Britain’s contribution is measured at £950 million ($1.21 billion) short despite the global twofold increase agreed on at Cop26 in the UK.
The report funded by insurer Zurich says the US should provide an extra $13.8 billion to meet its rightful share of the bill.
Adaptation finance helps countries prepare for life on a warmer planet by making them more resilient to floods, drought and other effects.
It is one of several cogs in the wheel of climate finance, which includes funding for cutting emissions and dealing with disasters.
The UAE’s presidency of Cop28 has called for developed countries to “ensure that the goal of doubling adaptation finance by 2025 is on track”.
The summit’s President-designate Dr Sultan Al Jaber, Minister of Industry and Advanced Technology, is also calling on development banks to “continue putting a stronger emphasis on scaling adaptation finance”.
A 19-nation UN finance committee has asked for submissions as it checks progress on adaptation funding, which would rise from $20 billion in 2019 to $40 billion in 2025 if the goal is met.
The draft findings submitted by the Overseas Development Institute and Zurich’s Flood Resilience Alliance say $28.3 billion was raised in the most recent annual figures for 2021, suggesting the goal is achievable by 2025.
The 11 countries already on track, led by Sweden, France, Germany and Norway, “could be commended for already providing their fair share”, the report’s authors said.
“This suggests they may be making a concerted effort to respond to the needs and priorities of developing countries.”
In contrast, Australia, Spain, Canada, the UK and Italy “are also large economies that pay less than their fair share of adaptation finance”, the 15-page paper said.
It said the UK and Italy were among the “more generous” of the group in overall climate funding but were not putting enough towards adaptation.
“Countries not paying their fair shares are arguably the countries that should be the focus of diplomatic and advocacy efforts seeking to increase the total volume of climate and adaptation finance,” the authors said.
Researchers acknowledged a “lively debate” over how the $40 billion bill should be split. They used three measures – population, economic strength and cumulative CO2 emissions.
By these measures, the US should come up with $17.4 billion “given its population size, economic heft and historical responsibility for climate change”, they said.
The paper said the UK should generate £2.35 billion. It provided £1.4 billion in 2021, according to the report.
In a separate submission, the UK said public funding “will not be sufficient to close the gap” in adaptation needs. The Zurich-funded report did not cover privately mobilised funds but said their current volume was small.
Britain has repeatedly touted the pledge to double adaptation funding as an achievement of its Cop26 presidency. The baseline of $20 billion raised in 2019 was double that in 2013.