Last year, the world reached another global population milestone when the number of people on the planet passed the eight billion mark.
As World Population Day is marked today, growth continues, with the UN's latest World Population Prospects report forecasting that, by 2050, our increasingly crowded planet will host 9.7 billion human beings.
While there are numerous projections, there is agreement that the population is likely to continue growing for several decades before it peaks.
UN demographers predict that at the end of this century, the world population will be 10.4 billion.
professor of demography at the University of Bologna in Italy
It raises the question of whether population growth, by leading to greater energy demands, higher rates of consumption and travel as well as agricultural expansion will derail efforts to combat climate change.
One factor cited by analysts is that population growth – which happens because of the lag between infant mortality falling and people having fewer children – is fastest in parts of the world where consumption is lower.
More than half of world population growth until 2050 is expected to occur in Sub-Saharan Africa, where the most recent World Bank figures indicate that average annual carbon emissions are about 0.7 tonnes per person per year, compared to the global average of 4.3 tonnes.
As a result, population growth in the coming decades may have less of an impact than it would have had, had it been happening in richer regions.
Raya Muttarak, professor of demography at the University of Bologna in Italy, said the real challenge of dealing with climate change is reducing consumption in richer parts of the world.
"What’s really interesting is that the lowest-income group in the US still emits carbon more than the highest-income group in Africa," she said.
Lisa Schipper, professor of development geography at Bonn University in Germany, also noted that "the main population growth is not happening" in the most carbon-intensive regions in the world.
"The more people, there’s more pressure on resources, but you cannot compare somebody coming out of poverty living in Ethiopia with somebody living in the UK, for example," she said.
"There’s going to be significantly more emissions in the UK because of the kind of networks and resources they use on a daily basis."
Slower rates of population growth
However, some researchers have argued that achieving slower rates of population growth could be part of a strategy to control carbon emissions.
In a 2017 paper in Environmental Research, economists Gregory Casey of Williams College in the US and Oded Galor of Brown University looked at population growth forecasts and carbon emissions in Nigeria.
"We find that by 2100 moving from the medium to the low variant of the UN fertility projection leads to 35 per cent lower yearly emissions and 15 percent higher income per capita," they wrote.
"These results suggest that population policies could be part of the approach to combating global climate change."
Much of the population growth to 2050 will be, the UN stated in the World Population Prospects report, a consequence of past growth "embedded in the youthful age structure of the current population". But actions by governments to reduce fertility could have an effect.
"The cumulative impact of such changes could contribute to a more substantial reduction of global population growth in the second half of the century," the organisation said.
Policies that promote gender equity are seen as one way to limit population growth, because women with greater freedom to choose typically have fewer children.
Organisations such as the Centre for Biological Diversity, a US charitable organisation, say that while this is often seen as applicable to poorer nations, greater gender equity in the US, too, "could have a substantial environmental impact".
Food production at risk
Dr Muttarak said that the key issue is not so much total food production, but a lack of equity distribution.
"We have the problem of over-consumption and under-consumption", with climate change set to create further disruption, she said.
"Conflict, climate change, it will disrupt food production. That’s something we have to worry about. But climate change in certain areas can make agricultural production better, for instance in the UK and Northern European countries," she added.
In other areas, such as the Sahel, South Asia - home to India, the world's most populous country with 1.43 billion people - and South-East Asia, she warned that food production could be hit by climate change.
This is noted by the UN's Intergovernmental Panel on Climate Change, which said that if global temperatures reach or exceed 2°C above pre-industrial levels, there could be malnutrition and deficiencies of micronutrients, especially in regions including South Asia, Sub-Saharan Africa, and Central and South America.
"Global warming will progressively weaken soil health and ecosystem services such as pollination, increase pressure from pests and diseases, and reduce marine animal biomass, undermining food productivity in many regions on land and in the ocean," the IPCC wrote in a report last year.
Climate change, in part thanks to effects on agriculture of increasing temperatures and more weather extremes, including droughts, could significantly increase migration. One forecast suggested there could be one billion "climate migrants" by 2050.
A study published in May found that because of climate change, about 9 per cent of the world’s population - about 600 million people - live outside the "human climate niche", the conditions in which people have historically thrived.
By the end of this century, if current policies cause global temperatures to increase to 2.7°C above pre-industrial levels, one third of people could live outside the niche, the authors warned.
"Exposure outside the niche could result in increased morbidity, mortality, adaptation in place or displacement [migration elsewhere]," they wrote in Nature Sustainability.
"When we look at migration, most of this will be within a country," Dr Schipper said. "That’s going to place huge pressures. There’s going to be migration to all sorts of areas, but primarily to urban areas."
Hope for breakthroughs
While there is concern that climate change will increase migration, Ilya Kashnitsky, assistant professor of demography in the Interdisciplinary Centre on Population Dynamics at the University of Southern Denmark, noted that technological breakthroughs may help people to cope with some of the worst effects.
He cited Israel's achievements with water management. The country has developed advanced desalination technology, recycles most of its wastewater and farms use dew to create water for irrigation.
Some forecasters predict that by the end of the decade, this largely desert nation could become a net water exporter.
"So it may be that not all the apocalyptic scenarios will [happen], even with a failure to address climate change issues," Dr Kashnitsky said.
Another driver of migration is the ageing of populations in Europe and North America, as this generates a demand for incoming labour.
"For example, it’s observed in many countries that the cost of healthcare and the care for the elderly is increasing very fast," Dr Kashnitsky says.
"The most developed countries are in dire shortage of healthcare workers. It’s becoming a big issue. Many European countries solve this by importing foreign labour."
He added that migration plays "an important role in population replacement in the developed world", but it remains "difficult to say" what will happen when countries that are donors in population terms themselves grow old.
These countries, he said, will "need their healthcare workers there".
"It’s really difficult with migration to forecast anything," Dr Kashnitsky added. "Trends change, not only from population development, but from economic and political reasons."
