The imminence and severity of problems posed by the accelerating changes in the global climate are becoming increasingly evident. Heatwaves are increasing in severity, droughts and downpours are becoming more intense, the Greenland ice sheet is shrinking, sea levels are rising and the increasing acidification of the oceans is threatening to disrupt the marine food chain.

The window of opportunity for keeping atmospheric concentrations of greenhouse gases within an acceptable range is closing, while the costs of mitigation and adaptation are rising relentlessly. At the same time, there is also an increasing convergence of science, economics, technology and finance to guide joint international action to address climate change. A sustainable energy future is clearly both possible and affordable, but increased political will and greater collaboration between developed and developing countries are required. These steps must be built on a foundation of public understanding and support.

Scientists worldwide have spoken conclusively. The Intergovernmental Panel on Climate Change (IPCC), jointly established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP), is one of the most sweeping and successful scientific collaborations in history. Its fourth assessment of climate change science could not be any clearer: human activities are altering the atmosphere, and the planet is warming. Unless we act now, with a great sense of urgency, there is significant risk that the Earth's environmental systems will cross a tipping point, beyond which costly and disruptive impacts all over the world will be inevitable.

According to IPCC, if current emission patterns are not altered, global temperatures are expected to rise an additional 1.8° to 4° Celsius. Such warming would prove extremely harmful, if not catastrophic, for our environment, economy and society, and would disproportionately affect the world's poor, whose livelihoods are most closely tied to agriculture and other natural resources.

Climate change is certainly the greatest environmental challenge facing humanity and may also be the greatest economic and political challenge. Forging global consensus on cooperative strategies for mitigating and adapting to climate change is a formidable challenge. But the world's scientific community has spoken unambiguously. If we fail to act with urgency and forthrightness, global climate change will have dangerous effects on the world economy and security. It is now time to hear from the world's policymakers, particularly on the issue of energy.

We need to change the way we produce, use and conserve energy. We have the technology to do it. Transforming the global energy economy to harness new technologies can be the engine that drives a new era of international economic development. The benefits of seizing this enormous opportunity would be significant for all countries, especially for the poorest in the world, many of which have lacked the modern energy services they need to compete in today's economy. Making better use of available energy and increasing the use of clean, renewable energy sources, such as solar, wind and biofuels, are the future. By developing those opportunities, we will create new economic growth and start to heal the planet. The scientific research society Sigma Xi cogently outlined such a road map in a 2007 report to the UN Commission on Sustainable Development, titled Confronting Climate Change: Avoiding the Unmanageable and Managing the Unavoidable.

While all nations must help to avoid catastrophic climate change, those that have contributed most to the problem have special responsibilities. The United States, which is responsible for almost 25 per cent of global emissions, will have an influence on how the world's energy future transforms. But, to date, despite its ratification of the 1992 United Nations Framework Convention on Climate Change, the United States Government has rejected the Convention's implementing agreement, the Kyoto Protocol, and has blocked further progress on international climate negotiations.

Carbon is the basis of all organic substances, from fossil fuels to human cells. On Earth, carbon is continually on the move-cycling through living things, the land, oceans and the atmosphere. Philippe Rekacewicz, UNEP/GRID-Arendal

However, the domestic political support for action on global warming has increased dramatically in the past year. It appears inevitable that the next President of the United States, who will take office in 2009, will bring the country back into the global process. The reasons for this political shift are threefold: increased understanding of the issue, driven substantially by former Vice President Al Gore's superb film, "An Inconvenient Truth"; increased recognition of the risk, brought home by the devastating impact of Hurricane Katrina; and a remarkable upwelling of activity at the state and local levels and among constituencies not normally thought of as environmental-especially businesses, investors and farmers.

The signs of support for a new approach to climate and energy are numerous. Seven northeastern states are using a regional cap and trading system to reduce carbon dioxide emissions. California's legislature has required the state to cut its greenhouse gas emissions by 80 per cent by 2050, and Governor Arnold Schwarzenegger has issued an executive order requiring California's petroleum refiners and gasoline sellers to cut their global warming emissions by 10 per cent by 2020. The United States Climate Action Partnership, an important new group of business leaders, is calling for mandatory controls on greenhouse gas emissions. They see the risk of global warming, as well as the enormous economic opportunities that will be created by a fundamental change in the world's energy systems. The economic impact of the energy revolution of the twenty-first century will be as significant as the digital revolution of the twentieth century.

In 2004, General Electric set a goal of reaching $20 billion in annual sales of energy-efficient products by 2010; by 2005, it was already halfway there with sales reaching $10.1 billion. Giant retailer Wal-Mart recently set goals of increasing the energy efficiency of its vehicle fleet by 25 per cent over the next three years and doubling its efficiency in ten, by reducing energy used in their stores by 30 per cent, and investing $500 million in sustainability projects. Economic gain and environmental protection go hand in hand. Numerous other companies are pursuing similar goals in sectors as diverse as banking (Goldman Sachs, JP Morgan Chase, Citigroup), chemicals (DuPont, Dow) and insurance (Swiss Re, AIG). In addition, venture capital is suddenly flowing to clean energy companies in such areas as biofuels, wind and solar energy, whose sales are accelerating from an existing base of double-digit growth. All of these are win-win strategies for business and society; they are good for the bottom line, for investors, consumers, the company's image, national security and the environment.

In March 2007, dozens of institutional investors, responsible for more than $4 trillion in assets, called on United States lawmakers to enact strong federal legislation to curb pollution-causing climate change. Joined by a dozen leading American companies, the group emphasized the need for greater investment certainty, as they called for a strong national policy to reduce emissions enough to avoid the dangerous impacts of global warming. More than 400 organizations-farm and environmental groups, and major businesses-have endorsed an agriculture-based initiative that calls for producing 25 per cent of United States energy from renewable resources, such as wind, solar and biofuels by 2025. Farmers and other landowners realize that producing more energy from biomass and wind will benefit them directly and spur rural economic growth. The American people overwhelmingly support this vision of "25x'25"-98 per cent of voters say it is an important goal for the country.

This success in forging new partnerships offers a template for global action. Toward that end, the Global Leadership for Climate Action initiative has been launched by the United Nations Foundation and the Club of Madrid, an independent organization comprised of 66 former Heads of State and Government from some 50 democratic countries around the world. This partnership will attempt to forge consensus on a new framework that could help guide future negotiations toward a practicable and enforceable international agreement for the post-2012 period, after the Kyoto Protocol expires, which is built around national commitments to constructive change in the production and use of energy. Internationally, responding to climate change presents opportunities to achieve common economic aspirations. For example, there is a huge and growing unmet demand for farm products that can be made into biofuels to substitute gasoline and diesel. Even more exciting prospects involve the use of non-food crops, such as fast-growing grasses and non-edible oils. Because the demand for transportation fuels is so large, biofuels offer farmers huge market opportunities. If transportation fuels are produced in an environmentally sustainable way, they can dramatically cut greenhouse gas emissions.

While all nations and farmers will profit, poor countries stand to benefit most, because they suffer disproportionately from the high price of oil. This price increase in recent years has cost poor countries three to five times as much as they gained from hard-won debt relief. By investing in biofuels, these nations could produce their own transportation fuels, cut energy costs, create new jobs in the rural economy and ultimately build export markets. However, impediments to this course should not be ignored, including the difficult issues of land ownership, governance and infrastructure. But bioenergy development offers poor countries the chance to reduce their costly oil dependence and better attract the kind of foreign investment that can modernize their farming practices and increase their food production.

Energy efficiency improvements are an example of "low-hanging fruit" of greenhouse gas reductions. With a fresh approach and concerted effort, it would be possible to double the rate of energy-efficiency improvement in such sectors as transportation and buildings, by accelerating deployment of existing technologies and altering incentives to make energy efficiency a bigger consideration in consumer and business decisions. This would not only result in environmental gains, but also in reduced energy costs-one reason why some prefer to call this approach investing in energy productivity.

We believe there is hope for protecting the earth's climate if we aggressively and collaboratively pursue innovative opportunities to transform the world's antiquated and inefficient energy systems. Building public and political support for such action, beyond the scientific and environmental communities, is possible if the ancillary economic and social benefits are recognized and celebrated. But we cannot afford to wait any longer. For the good of the planet, and to give our children and grandchildren the kind of world that we have enjoyed, we must take action now.