Shaking off the northern bias in temperature reconstructions

It’s certainly understandable that most reconstructions of historical temperatures have been dominated by data from the Earth’s Northern Hemisphere. The South, with its immense oceans and intimidating frozen continent of Antarctica, is just less accessible to researchers.

These northern-biased reconstructions – which are based on studies of tree rings, coral, ice cores, subfossil pollen, boreholes and lake sediments – have played a decisive role in our ability to separate out natural from human-caused global warming. But what about the other half of the planet?

New data on Southern temperatures show the importance of considering the entire globe when making climate predictions.

Of the roughly 25 hemispheric temperature reconstructions that were published over the last few decades, just three covered the Southern Hemisphere. Even these either used small sets of data or were just a part of larger Northern hemispheric or global reconstructions.

Paleoclimatologist Kim Cobb of the Georgia Institute of Technology wrote in a recent commentary in Nature Climate Change that this represented an “inconvenient truth” for the climate science community.

Speaking to Road to Paris, she said, “We have far fewer high-resolution marine records from the Northern Hemisphere than we do terrestrial records, so it follows that when you get to the watery Southern Hemisphere, you’re going to need a lot more marine records to compile a reconstruction.”

Now, a team of 17 climate scientists, including researchers from Australia and South America and a number of Antarctic specialists, has reported a truly comprehensive reconstruction of temperature variations for the Southern Hemisphere. The landmark research, led by Raphael Neukom of the University of Bern, appeared in Nature in March.

With a robust new network of Southern terrestrial and ocean-based paleoclimate proxy records from more than 300 different locations they were able to estimate Southern hemispheric temperatures over the past millennium.

At long last, the Southern Hemisphere is getting the climate recognition it deserves. But this clearing up of our Southern blind spot has only resulted in further unease.

The researchers were surprised to find that only at two points in the last thousand years did the Northern and Southern Hemispheres show atypical extreme temperatures at the same time. The first was the global Little Ice Age from 1594 to 1677. And the second? The warming experienced globally since the 1970s.

In the South, current warming era kicked in about 25 years after it did in the northern hemisphere

Meanwhile, they found that many times over the past 1,000 years the temperatures in the two hemispheres headed in wildly different directions for decades at a time. Intriguingly, in the South, the current warming era kicked in about 25 years after it did in the Northern Hemisphere.

This new Southern temperature reconstruction backs up parallel findings last year that the two hemispheres diverge significantly in their patterns of deglaciation and that evidence shows greater temperature coherence within each hemisphere than between them.

Neukom and his colleagues argue that current climate change models cannot explain these differences because they rely on data that is not truly representative of the global climate system. They conclude that without fully accounting for the relationship between the oceans and the atmosphere in the South, predictions will be less reliable.

Speaking to the importance of the study, Cobb said: “It points out the rough and ugly edges of our knowledge about climate, even of the last millennium, right on the heels of the IPCC (Intergovernmental Panel on Climate Change) report’s polished graphics of model-data coherence. I think more people in the climate community need to appreciate where we are at with paleoclimate networks in some under-sampled regions of the world.”

A superficial reading of the tale of the two hemispheres might lead one to think it a boon for climate sceptics. The findings certainly challenge researchers to step up their game when it comes to the predictability of the climate system. But two of the leading sceptic websites, Climate Audit and Watts Up With That, were quick to pooh-pooh the study.

Why don’t they like it? The thread that runs through most sceptics’ views is that there is nothing unusual about the current warming. But Neukom and his colleagues found that we are living through one of those rare periods where there is indeed a synchronicity of extreme temperature changes in both hemispheres.

While the researchers say more accurate predictions require more study of the South in concert with study of the North, nevertheless, the addition of this valuable information on Southern Hemisphere temperatures demonstrates how profoundly exceptional the last few decades of warming have been.

“The research lends strong support to the idea that the 20th century warming we have seen is unusual in the context of the last millennium – hardly welcome news for anyone seeking to cast doubt on the reality of anthropogenic climate change,” said Cobb. “But it is my belief that as scientists, we must not sugar-coat the uncertainties where they do lie.”

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