For Americans it was the summer of 2005 with Katrina and all the other hurricanes — along with record heat. For Europeans it was the heat wave of 2003 that killed 20,000 people.
"Sometimes it takes a triggering event like that to really get people's attention," says Jerry Meehl, a senior scientist at the National Center for Atmospheric Research in Boulder.
That attention is a major reason why so many people now accept the idea that carbon dioxide and other human-produced greenhouse gases are heating up the planet far faster than nature could
A big factor is that scientists at facilities like the one here in Boulder are now much more confident about their predictions.
Computer models now accurately explain past climate and give an ever more certain forecast of the future.
"It's going to get warmer," says Meehl.
'Flabbergasted' about melt
Mark Serreze studies the ice that covers the oceans of the Arctic and the land mass of Greenland. "I'm flabbergasted at the changes that we've seen in the Arctic, not just the Greenland ice sheet, but the Arctic as a whole."
There is no doubt the ice is melting fast. In the Arctic, an area twice the size of Texas has disappeared in a decade. The ice over Greenland alone contains so much water if it were to all melt the oceans would rise 20 feet. The ice over the oceans is critical because it reflects the sun's heat, so when it disappears the warming accelerates.
"We tend to think of climate change as this slow process, but the emerging view now is not a slow process — it can be a very rapid process," says Serreze.
The major international scientific body on climate change predicts an average warming of 2 to 8 degrees Fahrenheit in this century. That may not sound like much, but Meehl notes that "the thing about the climate system is that a very small change in the average climate actually produces fairly large changes of extreme events."
Those include torrential rain in some places, drought in others, rising sea levels, more tornadoes and stronger hurricanes. But the scientists cannot yet say exactly where and when.
"It is not so much of what we know that worries us," says Serreze. "It is what we don't know that worries us. What are the surprises that are in store for us?"
Business attitude shifts
Susanne Moser studies attitudes about climate change. She says most Americans now accept the idea that humans cause global warming but don't believe they personally can do anything. But business, she says, is a different story.
"There's a definite shift in the business community," she says. "In part because they see it might become a liability if they don't take climate change seriously."
Indeed, the Boulder area is becoming a Silicon Valley of climate change, with lots of government money for research and startup companies trying to sell solutions.
"It's a hot spot for climate science and for some of the technologies that one might look at to reduce emissions," says Moser.
The scientists agree there will be no easy fix for global warming ad that it will get much worse before there is any chance it will get better.