Mukund Palat Rao from Columbia University, US, and his colleagues studied mortality data from 21 Mongolian provinces, known as "aimags", between 1955 and 2013. They compared this information to climate data gathered over the same period. Livestock mortality was most strongly linked to exceptionally cold winters (November to February), the team found. But winter was not the only factor. The researchers also found that a dry summer (July to September) prior to a cold winter further increased the chances of a dzud. Such climate effects could explain nearly half of the livestock deaths.

"The exact mechanism for how climate causes mortality is complicated, but can include things like a lack of access to forage because of snow, or a weakened immune system after a summer drought," said Palat Rao.

Worse still, it appears that these harsh climate events often occur several years running. "Looking at the mortality time series more closely, we see that high mortality years often cluster together, for example 1966, 1967, 1968, [and] 2000, 2001, 2002, and there could be long stretches of time where there is very low mortality, [e.g.] 1984–1999," said Palat Rao.

The two most recent dzud events (2000 to 2002 and 2009 to 2010) were truly disastrous, wiping out 20 million head of livestock between them. Currently the Mongolian Index Based Livestock Insurance Program (IBLIP) uses a threshold of 6% mortality to trigger payouts. "This threshold is based on the natural background mortality rate (due to disease, predation and birth complications, for example)," said Nicole Davi from William Paterson University, US. Right now, this threshold is passed on average every four years.

But from the herders’ perspective, preventing the livestock deaths in the first place would be better. "During the Soviet era, when herding was collectivized, there used to be co-ordinated haymaking and other facilities like veterinary care, which have largely been done away with," said Palat Rao. "These helped tide over, or at least reduce, mortality."

However, the logistics of such programmes were extremely complicated, given that herders move over a vast region and are often one or two weeks away from the nearest urban centre. "An alternative strategy could be to provide herders with good weather forecasts, enabling them to make strategic decisions such as remaining at a sheltered camp until a storm had passed, or choosing to slaughter animals earlier based on summer weather conditions and forecasts," said Davi.

Looking ahead it isn't certain what lies in store for the nomadic herders. Future climate projections for this region are unclear, although it’s expected that summers will become hotter, possibly exacerbating future drought. Archaeological records show that herding has been a way of life for 4000 years or more, surviving both warm epochs – the Mediaeval Climate Anomaly between 950 and 1250AD – and cool times like the Little Ice Age between 1550 and 1850AD, as well as extended periods of drought.

"Perhaps more of a threat is competition for resources from industries such as mining, and the perception that nomadic herding is a 'backwards' way of life," said Davi.

But for now life rolls on. And with a greater understanding of the link between climate and dzud events, herders may be able to take some positive action. Meanwhile, insurance companies will be able to anticipate high mortality years based on climate alone, potentially speeding up the insurance payout process.

The researchers published their findings in Environmental Research Letters (ERL).

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