Acidification of the oceans is a major threat to marine life and humanity's food supply, Hilary Benn is to warn as the UN climate summit resumes.
The UK environment secretary will say that acidification provides a "powerful incentive" to cut carbon emissions.
Ocean chemistry is changing because water absorbs extra CO2 from the air.
Some believe this could be as big an impact of rising CO2 levels as climatic change, though it is rarely discussed within the UN climate convention.
The UN summit in Copenhagen, which started a week ago, is scheduled to conclude on Friday, when more than 100 world leaders will attend in an effort to agree a new global treaty on climate change.
'Really important'
 | OCEAN ACIDIFICATION Up to 50% of the CO2 released by burning fossil fuels over the past 200 years has been absorbed by the world's oceans This has lowered the pH value of seawater - the measure of acidity and alkalinity - by 0.1 The vast majority of liquids lie between pH 0 (very acidic) and pH 14 (very alkaline); 7 is neutral Seawater is mildly alkaline with a "natural" pH of about 8.2 The IPCC forecasts that ocean pH will fall by "between 0.14 and 0.35 units over the 21st Century, adding to the present fall of 0.1 units since pre-industrial times"  |
The science has come to prominence only within the last five or six years, and most of the details were not available when the convention was signed in 1992.
"We know that the increasing concentration of CO2 [in the air] is making the oceans more acidic," Mr Benn told BBC News.
"It affects marine life, it affects coral, and that in turn could affect the amount of fish in the sea - and a billion people in the world depend on fish for their principal source of protein.
"It doesn't get as much attention as the other problems; it is really important."
In September, the UN-backed study into The Economics of Ecosystems and Biodiversity (Teeb) concluded that the widely-endorsed target of trying to stabilise atmospheric concentrations of CO2 or their equivalent to around 450 parts per million (ppm) would prove lethal to much of the world's coral.