Twenty years ago this week, a then-unknown NASA scientist named James E. Hansen ushered global warming from the realm of climatology into the hot house of public policy.

He did it in testimony before the Senate Energy and Natural Resources Committee, delivered on a sweltering summer day.

Global warming is real, he told the senators. It is already under way, he said, adding that he was "99 percent certain" it was caused by humans.

Politicians and the public listened to Hansen's message, and for two decades they've listened as a mountain of scientific evidence emerged to strengthen and confirm his conclusions. A few skeptics remain, but even President George W. Bush has become a convert.

Unfortunately, politicians have done little to actually address the problem. A bill to create a so-called cap-and-trade program for carbon emissions stalled in the Senate earlier this month.

Hansen now is director of NASA's Goddard Institute. But he's perhaps best known for the efforts of a Bush administration political appointee to keep him from speaking publicly about climate change. It didn't work.

"Emissions are continuing, basically unfettered," Hansen said this week. "We're really running out of time."

One of the prevailing counter-narratives to global warming, invoked to great effect by industry apologists and professional skeptics, is that the scientists now warning about warming were not so long ago worried about world cooling.

In fact, concerns about warming were first raised in the 19th century. As far back as 1859, scientists knew that carbon dioxide was an important greenhouse gas.

But it wasn't until about 50 years ago that they began measuring levels of carbon dioxide in the atmosphere. Since 1958, it's increased 21.5 percent.

Carbon dioxide levels have grown by 8.4 percent since Hansen first addressed that Senate committee in 1988. The current level, 383 parts per million, is the highest in the past 650,000 years.

Evidence for warming now is "unequivocal," according to a United Nations-chartered scientific advisory group on global climate change. The Intergovernmental Panel on Climate Change says that we can avoid the worst effects of global warming only if we act now.

Somehow, that message isn't getting through. A survey released last year found that only 48 percent of Americans believe that most scientists agree about the cause of global climate change. Even fewer, just 40 percent, believe most scientists agree that warming is occurring at all.

Why the skepticism? Because change is going to be difficult and wrenching. Many folks would rather not bother.

These days, Hansen is pessimistic that we can stop burning oil to reduce carbon dioxide emissions. "You're not going to be able to tell Saudi Arabia and Russia, the countries that have oil, that they're not going to be able to sell" it, he told a Washington audience this week.

Instead, he wants to phase out the use of coal-fired power plants by 2030. That would pose a major challenge to states like Missouri; Missourians get about 85 percent of our electricity from burning coal.

That highlights an uncomfortable - some say inconvenient - truth about global climate change: Addressing it will require difficult choices and higher expenses. Hansen believes the best way to phase out the use of coal-burning power plants is to impose a carbon-emissions tax, with the proceeds going directly to citizens to offset higher energy prices. That idea isn't even on the table in Washington.

Unfortunately, Congress isn't likely to do anything about global climate change until next year, and then only if citizens send a strong message that they want action now.

We can't afford to wait. The only thing more expensive than acting now to blunt the worst effects of global warming would be doing nothing for another 20 years.

Reprinted from the St. Louis Post-Dispatch.