1) Back in 1988, Hansen first told Congress that man-made greenhouse gases were heating up the planet.
His testimony (pdf) was one of the first and clearest public statements on global warming. "It is time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here," he said at the time.
Hansen's 1988 projections for future climate change have been widely dissected over the years by supporters and skeptics alike. At the time, he offered up three scenarios for future temperature increases, depending on how emissions rose. Here's how those have panned out:
Greenhouse-gas emissions grew between 1988 and the present at a rate fairly close to Hansen's "Scenario B." Yet temperatures haven't risen quite as sharply as he thought they would. Why is that? Many scientists now think that climate sensitivity — basically, the amount that the Earth will warm in response to a doubling of carbon-dioxide in the atmosphere — is closer to 3°C than Hansen's early prediction of 4.2°C. (Though 3°C is still a lot of warming.)
All told, however, those 1988 predictions were quite useful. As NASA's Gavin Schmidt has pointed out, Hansen's early — and relatively crude — climate models forecast the future much better than models that assumed no global warming at all.
2) In 2007, Hansen warned that climate scientists were understating the risks of sea-level rise.
In 2007, the Intergovernmental Panel on Climate Change publicly predicted that sea levels could rise as much as 59 centimeters by 2100. Many observers looked at that number and said, "That's all?"
But that number was only a partial estimate, and Hansen helped write several papers arguing that scientists weren't properly communicating the full risk of sea-level rise due to "scientific reticence." In fact, he noted, data from the distant past suggested that ice sheets in Greenland and West Antarctica could melt quite quickly once they started warming.
In the years since, as more work has been done, many climate scientists now believe the ice sheets will melt more quickly than previously thought. For instance, one 2009 study predicted that sea-level rise would accelerate in the decades ahead, reaching an additional two to six feet by 2100
3) Hansen now thinks that international climate-change goals aren't nearly ambitious enough.
Throughout the various international climate talks, world leaders have aimed to limit global warming to 2°C or less. But in an attention-grabbing 2008 paper (pdf), Hansen and his colleagues argued that this 2°C target was overly optimistic and that we should be aiming for 1.5°C or less.
Here's Hansen's logic: For the past 7,000 years, sea levels have stayed remarkably stable, which has allowed human civilization to develop and prosper. But at plenty of other points in the geological record, sea levels have been quite volatile. The ice sheets at the poles can disintegrate rapidly once they start melting, and sea-level rises of a few meters per century are not unheard of. Hansen's research suggests that, in the past, it has not taken much to set off this process. Sea levels have been a couple meters higher when the world was only 1°C or 2°C warmer than it is today.
As such, Hansen argues, we need to rapidly bring carbon-dioxide levels in the air back down below 350 parts per million (we are currently at about 391 ppm and rising fast). And, Hansen contends, the only way to do that is for the world to phase out coal use quickly.
That paper led to plenty of debate among scientists. While Hansen's goal is considered politically difficult, his paper helped give rise to environmental groups like 350.org, which have pushed for rapid reductions in carbon-dioxide emissions.
4) Hansen argues that a carbon tax is the best way to deal with climate change, rather than cap-and-trade.
Hansen is a climate scientist, not an economist. But he has waded into economic policy debates by heavily criticizing the cap-and-trade mechanisms that Europe and California are using to curtail their carbon emissions. Instead, Hansen says, it would be better to have a simple carbon tax, with the revenue rebated to the public.
In his 2009 testimony (pdf) to the House Ways and Means Committee, Hansen laid out his basic critique of cap-and-trade: "(1) unpredictable price volatility, (2) it makes millionaires on Wall Street and other trading floors at public expense, (3) it is an invitation to blackmail by utilities that threaten “blackout coming” to gain increased emission permits, (4) it has overhead costs and complexities, inviting lobbyists and delaying implementation."
5) Hansen has become increasingly outspoken in recent years, criticizing former President George W. Bush's climate policies and protesting the Keystone XL pipeline.
From his perch at NASA, Hansen has often clashed with various administrations — in 1989 he complained that the Office of Management and Budget was trying to water down his testimony on global warming.
But his relations with the Bush administration were particularly fraught. In 2004, Hansen claimed that NASA Administrator Sean O'Keefe had tried to dissuade him from talking about human influence on the atmosphere. Hansen also criticized Bush appointees for editing scientific reports to make global warming seem less dire. He was especially vocal about the Bush administration's stance that more research was needed before dealing with climate change: "Delay of another decade, I argue, is a colossal risk."
During the Obama years, meanwhile, he has been arrested protesting the Keystone XL pipeline, which is still awaiting White House approval. “We have reached a fork in the road,” Hansen said, “and the politicians have to understand we either go down this road of exploiting every fossil fuel we have — tar sands, tar shale, off-shore drilling in the Arctic — but the science tells us we can’t do that without creating a situation where our children and grandchildren will have no control over, which is the climate system.”
--This is obviously just a brief look at some of his most attention-catching statements. But Hansen has done a staggering amount of scientific work on everything from the atmosphere of Venus to the effects of black carbon to monitoring the climate impacts of the Pinutabo volcano eruption in 1991. For a more in-depth overview, Wikipedia is a good place to start.