Skip to Content

The Messenger

The best scientists, scrutinizing atmosphere, ice, earth, and sea, say global warming is approaching a tipping point. But we still have time to keep it from reaching catastrophic levels.
July 1, 2006

Jim Hansen may be the most respected climate scientist in the world. He’s been director of NASA’s premier climate research center, the Goddard Institute for Space Studies (GISS), for 25 years and a member of the National Academy of Sciences (NAS) for 10. And he more or less single-handedly turned global warming into an international issue one sweltering June day in 1988, when he told a group of reporters in a hearing room, just after testifying to a Senate committee, “It’s time to stop waffling so much and say that the greenhouse effect is here and is affecting our climate now.”

Jim Hansen, director of NASA’s Goddard Institute for Space Studies (Credit: Ben Baker)

It took the rest of the scientific community about eight years to catch up with him on that point. He was ahead of the pack in 1988, and he remains so. He’s been accurately predicting the progress of global warming for 25 years. And as the science grows ever more solid, owing in no small part to his own work, Hansen’s predictions about an issue some see as the greatest threat civilization has ever faced are becoming ominously precise.

An attempt by the Bush administration to silence him early this year also helped turn global warming into one of the biggest news stories of 2006. It began on December 6, 2005, when Hansen declared in a talk at the American Geophysical Union in San Francisco that if our rate of fossil fuel burning continues to grow, we will eventually transform Earth into “a different planet.” He presented an analysis showing that existing technologies can significantly cut greenhouse emissions, and suggested that a global solution requires leadership by the United States.

On December 15, he and three colleagues posted a routine monthly analysis on the GISS website, summarizing data from thousands of weather stations around the globe. It showed that 2005 was coming in as the warmest year since the mid-1800s. He was interviewed about this by ABC News.


  • C02 and the 'Ornery Climate Beast'

  • Video: Scientists speak out about the threat of global warming and how to deal with it.

According to NASA memorandums provided by Hansen, senior political appointees at NASA headquarters in Washington quickly called career public-affairs officers at the agency and directed them to give headquarters advance notice of Hansen’s speaking schedule, his “data releases,” and his attendance at scientific meetings. The career officers also understood from the phone calls that the posting of all content on the GISS website, including scientific data sets, would now require headquarters approval; that no NASA employees or contractors could grant media interviews without approval; and that public-affairs officers had the right to stand in for scientists in all interviews. Hansen emphasizes that the political appointees made sure to leave no paper trail. But by throwing off this muzzle, Hansen propelled himself – and global warming – into the headlines. The story broke on the front page of the New York Times ; Hansen appeared on NPR and 60 Minutes , too.

Through it all, he remained productive scientifically. One week, he submitted a paper to the Proceedings of the National Academy of Sciences ; the next, he presented an invited talk at the NAS’s annual meeting; the next, he filed a brief in U.S. District Court in California, as an expert witness for the state in a suit brought by automobile manufacturers hoping to strike down a 2004 regulation by the California Air Resources Board that would eventually reduce greenhouse-gas emissions from vehicles sold in the state by about a third.

Hansen now starts off most public appearances by stating that he speaks as a private citizen, not a public employee, that his opinions are those of a climate scientist with more than 30 years’ experience, not of a government policymaker. Indeed, he asked to be interviewed not at his institute, on the campus of Columbia University, but at the small apartment he keeps nearby.

Over lunch in a tasteful but spartan living room on the top floor of a building that affords magnificent southwest views of the Hudson River and the western half of Manhattan, he sits in jeans and an untucked blue-checked shirt, without shoes, sipping his fourth or fifth coffee of the day.

He says he’s been muzzled before – during the Reagan and first Bush administrations – but that in more than three decades as a government employee, he has seen nothing to equal the recent clampdown. He is angry, but he expresses his anger calmly.

The Science

Hansen is a planetary scientist. He earned his doctorate from the University of Iowa’s department of physics and astronomy, when it was chaired by the legendary astrophysicist James Van Allen. For his dissertation, Hansen investigated the effect of atmospheric dust on the temperature of Venus; and it may be that this early work imparted a special knack for viewing Earth’s climate system as a whole. He joined GISS as a staff scientist in 1972 and was promoted to director in 1981. For more than 30 years, he and his dedicated research team have been producing work at the forefront of climate science.

He often employs a favorite quote from the late physicist Richard Feynman to explain his approach: “The only way to have real success in science … is to describe the evidence very carefully without regard to the way you feel it should be. If you have a theory, you must try to explain what’s good about it and what’s bad about it equally. In science you learn a kind of standard integrity and honesty.” Hansen invariably points out the shortcomings in his own arguments. When another scientist presents only the points that support his conclusion, Hansen will chide him for acting “like a lawyer.”

For about 25 years, however, the data have been telling him that Earth is getting warmer, humans are causing it, and this is bad news. In his view, moreover, the science has become so airtight in the last five years that the immense danger posed by greenhouse emissions can no longer be denied. This has placed him on a collision course with politicians and business leaders who want a different answer.

Hansen’s December talk was given in honor of greenhouse pioneer Charles David Keeling. Keeling monitored carbon dioxide on the summit of Hawaii’s Mauna Loa for almost 50 years, from 1958 until his death about six months before the meeting, and demonstrated that the concentration of atmospheric carbon dioxide had been rising the whole time.

Early in the talk, Hansen presented what may be the scariest graph in climate science, a 420,000-year record of carbon dioxide and temperature, inferred from a 3.6-kilometer ice core recovered at Russia’s Vostok station in Antarctica (see “C02 and the ‘Ornery Climate Beast,’” PDF, 631 KB). The graph puts the whole greenhouse story in a nutshell and demonstrates, as Columbia climatologist Wallace Broecker once put it, that “Earth’s climate system is an ornery beast which overreacts even to small nudges.”

Past atmospheric temperatures at Vostok may be inferred by measuring the ratio of deuterium to hydrogen, layer by layer, in the water molecules of the ice. Ancient carbon dioxide levels are recorded in the air bubbles trapped in the ice. These records show that temperature and carbon dioxide tracked each other for all but the last 200 years – both oscillating in a cycle that repeats about every 100,000 years, in step with minute changes in the shape of Earth’s orbit around the Sun. Dips in carbon dioxide and temperature correspond to ice ages, or “glacials,” and peaks to interglacials – such as the present warm period, which began about 12,000 years ago.

In the early 1800s, shortly after the start of the industrial era, carbon dioxide began to skyrocket, while temperature remained flat. Temperature began to spike only about 30 years ago. In contrast, temperature changes preceded carbon dioxide changes at Vostok until the beginning of the industrial era. Scientists believe these natural changes in carbon dioxide were a feedback response to initial, small changes in temperature, and that those changes – along with other responses – amplified the original temperature shifts.

The other responses included changes in the levels of other greenhouse gases, primarily methane; changes in the area covered by polar ice sheets and sea ice, which reflect sunlight back into space and cool the planet; changes in the levels of dust and airborne aerosols, which also cool by reflecting sunlight (the “parasol effect”); and changes in the mix of grassland, desert, and forest, which affect the reflectivity of the land.

The history of these factors is known. Besides the information about greenhouse-gas levels from the trapped air bubbles at Vostok, a sediment core from the bottom of the Red Sea indicates changes in sea level, which in turn give an approximation of ice sheet area. (The ice sheets grew and thereby drained the oceans during cold times; they melted to refill them during warm times.)

Using these and other geological records, Hansen can calculate Earth’s temperature at any given time in the past 420,000 years. He plugs in the data for greenhouse levels, sea level, and so on to produce a temperature estimate for the corresponding time. And as he showed his audience last December, these calculations match temperatures as recorded by the deuterium and hydrogen in Vostok’s ice quite precisely over the entire 420,000-year span.

Global-warming deniers like to complain that scientists base their predictions on faulty computer models. But Hansen’s calculations show that we don’t need a computer to know how temperature will respond to a given change in the greenhouse – or a change in dustiness, or forest cover, or the amount of ice on the Arctic Ocean. Solid geological field data give us everything we need – and provide a check for computer models. And lend credibility to Hansen’s predictions.

Besides demonstrating his firm grasp of the power of these various factors to change temperatures, this remarkable matching of theory to real-world data also tells us just how ornery the climate beast may be: the orbital changes that paced the ice ages were incredibly small. They had little effect on the total amount of sunlight reaching Earth in a single year – only its distribution over seasons and latitudes. Nevertheless, these minute redistributions led to swings in temperature of about 5 ºC and variations in sea level of more than 100 meters.

Greenhouse-gas levels, on the other hand, are more like a knob controlling the brightness of the sun. And the turning up of the rheostat that humanity has accomplished by adding about a trillion tons of carbon dioxide to the atmosphere thus far in the industrial era dwarfs the redistributions in sunlight that once switched the planet back and forth between glacials and interglacials. We are poking the climate beast in a way it has not been poked in the entire era of cyclical ice ages – at least two million years. As Hansen told his audience last December, “Humans now control global climate, for better or worse.”

A Slippery Slope

More than 20 years ago, Hansen also explained why global warming has lagged the greenhouse buildup. In 1985, he suggested that it should take between 50 and 100 years for the excess energy reaching the planetary surface to have its full effect on temperature, because the energy will first go to heating the oceans; only when they begin to warm will the atmosphere follow suit. Just last year, when studies demonstrating a global rise in ocean temperatures confirmed his thinking, Hansen began referring to the heating of the oceans as the “smoking gun” of global warming.

Another factor, which Hansen and GISS modeling specialist Andy Lacis have termed a “Faustian bargain,” also suppresses atmospheric warming. In 1990, Hansen and Lacis showed that traditional air pollution has produced a mighty parasol effect. We send dust and aerosols into the air from tailpipes and smokestacks, by burning the wood and dung that provide heat and light to hundreds of millions of the world’s very poor, and through slash-and-burn agriculture and other land use practices that have exposed vast tracts of dried-out, eroded soil to the blowing wind. The dimming of incident sunlight caused by reflection from these airborne particles now offsets about half the warming of the industrial age.

To continue offsetting our growing greenhouse emissions, we would have to maintain the rapid growth of traditional, noxious air pollution. But the United States and Europe have begun controlling it, and the dismal air quality in Beijing and Mumbai is convincing the Chinese and Indians that they must, too. Faust’s payment to the greenhouse is now coming due.

Owing to greenhouse changes we have already incurred, Hansen told his audience in San Francisco, Earth’s temperature will rise about 0.5 ºC in the next 50 years even if we stop burning fossil fuels today. We’re on a slippery slope: we could cross a threshold that leads to a drastically different planet, half a century before knowing that we’ve done so. Hansen believes we are horrifyingly close to such a threshold, and that we will cross it if we don’t change our greenhouse ways within the next few years.

Earth is now passing upward through the highest temperatures of the past 12,000 years, and the half a degree that is already in the pipeline will bring temperatures within half a degree of the high points they have reached only a few times in the past two million years. During a warm period about 120,000 years ago, for example, sea levels were probably five or six meters higher than they are today.

Running future emissions scenarios on a GISS computer model, Hansen finds that if we remain on the path he calls “business as usual,” temperatures will rise between two and three degrees this century, making Earth as warm as it was about three million years ago, when the seas were between 15 and 35 meters higher than they are today. There go many major cities and the dwellings of about half a billion people.

Evidence suggests that the seas could rise in a matter of decades or centuries; recent events in Greenland and Antarctica indicate that the process may already have begun. The last great ice sheet collapse, about 14,000 years ago, sent the seas up a total of 20 meters, at the rate of one meter every 20 years for 400 years. Just the first meter would obliterate New Orleans, force tens of millions of people in Bangladesh to emigrate, and inundate rice-growing river deltas throughout Asia, a major food source for our species.

A Solution

Yet Hansen continues to believe we can forestall disaster. In June 2000 his group charted a course for holding future temperatures below the danger level. They named this departure from business as usual the Alternative or A-Scenario.

His characteristic “whole systems” approach revealed some wiggle room on the hardest conundrum, carbon dioxide – which, as a direct by-product of fossil-fuel burning, is intrinsic to the global energy infrastructure. Though he warned that carbon dioxide emissions must be stabilized over the next few decades, he also suggested that significant progress could be made by reducing the emissions of other greenhouse gases, particularly methane and ozone – and that we must pay our Faustian debt involving air pollution.

Eliminating black carbon soot, which comes mainly from household heating systems, vehicles, and fires, would be a good place to start. Besides promoting asthma and other respiratory problems, soot heats rather than cools the air: it absorbs rather than reflects sunlight, owing to its color. Some soot emissions continue to heat the planet even after leaving the air, by settling on polar ice sheets and mountain glaciers, darkening their surfaces, and helping them melt. Carbon monoxide and ozone are powerful greenhouse gases as well as dangerous pollutants, ozone having an estimated impact on human health and crop productivity of $10 billion per year in the United States alone.

Still, the main greenhouse promoter in coming decades will be carbon dioxide. In 2001, Hansen assembled the “A-Team,” made up of GISS researchers and students and teachers from schools in the New York City area, to tackle the problem of providing for the world’s growing energy needs while adhering to the A-Scenario. They found that efficiencies based on existing technologies could buy time for a few decades, after which we must employ new technologies to cut global carbon dioxide emissions by 60 to 80 percent.

The A-Team found that growing emissions from coal-burning power plants and transportation posed the greatest threats. “Efficiency of energy end-use in the near term is critical for the sake of avoiding new, long-lived CO 2 -producing infrastructure,” Hansen notes. “Green” building codes, combined with energy-efficient lighting and appliances, would be sufficient to hold electrical needs – and the number of power plants – constant for many years. The team also developed an achievable plan for limiting vehicular emissions, a plan that starts by improving fuel efficiency with existing technologies. It is “technically possible to avoid the grim ‘business-as-usual’ climate change,” said Hansen last December. “If an alternative scenario is practical, has multiple benefits, and makes good common sense, why are we not doing it?”

He knew the answer from personal experience. Few remember that Vice President Dick Cheney chaired a cabinet-level climate-change working group in 2001, shortly after convening his infamous energy task force. Hansen briefed the group twice. He believed in those early days that the White House was open to a discussion of facts and potential solutions. But as he remembers it, Cheney picked only the cherry he liked from the Alternative Scenario: its emphasis on soot and the lesser greenhouse gases. He used this to justify ignoring carbon dioxide.

Indeed, the energy policy Cheney introduced poses a tremendous climatic danger. It relies on increasing supplies through oil drilling in the Arctic National Wildlife Refuge; opening other public lands to coal, natural-gas, and oil exploitation; and constructing more than 1,000 new power plants. So Hansen convened the A-Team. He had a second interaction with the White House in 2003, again with little effect. By the time of his talk in honor of Charles David Keeling, he had run out of patience. “It seems to me,” he said, “that special interests have been a roadblock wielding undue influence over policymakers.”

In his living room overlooking the Hudson, Hansen tells me that climatologists have now “made the science story much stronger than it was in 2000.” Yet, he says, “we have not been able to impact the U.S. position. And when you get to the further step, where not only do you have the information to make the story clear but you have this censorship, you know, that’s when you really get angry. I think the only way to get action now is for the public to get angry, [for] the public [to] see the frustration and … see that we have political leaders who are under the thumb of special interests. …

“No court of justice or court of international opinion will forgive us for what we’re doing now, because now we know the problem and we’re just pretending we don’t understand it. We are going to be responsible, but it will be our children and grandchildren that have to pay.”

Mark Bowen is the author of Thin Ice: Unlocking the Secrets of Climate in the World’s Highest Mountains.

CO 2 and the “Ornery Climate Beast”

How might today’s human-caused increases in atmospheric concentrations of carbon dioxide and other greenhouse gases change the planet? The past provides clues. Geological records show that in the past 400,000 years, atmospheric concentrations of carbon dioxide, average Earth temperature, and sea levels have risen and fallen roughly in tandem, in 100,000-year cycles paced by slight oscillations in Earth’s orbit. These oscillations affect the distribution of sunlight, hardly affecting the total amount reaching Earth; yet, scientists believe, this has been enough to set in motion chains of events that raise and lower temperatures, launch and end ice ages, and trigger vast changes in sea level.

What’s coming next? Carbon dioxide – the number one greenhouse gas – has much more power to affect Earth’s temperature than the orbital changes do. And in just the past 150 years, humankind has boosted carbon dioxide concentrations by 32 percent. NASA planetary scientist Jim Hansen says that if we continue to increase greenhouse-gas emissions, temperatures will rise between 2 and 3 ºC this century, making Earth as warm as it was three million years ago, when seas were between 15 and 35 meters higher than they are today. His predictions bear weight partly because he can verify his methods: using geological records, he has calculated past temperatures, and his results closely match the measured temperatures shown here.

By David Talbot

Keep Reading

Most Popular

DeepMind’s cofounder: Generative AI is just a phase. What’s next is interactive AI.

“This is a profound moment in the history of technology,” says Mustafa Suleyman.

What to know about this autumn’s covid vaccines

New variants will pose a challenge, but early signs suggest the shots will still boost antibody responses.

Human-plus-AI solutions mitigate security threats

With the right human oversight, emerging technologies like artificial intelligence can help keep business and customer data secure

Next slide, please: A brief history of the corporate presentation

From million-dollar slide shows to Steve Jobs’s introduction of the iPhone, a bit of show business never hurt plain old business.

Stay connected

Illustration by Rose Wong

Get the latest updates from
MIT Technology Review

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

Explore more newsletters

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at with a list of newsletters you’d like to receive.