In Pakistan, where just a third of married women use contraception, half of all pregnancies - 4.2 million each year - are unintended, according to the Washington-based Population Reference Bureau.

At the same time, the rising population in Pakistan – and elsewhere around the world - is creating more climate-changing emissions and putting more people in the path of extreme weather, food and water shortages, and other climate change pressures.

That suggests that giving more women who want it access to birth control to limit their family size – in both rich and poor countries – could be a hugely effective way to curb climate change and to build greater resilience to its impacts, according to population and climate change researchers and policy experts.

"We're not talking about population control. We're talking about giving people the choice to limit their family size and all the good things that go on from that" such as better health and education, said Baroness Jennny Tonge, chair of the All-Party Parliamentary Group on Population, Development and Reproductive Health, during an event at the UK Parliament Monday on linking population and climate issues.

Bringing together two politically contentious concerns – climate change and managing population growth – in an effort to build effective policy has been far from easy.

"They're both sensitive and it's difficult to make headway on either, much less both together," admitted Jason Bremner, a demographer and associate vice president of the Population Reference Bureau.

Still, an international coalition of experts on climate change, family planning and development aid are now pushing for universal access to family planning to be recognized as a part of "climate-compatible development" and included in new U.N.-backed Sustainable Development Goals set to be agreed in September.

Some countries, such as Ethiopia, already have included family planning among the activities they want to undertake on climate change, using international climate finance, according to an analysis by the London-based Population and Sustainability Network.

"They themselves identified population as a factor making it more difficult for them to adapt. We in the north are worried about, 'Is it fair to make this connection?' when people in the south are already making it," said Karen Newman, coordinator of the network.

Population growth has an impact on climate-related pressures as diverse as land availability, access to water, deforestation and migration, which often occurs "to coastal areas where vulnerability to climate change is very high", said Newman, a sexual and reproductive health and rights expert.

Family planning could potentially find a funding source in the Green Climate Fund, which was established as part of U.N.-led climate talks and which will later this year and early next begin its first distributions of about $10 billion in funds donated to help poor countries adapt to climate change impacts or adopt a lower-emission development path.

Money is key because "we can make all the policies in the world but if there isn't financing for both (climate change and birth control), neither are going to get any better", Bremner said.

But he admitted he had "not a lot" of confidence family planning projects would be supported by the climate fund, which faces a huge range of demands on its resources.