Ganesan, A. L., et al. (2017). “Atmospheric observations show accurate reporting and little growth in India’s methane emissions.” Nat Commun 8(1): 836. doi:10.1038/s41467-017-00994-7
Parallels Between the Carbon Cycle and Your Bank Account
Keeping track of how much carbon is in the atmosphere in the form of the greenhouse gases carbon dioxide and methane isn’t that different than tracking the balance in your bank account. Scientists who study the carbon cycle even use accounting terms to describe aspects of carbon cycling. For example, the gross carbon uptake of an ecosystem is the total amount of carbon dioxide plants breathe in during photosynthesis, taking it out of the atmosphere. The net carbon uptake is the photosynthetic uptake by plants minus the carbon dioxide emissions from the ecosystem. This is like the gross versus net income for a business, where the net income is the balance remaining after all of the business expenses have been subtracted from the gross income.
Another example of scientists using accounting terminology is when they talk about the carbon budget of an ecosystem, which is similar to a financial budget. A carbon budget details sources (income) and sinks (expenditures) of carbon between different systems (accounts). The systems could be a farm, a city, or the Earth’s oceans and atmosphere.
Knowing how much carbon dioxide and methane are in the atmosphere is important to track the role these heat-trapping gases are playing in climate change. Scientists also want to understand trends in emissions and feedbacks between emissions and warming to better be able to predict the future levels of these greenhouse gases in the atmosphere, and how they affect the climate. In addition, policy makers need to know the sources of greenhouse gases to be able to craft effective emission reduction strategies.
India’s Methane Budget
A recent study took a close look at India’s methane budget. Methane is the second most important greenhouse gas after carbon dioxide in terms of overall warming. Methane is produced when organic material is broken down under anaerobic (oxygen-free) conditions. It is also the main component of natural gas, and is produced during fuel consumption. The main human-generated sources of methane in India are cattle production, wastewater, landfills, rice agriculture, biomass burning, and fossil fuels. In the study, the authors evaluated methane emission estimates from several source-specific inventories (more on this below) using a combination of observations from satellites, aircraft, and on-the-ground monitoring.
Top-down vs. Bottom-up: Meeting in the Middle
Let’s revisit the banking analogy. Constructing an emission estimate by tallying up methane emission contributions from different sources or sectors is like keeping track of your bank account balance by adding up all of your purchases and deposits and subtracting them from your checkbook balance (that is, balancing your checkbook). Scientists who study greenhouse gas budgets call this a “bottom-up” approach, because it is based on the “ground level” measurements taken where the emissions are occurring. The alternative, known as a “top-down” approach, is to monitor how much of the greenhouse gas is in the atmosphere. This is similar to checking only your bank account balance, which gives you information on the overall impact of your spending on your account balance.
There are pros and cons to each approach. The main advantage to the top-down approach is its reliability. It is easier to accurately measure the increase in atmospheric methane due to the sum of all the inputs than it is to accurately inventory all of those inputs individually. This is like simply checking what your current checking balance is, as opposed to trying to balance your checkbook by tracking all your deposits and expenditures. The main drawback of the top-down approach for measuring methane emissions is that it does not help us more specifically pinpoint where the methane is coming from. The bottom-up approach to measuring methane emissions gives us more detailed information on sources, that is, where the methane is coming from, and how much methane is being emitted from a given source. This information is key in formulating effective policies to reduce methane emissions.
So, just like anyone who is serious about sticking to a budget knows: the best strategy is to combine both bottom-up and top-down monitoring. Track your purchases and deposits (bottom-up) so that you know how much of your money is going where, and make sure you aren’t missing anything important by verifying your account balance with what the bank confirms you have in your account (top-down).
Study Findings: Evaluating Bottom-Up Inventories with Top-Down Monitoring
The India methane study compared estimates of India’s national methane emissions from two different bottom-up inventories to top-down observations from satellites and aircraft. The two inventories were the Emissions Database for Global Atmospheric Research and India’s First Biennial Update Report to its National Communications. Both of these inventories combined estimates of methane emissions from different sectors into a national-scale total methane emission estimate for India. The top-down atmospheric observations indicated total annual methane emissions from India of 22.0 Tg per year, which is equivalent to 24 million tons. This top-down estimate is closer to India’s First Biennial Update Report estimate than to the estimate from the Emissions Database for Global Atmospheric Research.
The study authors further checked the accuracy of their top-down assessment of methane emissions with other available information. As previously mentioned, cattle are one of the main sources of methane in India. One inventory reported a growth in cattle methane emissions, based on the assumption that the number of cattle increased over the inventory time period. However, India’s Ministry of Agriculture reported there was actually a decrease in cattle population over that same time period. This more reliable report from the Ministry of Agriculture indicates that the emissions inventory is likely over-estimating cattle methane emissions.
Takeaways from the India Study Findings
Overall, it is important for policy makers in any nation to have the best available information on greenhouse gas emissions in their geographic area. That information will allow them to figure out the most efficient and effective ways to reduce greenhouse gas emissions, including emissions of methane. According to the analysis in the study conducted on India’s methane budget, it is encouraging that those emissions have not increased over the last several years.
We need similar studies, based on sound science, conducted in other parts of the world so that governments, municipalities, and individuals will be able to put into place policies and practices to reduce methane and other greenhouse gas emissions, without compromising the economics or quality of life in that area.