by Judith Curry
Given the new information now available from the Southern Hemisphere, climate scientists must consider a larger role for natural climate variability in contributing to global temperature changes over the past millennium. – Kim Cobb
A paper recently published in Nature Climate Change:
Inter-hemispheric temperature variability over the past millennium.
Raphael Neukom, Joëlle Gergis, David J. Karoly, et al.
Abstract. The Earth’s climate system is driven by a complex interplay of internal chaotic dynamics and natural and anthropogenic external forcing. Recent instrumental data have shown a remarkable degree of asynchronicity between Northern Hemisphere and Southern Hemisphere temperature fluctuations, thereby questioning the relative importance of internal versus external drivers of past as well as future climate variability. However, large-scale temperature reconstructions for the past millennium have focused on the Northern Hemisphere, limiting empirical assessments of inter-hemispheric variability on multi-decadal to centennial timescales. Here, we introduce a new millennial ensemble reconstruction of annually resolved temperature variations for the Southern Hemisphere based on an unprecedented network of terrestrial and oceanic palaeoclimate proxy records. In conjunction with an independent Northern Hemisphere temperature reconstruction ensemble, this record reveals an extended cold period (1594–1677) in both hemispheres but no globally coherent warm phase during the pre-industrial (1000–1850) era. The current (post-1974) warm phase is the only period of the past millennium where both hemispheres are likely to have experienced contemporaneous warm extremes. Our analysis of inter-hemispheric temperature variability in an ensemble of climate model simulations for the past millennium suggests that models tend to overemphasize Northern Hemisphere–Southern Hemisphere synchronicity by underestimating the role of internal ocean–atmosphere dynamics, particularly in the ocean-dominated Southern Hemisphere. Our results imply that climate system predictability on decadal to century timescales may be lower than expected based on assessments of external climate forcing and Northern Hemisphere temperature variations alone.
Over the last 1000 years, temperature differences between the Northern and Southern Hemispheres were larger than previously thought. Using new data from the Southern Hemisphere, researchers have shown that climate model simulations overestimate the links between the climate variations across Earth with implications for regional predictions.
In 99.7 percent of the results, the warmest decade of the millennium occurs after 1970. Surprisingly, only twice over the entire last millennium have both hemispheres simultaneously shown extreme temperatures. One of these occasions was a global cold period in the 17th century; the other one was the current warming phase, with uninterrupted global warm extremes since the 1970s. “The ‘Medieval Warm Period’, as identified in some European chronicles, was a regional phenomenon,” says Raphael Neukom. “At the same time, temperatures in the Southern Hemisphere were only average.” The researchers ascribe these large differences to so-called “internal variability.” This term describes the chaotic interplay of the ocean and atmosphere within the climate system that leads to temperatures changing in one or the other direction. Regional differences in these fluctuations appear to be larger than previously thought.
The scientists discovered that most climate models are unable to satisfactorily simulate the considerable differences between the hemispheres. The models appear to underestimate the influence of internal variability, in comparison with external forcings like solar irradiation, volcanic eruptions or human greenhouse gas emissions. “Regional differences in the climatic evolution of the next decades could therefore be larger than the current models predict,” says Neukom.
Climate Audit and WUWT have written several posts critical of the study:
- Neukom and Gergis serve cold screened spaghetti Climate Audit
- Neukom’s science by Proxy WUWT
- Duke Neukom’s secret sauce WUWT
Without going into detail, the main problem seems to be ‘post hoc proxy selection’, otherwise known as ‘cherry picking’.
After looking at the IPCC AR5 Chapter on Paleoclimate, its not clear to me what is actually new in the Neukom et al. paper, but I find this perspective in Nature Climate Change by Kim Cobb entitled Paleoclimate: A Southern Misfit to be illuminating. Excerpts:
For far too long the climate science community has grappled with an inconvenient truth: the vast majority of the datasets used to constrain temperature trends of the recent past come from the Northern Hemisphere. Over a dozen reconstructions of Northern Hemisphere temperature spanning the past millennium exist and have played a critical role in distinguishing natural from anthropogenic climate change. However, the extent to which recent temperature variations in the Northern Hemisphere resemble those in the Southern Hemisphere remains unclear. Such information is critical to a complete understanding of the mechanisms of global, rather than hemispheric, climate change. Although the new reconstruction resembles the Northern Hemisphere reconstructions in some key aspects — the anomalous nature of twentieth century warming being one of them — it also suggests that temperatures in the two hemispheres may have differed more than they have agreed over the past millennium.
However, the new reconstruction of Southern Hemisphere temperature1 suggests that the climate model simulations of past climate systematically underestimate the magnitude of natural climate variability in the Southern Hemisphere. At first glance, the reconstruction contains the same basic features of the Northern Hemisphere family of reconstructions — a centuries-long cooling into the seventeenth century, and a twentieth-century warming of unprecedented duration and magnitude. But a close comparison between the Northern and Southern Hemisphere reconstructions reveals many intervals when the two series diverge for decades at a time. Notably, some of these differences occur following large volcanic eruptions, when the Northern Hemisphere cools significantly but the Southern Hemisphere does not, at least according to the new reconstruction1. The fact that many of these differences occur within the past 400 years, when the data networks from both hemispheres are most robust, makes it less likely that such temperature differences are artifacts of poor data coverage. That said, it is possible that small but cumulative age errors in single palaeoclimate records may smear out interannual variability in large-scale temperature reconstructions — currently the topic of vigorous debate.
If the new reconstruction of Southern Hemisphere temperature is accurate, then estimates of climate sensitivity — the response of global temperature change to a given amount of external radiative forcing — may be lower than those calculated based solely on Northern Hemisphere reconstructions. Indeed, instrumental temperature data suggest that warming in the Northern Hemisphere has been greater than that observed in the Southern Hemisphere over the past two decades — a feature reproduced in the current suite of climate models11. Therefore, this hemispheric asymmetry may be a fundamental feature of the climate system’s response to a change in radiative forcing, whereby the ocean-dominated Southern Hemisphere acts as a buffer of sorts to global temperature change on decadal to centennial timescales. On the other hand, Neukom et al. propose that divergent hemispheric temperatures arise from strong natural climate variability in the Southern Hemisphere, and have been a constant feature of the past millennium.
Given the new information now available from the Southern Hemisphere, climate scientists must consider a larger role for natural climate variability in contributing to global temperature changes over the past millennium. While the new reconstruction brings strong additional support to the phrase ‘anthropogenic global warming’, it also highlights the limits of our current ability to understand, and predict, global temperature variations from decade to decade. In other words, global temperatures will warm appreciably by 2100, but the road may be bumpy and full of surprises.
JC comments: The hemispheric asymmetries are quite interesting and largely unexplored – there is also a very intriguing see-saw between Arctic and Antarctic sea ice. Whatever the flaws in the methodology of Neukom et al., the paper serves to highlight the hemispheric asymmetries, the importance of internal variability, and concerns about overestimates of climate sensitivity.