Amid the crises and battles, both predictable and unforeseeable, that you will face over the next four years, one problem will stand out both for the economic and social dangers it poses and for the difficulty and cost of solving it. Whether you can develop a practical and sustainable strategy to address climate change—specifically, to begin lowering carbon dioxide emissions—will define the success of your new term as president. We do not make such a declaration lightly; we are keenly aware of the many other challenges you face. But the potential for global warming over the next decades threatens consequences so dire that they could overwhelm any progress you make toward other long-term economic, social, and political goals.

Altering the course of climate change is a task that will take decades. It will require innovative new technologies and overhauls of the world’s energy, agricultural, and transportation infrastructure. We don’t suggest that you can reverse the warming trend over the next four years, or even that you will be able to significantly decrease carbon dioxide emissions. But with the help of the world’s best economic, technical, and scientific minds, you can formulate a policy that will show the nation—and the world—how we can begin to make the changes necessary to ensure that the concentration of carbon dioxide in the atmosphere stabilizes at a safe level. Indeed, it is critical that you do so.

Four years ago, you made a remarkable start. The $90 billion in your 2009 stimulus bill for energy projects and research breathed new life into the search for cleaner sources of energy. The appointment of prominent researchers such as Steven Chu, your secretary of energy, and John Holdren, your senior advisor on science and technology, sent a signal that your administration was committed to making decisions based on facts and science. Most important, you made it clear that the government would play a vital role in encouraging the innovation needed to develop these new energy sources.

But you also made several painful mistakes that doomed much of the progress you had hoped for. Perhaps most damaging, you justified much of the spending by holding out the prospect of “green jobs” and suggesting that the creation of a new clean-energy industry could jump-start the economy. In the May/June 2009 issue of this magazine (see “Can Technology Save the Economy?”), we cautioned against conflating economic stimulus with a sustainable and effective energy policy. Leading economists noted that job creation needed to happen quickly, while transforming our energy infrastructure would take decades. And much of the energy spending in the stimulus bill, suggested one, resembled “pork-barrel politics” to satisfy the immediate need for jobs. The rush to fund energy projects meant that the choices made were not always wise. As another economist warned, “The cost here is not only the dollars. It may also be the dog that doesn’t bark—the truly important program that we could put in place if we went about encouraging innovation in a thoughtful way.”

Many projects that received large investments in the 2009 stimulus legislation were not (in the term of those days) “shovel-ready”—they were still just promising startups. And yet, because of the emphasis on job creation, hundreds of millions of dollars went into building big manufacturing facilities as quickly as possible. The result set companies like Solyndra, A123 Systems, and Abound Solar on a course that ended in bankruptcy. Each of these companies had interesting technologies, but none was ready for the challenge of building commercial products and selling them in highly competitive energy markets. The outcome, which we foresaw in our 2009 article, was an entirely unnecessary black eye for the clean-energy effort.

Renewable energy sources, like solar and advanced biofuels, are simply not yet ready to compete with fossil fuels. Solar power, for example, still generates less than 1 percent of our nation’s electricity and, under most circumstances, remains much more expensive than electricity generated from fossil fuels. We need new and far more advanced technologies. Creating cleaner ways to produce energy will require inventions in physics and chemistry labs and innovations in how we scale up and test those inventions. And it will require market incentives, such as a tax or some other price on carbon dioxide emissions, to encourage consumers and industry to use clean energy. Your administration can play a critical role in each of these areas, from increasing funding for energy R&D to helping establish facilities where companies can share the costs and risks of testing new technologies. Perhaps most important, you will need to rally the nation around the issue of tackling climate change. Only with broad public support can you hope to push a recalcitrant Congress into passing legislation that will establish some form of carbon pricing.

Slowing down global warming won’t be cheap. You have often stressed the economic benefits of choosing new energy technologies. You make a valid argument that moving away from fossil fuels will have positive implications for many businesses. And certainly new technologies will provide jobs and other economic opportunities. But we can no longer pretend that addressing climate change will be without real costs. Economic studies show that it is likely to cost trillions of dollars worldwide, though those analyses also present evidence that the price will grow higher the longer we wait.

Adding to our difficulties is the recent boom in our country’s production of fossil fuels, including natural gas and related deposits of shale oil. Already, the glut of cheap natural gas created by advanced drilling technologies and by the nation’s vast supply of shale gas has made it difficult for renewable energy to compete on price. The inexpensive energy made available by these drilling activities is good news for the overall economy, but it is also a stark reminder that the motive for adopting nonfossil fuels is not market-driven but is—and always has been—a simple one: we must do it to reduce carbon dioxide emissions and begin stabilizing our climate.

It’s time to acknowledge that green jobs were always just political cover for that motive. You must say unambiguously that the real reason to transform our energy system is to avoid the most catastrophic effects of global warming.

This is a deeply unpalatable political message. It means immediate spending and economic sacrifice by present-day voters in order to achieve benefits that will be realized decades from now. And it must be done while millions of Americans are still skeptical that global warming is taking place or that it is caused by human activity. But as extensive and exacting analyses over the last decade have shown, we can no longer wait without risking dramatic upheavals in global security and the health and welfare of hundreds of millions of the world’s inhabitants.

The International Energy Agency reports that global emissions of carbon dioxide from fossil-fuel combustion reached a record 31.6 metric gigatons in 2011. To have a decent chance of limiting the average global temperature increase to 2 °C and avoiding the most devastating effects of climate change, we will need carbon emissions to peak at no more than 32.6 metric gigatons, and to start falling no later than 2017. The president who takes office that year will thus be facing a far more urgent problem—probably, like you, with no political consensus on how to solve it. But as a president in his final term, you have a chance to take risks. You have the power and the opportunity to lay the groundwork for a new clean-energy policy that will help us avoid the worst consequences of climate change. It is quite possible that if this is not done over the next four years, it will be too late.