In the letter, Bill and Melinda Gates say they want to push for more research on an "energy miracle" to bring clean electricity to everyone on the planet, inspired by the UN's COP 21 climate conference in Paris, France, last December. "If we really want to help the world's poorest families, we need to find a way to get them cheap, clean energy," they write.
But the move is eyed critically by some observers of the Gates Foundation's work. Linsey McGoey, a sociologist at the University of Essex in the United Kingdom and the author of a book on the Gates Foundation, says it is unclear what kind of funding the foundation would provide for energy research, and how this would tie in with other Gates' projects related to curbing climate change.
"The Gates Foundation is canny, well meaning and astute enough to realise global climate change is the fundamental challenge of our era," she says. "Yet how they will try to tackle it is still evolving."
Bill Gates described the proposed "energy miracle" as some form of technology whose 24-hour cost is competitive with fossil fuels. To find this solution, more funding of innovation in the field is needed, he said.
Gates also states that such developments in energy provision would benefit poor farmers, who, he says, will be hit hard by climate change. The Gates Foundation has strongly focused on funding agricultural projects in recent years.
Graham Brown, an international development researcher at the University of Western Australia, tells SciDev.Net there is a tension between the lofty goals just announced and the foundation's work tackling the nitty-gritty problems of the developing world.
"[The foundation] is moving from development challenges to global challenges, but in doing so, they are removing their gaze from the scale of the problems the developing world is facing," he says.
Brown says it will not require a "miracle" to bring energy to the poorest as alternative technologies have existed for decades. "The challenge is not in getting efficient fuels to remote areas, but in getting a massive transformative effect and the foundation is not willing to touch that," he says.
But Alex Reid, head of global programme communications at the foundation, says the 2016 letter was "not a foundation annual letter but Bill and Melinda talking about issues they focus on outside the foundation and are investing in" - unlike previous letters.
Others have also challenged the technological focus. Last month, the NGO Global Justice Now released a report saying the foundation's emphasis on such an approach often ignored real solutions involving social and economic justice.
"This cannot be given by donors in the form of a climate-resilient crop or cheaper smartphone, but must be about systemic social, economic and political change - issues not represented in the foundation's funding priorities," the report says.
Polly Jones, head of policy and campaigns at Global Justice Now, says: "We would argue [that] the Gates Foundation needs to evaluate what methods are needed to be effective."
Mark Curtis Gated development: Is the Gates Foundation always a force for good (Global Justice Now, January 2016)