driver of the hiatus.

“The new study extends this earlier modeling study by relying on observations that go back to 1920,” said Xie, “We show that over nearly 100 years, the observed deviations in global mean temperature from the anthropogenically forced climate response are nearly all due to IPO.”

The large model-versus-observation discrepancy in the pace of global warming has attracted considerable attention in climate research and in news media since it has the potential to undermine the credibility of climate models and their projections of future climate changes.

“Naturally, people would ask the question: if the models cannot simulate the current global warming rate, how can we trust their projections of future climate change? This is a very reasonable question that deserves a satisfactory answer from the climate science community,” said study lead author Aiguo Dai, an associate professor at the University of Albany. “The global warming hiatus has also been used to dismiss climate science entirely by some deniers of global warming. Thus, explaining the warming hiatus has become an urgent task for climate scientists.”

The study, “Decadal modulation of global surface temperature by internal climate variability,” was published online April 13, 2015 in Nature Climate Change. Co-authors besides Dai and Xie include researchers J. C. Fyfe of Environment Canada, and Xingang Dai of the Institute of Atmospheric Physics/Chinese Academy of Sciences (IAP/CAS).

A key new aspect of this study was the use of simulations to estimate the forced global warming and separate its effects from natural climate variations. Their analyses show that the Inter-decadal Pacific Oscillation, which causes variations in Pacific sea surface temperatures that last several decades, has been associated with large temperature anomalies over both ocean and land. The spatial patterns of the IPO-induced temperature changes match observations but differ from those associated with greenhouse gas-induced warming or aerosol-induced cooling.

“Recent history suggests that the IPO could reverse course soon. Should that happen, we may see accelerated global warming rates in the coming decades,” said Dai.