President Biden has promised to double the amount of money the United States is providing to help poorer countries deal with climate change.
His speech at the United Nations came a day after Boris Johnson said it was going to be 'tough' to meet climate finance goals in advance of the 26th UN Climate Change Conference (Cop26), which takes place in Glasgow in November.
The developed world had pledged to provide $100bn (£720m) a year by 2020 to poorer countries, to help them cope with climate change, but this has still not been achieved.
What is the money for?
It is going to be hard enough for rich countries to adjust to the need to remove fossil fuels and carbon from their own economies.
But it is going to be a lot more challenging in developing nations, where there is far less money to pay for new infrastructure and technology.
And there are an awful lot of people under threat.
So funding is needed for:
- adaptation - adjusting to the growing effects of climate change
- mitigation - reducing the release of greenhouse gases into the atmosphere
How much has been promised?
As long ago as 2009, the developed world agreed it would provide $100bn a year by 2020 to help poorer countries:
- deal with the effects of climate change
- build greener economies in the future
But, although official figures have not yet been released, an expert report commissioned by the United Nations concludes the target has not been reached - even though a new and more ambitious target is now supposed to be set for 2025.
"The $100bn commitment should be seen as a floor not a ceiling," lead author Amar Bhattacharya, from the Brookings Institution, says.
"Some progress has been made - but a lot more needs to be done."
For many countries, this is the biggest issue to resolve in the run-up to Cop26 - and the very poorest are demanding action.
How far short are the pledges?
It is quite hard to calculate what money should be included in the overall figure, because it is a complicated mix of money from governments, international lenders and private companies.
But the UN and the Organisation for Economic Co-operation and Development (OECD) estimate that by 2019 the figure had reached just over $79bn, only a small increase on the previous year, and they say it won't have got to $100 billion by 2020.
Between 2016 and 2018, 43% of the funding went to Asia, 25% to Africa and 17% to the Americas - a lot of it spent on green energy or transport infrastructure.
Who is not paying enough?
The rich countries recognise they have not yet met the target they set themselves.
"Within the G7 [group of rich countries], the three countries that have been the leaders are Germany, Japan and France in that order," Mr Bhattacharya says.
The UK and Canada are slightly behind them - but the two big laggards are the United States and, in particular, Italy.
In April, the US announced it would double its 2016 climate-finance contributions to $5.7bn by 2024.
In his UN speech, President Biden said he would go further.
"Today I'm proud to announce that we will work with the [US] Congress to double that number again. This will make the United States the leader in public climate finance".
Critics point out that even if another $5.7bn is approved, it is still a relatively modest amount given the size of the US economy.
Meanwhile, Italy provides only about $0.6bn per year.
Both countries have signed up to a big push to increase the overall amount of money provided, but - as Mr Biden acknowledged - that will need domestic political approval.
Ministers from Germany and Canada have already been tasked with developing a "credible delivery plan" to make sure the $100bn figure is achieved. But experts say that should be only the beginning.
Dr Alina Averchenkova, from the Grantham Research Institute on Climate Change and the Environment, says: "$100bn isn't going to do it - we need to move trillions in both public and private money.
"The pandemic has shown us it can be done when there is the political will.
"Unfortunately, climate change is quickly becoming the same kind of emergency - and it will be with us for the long term."
Are there any strings attached to the money?
Yes. By 2018, about three-quarters of the government money made available for climate action in developing countries was in the form of loans that need to be paid back, rather than grants that do not.
And that is a big problem in countries, many already heavily in debt, where Covid has made access to international funds even more pressing.
"Developing countries cannot just rely on loans, so it is going to be really important that more climate finance is provided in grants," Dr Averchenkova says.
"It's never going to be the whole amount - but it needs to be more."
So it is the quality as well as the quantity of funding that matters.
And the message from the world's poorer countries is pretty simple - if you want ambitious climate targets, you are going to have to pay for them.