The Atlantic Ocean's current system, an engine of the Northern Hemisphere's climate, could be weakening to such an extent that it could soon bring big changes to the world's weather, a scientific study has found.

The Atlantic Meridional Overturning Circulation is a large system of ocean currents which transports warm water from the tropics northwards into the North Atlantic.

As the atmosphere warms due to increased greenhouse gas emissions, the surface ocean beneath retains more of heat.

A potential collapse of the system could have severe consequences for the world's weather systems.

Climate models have shown that the AMOC is at its weakest in more than a 1,000 years.

However, it has not been known whether the weakening is due to a change in circulation or it is to do with the loss of stability.

The study, published in the journal Nature Climate Change, said the difference is crucial.

"The loss of dynamical stability would imply that the AMOC has approached its critical threshold, beyond which a substantial and in practice likely irreversible transition to the weak mode could occur," said Niklas Boers at the Potstdam Insitute for Climate Impact Research and author of the study.

By analysing the sea-surface temperature and salinity patterns of the Atlantic Ocean, the study said the weakening of the last century is likely to be associated with a loss of stability.

"The findings support the assessment that the AMOC decline is not just a fluctuation or a linear response to increasing temperatures but likely means the approaching of a critical threshold beyond which the circulation system could collapse," Mr Boers said.

If the AMOC collapsed, it would increase cooling of the Northern Hemisphere, sea level rise in the Atlantic, an overall fall in precipitation over Europe and North America and a shift in monsoons in South America and Africa, Britain's Met Office said.

Other climate models have said the AMOC will weaken over the coming century but that a collapse before 2100 is unlikely.

Climate change raises risk of crop disease

Climate change may boost crop yields in Europe and countries further away from the equator, but the produce could also face an increased risk of infection by pests, research suggests.

Scientists at the University of Exeter have developed models to predict what will happen to crops in the future as the planet's temperatures rise.

The findings, published in the journal Nature Climate Change, indicate that tropical areas such as Brazil, sub-Saharan Africa, India and South-east Asia, may see the impact of crop diseases fall.

But, the team added, disease risk will grow in areas with higher latitudes, with Europe and China being "particularly vulnerable" to around 80 fungal and fungi-like plant pathogens.

Study author Professor Daniel Bebber, of University of Exeter's department of biosciences and the Global Systems Institute, said: "Plant pathogens already cause devastating production losses globally.

"Our previous research has shown that crop pests and pathogens are moving away from the equator, and this new study estimates risks from pathogens in the coming decades.

"Our results show that climate-driven yield gains in temperate regions will be tempered by the increased burden of crop protection.

"Rapid global dissemination by international trade and transport means pathogens are likely to reach all areas in which conditions are suitable for them."

The authors compared current yields and future yields projections for 12 major crops.

Thomas Chaloner, a PhD student at the University of Exeter and first author on the study, said: "Agriculture has to plan and prepare for the future - and that future is almost here.

"We have only got a few decades, and crop breeding can take a long time, so we need to think about resistance to pathogens that haven't arrived yet.

"A lot of pathogens - especially those currently found in tropical areas - are seriously under-researched.

"We need to invest in understanding these diseases, which could become increasingly prevalent in the key crop-growing areas of the world."