Sitting in a New York restaurant with several top climate scientists on a beautiful summer evening, it's difficult to remember that, nine months ago, this part of Manhattan was without power for six days as a result of a Hurricane Sandy storm surge. But the climate scientists around the table had not forgotten. They warned it could get much worse as average global temperatures rise - problems so severe as to be flagged profound national security challenges.

Scientists have been wrestling with this problem for a long time. The atmosphere is trapping more heat, largely as a result of human activity, causing a build-up of CO2. Modelling the effects is difficult, they explained, because the additional heat is fed into a dynamic, finely balanced system that resists change - up to a point. Oceans act as heat sinks, forests expand to soak up carbon, glaciers melt, and so forth.

And there are still elements of the system we can't fully model, such as the melting rate of Greenland's disappearing icecap or the heat exchange processes that occur within the deep ocean. There are also adverse feedback mechanisms - such as the increased absorption of solar heat when ice cover shrinks, and the release of methane from long-frozen, decayed vegetation in the permafrost, which can be seen bubbling up in puddles in the Arctic. These have made previous modelling efforts understate the pace and severity of prospective climate change.

But as former Senior Official of the National Science Foundation Bob Correll explained, the conclusion is inescapable: Earth is going to warm substantially, and soon. Perhaps, 3.88 to 5 degrees Celsius (7 to 9 Fahrenheit) over the next 80 years.

For context, this is more than twice the temperature range change since the most recent Ice Age, when parts of Europe and North America were covered by more than a mile of ice. And the consequences will likely shake the foundations of global civilisation as we know it today.

Water shortages will become endemic in some regions previously supplied by glacial streams - affecting some two billion people in Asia. Rainfall patterns will change, drying up large parts of the most fertile agricultural lands, including in Africa. Water scarcity is likely to affect up to half the world's population by 2030. Sea levels will rise by more than a metre, some islands will disappear, and coastal cities - such as New York - which house more than 60 percent of humanity, will be increasingly at risk in storm surges, or even partially submerged.

Storms will be more violent; rainfall more intense, heat waves and drought more destructive. Climate models are able to predict the general areas that will be affected by these changing weather patterns. Africa and the American Southwest will be drier, the Arctic much warmer, with polar ice rapidly disappearing, and northeast Europe - at least temporarily - will be colder. And all of this in a world with a rapidly growing population and increased pressures on natural resources and energy sources. The UN is predicting 50 million environmental refugees by 2030, as regional climate-related stresses exacerbate existing political conflicts.

I have spent most of my life in the national security business as a US Army officer. Seeing the causes and consequences of war and civil conflict - failed states has become almost second nature - these climate scientists’ talk raises profound concerns. The political and economic map of the world simply cannot cope with these stresses, without real change in the way nations plan, govern and commit resources. Disaster relief will likely be inadequate, insurance funds will probably fail, and vast dislocations of supplies and services are to be expected. Having a capable army and rescue services will not be enough. The threat of mass casualties, political upheaval, and conflict within and between states will certainly increase.

This doesn't argue for spending more on weapons. It argues, first, for clear public understanding of scientific assessments, followed by cooperative action on a global scale. It is already too late to prevent substantial impact, but not too late to avert a civilisation-altering catastrophe. These preventive measures fall into three categories: first, reduce the amount of carbon dioxide and other heat-trapping gases we release into the atmosphere; second, find ways to remove the gases from the atmosphere in large scale; and third, prepare infrastructure, agriculture and public security for the inevitable challenges ahead. In general, we already know what to do, and have been discussing it for more than two decades. But do we have the will to do it?

Since the dawn of history, humanity has organised to prepare for conflict and struggle. Often only the prospect of life-threatening destruction - or perhaps life-changing conquest - could cause people to unite, plan ahead, and work together for common cause. Today, the challenge of climate change is both these things - life-threatening and life-changing. Humanity is largely responsible for causing it, and only by working together can we succeed in dealing with its impact. Surely, this ranks as a national security challenge of the first order; it could well become the most important shared challenge of the world's security establishments.

It will take leadership of the world's great powers - including both the US and China. But it will also take the voices of the nations most affected - island states such as the Maldives and developing countries in Africa, which can already feel the impact of rising sea levels and altered rainfall patterns. It will take support from wealthy oil producing states that have the disposable income to invest in studies, organisations and technologies to help humanity work through these challenges.

And, of course, it will place great demands on international organisations such as the United Nations and the African Union - not just to respond to the inevitable crises and disasters, but to foster collective investments in infrastructure, water, agriculture, clean energy and energy efficiency which can head off the more threatening scenarios ahead.

The issue isn't whether we can afford to do this; rather, how can we afford not to do it?

Wesley K Clark, a retired Army general and former NATO supreme allied commander for Europe, is a senior fellow at the Burkle Center for International Relations at the University of California, Los Angeles.