It's frustrating to feel you're part of the problem when you want to be part of the solution. But there isn't much choice in a fossil-fuel driven world. Unless you practice a severe form of back-to-nature self-sufficiency, you contribute to global warming, like it or not.

There's no avoiding it -- not even if you drive a Prius, perfectly insulate your home, replace all your incandescent bulbs with compact fluorescents, use rechargeable batteries and follow every other precept in the energy-savers rulebook. No, not even if you power your home with your very own solar panels and bike to work each day (though kudos to you if you do).

The reason is that personal emissions -- the ones from home energy use and driving that you're directly responsible for -- account for just 40 percent of your total. The larger part comes from everything else you buy and do. Your clothes, for instance. The songs on your iPod. The food you eat. For all of these things are made, grown or transported with the help of fossil fuels. So is the bike with which you may idealistically pedal to work. So are the solar panels.

But let's get real. If you're like most people, these indirect emissions are beside the point. You do depend on a car for transportation and the "grid" for power and are not about to overturn your whole life in a quixotic attempt to fight a global problem single-handedly. Still, you wish there were something reasonable you could do. And there is: buy carbon offsets to cancel out your emissions.

Carbon offsets are projects that reduce or prevent the accumulation of global warming gases in the atmosphere to make up for the gases that you have inadvertently put there. They achieve this either by increasing the availability of renewable energy, supporting energy-efficiency improvements by industry or capturing and sequestering emissions.

Of course, you don't really buy these projects. What you do is contribute to them. Depending on who you do it with, the contribution may or may not be tax-deductible. What it goes towards will also vary. Some organizations and companies focus on just one thing, such as buying renewable energy certificates. Others make a point of funding different types of projects, much as a mutual fund would. Certain groups choose projects that not only help with global warming, but other environmental or social problems as well, such as forest degradation or poverty.

Here are some of the major players in the United States:

CarbonFund.org is a non-profit that funds renewable energy, efficiency and sequestration projects. You can choose among them on the contribution form. The site has an easy-to-use calculator on the homepage for estimating your emissions, both direct and indirect. However, it only factors in carbon emissions, not those from other global warming gases. If this bothers you, contribute a little extra.

CarbonCounter.org is a joint project of The Climate Trust and the international relief organization Mercy Corps, which has gotten involved out of a desire to forestall humanitarian disasters stemming from climate change. CarbonCounter.org funds energy efficiency, renewable energy, cogeneration, transportation efficiency and reforestation projects. For project details, see The Climate Trust site.

Native Energy, a privately held Native American energy company, helps build new wind farms and biomass generators owned by Native Americans and/or farmers. Native Energy also offers renewable energy credits. In addition to an all-purpose calculator, it has a travel calculator that factors in all kinds of transportation (including train and bus) as well as accommodations. Unfortunately, the site itself is a little confusing, but not so bad you can't make sense of it.

Bonneville Environmental Foundation is a non-profit that sells renewable energy credits to offset emissions. Bonneville's site has several calculators -- one is all-purpose, another is for special events and a third is for car and air travel.

Terrapass is a for-profit company that offers a simple 1-2-3 process for offsetting your car emissions. Funds go toward a variety of wind, biomass and efficiency projects.

Solar Electric Light Fund provides rural villages in developing countries with solar power. A gift to this group isn't an offset per se, but a way of extending the benefits of electricity -- in a climate-neutral way -- to some of the two billion people on the planet who lack it.

Each of these groups, other than the Solar Electric Light Fund, takes pains to get third-party certification for the projects it funds. Unfortunately, certification criteria aren't uniform, so you can't be sure all projects measure up. Renewable energy certificates are the exception. You can feel confident they conform to high standards if they have the Green-e seal of approval.

Some people question whether buying offsets isn't like paying for the right to pollute. They think we should focus on bringing our individual emissions down instead. I don't see it that way. Sure, we should do what we can in our personal lives. But dealing with global warming requires something more -- a change in the technologies that power our world. In my view, offsets will get us there quicker.

Besides, it's not an either/or choice. We can reduce our energy use and buy carbon offsets at the same time. The more we do -- and the sooner -- the better. Time is short.

—Sheryl Eisenberg

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Why carbon calculators give different answers. All the calculators at the sites listed here use government data, but do their accounting differently. For example, some factor in all global warming gases; others, just CO2. As a result, your total will differ with each. Don't fret over which is "right." The point is just to get some sense of the amount of emissions you're responsible for. As all the calculators will show you, the answer is, literally, tons.

Why offset prices vary. There is no single solution to our global warming woes. Rather, there are many, all of which will have to be tried if we are to shift our modern economy, and way of life, away from fossil fuels. The variety of solutions is reflected in the range of offset projects offered, and the range of prices. Different accounting methods also explain some of the variation, as does the fact that some groups use some of the money for other purposes -- charitable, educational and operational. Rather than look for the group that offers the cheapest offsets, I suggest you choose the one whose solutions seem most promising to you.

Even NRDC does it. NRDC's offices are energy-efficient, but still need electricity and heat. To offset the emissions, NRDC buys renewable energy credits. In addition, it's begun to inventory emissions associated with employee travel and is considering how to track those from commuting, paper use and waste generation, with the idea that all could eventually be offset.