By Nigel Holmes/Explanation Graphics, 2009; Published in the National Geographic magazine, December 2009. See the full illustration.

The U.N. Climate Change Summit in Cancun, Mexico just concluded with some agreements about how to cut harmful carbon emissions, although most acknowledge that they fall short of what is necessary to stop global warming. Read a straightforward explanation of how carbon emissions work in "The Carbon Bathtub" and the effect of the global thaw in "The Big Melt." Follow along as Peter Miller goes on a carbon diet and reports on the results. Then take a quiz (one answer can be found below) and find out how much you know about carbon emissions and how to reduce them.

• 2010 April -- National Geographic magazine. The Big Melt. 60-79. The Tibetan Plateau is often called the roof of the world and the glaciers there have long functioned as a storehouse of freshwater which feed Asian rivers. Brook Larmer reports on studies showing the glaciers are shrinking, which one Chinese glaciologist predicts will lead to an ecological catastrophe. Photos by Jonas Bendiksen illustrate the changing landscape and the impact on residents.
• 2009 December -- National Geographic magazine. The Carbon Bathtub. The Big Idea (department). How does global warming occur, and what is the fundamental problem in grasping the concept? Robert Kunzig describes Professor John Sterman's theory and his explanation of carbon production and draining in the atmosphere, using water running into a bathtub as an analogy.
• 2009 March -- National Geographic magazine. Saving Energy: It Starts at Home. 60-81. The average U.S. household produces 150 pounds of CO2 a day. Author Peter Miller enlists his wife and neighbors in a carbon diet to cut their footprint to a level the planet can handle. He reports on the results and considers what impact the efforts of individuals can have. Photos by Tyrone Turner.

Native to warm climates, soapnut trees produce clusters of cherry-like fruits. After the ripe fruits are harvested and pitted, the skin and pulp are dried in the sun. The resulting hollow, leathery globes make suds when added to water that’s churned by hand or machine.

What if you knew exactly which appliances in your home are the worst energy hogs—that the hot tub pump kicks in overnight when it shouldn't, or that the flat-screen TV is sucking energy even when it isn't "on"? Would that change the way you use—and save—electricity?

Residents of Boulder, Colorado, are finding the answers to such questions, thanks to a new "SmartGridCity” program that allows them to track electricity usage in near-real time. Instead of conventional electricity meters that require a reader to come and take a monthly reading, customers of utility Xcel Energy are outfitted with newfangled digital sensors that send real-time usage information back to Xcel’s computers. That data helps the company better avoid power outages—and also lets customers log on to their utility accounts online, where they can track when their usage spikes, then make adjustments.

Although still in its early stages, Boulder’s SmartGridCity program has already yielded a bevy of ways for residents to cut their bills:

1. Kill Your "Vampire" Loads. Those ‘stand-by’ modes that keep your flat-screen TVs, gaming systems, cordless drills, and the rest at the ready are mighty convenient. But they suck up so much electricity that they can add 10 percent to your monthly bill. To save electricity, Boulder, Colorado, resident Tim Hillman did the same thing with the power strips he plugs his electronic gadgets into that some folks do with their living-room table lamps: He attached the strips to automatic timers that switch off overnight. The resulting tangle of timers, strips and cords might look like deranged cootie toys, but they’ll slice your electricity bill.

2. Swamp Your Air Conditioner. Anyone who's hung out by a cold river on a hot day knows that the cold water creates its own microclimate, cooling the air that surrounds it. Evaporative coolers, commonly called "swamp coolers," operate on the same principle. A cold water line is attached to the cooler's reservoir. A filter then sucks up the cold water while a fan blows air through it. The result: a cool, nicely humidified house along with an air-conditioning bill that's as much as 90 percent lower than what it would cost to use conventional air conditioning— the biggest energy hog in the average home. Even better, most swamp coolers (available at appliance stores) can be had for $100 and up—about two-thirds the cost of equivalent A/C units. One caveat: swamp coolers are great for low-humidity climates like Arizona but are far less useful in muggy places like Washington, D.C.

It began with a book. Not a famous book or a best seller but a science textbook, on the shelf of a community library in Malawi—one of 300,000 volumes donated to locations across Africa through the American Institutes for Research. Using Energy, by Professor Mary Atwater of the University of Georgia, had a picture on its cover that captured a 14-year-old William Kamkwanba's imagination, inspiring him to feats of invention. It was the image of a windmill.

In 2002 Kamkwamba had gone to the library in a stubborn attempt to continue his education. A drought had cut his family's food supply so he couldn't afford the fees necessary to enroll in secondary school. He knew little English and couldn't read most of Using Energy. But being the kind of guy who takes apart broken radios and fixes them, he was able to learn a great deal from the illustrations. He was sure he could build his own windmill using scrap from junkyards—an old bicycle frame, PVC pipes for blades. And he did.

To the amazement of fellow residents in the little town of Wimbe, when Kamkwamba hooked his windmill to a dynamo of the sort used to run a bicycle light off a rider's pedaling, his invention generated electricity.

Soon, Kamkwamba built another windmill to pump water from underground. A newspaper noticed. Then a blogger (although Wimbe did not have Internet access, and Kamkwamba had yet to learn the meaning of the world "Google"). Kamkwanba was invited to a TED conference and then himself became the subject of a new book, The Boy Who Harnessed the Wind, written with journalist Bryan Mealer.

We are getting closer to the point where we may have to employ emergency engineering solutions to cool the planet, according to panelists at a geoengineering session during last week's Aspen Environment Forum.

The economic climate is right for redefining the automobile industry, Elizabeth Lowery, vice president for environment, energy, and safety policy at General Motors said last week at the Aspen Environment Forum.

Electric cars are the short-term solution to wean the world off of gas and oil and in return reduce the carbon dioxide (CO2) emissions that are driving climate change, according to a panel at the Forum on the future of transportation.

Just six days into the job, Jane Lubchenco, the new head of the National Oceanic and Atmospheric Administration (NOAA), tells National Geographic "there is a great urgency in addressing [ocean] acidification by reducing CO2 emissions."

"The decisions that individuals make every day add up to affect our global climate," Lubchenco added. "The changes we are seeing now are influenced by our energy choices and uses over the last couple hundred years."

Oceans serve as a carbon sink, absorbing about a third of human-generated carbon dioxide (CO2) emissions. The additional CO2 changes the chemical composition and lowers the pH of the seas. Acidic waters can prevent some marine life from producing calcium carbonate needed for shells and exoskeletons.

Lubchenco, a marine biologist and former Oregon State University professor, was at the Aspen Environment Forum in Colorado yesterday to talk about climate change politics and science.

On a Rocky Mountain day that saw cloudy skies and several inches of powder, solar energy experts gathered at the Aspen Environment Forum encouraged conference participants to turn sunlight, normally abundant in Colorado, into profit. 

Solar power is expected to be a growth market, in both developed and developing countries.

While captured sunlight will never account for the bulk of energy on a global or regional scale, it could provide up to 25 percent of U.S. energy needs and play an important role in delivering energy to poorer countries, said Neville Williams, founder of several solar companies and the author of Chasing the Sun.

According to the U.S. Energy Information Administration, in 2007, solar energy accounted for less than one percent of total American energy use.

U.S. Environmental Protection Agency (EPA) Administrator Lisa Jackson outlines her priorities and tells National Geographic that the EPA is back, ready to protect human health and the environment, despite the bumpy road ahead.

Jackson told a crowd of more than two hundred Aspen Environment Forum participants last night that EPA's top strategy for tackling climate change is to work with Congress on legislation, instead of focusing on amendments to the Clean Air Act that would allow regulation of carbon dioxide (CO2) and other greenhouse gases.

The 2009 Aspen Environment Forum—focused on sustainable energy—kicks off today in Colorado.

Wal-Mart executives, green building experts, climate scientists, Economist and Washington Post reporters, and government officials from Mozambique, Panama, and the U.S. Environmental Protection Agency, among many others, will mingle among the mountains as they discuss climate change, energy extraction and use, innovation and technology, efficiency, and conservation.

Photographer Tyrone Turner photographed the March 2009 coverage on energy efficiency and produced a striking set of images using a thermal camera. I had a chance to ask him about the challenges of the assignment.

“There is no feast which does not come to an end,” a Chinese proverb warns, and this month’s story on Canadian oil sands is a cautionary tale about the consequences of large appetites. With the decline of conventional oil reserves and the rising price of oil extraction, sources like oil sands—layers of tarlike bitumen mixed with clay, sand, and water—are increasingly attractive as a way to satisfy the world’s craving for hydrocarbons. The catch: Extracting them is messy and costly to the environment.