With the developed world holding the largest historical responsibility for climate change, climate finance to help developing nations mitigate and adapt to the impacts of climate change is crucial to reaching an international agreement to limit global warming.

The sum of US$100 billion a year has been promised by richer nations to help with this, and real progress is finally being made.

A new report shows the majority of the promised US$100 billion is now materialising, with countries including China, France and the UK, as well as development banks making significant financing pledges, and the private sector  increasingly stepping up.

However, a significant gap of nearly US$40 billion remains, and vulnerable nations are joining forces to call on developed governments and institutions for better access to international climate change finance to help plug it.

Activists say governments need to provide a credible roadmap showing how the full pledged amount will be met by 2020, and ramped up afterwards. In this way, leaders will build the bridges of trust between developing and developed countries that are necessary to reach a meaningful, lasting and ambitious international climate change framework in Paris this December.

Crucially, climate finance has its own dedicated work stream at the G20 in Turkey this year, for the first time. This followed failed efforts by the previous host, Australia, to somehow try and pretend that climate change was not an economic issue.

A total of $5.8 billion has been pledged for the Green Climate Fund, however G20 countries have spent $458 trillion on fossil fuel subsidies in 2015.

These subsidies are 789 times higher than Green Climate Fund pledges and 848 times higher than the United Nations proposed budget of $5.4 billion for 2014 -2015.

In 2015 G20 countries have a unique historical and political opportunity to shift public spending from the perverse incentives of fossil fuel subsidies, towards meaningful climate finance activities like that of the Green Climate Fund.

Turkey is crucial in building this momentum during the G20 Leaders Summit.