Senators returned from their August recess with lots of ideas about how they would like to change the comprehensive clean energy and climate protection bill the House passed in June. This is a normal, and in many respects, healthy part of the legislative process. Some of these ideas, such as devoting more resources to building a sustainable transportation system, will be constructive; others, such as delaying consideration of enforceable pollution limits and proceeding with just a stripped down energy bill, not so much.

One thing we know for sure is that the Senate will not rubber stamp the House bill. That’s good. While the House bill is an excellent starting point, it definitely has room for improvement, as I have discussed before. With dozens, if not hundreds, of proposed changes, big and small, flying around as the Senate gets down to business, how do we sort out the good, the bad, the ugly, and the inconsequential?

The bottom line for me comes down to tons. How many fewer tons of global warming pollution will end up in the atmosphere as a result of the legislation? The answer for the House bill according to the World Resources Institute (WRI) is 2 billion. Specifically, WRI’s analysis shows that the House bill will likely reduce emissions by 2 billion metric tons compared to 2005 levels in 2020. This is equivalent to the annual output of about 600 conventional coal-fired power plants.

The House bill accomplishes this primarily through limits on the total amount of global warming pollution that can be emitted by major sources, but other aspects of the legislation also contribute. For example, the overall pollution limits in the House bill cover about 85 percent of emissions, but the EPA is directed to set performance standards for most of the remaining smaller sources. Substantial additional pollution reductions would be achieved by dedicating a portion of the revenue from auctioning pollution permits to a fund for reducing tropical deforestation.

In applying the 2 billion ton test, one nuance we need to keep in mind is that the expected pollution reductions from any legislation are not all equally certain to materialize. For example, the House bill provides a lot of flexibility for regulated polluters to “offset” some of their emissions by securing pollution reductions from unregulated sources. That’s OK because CO2 emissions have the same impact on our climate no matter where they come from, but only if the offsets represent real and additional pollution reduction beyond business-as-usual.

Assessing the quality of offsets is inherently more uncertain than measuring direct reductions in emissions from regulated sources. Unfortunately, some of the amendments adopted on the House floor would make it more difficult to enforce offset quality, so changes in the Senate that improve oversight of the offsets program can increase the likelihood of achieving the 2 billion ton target, even if the target itself is not altered. Similarly, complementary measures to promote energy efficiency and renewable energy deployment generally don’t change the emission reduction target, but by lowering costs and reducing reliance on offsets they increase the likelihood of achieving it.

Given these nuances, is the 2 billion ton test really useful in practice? Here is an example of how it applies to a concrete proposal released on Friday:

A Senate working group on coal issues made a number of proposals for changes to the House bill. Some of these are fine, others risk wasting valuable resources, but one goes directly to the bottom line: The working group recommended exempting coal mine and landfill methane from performance standards and instead making all emission reductions from these sources eligible as emission offsets. The net effect of this proposal would be to increase global warming pollution by about 100 million tons in 2020. Were the Senate to accept this proposal it would have to come up with strengthening changes that compensate for these excess emissions in order to pass the 2 billion ton test.