* Oil major says fossil fuels will be needed through 2050
* Calls "carbon bubble" theory flawed in letter to shareholders
* Says decarbonisation of energy sector will take decades
* Predicts it could take rest of century to resolve climate change
LONDON, May 19 (Reuters) - Royal Dutch Shell has dismissed the possibility that its proven oil or gas reserves will become unusable as a result of climate change regulation, saying fossil fuels will play a key role in global energy to 2050 and beyond.
Environmental campaigners, activist investor groups and some lawmakers have warned that financial markets could be overvaluing companies with large fossil fuel assets, such as Shell, thereby creating a "carbon bubble" and putting at risk trillions of dollars in pension funds.
Shell, however, played down such claims last week in a letter it said was in response to shareholder inquiries on the issue of "stranded" assets, referring to large investments in fossil fuel reserves that could become unprofitable if governments pass laws to curb runaway growth in greenhouse gas emissions in an attempt to reduce the impact of climate change.
"While the 'stranded asset' notion may appear to be a strong and thought-through case, it does have some fundamental flaws," JJ Traynor, Shell's executive vice president of investor relations, said in the letter posted on the company's website, dated May 16.
Traynor maintained that the world will need oil and gas for many decades to come, supporting both demand and prices.
"As such, we do not believe that any of our proven reserves will become 'stranded'," he wrote.
"There is a risk that focusing on 'stranded assets' or the concept of the 'carbon bubble' distracts attention away from the reality of a growing population, increasing prosperity and growing energy demand."
Shell said it believes that climate change will continue to rise up the public and political agenda, but it estimated that energy demand growth would allow fossil fuels to continue play a major role and account for 40-60 percent of global energy supply through to 2050 and beyond.
Shell also pushed carbon capture and storage - a technology that critics say has not yet been proven - as a key tool to help to keep global warming below 2 degrees Celsius, a threshold deemed dangerous by scientists.
While much of the world has yet to put a price on greenhouse gas emissions, Shell said it employs a cost of $40 per tonne of CO2 when calculating the financial viability of its projects.
In its 2013 annual report, Shell warned that tougher rules on greenhouse gas emissions may lead to higher operating costs, delayed projects and reduced demand for its products.
But on Friday the company said that the long-lived nature of the global energy system's underlying infrastructure and the many assets within it mean that any regulatory-induced change would "inevitably take decades".
"The world can tackle and resolve the climate issue over the course of this century, but not in less time than that," it added.
Exxon Mobil, the world's largest publicly traded oil company, said in March that it was confident that none of its reserves will become stranded if governments act to bring about a drastic reduction in emissions. (Reporting by Michael Szabo; Editing by David Goodman)