Gap, Nike, Levi Strauss and other apparel companies made headlines in September when they committed to setting science-based greenhouse gas reduction targets.

The clothing makers joined about 300 other companies around the world that have pledged to set science-based climate targets, which differ from other targets as they focus on achieving measurable reductions that aim to keep global temperatures from rising more than 2 degrees Celsius. That's according to the Science-Based Targets Initiative, a partnership of CDP, formerly the Carbon Disclosure Project, and the World Resources Institute and World Wide Fund for Nature.

What makes science-based targets different from other emissions-reduction targets? The short answer is that they align with the aggressive emissions reduction pathways outlined by the Intergovernmental Panel on Climate Change.

Science-based targets also adhere to the standards that CDP uses to assess and score a corporation’s environmental impact, including greenhouse gas emissions, water usage and impact on forests.

CDP’s criteria for judging companies’ efforts to cut emissions have become more stringent over the years. The organization plans to beef them up even more between now and 2020, to measure each company’s commitment and goals as part of a holistic review of each sector’s carbon footprint, as well as individual companies’ progress in meeting previous environmental goals and setting new, more ambitious ones.

Targets based on science also drive innovations that are needed for the world to transition to a low-carbon economy, according to CDP.

CDP’s criteria for judging companies’ efforts to cut emissions have become more stringent over the years.

Companies that spend the time and effort to establish best practices, measure and reduce their impact, and disclose this information to CDP -- which in turn shares it with investors — are rewarded with top grades. The very best are showcased on CDP’s "Climate A List," which included 193 companies in CDP’s most recent, 2016 corporate climate action report

Companies that report their emissions and activities to CDP get extra points if they have science-based targets, which can help them win a spot on the A List, said Cynthia Cummis, director of private sector climate mitigation at the World Resources Institute.

"The CDP survey score is very important to companies and the staff who fill out those surveys," she said in an interview. "Companies are competitive with each other and they want to outperform their peers."

Companies are increasingly under pressure from their investors and customers to demonstrate that they are being environmentally responsible and also that they are planning ahead to ensure that they will be competitive in a low-carbon economy, she added.

To set science-based targets, companies first submit a commitment letter to the Science-Based Targets Initiative, then work to develop a target using tools developed by the initiative partners, then submit the target to the initiative for validation.

Of the roughly 305 companies that have committed to setting science-based targets, 72 have completed their targets and had them approved, Cummis said.

The initiative in August approved Las Vegas Sands’s science-based climate target to cut resort operations emissions by 9 percent from 2015 levels by 2021, and reduce emissions from its Hong Kong ferry operation by 19 percent by 2030.

Climate A List companies, which are a bit further ahead, include U.S. manufacturer Stanley Black & Decker, which makes battery-operated outdoor equipment that reduces greenhouse gas emissions for its customers; has sponsored lawn-mower exchanges that have removed more than 20,000 gasoline-fueled mowers from U.S. lawns; and which has established science-based carbon reduction targets and an internal price on carbon, according to the CDP 2016 climate change report.

Companies are increasingly under pressure from their investors and customers to demonstrate that they are being environmentally responsible and also that they are planning ahead.

Stanley Black & Decker aims to cut its energy and water use by 20 percent, and cut its waste and greenhouse-gas emissions by 20 percent all by 2020, according to the company

Stocks of A List companies are included in the Stoxx Global Climate Change Leaders Index which provides a way for investors to buy shares of companies that are taking action on climate change. The index was valued Thursday at $229.96, up 21 percent year-to-date.

The apparel companies that committed to science-based targets in September already had greenhouse gas reduction and sustainability programs on the books.

Levi Strauss aims to cut emissions at its own facilities by 25 percent by 2020 and to use renewable energy to power 20 percent of its operations by that date. The San Francisco-based company also plans to cut the amount of water it uses to make its products and switch to 100 percent sustainable cotton by 2020.

Cotton is very water intensive. For example, the cotton used to make one shirt requires 2,700 liters of water, an amount the average person drinks in 2.5 years, according to World Resources Institute.

Nike plans to cut its environmental impact by 10 percent by 2020, and use 100 percent renewable energy by 2025. The company has been working to use more recycled polyester and sustainable cotton to cut energy and water use. Nike also is developing new low-carbon impact materials, including materials made with recycled waste products.

Gap plans to cut greenhouse gas emissions at its global operations in half by 2020, and divert 80 percent of the waste from its U.S. operations away from landfills by that date.

The companies suggested that by taking the extra effort to make their targets science-based, they were demonstrating their strong commitment to fighting climate change.

"We believe that the private sector has a critical role to play in the transition to a low-carbon economy," Melissa Fifield, Gap’s senior director of sustainable innovation, said in a September statement. "By joining the Science-Based Targets initiative, we’re strengthening our commitment to tackling climate change and working collaboratively, building on the progress we’ve made in our owned and operated facilities."

Other companies that have announced climate goals on their websites or in other venues have not taken the effort to report their emissions to CDP or set science-based targets.

Companies that have not responded to CDP’s survey inquiries include Amazon.com, Facebook and Tesla Motors, according to CDP.