Friday, 23 January 2015

How much CO2 are humans producing, anyway?

By Jocelyn Turnbull, Senior Scientist, GNS Science

Jocelyn Turnbull collecting air samples
downwind of the Kapuni natural gas plant
in Taranaki in October 2014. 
Photo credit: Jessica Mills, GNS Science
With the historic news that China and the USA have agreed to limit their greenhouse gas emissions and the progress at the latest round of climate talks just finished in December 2014, it is worth thinking about how we determine what those emissions actually are, and how we will know if they (and we!) are meeting emission goals.

We know what the current emission levels are because governments and industry have agreed to report them.  For the energy sector, industries and governments track usage of coal, oil and natural gas, and governments tally up the totals and report them to the United Nations Framework Convention on Climate Change, following a detailed set of guidelines.  As far as we know, those reports have been made in good faith, and reports undergo international peer review, but in a future where we agree to regulate emissions, how can we establish greater trust and be more confident that other nations are meeting their obligations?

Atmospheric scientists are working hard to provide an alternative method for measuring fossil fuel carbon dioxide emissions – observing them through “top-down” atmospheric measurements.  This is a challenging problem, because it requires us to overcome two major hurdles. First, how do we distinguish the carbon dioxide that comes from fossil fuel burning from the “natural” carbon dioxide in the atmosphere?  Once we’ve solved that problem, how can we use these atmospheric observations of carbon dioxide concentrations to calculate the emission rates from power plants and other carbon dioxide polluting sources?

To solve the first problem, we use a novel application of radiocarbon dating.  Normally, radiocarbon dating is used to determine the age of organic materials, but here we turn the method on its head.  We date carbon dioxide in the air, but we know that it is made up of natural carbon dioxide that is modern, and fossil fuel carbon dioxide which has no radiocarbon because it is extremely old (fossil!).  The age we measure in the air tells us how much of the carbon dioxide comes from fossil fuel burning.  This idea is not new, it was first discovered by Dr. Suess (not THAT Dr. Seuss!) in the 1950s, but wasn’t until this century that the measurement techniques caught up and allowed us to make detailed enough measurements to be useful.

So, now to the second problem.  I like to think of it as the “who farted” problem.  You know, you’re in a crowded room, and suddenly you smell it.  Of course you look around to see who did it.  How do you figure it out?  Was it a little one from nearby, or a whopper from further away? This is the same problem we deal with in the atmosphere, except that we are sniffing carbon dioxide.  To figure out where it comes from, we need to know how the air moves from one place to another, and we use atmospheric transport models to do this.  The models range from very simple to very complex, but have one thing in common – none of them are perfect.  These models are the most challenging part, and mean that thus far, the best “top-down” fossil fuel emission measurements are accurate to within about 20%.  Scientists are working hard on improving the accuracy, partly by improving the models, and also by thinking about clever sampling strategies that are less reliant on these models.

A great example of this is a collaborative project involving researchers at GNS Science and in the USA, China and South Korea to examine whether the massive Chinese economic growth in the 2000s really resulted in the doubling of Chinese fossil fuel CO2 emissions that has been reported.  We measured the radiocarbon content of air in flasks collected from 2004 to 2010 near Beijing and on the west coast of South Korea where air flows out of China, then compared the measurements with model simulations.  We found that the reported increase in Chinese emissions was indeed correct, making China the largest emitter of fossil fuel CO2 in the world.  As a bonus, we also measured the air pollutant gas carbon monoxide, and found that over the same time period, carbon monoxide emission totals stayed roughly constant, indicating that Chinese efforts to improve combustion efficiency and reduce air pollution have been quite effective – although China clearly has a lot more work to do to improve air quality.  Other projects we are currently working on include using the Kapuni natural gas plant in Taranaki as a demonstration site for measuring emissions from power plants, measuring urban emissions in Indianapolis in the USA, and reconstructing Wellington’s fossil fuel CO2 emissions over the last 50 years using tree rings. 

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