Developing countries may need up to $500 billion per year by 2050 to adapt to the ravages of climate change, dwarfing previous estimates, a UN report said Friday.

The figure was about 20 times today's public spending on climate adaptation, according to the United Nations Environmental Programme (UNEP) that warned of a "significant funding gap after 2020."

And the number could be further inflated if countries fail to meet the UN target of limiting global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.

"The impacts of climate change are already beginning to be factored into the budgets of national and local authorities," UNEP executive director Achim Steiner said in a statement.

"The escalating cost implications on communities, cities, business, taxpayers and national budgets merit closer attention as they translate into real economic consequences," he added.

In 2012-13, the amount of global public finance committed to adaptation was about $23-26 billion, of which 90 percent went to developing countries.

Adaptation support is a key sticking point at UN negotiations under way in Lima to hammer out the broad outlines of a new world pact to curb global warming.

Poor countries most vulnerable to climate-change-induced impacts -- extreme weather events, floods, droughts and sea-level rise -- are demanding that a rich nation commitment to adaptation and finance help be written into the pact.

But many developed countries insist the deal, due to be signed in Paris in December 2015 to enter into force by 2020, should focus on mitigation -- meaning efforts to curb planet-warming greenhouse gas emissions.

Steiner said the new report "underlines the importance of including comprehensive adaptation plans in the agreement."

- 'Significant underestimate' -

The UN's top climate science body, the Intergovernmental Panel on Climate Change (IPCC), has projected adaptation costs in developing countries to reach $70-100 billion per year by 2050, based largely on World Bank figures from 2010.

But the new UNEP report said this was likely a "significant underestimate", even if warming can be limited to two degrees Celsius this century -- which many scientists say is unlikely.

Data gathered by research institutions, based on a wider and more detailed database, found that "at a minimum, the costs of adaptation are likely two to three times higher," it said.

And on some calculations, based on national-level rather than global-level studies, "adaptation costs could climb as high as $150 billion by 2025/2030 and $250-500 billion per year by 2050" -- and double that if the global average temperature rise is allowed to approach 4 C.

Senior climate change advisor Mohamed Adow of Christian Aid said some developing countries were already at their financial limit for climate adaptation.

"It's a cruel irony that it is the rich countries whose carbon emissions helped create these climate change impacts that don't want adaption to be a central part of the Paris agreement," he said.

Sandeep Chamling Rai, adaptation policy advisor to green group WWF, added that the report "opens up a window onto a nightmarish future, where the global economy is crippled and the most vulnerable countries are even further disadvantaged.

"This is not a gap, it's an abyss. We can avoid falling into it, but we're running out of time."

Gathering 195 states and the European Union bloc, the 12-day Lima meeting has as one of its tasks to draft guidelines for nations when they make emissions-cutting pledges next year -- commitments that are at the heart of the new pact.

The climate negotiations have been bedeviled for years by rifts between rich and poor nations over who should shoulder the burden of emissions cuts, which require a politically and financially difficult shift from cheap and plentiful fossil fuel to cleaner energy sources.

Finance remains a sore point, with developing nations insisting that rich economies must show in Lima how they intend to honor promises to muster up to $100 billion in climate finance per year from 2020.

To date, nearly $10 billion in startup capital had been promised for the Green Climate Fund, the main vehicle for channeling the money.

Norway on Friday said it will provide $258 million to the fund over the next four years.