Crop production in Britain will fall dramatically if climate change causes the collapse of a vital pattern of ocean currents, new research suggests.
The Atlantic Meridional Overturning Circulation (AMOC) brings heat from the tropics, making Britain warmer and wetter than it would otherwise be.
University of Exeter scientists show that, while warming Britain is expected to boost food production, if the AMOC collapses it would not just wipe out these gains but cause the "widespread cessation of arable (crop-growing) farming" across Britain.
Such a collapse - a climate change "tipping point" - would leave Britain cooler, drier and unsuitable for many crops, the study says.
The main problem would be reduced rainfall and, though irrigation could be used, the amount of water and the costs "appear to be prohibitive".
"If the AMOC collapsed, we would expect to see much more dramatic change than is currently expected due to climate change," said Dr Paul Ritchie, of the University of Exeter.
"Such a collapse would reverse the effects of warming in Britain, creating an average temperature drop of 3.4°C and leading to a substantial reduction in rainfall (?123mm during the growing season).
"These changes, especially the drying, could make most land unsuitable for arable farming."
The study examines a "fast and early" collapse of the AMOC, which is considered "low-probability" at present - though the AMOC has weakened by an estimated 15% over the last 50 years.
Professor Tim Lenton, Director of the Global Systems Institute at the University of Exeter, said worst-case scenarios must be considered when calculating risks.
"Any risk assessment needs to get a handle on the large impacts if such a tipping point is reached, even if it is a low-probability event" he said.
"The point of this detailed study was to discover how stark the impacts of AMOC collapse could be."
The study follows a recent paper by Lenton and colleagues warning of a possible "cascade" of inter-related tipping points.
The new study reinforces the message that "we would be wise to act now to minimise the risk of passing climate tipping points" said Lenton.
Growing crops is generally more profitable than using land as pasture for livestock rearing, but much of northern and western Britain is unsuitable for arable farming.
"With the land area suitable for arable farming expected to drop from 32% to 7% under AMOC collapse, we could see a major reduction in the value of agricultural output," said Professor Ian Bateman, of Exeter's Land, Environment, Economics and Policy Institute.
"In this scenario, we estimate a decrease of £346 million per year - a reduction of over 10% in the net value of British farming."
Speaking about the expectation that moderate warming would boost agricultural production in Britain, he added: "It's important to note that the wider effects for the UK and beyond will be very negative as import costs rise steeply and the costs of most goods climb."
The study focusses on agriculture, but AMOC collapse and the resulting temperature drop could lead to a host of other economic costs for the UK.
The AMOC is one reason that average temperatures in Britain are warmer than those of many places at similar latitudes. For example, Moscow and the southern extremes of Alaska are further south than Edinburgh.
The paper, published in the inaugural issue of the journal Nature Food, is entitled: "Shifts in national land use and food production in Great Britain after a climate tipping point."