Met Office's Laki volcano impacts study due soon
The UK Met Office aims to deliver its report in June on the consequences of a future Laki-style eruption.
The Icelandic volcano caused widespread disruption and death on the island, in Britain, across Europe, and beyond when it let rip in 1783.
Ministers want the UK to be prepared and resilient should an event of similar magnitude ever occur again.
To that end, the Met Office has been asked to model where the gases from such an eruption might travel.
And here in Vienna at the European Geosciences Union General Assembly, the meteorological agency has been giving an update on its progress.
Researchers are using various numerical tools to simulate a range of outcomes from a Laki repeat.
The 1783 event was very different from the Eyjafjallajokull eruption of 2010 which grounded Europe's airliners.
That was an explosive event whose impacts were dominated by the wide dispersal of ash particles.
Laki, on the other hand, was an effusive eruption. It oozed 600 sq km of lava on to Iceland's surface, but critically also sent waves of gas, particularly sulphur compounds, in the direction of Europe.
These sulphur species reacted with the water in the atmosphere to produce tiny droplets, or aerosols, which then cooled the climate. And when the sulphur washed out of the sky, it fell as acid rain.
The deaths in Britain probably numbered in the thousands - from respiratory complications, from the harsh winter that followed the eruption, and from famine.
A modern UK would hope to be more robust and better prepared, but the Cabinet Office needs to have a good grasp of the challenges.
The Met Office is modelling the possibilities using much of the same knowledge it deploys to forecast evolving weather systems.
"The big difference between ash and gas is that the ash is heavy, and it falls out of the atmosphere under gravity, under its own weight. This will influence which winds will pick it up and transport it," explained Dr Claire Witham, who runs the agency's effusive eruptions modelling project.
"The gases and the fine aerosol particulates, on the other hand, don't fall in the same regime; they don't have the same mass component. What this means is that if you get ash and gases, they won't necessarily move in the same direction."
The 1783 Laki event went on sporadically for eight months, but to constrain their simulations and make the whole endeavour manageable, the Met Office is assuming a five-week eruption. The model runs incorporate 10 years of weather data.
"You can start the five-week period in a January, a February, a March, or an April, etc. The seasonality is important, but so too is the natural variability in the climate," said Dr Witham.
"Sometimes we have hot summers; sometimes we have washouts. We want to test the Laki scenario for all those different types of weather."
The plot at the top of this page gives an idea of what is being done. It shows a concentration for sulphur dioxide in the atmosphere.
It is just a one-hour snapshot from a 10-year sequence, illustrating the size and complexity of the computational task.
And it is not just the transport direction that matters; the project is trying to get a grip on the evolving chemistry of the cloud because reactions within it will be changing its composition as it moves.
The final report will deliver some "reasonable worst-case scenarios" to the Cabinet Office. For disaster planning purposes, this is what the office needs to work with.
Certainly, conditions in 1783 were bad enough.
Science journalists Alex Witze and Jeff Kanipe have just published an account of the event in their book Island On Fire - The Extraordinary Story Of Laki, The Volcano That Turned 18th Century Europe Dark - and also attended the EGU meeting.
They relate the immediate impacts on Iceland itself - 10,000 deaths, which would have been a fifth of the population, along with the destruction of 50% of the cattle herd and 80% of the sheep.
"But this wasn't just some little volcano in some remote country; it changed the lives of everyone around it, and everyone across the Northern Hemisphere," says Alex Witze.
"A toxic haze went all across Europe, from Iceland to Great Britain, Scandinavia, Continental Europe and beyond.
"It poisoned people, it poisoned livestock, it caused climate change around the Northern Hemisphere for more than a year.
"In the end, depending on how you count it, Laki could have killed as many as one million or even as many as six million people."
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos