Basic chemistry teaches that dissolving carbon dioxide in seawater will increase acidity. With atmospheric CO2 levels rising—touching 400 parts per million for the first time in millennia this past May—it is therefore a safe bet that the world's oceans are becoming more acidic. But just how much more? And how much do those levels change from place to place—at the coast or out in open waters, or at the surface versus in the depths?

Those questions have few answers at present, mostly because acidity tests are difficult and expensive to conduct, and therefore infrequent and extremely limited in scope. A new prize aims to change that by offering a $1-million reward to inventors who can devise a cheaper and more accurate test of ocean acidity, which is measured in pH, a gauge of the concentration of ions in a solution. "We know nothing of pH at depth, which is a real concern," says biologist Paul Bunje, a senior director for oceans at the X PRIZE Foundation and the administrator of the new contest. "We have a spotty picture of what ocean acidification looks like around the world."

The new prize—dubbed the Wendy Schmidt Ocean Health X PRIZE in honor of its funder, who previously funded the successful oil spill cleanup prize that offered solutions to disasters like Exxon Valdez or BP's Macondo blowout in the Gulf—offers $1 million to the team that invents the most accurate sensors as well as another $1 million to the team that devises the most affordable and easy-to-use sensors. The sensor’s technical goals are threefold: to operate at depths of at least 3,000 meters, take measurements that are precise to the level of the annual pH change (roughly 0.002 on an acid-to-base scale that stretches from 0 to 14) and avoid the need for frequent recalibration. In addition, the sensors should be cheap so that they can be used widely: "pH sensing should not be limited to those scientists out on $40,000 per day research cruises," Bunje says. "These sensors need to be deployed globally, including in places like developing countries."

Present sensors rely on electrochemistry or dye-based tests (the most common of which is perhaps the litmus test), and can cost more than $5,000 per device. These sensors must also be sent back often to the manufacturer for recalibration to ensure accuracy. The free market offers little incentive for companies such as Honeywell to improve their pH sensors because the market—ocean researchers and the beverage industry—is relatively small and has conflicting needs; a sensor that may be perfectly adequate for cola carbonation may not work well in the deep Pacific. The goal of this X PRIZE, according to Bunje, is to turn ocean acidity readings into valuable and ubiquitous information as is the case with temperature data, and he hopes to inspire research spending well in excess of the prize money put up. For "every X PRIZE so far, the amount of money invested in winning that purse is 10 times the prize itself," Bunje notes. This prize may have a head start, thanks to the desire for cheap and accurate blood-pH monitors in medicine.

Researchers are excited by the idea of an inexpensive pH sensor. Marine biologist John Bruno of the University of North Carolina at Chapel Hill calls current ocean acidification data "trivial, because you can't buy cheap little devices to monitor [in place]." That's what is already done to measure temperature—logging local temps when attached to a rock underwater as often as every second—to register very local changes that can have big impacts. And ocean acidity levels vary even more than temperature in both location and time. "Having a cheap, simple device that you can purchase commercially—not build—would revolutionize the science of ocean acidification," Bruno adds.

Yet, some scientists question prizes as a way to incentivize such innovation. "People want to get some magic payoff from a research program without actually paying for the research program," says climate modeler Ken Caldeira of the Carnegie Institution's Department of Global Ecology at Stanford University, who has worked on some of the initial models for the impacts of ocean acidification, noting that the funds devoted to such sensor research have to come from somewhere. "We know what we need to do to protect our oceans, and that is: stop using the atmosphere and oceans as a waste dump. Measuring pH allows us to document the ocean's decline but is largely irrelevant to saving the ocean."

The Ocean Health X PRIZE will run for the next 22 months. The first 12 months will be given to interested teams to develop their concepts. Following that, entrants will be evaluated in lab trials and, ultimately, tested for a month in Puget Sound, an inlet of the Pacific on the Washington State coast. "We will subject all these sensors to all the great wonderful stuff in the water: diurnal changes in pH, temperature and salinity, and all the biofouling that happens," Bunje says. "Puget Sound is one of the more difficult places these things will have to work."

Those finalists for the accuracy prize that survive Puget Sound will then be sent to sea on a research vessel and tasked with assessing pH at a depth of 3,000 meters, among other rigors. The winner or winners will then reap the rewards in summer 2015, by Bunje's calculations. "We can say there is a problem with ocean acidification right now, but that doesn't mean we know what's going to happen when, and certainly not where," he adds. "For everybody from aquaculture to those who monitor coral reef health for tourism, there is real value in having this data."