Two subway cars on Washington, D.C.'s Red Line - which I usually ride to work - recently collided. It was the worst accident in this subway's history, killing nine D.C. residents and injuring scores of others. The National Transportation Safety Board's advice to the local transit authority soon came to light: Replace older-model subway cars, including the ones that crashed. The NTSB had said this three years ago, but the transit authority hadn't had the money to do it.

The Metro disaster has security implications that extend beyond the safety of subway passengers like me. Developing clean mass transit is a key piece of the solution to the most serious security challenge of our time.

World leaders will gather in Copenhagen in December to try to agree on a plan to stop climate change. If they fail, the consequences will include large land masses around the world rendered uninhabitable by drought in some areas and by flooding in others. The U.S. military has begun to see these consequences as not merely a massive human and planetary tragedy, but a major potential cause of increased violent conflict.

Climate change, in other words, is a security challenge as well as an environmental problem. Developing transportation systems that reduce greenhouse gas emissions is part of the solution.

Doing so is an expensive proposition. As the subway wreck suggests, the money hasn't been there to do it.

The money has been there, though, for military security. Since 2001, U.S. military spending has ballooned by 70 percent, to nearly $700 billion a year. Although the Obama administration has proposed the most ambitious set of spending cuts in unnecessary weapons programs since the early 1990s, it also proposed an even larger military budget overall than any of the previous Bush administration models. And while the wreckage of the subway crash was being cleared away, a congressional committee was voting to add more money to this budget, to build advanced fighter jets that the military itself says we don't need.

Unlike its predecessor, the Obama administration takes the threat of climate change seriously. At the heart of its vision of economic reform is the construction of a sustainable economy, with green technology - renewable energy, energy efficiency and clean transport - as one of its principal drivers. Though the climate change bill working its way through Congress is seriously flawed, the administration has made a down-payment on a green economy through its spending plans so far.

In the last fiscal year, the Bush administration spent $88 on security by military force for every $1 it spent on climate security. The Obama administration's spending plans would narrow that gap to $9 for the military for every $1 spent on climate - a huge improvement.

But there's a catch. Of the $79 billion it budgeted for green investment, the great majority - 87 percent - comes from the stimulus package Congress passed in February. The regular budget includes only about $3 billion more in green investment than the Bush administration spent in 2008. This barely narrows the gap at all: taking the military's environment ratio from 88:1 to 85:1.

You can't build a low-emissions economy with a one-time investment. The security of my fellow subway riders depends on changing our long-term investment strategy. The Earth's security depends on that too. We need to take money from weapons systems we don't need and use it to build the green economy we do. This economy must be viewed as, among other things, a conflict-prevention device, because it can prevent the climate-change-driven violence that our military forces will be powerless to stop.

This op-ed was distributed by Minuteman Media.