Two landmark conferences of the 1990s really seemed to get the links between human population and the environment. The 1992 Rio Declaration on Environment and Development noted that "human beings are the centre of concern for sustainable development." Building on this two years later, the Cairo Programme of Action included the objective "to reduce both unsustainable consumption and production patterns as well as negative impacts of demographic factors on the environment in order to meet the needs of current generations without compromising the ability of future generations to meet their own needs."

But in the following years, population started to fall off the map. In 2002, after several preparatory meetings for the Johannesburg Summit (the UN's World Summit on Sustainable Development), population as a key component of sustainable development was still absent from the agenda. As a response, Wolfgang Lutz of the International Institute for Applied Systems Analysis (IIASA) and 34 other distinguished scientists from various disciplines and regions organized the Global Science Panel on Population and Environment, calling for population to be included at the core of the agenda. Though the panel successfully got the message out, participating governments eventually decided to leave population out of the negotiation process.

Population was at the center of public discussion, many national policies, and almost all international conferences and agreements from the late 1950's to the early 1990's. The sudden shift away from this issue was unexpected for many people, and just as population, family planning, and reproductive health were left out of the Millennium Development Goals, population has been largely absent from the response to climate change, potentially the greatest environmental threat we have ever faced.

Population has been overlooked by the Intergovernmental Panel on Climate Change (IPCC), which sets the gold standard in climate research. Its 1995 report devoted only a few pages to the role of population in this 2,000 page document; the companion summary for policymakers did not even mention population. Population was also forgotten in the 1997 Kyoto Protocol, aimed at limiting carbon dioxide emissions and reducing the threat of global climate change.

John Bongaarts, a leading demographer at the Population Council, and Brian O'Neill, a scientist at the National Center for Atmospheric Research, argue that three common misconceptions can explain why population has been pushed to the fringe: the beliefs that (1) the real problem is consumption, not population; (2) not much can be done about population, and (3) strengthening population policies leads to coercion.

Emphasizing the issue of consumption rightly points out the importance of curbing rich countries' consumption of fossil fuel in combating climate change. However, population growth in both developed and developing nations is expected to play a very important role in global greenhouse gas (GHGs) emissions. In a paper published in Population and Development Review, Fred Meyerson, assistant professor at the University of Rhode Island, writes:

[For developed countries,] per capita carbon emissions have stabilized or even decreased in the last two decades. This means that emission increases in the developed world are now primarily driven by population growth.

Richard York, a professor at the University of Oregon, has also found that

[developed] nations with higher expected population growth rates negotiated higher carbon targets (in global GHGs emission treaties) than other nations and were ultimately less likely to ratify the Kyoto Protocol.

Population matters to climate change, in both developed and developing countries, for different reasons and to different extent. It is not only because of total population size and fertility, but also because of other demographic trends, particularly aging in the developed nations and urbanization in the developing world.  First, even though it is more important in the developing world, change of total population size still plays a significant role in climate change in developed countries where fertility rate reached or declined to below the replacement level, due to population momentum and net immigration.

Second, in the developed world, the more important demographic trend relevant to climate change is the changes in population age structure.
Michael Dalton, economist at the National Oceanic and Atmospheric Association, and his co-authors have shown that future changes of population age structure -- the comparative size of specific age groups relative to the population as a whole -- under a scenario of low fertility, will drive U.S. carbon emissions down by 40 percent by the end of the century. This effect, under certain circumstances, would be even more significant than technological advancements. While this does not imply that population growth is more important than other factors for climate change, it surely illustrates that demographic factors do affect environmental consequences and the feasibility of social choices.

While it is important to be vigilant against them, coercive population policies are highly criticized by the global community. Voluntary family planning programs, on the other hand, have produced multiple benefits for society and the environment. High quality voluntary services, along with efforts to improve female education and promote gender equity, have already helped individuals achieve their reproductive preferences and reduce the population size of less developed countries by half a billion.

Recently, population issues have gradually regained the spotlight -- probably partially due to the pressing issue of global warming (the top 11 warmest years on record have occurred in the last 13 years), the increasing impact from emerging economies' energy consumption (especially highly-populated countries such as China and India), and the global food crisis.

The IPCC has been looking to population more as well. Its Fourth Assessment Report, published in 2007, calls for greater focus on population dynamics in future research, and the organization has founded a consortium to develop new demographic and socioeconomic scenarios to help improve our understanding of how demographic trends (such as aging, urbanization and changes in household structure) will interact with economic growth and technological improvement to determine the global climate future.

It is indisputable that economic development, improved standard of living and energy poverty alleviation are necessary to improve the lives of the global poor. Even though the poor still account for a very small amount of overall GHGs emissions (disproportionately less than their population share), rapid economic development is taking place in much of the developing world. The proportion of global emissions originating in the developing world is growing as well, and is anticipated to overtake that of the more developed nations in the next one or two decades.

To fight global warming, international, collective actions are needed -- including financial support and technological transfer to developing countries. It is acknowledged that rapid implementation of multiple strategies is needed to curb global warming -- the international community is working on multiple clean and energy efficient technologies, carbon capture and storage, and prevention of deforestation, among many others.

It is important that the scientific community also consider population factors -- in all their complexity -- in climate change research. Moreover, the sexual and reproductive health community needs to be aware of and engaged on this issue -- to add their expertise, and to ensure that these issues get back on the agenda, where they belong.