I'm not trying to start a nerdy parlor game, but if we were to list the most important climate scientists of the past fifty years, James Hansen would have to be in the mix. Three decades ago, he helped create one of the world's first climate models to predict how the Earth would heat up in response to rising greenhouse gases. (Many of his early forecasts held up well.) He stepped into the spotlight again in 1988, issuing one of the first climate warnings to Congress. And, in the 2000s, when Bush appointees tried to downplay the severity of global warming, Hansen was the one blowing the whistle. The director of NASA's Goddard Institute for Space Studies is hardly infallible, but as would-be Cassandras go, his record has proven awfully solid.
So what are we to make of his latest turn, which has consisted of one ultra-terrifying prediction after another? In recent years, Hansen has warned that rising temperatures pose a far greater threat to civilization than is commonly believed, that scientists are understating the problem, that we have already passed the danger point, that we need to stop burning coal immediately, that the climate bill in Congress is a horrible scam.... Hansen has been saying these things for some time now, in technical papers, in e-mails to colleagues, and in interviews with reporters. But in his new book he has compiled his arguments at length for the first time. And the portrait that emerges is mixed. While Hansen is still a top-flight climatologist—very much worth listening to even at his most heretical—his recent foray into politics leaves a lot to be desired.
Like many global-warming skeptics, Hansen has sharp disagreements with the mainstream scientific consensus on climate change. Unlike the deniers, however, he believes that the consensus view actually downplays the problem. The computer models used to project future warming, for example, seem to be too conservative: they failed to predict, among other things, the rapid collapse of summertime Arctic sea ice since 2007.
Instead, Hansen argues that we should look to Earth's past for clearer answers. We know from basic physics that, all else being equal, an increase in greenhouse gases will warm the planet. But by how much? From studying a wealth of prehistoric data—the movement of ice sheets, samples of atmosphere trapped in ice cores, changes in the sun's brightness—we can assemble snapshots of the Earth's "energy balance" over millions of years, and pinpoint the factors that altered it. The past is full of huge temperature changes: fifty million years ago, for instance, Alaska had tropical vegetation and crocodiles. And as it turns out, the biggest factor affecting prehistoric temperature shifts has been fluctuating levels of carbon dioxide in the air. (Previous changes in carbon dioxide levels were due to natural factors, such as rock weathering, and occurred very sluggishly, over many millenia.) Historically, a doubling of carbon dioxide has raised global temperatures by about 3°C. There is every reason to believe that, by burning fossil fuels, humans are charting a similar course today—only this time in fast motion.
One consequence of these past temperature swings has been dramatic changes in sea levels, and this is where Hansen gets unnerved by his data. For the past seven thousand years, sea levels have stayed remarkably stable, a happy event that allowed human civilization to develop and prosper. But at plenty of other points in the geological record, sea levels have been extremely 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.
Now, at many global climate conferences, a 2°C rise is considered a best-case scenario. Hansen is saying that even our most ambitious targets are flirting with catastrophe. To preserve a livable climate, we need to rapidly bring carbon-dioxide levels in the air back down below 350 parts per million (we are currently at about 387 ppm and rising fast). And, Hansen contends, the only way to do that is for the world to phase out coal use quickly. Oil doesn't get the same harsh treatment because Hansen thinks the idea of weaning ourselves off of oil in such short order is unfeasible. What’s more, there is probably not enough conventional oil left to tip the climate over the edge. ("Unconventional" oil from shale or tar sands is a different matter.) There is, however, more than enough coal left in the ground to send temperatures soaring if we keep burning it. So, he says, we should put a moratorium on any new coal-fired plants unless they can capture and bury their emissions.
Up to this point, Hansen has built a sturdy case. His discussion of the science is heavy-going, if only because paleoclimatology and radiative physics can make for a difficult slog, but this is still an excellent primer. Hansen walks the reader step by step through the detective's tale of how he came to his conclusions, laying out uncertainties and addressing objections. Yes, many climatologists would disagree with his views on sea levels and the direness of our current situation. But it is also true that past forecasts from "consensus" groups like the IPCC appear to have been too rosy on matters like ice-sheet loss—a reason to listen carefully when Hansen speaks.
But when Hansen steps out of the scientific arena and into politics, he starts to stumble. Politics, after all, is an area of expertise like any other. Just because you are a world-class scientist doesn't automatically make you an authority on how Congress works. Hansen keeps insisting that "special interests" rule Washington, and that lobbyists have weakened the cap-and-trade bill before Congress beyond repair. Instead, he proposes, Congress should discard the whole thing and pass a simple tax on carbon-dioxide emissions. Problem solved! Except that if Congress is festering with lobbyists and special interests, wouldn't they just rip apart a carbon tax, too? As it turns out, that is what happened to Bill Clinton's ill-fated BTU tax in 1993. If there is one thing lobbyists are good at, it is scraping out exemptions in tax bills. What's more, even after House Democrats bent over backward last year to accommodate electric utilities and other polluters, Congress only barely squeaked out the votes for a cap-and-trade bill. Why would it be any easier to pass a robust carbon tax?
There is nothing wrong with demanding more of Congress. Tackling global warming is going to be a massive endeavor, and outsized reforms always require both outsiders who ask for the moon and insiders who know when to compromise to get something passed. Yet Hansen seems to deny that compromise is ever necessary. In recent months, he has spent his time trying to poison all ongoing efforts to tackle climate change. At the Copenhagen summit in December, he was rooting publicly for the international talks to collapse. But what would happen if they did? Would everyone start over again and magically arrive at a better agreement through sheer willpower? He seems to believe so. In his book, Hansen presumes that China and the United States would agree to a global carbon tax "once both countries realize they are in the same boat and will sink or survive together." This sounds like Obama's critics on the left, who in recent weeks have lobbied Congress to scuttle its health care bill and try again for an even more ambitious package, even though killing the current efforts could wipe out all further momentum for reform.
To be fair, health care and climate change are not entirely analogous. When Democrats scrapped the public option, the health care bill got worse, yet still remained a landmark piece of social legislation. But if you take Hansen's research seriously, half-hearted attempts to curb greenhouse gases really might mean the end of the world, at least as we know it. My own view is that relentless incrementalism may well be the planet's only shot. Environmental legislation in the United States has always started out relatively weak and been strengthened over time, through ceaseless pressure. Hansen has persuasively argued that we need to take radical steps on climate; but without realistic means of getting there, all the warnings in the world will mean nothing.
Bradford Plumer is an assistant editor at The New Republic. He blogs about the environment at www.tnr.com/blogs/the-vine.