Natural variability in surface temperatures - caused by interactions between the ocean and atmosphere, and other natural factors - can account for observed changes in the recent rates of warming from decade to decade and these "climate wiggles" can slow or speed the rate of warming from decade to decade, or accentuate or offset the effects of increases in greenhouse gas concentrations.
Unlike many studies, the new one in Scientific Reports uses empirical data rather climate models to estimate decade-to-decade variability. If not properly explained and accounted for, the authors note, the climate wiggles may skew the reliability of climate models and lead to over-interpretation of short-term temperature trends.
The team examined whether climate models, such as those used by the IPCC, accurately account for natural chaotic variability that can occur in the rate of global warming as a result of interactions between the ocean and atmosphere, and other natural factors. To test how accurate climate models are at accounting for variations in the rate of warming, the researchers created a new statistical model based on reconstructed empirical records of surface temperatures over the last 1,000 years.
"By comparing our model against theirs, we found that climate models largely get the 'big picture' right but seem to underestimate the magnitude of natural decade-to-decade climate wiggles," said Patrick T. Brown, a doctoral student in climatology at Duke University's Nicholas School of the Environment. "Our model shows these wiggles can be big enough that they could have accounted for a reasonable portion of the accelerated warming we experienced from 1975 to 2000, as well as the reduced rate in warming that occurred from 2002 to 2013.
"Statistically, it's pretty unlikely that an 11-year hiatus in warming, like the one we saw at the start of this century, would occur if the underlying human-caused warming was progressing at a rate as fast as the most severe IPCC projections. Hiatus periods of 11 years or longer are more likely to occur under a middle-of-the-road scenario."
Under the IPCC's middle-of-the-road scenario, there was a 70 percent likelihood that at least one hiatus lasting 11 years or longer would occur between 1993 and 2050, Brown said. "That matches up well with what we're seeing."
Citation: "Comparing the Model-Simulated Global Warming Signal to Observations Using Empirical Estimates of Unforced Noise," Patrick T. Brown, Wenhong Li, Eugene C. Cordero and Steven A. Mauget; Scientific Reports, April 21, 2015. DOI: 10.1038/srep09957. Funding came from the National Science Foundation (Faculty Early Career Development Program grant #ATM-0449996 and NSF grant #AGS-1147608) and the National Institutes of Health grant #NIH-1R21AGO44294-01A1.