Migration has always been a way of life in the Sahel, an arid belt of land that stretches across Africa just south of the Sahara. Many of the region's 100 million inhabitants lived for millennia as nomadic pastoralists who moved with their herds in search of water and pasture.

But recently, changes in rainfall patterns and rising temperatures have led to a disturbing form of population movement: climate displacement.

In June, I visited poor communities in Burkina Faso and Niger, at the heart of the Sahel. People described severe rainfall anomalies in recent years, which can be linked to climate change. In every village, the refrain was the same: "The rains are no longer predictable"; "They come too early and then end too soon"; "There is too much rain, or too little."

In the small town of Bighin in Burkina Faso, unpredictable weather has battered residents year after year. In 2010, there was not enough rain to grow millet or replenish pasture. Then, in 2011, sudden downpours caused heavy flooding that destroyed houses and schools – something people there had never seen before. But after the flooding, the rains largely ceased and crops withered, resulting in a region-wide crisis that left 18 million people without enough food and a million children at risk of starvation.

These recurrent crises have pulled the most vulnerable Sahelians into a downward spiral wherein there is insufficient time to recover before the next shock hits. Repeated droughts and floods, combined with land degradation, have lowered crop yields and wiped out people's limited savings.

Harvests that used to feed a family for almost a year run out after four or five months, forcing people to find other ways to survive. Some sell animals or go into debt, becoming poorer and more vulnerable with each passing season. Eventually, many have no choice but to leave their villages. According to one humanitarian organisation with long experience in the region, about 30% of households in part of Burkina Faso's Sahel zone have moved away in the past 20 years because they could no longer survive.

In these cases, leaving home has become a negative coping mechanism – a distress signal. Having exhausted their assets, these vulnerable households cannot afford to migrate internationally in search of new skills or higher wages.

Rather, as a last resort, they seek out more productive farmlands to the south, or move to fast-growing urban slums where they engage in petty trade or, in the worst cases, beg. What is propelling people to move is not so much the pull of economic opportunity as the push of a changing climate.

For poor Sahelians who must "leave or die here" – as one woman described her dilemma – laws and policies offer limited protection, and few long-term solutions.

Though climate-related changes have made it impossible for them to stay in their homes, they are not considered refugees because the term, as defined by the UN refugee convention, applies only to those who cross an international border while fleeing persecution.

Those who are forced to leave their homes because of climate are not granted the same protection. And national governments have not distinguished this form of population movement from economic or labour migration, despite its wholly different character.

People who must flee their homes for any reason inevitably face risks, including exploitation and extortion, insufficient access to employment and public services, gender-based violence, and damage to family and community ties.

Climate displacement may not fit easily within existing definitions or categories, but that does not mean the international community can ignore it.

With temperatures in the Sahel anticipated to increase by 3-5C (pdf) by mid-century, changing conditions will undoubtedly cause more displacement and further strain a chronically poor and unstable region. Major donors such as the US and EU, along with regional governments and aid agencies, must do more to understand and respond to climate displacement, whether it occurs in the Sahel or other parts of the globe.

However, any effort will be for nothing unless we address the underlying problem: our rising greenhouse gas emissions. Building resilience to climate change should not distract us from the need to urgently slow and reverse it. Developed and fast-developing countries whose emissions drive global climate change must curb these before it is too late, while also mitigating the devastating impacts on people – such as those in the Sahel – who are the least responsible.

Alice Thomas is climate displacement programme manager for Refugees International