The warming of the world's oceans is spreading dangerous diseases and affecting fish stocks and crop yields, a major new report has said.
Conservationists warned the world is "completely unprepared for" the impacts of warming oceans on wildlife, natural systems and humans, some of which are already being felt.
Even with action to significantly reduce the greenhouse gas emissions which are causing ocean warming, there will still be a high risk of impacts, according to the report launched by the International Union for Conservation of Nature (IUCN).
IUCN director general Inger Andersen said: "Ocean warming is one of this generation's greatest hidden challenges - and one for which we are completely unprepared.
"The only way to preserve the rich diversity of marine life, and to safeguard the protection and resources the ocean provides us with, is to cut greenhouse gas emissions rapidly and substantially."
As part of the report, findings from University of Plymouth professors Camille Parmesan and Martin Attrill show that marine-related tropical diseases and harmful algal blooms are spreading to colder regions for the first time.
Outbreaks of Vibrio vulnificus, a relation of the bacteria causing cholera and which causes death in between 30 per cent and 48 per cent of cases, have been newly diagnosed further north than previously recorded.
The disease has previously been a problem in warm waters such as the Gulf of Mexico where mostly it has been contracted by eating infected oysters, but cases have recently occurred in the Baltic and Alaska, the report warns.
Warming sea surface temperatures in fishing grounds can also cause toxins from algal blooms to enter the food chain.