After spending what is likely a fairly large amount of money to buy carbon offsets, Google can now say it has "neutralized" all of the company's emissions from 2007 and some from 2008.

But the money wasn't the tough part. The more difficult aspect was finding carbon certificates that could definitely be tied to projects-like the methane capture project it did buy credits from – that actually reduced greenhouse gases.

Bill Weihl, who carries the title "Green Energy Czar" at Google, announced this milestone on a blog post yesterday. In 2007, the company vowed to become carbon neutral by the end of that year. It charted out three ways to achieve this goal: use energy more efficiently, use renewable energy and buy carbon offsets.

Getting there obviously took a lot longer than Google had expected, and Weihl's blog post provided a glimpse why. Selecting which carbon offsets to buy is a time-consuming task. The process involves evaluating projects, verifying that the projects meet certain criteria and validate that the projects were completed as planned.

"Current standards for offsets require a significant amount of work to evaluate the quality of each offset project and ensure that projects go beyond 'business as usual.' Stronger additionality standards – that are more stringent, clear, and objective – would also make it simpler for corporations like Google to use offsets as part of an overall strategy to neutralize emissions," Weihl wrote.

Weihl didn't discuss the standards Google followed. The reality of the carbon-trading world is there is no national standard or even an industry-wide standard. The rules that are set largely depend on which carbon trading companies and consultants a company uses to obtain carbon credits.

He didn't discuss prices either but they have been going up. In the Regional Greenhouse Gas Initiative auctions, the price has gone from $3.07 per allowance last September to $3.51.

In some cases, the buyers get a blend of carbon credits and have little knowledge of what projects were involved in generating the credits and whether those projects really accomplished the emissions reduction goals (see A Shopping Mall for Carbon Credits and American Carbon Wants Your Business).

"There is a spectrum of projects, and you have high quality and lower quality. It makes sense for Google to find the right projects," said Justin Felt, senior analyst with Point Carbon, a market research firm in Washington, D.C. "There are several standards out there providing some clarity and market transparency, but if you want to go outside of those registries, then you have to do your homework."

For companies in industrialized countries that abide by the Kyoto Protocol, they could go through a program called Clean Development Mechanism (CDM) to get offsets by investing in cleantech projects in developing countries. The program has received strong criticism for its bureaucracy and ability to enforce rules. 

Earlier this week, Reuters reported that some hydropower projects in China that are set to receive investment funds in exchange for carbon credits under CDM might not qualify after all. CDM rules say projects must show that they couldn't be completed without investments from the industrialized world. Friends of the Earth and other groups have contended that this kind of projects creates what they called "subprime carbon" and don't make a big dent in efforts to curb global warming.

Weihl didn't discuss what types of cleantech projects produced the carbon credits it bought, where those projects are located or how much money it has sunk into these offsets. Weihl did point out that the company relied on Winrock International to verify Google's portfolio of carbon credits and its emissions calculations.

Winrock runs American Carbon Registry, which announced last November that Google bought some of the credits produced from a methane capture project at a landfill in New York.

While Google doesn't want to rely on buying carbon credits to offset its emissions in the long run, this approach remains a key measure for the company to make some sort of carbon-neutral claims.

Over the past five years, the company has implemented measures to directly cut its emissions, such as developing energy efficient data centers and using renewable energy. Those measures have eliminated half of the emissions that Google would otherwise produce, Weihl wrote. Carbon credits took care the other half, he added.