Nine gigatons! That’s big! Ten gigatons! Even bigger! What is a gigaton, anyway?
After reading this or that GHG number slung about, I realized that I had no idea what any of the numbers really meant. Now I know enough to be dangerous. This is what I found.
My mission: Giving you the ability to recognize the relative meaning of some of the big numbers relating to Greenhouse Gas (GHG) pollution, so they make some kind of sense when you read about them.
I confess, I’m a numbers guy. Numbers matter. Not crazy exact, prove me wrong out the 4th decimal kind of numbers. Just the kind of round numbers that give you some kind of handle on what’s going on. One significant digit. Maybe two. The kind of numbers that will allow you to read something and say either “Oh crap, that’s big” or “piffle, don’t bother me with mere kilotons of carbon.” The kind of numbers that tell you when someone is trying to blow some Koch up your ass.
When it comes to GHG pollution, first order of business is to organize the fruit so you are comparing like ones. What kind of emissions are you talking about?
1) CO2 emissions. This number is actual emissions of Carbon Dioxide molecules in the atmosphere. For instance, one gigaton (one billion tons) of actual CO2 emissions.
2) Carbon emissions. This number is the carbon component of carbon-containing emissions. In the example of one gigaton of CO2 emissions, the carbon emissions are 270 megatons (270 million tons). Although the CO2 emissions are reduced to about 27% when converted to carbon, non-CO2 GHG emissions also get counted in this total.
3) CO2-equivalent emissions. This number is an climate impact-weighted measure of pollutant emissions, where the reference standard is based on CO2 emissions
a. A reference standard is a baseline that can be used to normalize and thus compare impacts of a disparate collection of things. For instance, one Koch is a standard unit of harm, which can be used to reconcile harms as disparate as invading another country, supporting a dictator, or eliminating environmental protections. Similarly, CO2 equivalents are used to reconcile and add together the impacts of various GHG pollutants.
b. This index is has some tricky aspects because GHG pollutants have different residence times in the atmosphere. Methane has 19 times the climate impact of CO2 in the short term, but it leaves the atmosphere more quickly. So, is it really 19x CO2 equivalents? (Answer: Sort of, it's complicated)
Anyway, the first order of business, any time you read about GHG pollutant emissions, is to understand which of these numbers you are reading.
Here are some big numbers:
- 9 GT per year current world wide carbon emissions (so that’s measure #2 above)
- About 130 GT worldwide carbon emissions in the last century
Here’s an example of trying to use the big numbers to make some kind of sense of other numbers you might read about.
FishoutofWater’s recent diary about permafrost melting, a chart (apparently from the cited article, where the whole text of the original is behind a paywall) showed modeled future emissions from the melting, where the unit of measure was gigatons of Carbon (choice 2 above). That is a good choice for the chart, because the carbon in question is a mix of CO2 and methane. However, it’s very difficult to tell from the chart what the actual climate impact would be. Probably nobody knows, other than the quoted “it’s a lot of carbon”.
So, other than “a lot”, can we figure out how big this really is? To put these numbers in context, it’s useful to consider the numbers as they compare to current GHG emission rates (converted to a standard unit or units, of course). The modeled contribution of permafrost melting peaks at 1.8 gigatons of carbon a year and spends a lot of years (about 2036 through about 2135) north of 1 gigaton per year (let’s call it 1.4 GT per year over that time).
On a carbon to carbon basis, this is an additional kick in the ass but it is far from a doomsday scenario – about a 15% adder to our current carbon emission rates of 9 GT/year. The monster variable is how much of the carbon is methane. At low percent methane, it’s a really big number but still smaller than our impacts to date. At 50% methane (where methane is 19 x CO2 in short term impact), it’s beyond a disaster.
So what did that numbers game accomplish?
1) It helped put an enormous number into some kind of perspective
2) It identified the controlling variable which really matters, in this case the % methane of the emissions.
3) Maybe, it even identified a mitigation strategy. Fire, anyone? You heard it here first. Ironically, in a case where recklessly combusting hydrocarbons has brought us to the brink of disaster, the largest combustion exercise in history might be helpful. Go figure. Mid 21st century job: I’m a Siberian fireman, in a sort of post-post-Bradbury sense.
Ok, here’s another one. You’re discussing global climate change with someone who pulls out this old but good one:
Natural sources of carbon are ten times greater than emissions. So, you may as well try to stop wild animals from breathing, or farting. Answer me that! Huh? Huh?
The first thing to realize about this one is: It’s true! True, but incredibly misleading. Yes, natural sources of carbon are greater than artificial. But, it doesn’t matter.
Picture a bathtub, with some water pouring in from the faucet and some draining away. The amounts are equal and the water in the tub stays level. So far, so good.
So, what happens if you increase the amount of water pouring in by 10% (effectively what we have done with GHG emissions)? That water level is going to go up.
Then, what happens if you clog up the drain, not totally, but just by 10% or so (effectively what we have done by eliminating natural sources of carbon absorption). Guess what, the water level is going to rise twice as fast.
So, the way the GHG pollution actually increases is not just about the gross quantity of emissions. It’s the change: emission minus absorption. The term carbon flux is also used for this.
For the next numbers game, I wonder how many gigatons of carbon (emitted as CO2) that it takes to move the earth’s average CO2 concentration by a ppm. After all, that’s the bad result of the emissions.
Again, just round numbers to get the idea. The real math is doubtless too complex for any mortal to understand.
Current CO2 concentration is going up by about 2 ppm per year. Emissions are 9 GT carbon per year. So, to one significant digit, 1 GT raises CO2 by 2/9 = .2 ppm.
Next logic step: How many ppm result in an average temperature rise of 1C?
STOP. That is not a useful number. Carbon pollution has so many ill effects, including local severe weather effects, local warming of 6 or more times the global average, and ocean acidification, that thinking in terms of an average temperature rise of 1C just blands the impacts away. A global rise of 1C doesn’t sound too bad, until you realize that really is a 6C increase in certain places, plus many other harms.
It appears that sticking with CO2, or CO2 equivalent, concentration is the best end point measure to focus on. Original 280 ppm. Now 385 ppm. Rising 2 ppm/year and accelerating. More than 450 ppm, very very bad. More than 450 ppm plus lots of methane. Even worse.
Okay, that’s a lot of words and a lot of numbers.
Leaving the numbers for a moment, an appeal as always - your contribution matters. Recycling reduces GHG pollutant emissions, are you recycling all you can? More efficient light bulbs matter (and save you money). Next time you buy a vehicle, if you base your mileage choice on the concept that gasoline has a fully loaded cost of about $10 a gallon ($3.50 out of your pocket and $6.50 of collateral damage to you and everyone else), that's a good way to look at it.