Victims of the Black Death in the 14th century, whose remains were discovered during excavations for London's new Crossrail railway line. Photo: Crossrail Ltd
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Gerbils and squirrels, not rats, may have been responsible for the Black Death

New research has found that rats alone can't have been responsible for Europe's medieval plague outbreaks - and giant Central Asian gerbils may have been an alternative accessory to the crime.

The political and cultural impact of the Black Death, which is usually seen as beginning in the period 1347-1353, on European life was immense. At least a hundred million people were killed (and possibly more than twice that by some estimates), reshaping the economy of feudalism and influencing the wars and politics that followed for at least the next four centuries, as plague outbreaks struck cities and villages over and over again. The bacteria responsible, Yersinia pestis, has long been believed to have originated in China, and to have been carried from east to west in fleas living on black rats that migrated with the merchants moving along the Silk Road, first by camel across central Asian steppes and deserts, and then across the Black Sea and Mediterranean by ship to major trading ports like Venice.

This distributional jigsaw may be missing some pieces, however - a new study by Norwegian and Swiss bioscientists and climatologists instead points the finger at some different rodents as bearing some of the responsibility for spreading plague: grey marmots, long-tailed squirrels, camels and Mongolian gerbils.

In the paper, published in the Proceedings of the National Academy of Sciences, the authors looked at 4,119 records of historical plague outbreaks across Europe, and cross-referenced them with 15 climate records derived from analysing tree rings within the same regions as the outbreaks. They were testing the hypothesis that the plague, once it arrived in Europe, dug itself in by living among rat populations (or "rat reservoirs" as the authors call them), which then experienced regular population swellings and shrinkings as the climate adjusted to become more or less rat-friendly over the centuries. They expected to see that plague outbreaks correlated neatly with warmer, drier summers, and that periods with fewer outbreaks were wetter and colder - yet it didn't quite work.

Most of the outbreaks in the records could be traced to other outbreaks nearby, but there were a select few - 16 in total, all in port cities like London, Lisbon and Hamburg - which had no connection with other outbreaks nearby, and where brand new imports of Y pestis via trading ships must have been responsible.

This, in itself, isn't too remarkable, but the researchers did notice a strong correlation between the climate records of east Asia and these 16 outlier European plague outbreaks. For each of them, certain climate conditions in east Asia would, 15 years later, be followed by a large outbreak. Something was linking the two sides of the Eurasian continent, and the researchers see a number of different rodents as responsible.

Consistently, a warming climate followed by a sudden cooling seemed to kickstart the transmission of plague bacteria across the Silk Road. The researchers theorise that warmer summer made it easier for fleas to find hosts to live and feed on - but the crash in temperature kills those same hosts, meaning fleas jump to new species, including humans. The model they propose has this happening over the first couple of years after the climate event; then, as those fleas move through the human population, they hitch onto humans on the edge of the Silk Road trading networks of central Asia.

From there, the plague moved slowly over a decade or so along trading routes, surviving in the large populations of rodents that live in those areas. Once the migration hit the eastern edge of the Black Sea, the journey was then sped up thanks to the speed of ships - within two or three more years, the plague will have successfully travelled from one side of the world to the other, to infect those living in European cities. "This association strongly suggests that the bacterium was continuously reimported into Europe during the second plague pandemic," writes lead author Boris Schmid of the University of Oslo, "and offers an alternative explanation to putative European rodent reservoirs for how the disease could have persisted in Europe for so long."

This doesn't get the black rat (Rattus rattus) off the hook - it's still the key vector for plague transmission once the bacteria arrived in Europe with each new wave. But it does mean that other members of the rodent family lose some of their ability to act all cute and innocent.

Ian Steadman is a staff science and technology writer at the New Statesman. He is on Twitter as @iansteadman.

Artie Limmer/Texas Tech University
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Meet the evangelical Christian persuading believers that climate change is real

Katharine Hayhoe's Canadian missionary parents told her science and God were compatible. Then she moved to Texas. 

During Donald Trump’s presidential campaign, alarm rose with each mention of climate change. Denial, dismissal and repeated chants of “hoax” left no doubt as to his position.

Now President Trump’s withdrawal from the Paris Agreement has been seen as a seminal moment in the fight against climate change - one which many fear could lose the battle ahead of humanity.

But one scientist has been fighting a war of her own on the ground, against those who typically doubt the facts about global warming more than most - the evangelical Christian population of America.

And to make matters even more unusual, Katharine Hayhoe herself is an evangelical Christian who lives in the indisputably "bible belt" of Lubbock, Texas.

The atmospheric scientist has been named one of Time magazine's 100 most influential people and one of Politico’s 50 thinkers transforming American politics. Now she is using her considerable heft to speak to those who are hardest to convince that there is a manmade problem that threatens the Earth’s future.

I meet her at the science and music festival Starmus in Trondheim, Norway, where she is to address the attendees on Thursday in a talk entitled "Climate Change: Facts and Fictions".

Hayhoe was born in Canada, to missionary parents. Her father, a former science educator, showed her that there was no conflict between the ideas of God and science. However, it was something of a surprise to her when she discovered her pastor husband, whom she married in 2000, did not feel the same about climate change. It took her two years to convince him.

What started as a conversation became an organised project when she moved to America's South in the mid 2000s. 

“Moving to Lubbock was a culture shock," she tells me. "When I moved there I wasn’t doing much outreach, but it moved me in that direction.

“Lubbock is very conservative. It’s small and isolated.

“I would say the majority of people in Lubbock are either dismissive or doubtful about climate change. I was surrounded by people - neighbours, parents of friends, people at church, colleagues down the hall in the university - who weren’t convinced.”

So Hayhoe, who works as an associate professor and director of the Climate Science Centre at Texas Tech University, set to work. She began to collect the responses she was seeing to the climate change discussion and prepare her counter-argument.

“When I talk to people who are doubtful, I try to connect with the values they already have," she says. “The greatest myth is the myth of complacency - that ‘it doesn’t really matter to me’.

"But I would say that the second most insidious myth is that you only care about this issue if you’re a certain type of person. If you’re a green person, or a liberal person, or a granola person."

The stereotypes mean that people outside that demographic feel "I can't be that kind of person because that's not who I am", as she puts it.

Hayhoe convinced her husband using data, but rather than repeating a formula, she tries to find out what will resonate with different people: "For many groups, faith is a core value that people share.”

Whether she’s speaking to city planners, water company managers, school kids or Bible believers, Hayhoe says her hook is not the facts, but the feelings.

“I recently talked to arborists," she says. "For them, trees and plants are important, so I connect with them on that, and say ‘because we care about trees, or because we care about water or what the Bible says then let me share with you from the heart why I can about these issues because it affects something that you already care about’.

“My angle is to show people that they don’t need to be a different person at all - exactly who they already are is the kind of person who can care about climate change.”

Hayhoe came to public attention in the United States after appearing in a Showtime series on climate change. She has appeared on panels with Barack Obama and Leonardo DiCaprio, and launched a web series. As well as plaudits, this level of fame has also earned her daily threats and online abuse. 

“My critics think they’re coming from a position of religion, but they aren’t," she says. "They’re actually coming from a very specific political ideology which believes that the government should not have control over people’s lives in any way shape or form - very libertarian, free market, free economy, Tea Party."

She believes that in the United States, faith and politics has been conflated to the point "people can no longer tell the difference". 

“Now it’s conservatism that informs religion," she elaborates. "If the two are in conflict - like the Bible says God has given us responsibility over everything on this earth - then people say ‘oh, we can’t affect something as big as this Earth, God will take care of it anyway’."

Around half of those who attack her on social media identify themselves as Christians, she notes, but almost all call themselves conservatives. 

As a scientist, she’s been preparing data herself - naturally - on her online attackers, with depressingly familiar results.

“As soon as you stick your head out of the trench, you get it. There have been papers published showing that white men disproportionately form up that small group of dismissives. They’re almost all men. When I track my social media comments, I would say that 99.5 per cent of them are white men.

“Out of 1,000 negative comments, I have maybe five from women.”

After the climate change argument moved up a gear - following the Paris withdrawal - Hayhoe admits that she and her fellow scientists are concerned, although she pays tribute to the businesses, cities and states from the US that have committed to following the Paris agreement themselves.

On the subject of the chief white male denier, Trump himself, Hayhoe says she has a discussion point which she feels may convince him to think carefully about his role in the fight against global warming’s impact on humanity.

“I would attempt to connect with the values that he has and show him how acting on this would be in his best interests," she says.

“One guess would be ‘what do you want your legacy to be? What do you want to be known as, the man who destroyed the world, or the man who saved it?’”

Katharine Hayhoe is speaking at Starmus on Thursday June 22. For more details, visit Starmus.

Kirstie McCrum is a freelance journalist. Follow her @kirstiemccrum.

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