Climate change may seem too large a problem to tackle, but it's a treatable problem. It won't be easy, and the longer we delay, the more difficult and expensive it will be.

There are two facets to combating climate change:

• Adaptation (adjusting to the changing conditions) and
• Mitigation (reducing greenhouse gases)

Adaptation includes strategies such as growing different crops that are suited to a different climate, building walls to hold back the rising seas, or moving people to safer locations. Adaptation is not optional because of the greenhouse gases already in the atmosphere. But there is still a choice. Will the adaptation be planned, or will it involve strife and large loss of life?

Reducing greenhouse gases—the mitigation part of addressing climate change—is necessary if we're going to limit the damage.

Here is the way one team of Princeton researchers sees it: Our current path is toward doubling CO₂ emissions in the next 50 years, with even greater increases beyond that. In order to get off this path, we need to find ways to keep emissions constant for the next 50 years and then reduce them during the second half of the century. This would limit atmospheric CO₂ to about 570 ppm—still greater than the roughly 380 ppm in the atmosphere today, but enough to avoid the worst predicted consequences.

In order to hold carbon emissions constant over the next 50 years, we need to find some combination of ways to cut 8 billion tons of carbon emissions per year. In the graph, the difference between where we are and where we'd like to be forms a triangle with a height of 8 billion tons in 2055. This triangle can be divided into 8 wedges representing one billion tons each.

What Do You Think?

Below are some technologies that could reduce one billion tons, or one wedge, of carbon. Some of these we could do right away, while others are based on technologies still being studied, such as capturing and storing carbon. Note also that this list represents only some of the possible strategies. However, the longer we wait to reduce emissions, the higher the target will need to be, and the more adaptation will be necessary. In 2004, when the wedges concept was first introduced, the target was only 7 billion tons.

Increased Efficiency & Conservation

• Efficient vehicles: Double car fuel efficiency in 2055 from 30 miles per gallon (mpg) to 60 mpg
A typical 30 mpg car driving 10,000 miles a year emits a ton of carbon annually. Projections are 2 billion passenger vehicles will be on the road by 2055. Doubling fuel efficiency would save a wedge of emissions. Car size and power could probably decrease in order to achieve this. Decarbonization of airline travel will likely prove more difficult, and aviation is the fastest growing component of transportation today.

• Reduced vehicle use: Halve the miles traveled by the world's cars in 2055
In order to achieve this wedge, society may have to invest in increased public transport and urban design changes. Because reducing vehicle use often means increased use of mass transit, which also emits carbon, some of the reduced emissions from this category may be double-counted.

• Efficient buildings: Cut emissions by 25% in all buildings
Significant emissions come from heating and generating electricity for buildings and industry. Two wedges are available in this category: one from residential and another from commercial buildings. In both the residential and commercial buildings, almost half of the savings are to be found in developing countries. The trend toward large houses and many appliances is a challenge to achieving this wedge.

• Efficient electricity production: Double the efficiency of coal-fired power plants
Today's coal-burning power plants produce about ¼ of the world's carbon emissions. The emission savings for this wedge come from converting the fuel to electricity more efficiently at the power plant. Utilities would have to spend more money to achieve this wedge, potentially raising prices for consumers.

Carbon Capture & Storage (CCS)

• CCS Electricity: Capture and store carbon from 800 large coal or 1600 large natural gas power plants
In CCS, the CO₂ power plants normally release to the air would be captured and stored underground in geologic formations. The global storage capacity in oil and gas reservoirs is estimated at 10 to 20 wedges worth, but the method is currently experimental. Demonstration projects store about 1 million tons of carbon underground per year; it would take 3,000 similar projects to achieve one wedge. A main question is safety—small leaks from storage reservoirs would simply weaken the system, but a large leak could be deadly for any people or animals nearby.

• Capture CO₂ at hydrogen plants: Increase hydrogen fuel production from coal by a factor of 10 and store the captured CO₂
Hydrogen could be used as a substitute vehicle fuel. When it's burned, the product is water. But making the hydrogen from fossil fuels itself produces CO₂. This CO₂ can be captured and stored relatively efficiently at a central production plant, but new infrastructure would be needed to get the hydrogen from production facilities to small-scale users. Hydrogen is currently costly to produce, and there are safety issues associated with handling and storing it.

• Capture CO₂ at coal-to-synfuels plant: Capture and store carbon from 180 coal-to-synfuels facilities
By mid-century, the world could be out of cheap oil and may turn to making synthetic fuel from coal. However, this process uses much more carbon than making fuel from crude oil. Carbon capture and storage could mitigate some of the emissions if carbon is captured and stored during the production process. However, carbon would still be emitted when the fuel is used by vehicles or buildings, so synfuels would only break even with emissions from gasoline and diesel. Achieving this wedge would require capturing emissions from 180 plants of the size of a synfuels plant in Secunda, South Africa, that produces 165,000 barrels of synfuel a day. As of 2008, this plant was the largest point source of atmospheric CO₂ emissions in the world since it is not currently capturing and storing this gas.

Fuel Switching

• Nuclear electricity: Triple the world's current nuclear capacity
Nuclear power provides about 17% of the world's electricity and produces no CO₂. One wedge would be saved if the current nuclear production tripled and replaced fossil fuel power plants. Perceptions about safety problems and waste storage issues make this a controversial energy source in the U.S.

• Natural gas electricity: Replace 1,400 large coal-fired power plants with natural gas plants
Because natural gas has a lower carbon content and natural gas plants are more efficient, switching to this fuel would result in about half the emissions of coal. Achieving this wedge would require generating about four times the global production from natural gas power plants operating in 2000. The availability and price of natural gas will be a challenge to achieving this wedge.

• Wind electricity: Increase wind-generating capacity 30 times
Wind energy currently makes up less than 1% of global electricity, but it is growing about 30% per year. An emissions wedge could be saved by displacing coal-based electricity through expanding current wind capacity by a factor of 30. This would require a combined area roughly the size of Germany, but the land can be used for other purposes such as crops or pasture. Difficulties include the availability of storage systems to compensate for the intermittency of the wind. Another limitation is the Not In My Back Yard phenomenon, in which people protest the siting of wind turbines nearby because they view them as an eyesore, a threat to property values, or a danger to creatures such as birds and bats.

• Solar electricity: Increase solar capacity 700 times
Photovoltaic solar cells can be expensive and can't collect energy at night. Solar panels also require significant surface area— a combined area of about the size of New Jersey to produce one wedge. However, the arrays could be located on both dedicated land or multiple-use surfaces such as building roofs and walls. This is one-fifteenth of the land required for a wedge from wind power, but land with only panels on it may or may not be reusable for other purposes as land with wind turbines is. Solar power currently provides less than 0.1% of global electricity.

• Wind hydrogen: Increase wind electricity 80 times and use it to generate hydrogen vehicle fuel
Wind power can produce hydrogen by splitting water molecules into hydrogen and oxygen. A wedge would require four million 1 megawatt wind turbines, which would take up significant land areas, though the land could also be used for farming or pasture. Hydrogen fuel can, however, be less safe to use and store than gasoline, and infrastructure for producing, delivering, and using the fuel would need to be created. As with wind power for electricity, finding acceptable locations for wind farms can be problematic.

Renewable Energy & Biostorage

• Biofuels: Increase biofuel production 30 times
Fuel made from plants still emits CO₂, but the CO₂ would have been released back to the atmosphere anyway as the plants decayed. Consequently, burning biofuels doesn't increase net atmospheric CO₂ concentrations. Achieving a wedge would require the use of about one-sixth of the world's cropland, or an area about the size of India. This could potentially drive up food prices. If biofuel croplands are created by razing tropical forests, biodiversity suffers and carbon is released by the decaying or burned trees. Land resources could be stretched by using crop residues like husks and cobs for biofuel production, or by growing biofuel crops on lands that aren't suitable for food crops. Scientists are working on producing plants and algae that are super-efficient photosynthesizers, and thus require less land to produce the same amount of energy.

• Reduced deforestation, increased reforestation: Halve global deforestation and double forest planting in 50 years
Today there is a net removal of carbon from the atmosphere by forests and soils, even though deforestation adds between 1 and 2 billion tons of carbon to the atmosphere. Halting global deforestation would provide one wedge of emission savings, but to achieve the same savings through reforestation would require new plantings over an area the size of the continental U.S. Presently, about 40% of tropical deforestation is in Latin America, while 30% takes place in Africa and 30% in Asia. In Latin America, forests are mainly cut to provide pasture for cattle; in Asia, cropland is number one; and in Africa fuel wood and cropland share primary responsibility. Therefore, preserving forest reduces future land available for food production.

• Conservation tillage: Apply carbon management strategies to all of the world's farm fields
Converting forest and grassland to cropland causes the loss of one-third to one-half of the carbon that had formerly been stored in the soil. This can be reversed by reducing the time the land is bare and by increasing cover crop planting. Conservation tillage, or less-invasive plowing techniques like no-till agriculture that reduce the aeration of the soil, are another effective approach. In fact, these practices can actually help the soil begin to store carbon, but the benefits are erased if the land is later deep-plowed.

As you can see, there are many ways society can approach the struggle to reduce carbon emissions, but there is no single solution. Many of the strategies mentioned are the realm of governments. And the ever-growing world population means that we'll have to work that much harder to reduce global emissions. But on an individual level, there are many things one can do to make a difference.

The IPCC is not finished with its work. A Fifth Assessment Report is planned for 2013-2014. Climate models also continue to improve. Oceanographers, for example, want to incorporate massive ocean eddies in models. Other scientists want to better model rain and snow's effects on water supplies to help predict droughts and floods. And we need to learn more about what drives the giant ice sheets of the world, how much farther they are likely to melt or slide, and how that will affect sea level.

Scientists are also working on regional models that can predict the effects of climate change locally. That information could be used, for example, to decide whether to put funding toward more firefighters in the West or toward research on how open summertime waters in the Arctic will affect national security.

Scientists are working to produce better datasets from both surface observations and satellites. For example, we need to get a better handle on methane—from both ground and satellite measurements—to improve our understanding of its contribution to global warming. Ultimately, better data will help produce better model results, and many scientists are now working toward a "climate information system" that can tell us more precisely how, where, and why the climate is changing and help us decide how best to respond.

In short, the puzzle is already complete enough to know we need to take action, but climate scientists will continue working to improve the picture. These efforts will provide more pieces of the puzzle to help decision makers target adaptation and mitigation actions to preserve our future.