Researchers from the Universities of Princeton, California, Tokyo, Kyushu and the Scripps Institution of Oceanography, say the recent hiatus in global temperature increase has led to a surge in climate science.
The global effort to understand the global warming hiatus they say has led to increased understanding of some of the key metrics of global climate change such as global temperature and ice-cover. Searching for an answer to the hiatus, they say, meant that the scientific community grappled with difficulties with these climate metrics, in particular the fact that they do not unequivocally portray the same story about global warming.
For instance, as the global surface temperature increase underwent an apparent slowdown, Antarctic sea ice expanded, and boreal summer Arctic sea ice declined rapidly, at least until 2007. Hot and cold extremes increased in northern hemisphere continents, and the Hadley circulation shifted poleward.
Many of the changes are not ones expected due to increasing greenhouse gas forcing. For some this called into question the viability of computer models of the climate and whether these changes indicated a fundamental lack in our understanding and ability to simulate radiatively forced changes, or indeed if internal climate variability alone is sufficient to explain the changes.
The researchers point out that since the hiatus was identified just over a decade ago it stimulated advances in our understanding of the multidecadal variability of these key metrics, providing insight into internal climate variability. As well as drawing attention to biases in the temperature record it has also improved our understanding of the role of the tropical Pacific Ocean in mean global temperature. Despite the research progress many challenges remain, especially due to the relatively short timescale of the observations. There are also limitations of climate models in simulating internal, multidecadal climate variability and the way radiatively forced changes interact with that inherent variability.
The short period over which we have reliable observations restricts the number of independent simulations of observed multidecadal variability that can be performed. Uncertainties in the data, such as sea ice extent are very large before the satellite era in the late 1970s.
Other datasets with records extending more than a century, such as global sea surface temperature, have large uncertainties in the first half of the 20th century particularly over the Southern Ocean due to sparse data. Because of these uncertainties, the scientists say, many existing sea surface temperature reconstructions may have underestimated the amplitude of early 20th century natural climatic variations, impeding our ability to understand associated climate changes such as accelerated Arctic warming.
Although recent climate models can simulate the basic structure of internal climatic variability, they underestimate the strength of some important modes of internal multidecadal variability, in particular the oceanic climate cycles. Specifically, the decadal-to-multidecadal component of the North Atlantic Oscillation which has important consequences for the Northern Hemisphere temperature.
As with the recent hiatus, they say, it is inevitable that internal variability will offset and possibly even temporarily reverse the radiatively forced trends for each of these metrics over decadal to multidecadal periods, that is possibly bring on a period of global cooling. The scientific community should be ready for this they imply.
Johnson et al writing a review article in Global and Planetary Change conclude that because of these multidecadal modulations, the trends of these metrics must be calculated over several decades to suppress the noise of internal variability. This is in contrast to the prevailing message before the identification of the hiatus which was the long-term climate warming signal was strong and the “noise” relatively weak.
The hiatus, they add, demonstrated that natural variability of global surface temperature can overcome the effects of radiative forced global warming over periods of about 15 years. For other more regionally confined metrics, this timescale tends to be even longer and may extend beyond available observational records. In other words, we do not have enough long-term data to evaluate natural climatic variability to place today’s change into their proper historical perspective.
Thus, these researchers lay bare some of the dominant memes of climate change regarding its ability to forecast the future. The lesson of the hiatus is that we do not understand internal climatic variability as much as many think we do, and our predictive power is less than many believe.