Greenhouse gases, which include carbon dioxide and methane, trap heat in the Earth’s atmosphere, driving global warming. A carbon offset is a reduction in greenhouse-gas emissions to compensate for an emission made elsewhere. When you buy an offset, you fund projects that reduce emissions by methods such as restoring forests, updating power plants and factories, or supporting renewable energy. You can pick offset investments that fund projects as diverse as providing clean cook stoves in developing countries and reducing methane emissions from local farms.

Carbon offsetting is a growing trend. Companies, governments, and individuals voluntarily spent just under $4.5 billion over the last decade to purchase nearly a billion carbon offsets, according to Ecosystem Marketplace. But offsetting isn’t a panacea for global warming. For one thing, there are already enough greenhouse gases in the atmosphere to drive significant climate impacts. Further, organizations that use carbon offsets without taking meaningful steps to reduce their emissions are open to the charge of greenwashing and the backlash that can come with it.

What are the hallmarks of a quality carbon-offset investment? According to the Environmental Defense Fund, the most important criteria are making sure that the reduction wouldn’t have occurred in the regular course of business operations without the help of the offset investment, that the amount of reduction is measurable and verifiable, that the savings are permanent and not likely to be reversed, and that legal recourse can be taken if the offset provider fails to see the project through to completion.

Carbon-offset brokers, such as Carbonfund.org and TerraPass, can help take the work out of finding quality offset projects by providing a portfolio of vetted options. TerraPass also offers a free carbon calculator that includes a section for tracking the carbon footprint of events. The calculator is a valuable tool for meeting planners not only to measure their events’ footprint but to see where cost savings can be made, according to Nancy Bsales, TerraPass’ manager of carbon solutions. “Energy is money,” Bsales said, “and if you can see that you are, for example, spending a lot of money on water usage or electricity at one meeting and not another, you can make more informed decisions about how to make reductions.”