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% and 48% of cases, have been newly diagnosed 1,000 miles 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, including ciguatera, which can cause severe and sometimes lethal gastric and neurological damage - though very hot seas may prevent spread.
Prof Parmesan warned new healthcare strategies will be needed to deal with and treat tropical pathogens where historically they have not been needed.
The report, by 80 scientists from 12 countries also found that groups of species such as jellyfish, turtles, seabirds and plankton are being driven up to 10 degrees north by warming oceans, affecting the breeding success of marine mammals.
Fish are moving to cooler waters and warming is damaging habitats, affecting fish stocks and potentially leading to smaller catches in tropical regions, the report said.
In east Africa and the West Indian Ocean, warming seas have killed parts of the coral reefs fish depend on, reducing populations already hit by overfishing and destructive techniques such as dynamite fishing.
In south-east Asia harvests from marine fisheries are expected to fall by between 10% and 30% compared with the period 1970 to 2000, if greenhouse gas emissions carry on at current levels.
Warming oceans are also affecting weather patterns, with the number of severe hurricanes rising by around 25%-30% with each degree centigrade of warming.
The changes to the world's seas are leading to more rainfall in some areas, including those with monsoons, and less rain in sub-tropical areas, which is set to affect crop yields in important regions such as North America and India, it said.
The IUCN's Dan Laffoley, one of the lead authors of the report, said: "Most of the heat from human-induced warming since the 1970s - a staggering 93% - has been absorbed by the ocean, which acts as a buffer against climate change, but this comes at a price."
The report calls for rapid and substantial cuts to greenhouse gases, expansion of marine protected areas, legal protection of the high seas, and recognition of the severity of the problem and efforts to fill in gaps in scientific knowledge.