Companies are snapping up carbon offsets, but the patchwork of informal markets serving that appetite lack consistent standards and oversight, some businesses say, making their effectiveness in cutting emissions hard to gauge. …This year, the value of carbon-offset markets is forecast to more than double to over $1 billion. The most active buyer of offsets this year is the energy sector, followed by consumer goods, finance and insurance. …Prices can vary widely, from just a few dollars for credits linked to activities like conserving grasslands to thousands of dollars for capturing carbon from the atmosphere to store in rocks. Those factors make it more difficult for executives, investors and consumers to gauge the effectiveness of projects that generate offset credits. …Companies buy carbon offsets voluntarily, unlike in the regulated mandatory carbon markets in places like Europe and California, where heavy-emitting industries are allocated allowances to emit carbon, which can be bought and sold. [a WSJ subscription is required to access the full story]