Human civilization will heat up the planet; the glaciers will melt and the seas will rise. It's a familiar refrain by now, with a familiar solution: stop pumping out the greenhouse gases that trap the sun's heat.
But even if we bring the greenhouse effect under control, says a Tufts astrophysicist, the earth will warm up anyway, thanks to a completely different source of heat that we create ourselves.
Over the next 250 years, calculates Eric J. Chaisson in a recent paper, the earth's population will start generating so much of its own heat - chiefly wasted from energy use - that it will warm the earth even without a rise in greenhouse gases. The only way to avoid it, he says, is to rethink how we generate energy.
His paper examines the planet's growing pool of waste heat, a widespread phenomenon that nonetheless has been little studied as a cause of climate change. Nearly everything that uses or generates energy - chiefly power plants, but also cars, snowblowers, computers, and light bulbs - squanders some energy as wasted heat. And the larger and more energy-hungry the human population grows, the more waste heat remains in our atmosphere.
"What this means for humans is that this is the ultimate limit to growth," said Dennis Bushnell, the chief scientist at NASA's Langley Research Center, who urged Chaisson to publish his idea. "As we produce more kilowatts, we have to produce more waste heat."
Chaisson's prediction suggests we need to change our energy policy - not just by keeping emissions low, but by shifting toward power sources that don't add new heat to the earth's system.
The culprits in the waste-heat problem are not only dirty fossil fuels like coal and oil, but also some "clean" power sources like nuclear and geothermal energy, which still add to the problem by pumping new heat into the atmosphere. The only way to stop waste heat-induced global warming, in Chaisson's view, is to rely on energy that already reaches the earth's surface: sunlight, and the wind and the waves that it powers.
Critics say Chaisson's paper describes a scenario so far in the future, and so dependent on projections, that there's simply no way to know if it will come to pass. They also say it could distract us from the far more urgent problem of greenhouse gases. But the idea has piqued the interest of several scientists from around the world who see an opportunity to avert a crisis before future generations have to face it. And in a broader sense, it also suggests a new framework for decisions, one that appreciates the long road we - and our planet - have traveled in our evolution from the microbes of the primordial oceans and the stardust of the cosmos.
That kind of long-range thinking is exactly what drew Chaisson's attention from astrophysics to the topic of global warming. His research typically focuses on the origins and evolution of stars and galaxies, and during a seminar on climate change in 2007, Chaisson asked himself: If we look at the earth as a spinning ball in space, reliant on the Sun, but where people plow new energy sources, how long will it be before all that new energy heats the planet up? He scribbled out some rough calculations to predict the eventual explosion of waste heat.
The concept was not entirely new to him. With his mentor and friend, the late astronomer Carl Sagan, Chaisson had often discussed the notion that no civilization on any planet could survive over the long term unless it relied on energy from "its parent star."
"It just came as a no-brainer," he said, of the waste heat idea.
Chaisson published his paper last year in a journal of the American Geophysical Union. Since then, he has received a flurry of e-mails from intrigued researchers and has spoken about it at conferences from the West Coast to Europe.
His predictions are based on a simple but fundamental law of science: Energy can't be perfectly harnessed, but tends to dissipate, usually in the form of heat. The concept, also known as entropy, is laid out in the inviolable second law of thermodynamics.
In practice, this means that any machine we run, whether a car engine or a power plant, not only does the work we're asking it to do, but emits heat - a lot of heat. Think of touching a lightbulb, or holding your hand over your exhaust pipe after your car has been running for only a few minutes. The heat you feel is energy radiating uselessly into the air. On a much larger scale, the same thing happens when we burn coal or oil, run a nuclear plant, even bring geothermal energy up from beneath the earth's surface.
Altogether, humans waste about two-thirds of the energy we produce on earth. More than half of the energy it takes to run a car's engine, for example, is squandered as waste heat, most of which pours out of exhaust pipes. Power plants regularly release more energy in waste heat that they make in electricity. Currently, all this heat makes little difference for our climate: it mostly dissipates into space. But the more energy humans use to fuel our societies and to feed our populations, the more waste heat we will emit. Eventually, as more economies industrialize, and as the population grows, that heat will become a significant problem.
Even if we capped all greenhouse gas pollution, Chaisson calculated, in roughly 300 years the planet will warm at least 3 degrees Celsius, if our non-renewable energy use increases in line with historical trends and UN projections of population growth. Though 3 degrees might sound small, such a rise in average global temperatures would lead to dire consequences, including significant sea level rise and mass extinction of species, according to scenarios outlined by the Intergovernmental Panel on Climate Change.
The impact of waste heat from power plants, cars, and factories can already be measured in certain ways. The lakes, rivers, and oceans around nuclear power plants heat up as the water used to cool reactors is discharged into them. And waste heat contributes to "heat islands" in cities. A 2007 study of Tokyo showed that summer temperatures in neighborhoods with office buildings are warmer by nearly 2 degrees Celsius when air conditioning units are running - because as the units cool the insides of buildings, they also pump heat into the air.
The only way to stop this wasted heat from warming our atmosphere is to avoid generating it in the first place, which means using the energy that naturally comes to the planet. When a wind or wave turbine spins, or a solar panel collects rays, they're using energy that's already part of the system.
Today, many climate scientists view waste heat as a negligible problem compared to greenhouse gas emissions. The heat-trapping effect of greenhouse gases has 100 times the effect on global warming that waste heat does, said Mark Flanner, a researcher at the National Center for Atmospheric Research in Boulder, Colo., who uses computer models to simulate waste heat. Flanner agrees with Chaisson, however, that it will play a much larger role over the long term.
"If we assume the current growth in nonrenewable energy use, the heat flux will be of equal magnitude to the greenhouse effect 200 years from now," he said.
Still there are several big unknowns in Chaisson's predictions. One is his assumption that power plants and engines will continue to be quite wasteful. History suggests that societies tend to become more, not less, energy-efficient as technologies improve. Today's machines are far more efficient than yesterday's, and if that trend continues, the problem of global waste heat could be slower to develop - although the laws of thermodynamics say it's impossible to reduce wasted energy to zero.
"We just don't know if there is a point at which energy use will level off," said Yangyang Liu, an atmospheric scientist at Brookhaven National Laboratory who was intrigued by Chaisson's paper, but thinks it may overestimate waste heat's contribution to long-term global warming. "The efficiency to convert energy to work will also probably improve over time."
Also uncertain is how much waste heat the planet's oceans can absorb before the overall temperature rises. And Chaisson's starkest scenarios assume that the human population will grow at certain rates, although it could drop sharply from widespread disaster or disease.
Since his paper was published in July, the most frequent criticism Chaisson has heard is that the waste-heat problem diverts attention from the far more urgent question of how to tackle greenhouse gas emissions. If we cook ourselves with carbon dioxide first, critics say, waste heat will make little difference.
"I can't show that the estimates are wrong," said John Merrill, an atmospheric physicist at the University of Rhode Island's Graduate School of Oceanography. "But I can say that there are many hurdles caused by climate change and other environmental and social challenges that we need to address a lot sooner than this set."
Chaisson concedes that in looking 300 years into the future, not just 50 or 100, his critique sits outside current debates over climate change. But he warns that taking a long-term view is vital to human survival - even if the coming environmental catastrophe is something that neither we, nor our children, are likely to see in our lifetimes.
"It's true, it's not important now," he said, while sitting below a timeline of the universe in his office at Tufts. "But from the point of view of an astrophysicist, we shouldn't be digging up any resources on our planet when we have plenty of energy coming from our parent star, the sun."
Long-range ideas like Chaisson's are uncommon in the realm of policy, where the pressure is great to deal with present-day challenges that affect people alive today. But such long-term thinking has been an important thread in the environmental movement, and holds an undeniable grip on the popular imagination. Rachel Carson's early and influential book "Silent Spring" opened by imagining a future in which pesticides had killed all the songbirds. More recently, last summer's animated blockbuster "Wall-E" set its story on a post-apocalyptic Earth, a wasteland heaped with trash.
In the hope that thinking far, far ahead will catch on among scientists and policymakers, Chaisson has been spreading his perspective. Foundation for the Future, a Seattle-based nonprofit dedicated to contemplating the "long-term future of humanity," invited him to present at a meeting in Paris hosted by the United Nations Educational, Scientific, and Cultural Organization.
Energy has been the key ingredient that has sustained human life, the growth of our intelligence and civilization, Chaisson told them. But the way we are making it could thwart our evolution, and our survival, if we do not adapt now. "Humans are part of an evolving universe stretching across billions of light-years of space, and billions of years of time," he said. Then he asked, "Who are we to think that change ought now to stop?"
Bina Venkataraman writes about science for the Globe.