Worsening climate change means the world is facing ‘a global health catastrophe’ that will hit the poorest people on earth the hardest, the British Medical Journal and the Lancet warn today. In an unusual move, the two journals simultaneously publish the same editorial calling for dramatic changes in policy and behaviour to greatly reduce carbon emissions. We reproduce it with the journals’ kind permission.

Expectations are running high for the United Nations climate change conference in Copenhagen this December. But will we get the global commitment for radical cuts in carbon dioxide emissions that the world so urgently needs?

The scientific evidence that global temperatures are rising and that man is responsible has been widely accepted since the Intergovernmental Panel on Climate Change’s report in 2007. There is now equally wide consensus that we need to reduce carbon dioxide emissions to at most 50% of 1990 levels by 2050 if we are to have even a 50% chance of preventing temperatures from exceeding preindustrial levels by more than 2 degrees, considered by many to be the tipping point for catastrophic and irreversible climate change.

The economic argument that taking action now rather than later will be cheaper has also been widely accepted since the Stern report in 2006. The election of President Obama has shifted policy in the US from seeking to block an agreement to seeking to find one.

So the chances of success should be good, but the politics are tough. The most vocal arguments are about equity: the rich world caused the problem so why should the poor world pay to put it right?

Can the rich world do enough through its own actions and through its financial and technological support for the poor to persuade the poor to join in a global agreement? The present economic climate doesn’t help, giving sceptics from the rich world arguments for not acting—or at least not acting now. And the sensitive issue of population stabilisation continues to slip off the agenda but is crucial to achieving real reductions in global carbon dioxide emissions.

These arguments need to be tackled head on. Climate change is global, and emissions know no frontiers. The necessary measures should be seen not as a cost but as an opportunity.

Coal-fired power stations and internal combustion engines pollute the atmosphere and worsen health, and deforestation destroys biodiversity, whereas saving energy helps hard-pressed household budgets, and drought-resistant crops help poor farmers. So even without climate change, the case for clean power, electric cars, saving forests, energy efficiency, and new agriculture technology is strong. Climate change makes it unanswerable.

The threat to health is especially evident in poorest countries, particularly in sub-Saharan Africa, as the recent Lancet and University College London report shows. These countries are struggling to meet the Millennium Development Goals.

Their poverty and lack of resources, infrastructure, and often governance, greatly increase their vulnerability to the effects of climate change. Warmer climate can lead to drought, pressure on resources (particularly water), migration, and conflict. The conflict in Darfur is as much about pressure on resources as the desert encroaches as about the internal politics of Sudan.

And the implications for the health of local populations are acute: on the spread and changing patterns of disease, notably water-borne diseases from inadequate and unclean supplies; on maternal and child mortality as basic health services collapse; and on malnutrition where food is scarce. And population stabilisation will not be achieved if, for want of resources, girls are not educated and contraceptives are unavailable.

Climate change is causing other kinds of extreme weather events too: storms, floods, and rising sea levels affecting coastal populations and islands. Every such event has adverse consequences for health. The poorer the country and its infrastructure, the worse are the consequences and the poorer the chances of meeting the Millennium Development Goals.

Crucially for winning hearts and minds in richer countries, what’s good for the climate is good for health. The measures needed to combat climate change coincide with those needed to ensure a healthier population and reduce the burden on health services. A low-carbon economy will mean less pollution. A low-carbon diet (especially eating less meat) and more exercise will mean less cancer, obesity, diabetes, and heart disease. Opportunity, surely, not cost.

This is an opportunity too to advance health equity, which is increasingly seen as necessary for a healthy and happy society. If we take climate change seriously, it will require major changes to the way we live, reducing the gap between carbon rich and carbon poor within and between countries.

The Commission on Social Determinants of Health said that action to promote health must go well beyond health care. It must focus on the conditions in which people are born, grow, live, work, and age, and in the structural drivers of those conditions—inequities in power, money, and resources. These insights give further confirmation that what is good for the climate is good for health.

A successful outcome at Copenhagen is vital for our future as a species and for our civilisation. It will require recognition by the rich countries of their obligations to the poor; and recognition by the poor countries that climate change is a global problem that requires a global solution in which we all have to play a part.

It will require a new mindset: that the measures needed to mitigate the risks of climate change and adapt to its already inevitable effects provide an opportunity to achieve goals that are desirable in their own right – the achievement of the Millennium Development Goals in the poor countries and a healthier more equal society in the rich world and globally. Failure to agree radical reductions in emissions spells a global health catastrophe, which is why health professionals must put their case forcefully now and after Copenhagen.

• Michael Jay, chair, Merlin; Professor Sir Michael Marmot, director, International Institute for Society and Health