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Tuesday, April 26, 2022

Charts of Atmospheric CO2, Carbon Isotopes, Oxygen and Methane

 I started making charts of atmospheric CO2 in 2009, when the global average CO2 concentration was 386 ppm.  I updated my charts in 2012, at 392 ppm, and in 2017, at 405 ppm, and at the end of 2021, at 418 ppm. 

The monitoring stations are located from the far north, at 82° N in Canada to the South Pole.  Scripps Institute has managed most of these stations since the 1950s, first under the direction of Charles Keeling, and later under his son, Ralph Keeling.  I also included records from a few obsolete legacy stations that were operated by foreign governments.  I standardized my chart displays using cool colors to represent the Northern Hemisphere, and warm colors for the Southern Hemisphere.

The amplitude of the CO2 seasonal cycle varies with latitude, from high amplitude in far northern latitudes to very little amplitude at the South Pole.  The seasonal cycle is driven by seasonal plant growth and decay on lands with temperate climate, which are concentrated in the Northern Hemisphere.  Agriculture, which is also concentrated in the Northern Hemisphere, also contributes to the seasonal cycle.  I took advantage of this for my standard display, overlaying low amplitude over higher amplitude traces, so that all traces can be seen.

In general, CO2 concentration in the atmosphere is growing exponentially, a fact noted by Isaac Asimov in 1959.  In 2009, I made an exponential function, beginning at the pre-industrial CO2 concentration of 280 ppm in 1800, with an eyeball-fit to the data from 1957 to 2009.  Here’s the function, and the chart beginning in 1800, updated with CO2 data through 2021.  This chart has the “hockey stick” impression that characterizes many climate-change charts.

CO2 concentration, ppm = e(n*0.001854) + 280, where n = the number of months since Jan. 1800

This function would predict that global CO2 would pass 450 ppm in January, 2032 (ten years from now), and pass 500 ppm in August, 2043.

The exponential function seems to be slightly overstating the rate of CO2 growth since 2009, so I tried an alternate formula for the forecast in coming decades, a second-degree polynomial with a least-squares fit to the global average CO2 from 1974 to 2009.  That formula is CO2 in ppm = 0.000104*x2+0.0897*x+331.66, where x is the number of months from July, 1974.  This formula predicts global CO2 will pass 450 ppm in June, 2034, and pass 500 ppm in July, 2050. 

Certainly, these forecasts are simple extrapolations, and include none of the analysis of policies and economics which should be the basis of forecasting.  But it’s worth noting that my exponential forecast from 13 years ago is pretty much right on the money, overshooting by only one or two parts per million.  The last thirteen years has seen unprecedented growth in renewable energy technologies, but so far without significant impact on the rate of CO2 growth.  Here are the two forecasts on the same chart.

The seasonal cycle can easily be filtered from the data, leaving the long-term trend at each station.  From this, it’s easy to see that the Northern Hemisphere leads the Southern Hemisphere in rising CO2.  About 90% of fossil fuel burning happens in the Northern Hemisphere, and CO2 accumulates in the far north, while dispersing to the south. 

The difference in concentration from the far north to the South Pole has been increasing as larger volumes of fossil fuels are burned each year, from about 3 ppm in the 1980s to over 5 ppm now.  The chart below shows the difference in the one-year time-averaged CO2 concentration measured in Alert, Canada, at latitude 82° North, and the South Pole. 

The amplitude of the seasonal cycle has also been increasing in the far north.  The amplitude of the cycle increased from 15 ppm to 20 ppm since the mid-1970s.  This probably reflects increased agriculture and farm productivity in the Northern Hemisphere as world population has doubled.  Previous work showed that seasonal fossil-fuel use is volumetrically inadequate to produce the change in the atmospheric CO2 seasonal cycle.

Carbon comes in two common naturally occurring isotopes, C12 and C13.  Various processes, including life processes, sort the isotopes, favoring the accumulation of one or the other isotope.  Photosynthesis favors C12, so everything with carbon derived from plants, including lumber, your mashed potatoes, you, me, and fossil fuels is enriched in C12.  Scientists use a measure of the C13/C12 ratio written as d13C , and called delC13.  As fossil fuels are burned the C12-enriched carbon in CO2 changes the ratio of these isotopes in the atmosphere, lowering the value of delC13.  DelC13 continued to fall from 2009 to 2021, reflecting a growing fraction of carbon from fossil fuels in the atmosphere. 

Carbon isotopes in the atmosphere are also affected by the seasonal cycle of plant growth on the temperate land mass of the Northern Hemisphere.  As plants grow during the northern summer, the lighter isotope C12 is preferentially removed from the atmosphere, and returned during the winter months as plants decay.

After filtering the seasonal cycle, we see that the Northern Hemisphere leads the Southern Hemisphere in falling DelC13.  As an aside, the residual fluctuations in the trend have a strong correlation to the Oceanic Nino Index (ONI), reflecting sea surface temperatures in the Pacific.

Interestingly, if all of the carbon released by fossil fuels stayed in the air, the DelC13 value would be much lower, about -13, instead of -8.5.  The measured dilution of carbon with the isotope signature of fossil fuels provides a way of estimating the volume of all carbon reservoirs exchanging carbon with the atmosphere.  Currently, the reservoirs freely exchanging carbon with the atmosphere have a carbon mass of about 5200 gigatonnes, before accounting for additional carbon in the system from new burning of fossil fuels.  That’s about 6 times the mass of carbon currently in the atmosphere.

Atmospheric oxygen is also influenced by burning of fossil fuels.  Oxygen is consumed, causing atmospheric O2 to fall.  The atmosphere is about 21% oxygen, and the decline is only about 0.08%, so there is no threat to breathing.  Still, the decline can be measured precisely.  The decline in oxygen is reported in units per meg, which is equivalent to ppm in this range of values.

After filtering the seasonal cycle, we see that the Northern Hemisphere leads the Southern Hemisphere in oxygen decline, because most fossil fuels are burned in the Northern Hemisphere.  The total volume of oxygen decline is very close to the expected consumption of oxygen considering the reported volumes of fossil fuels burned and deforestation, as reported in this previous post.

Atmospheric methane (C4) is also increasing as a result of human emissions.  Methane is a much more powerful greenhouse gas than CO2, but has a shorter lifespan.  CO2 has a half-life of 120 years, while methane has a half-life of about 10 years.  This is why the climate scientists use the parameter GWP (global warming potential) to represent the different strength of various greenhouse gases over an effective time frame.  The GWP of CO2 equals 1, by definition, for all time intervals.  For methane, the warming potential over 20 years (GWP-20) is 84 – 87, and over 100 years is 28 – 36.  Over shorter intervals, methane is an even stronger greenhouse gas.  Currently, methane concentration in the atmosphere is about 1.9 ppm (i.e. 1900 ppb).  In absolute terms, methane warmed the earth by about 0.52 W/m2, compared to 2.11 W/m2 for CO2, for the latest year reported by NOAA, 2020.  All other greenhouse gases combined contributed another 0.55 W/m2.  Methane also has a seasonal cycle in both hemispheres with high values in the summer and low values in the winter, but I don’t know the explanation for the seasonal cycle. 

As the concentrations of CO2 and methane in the air rise, the atmosphere will absorb heat at a faster rate, leading to destructive climate change.  Temperatures and climate change will not stabilize until carbon emissions reach zero.  I will update my charts on carbon emissions when summary data for 2021 is released in the BP Statistical Summary of World Energy in July.  Apart from a small pandemic-related decline in emissions in 2020, the world continues to add CO2 to the atmosphere at an ever-increasing rate.  If the world had acted to reduce emissions three decades ago, simply reducing emissions might have been a reasonable policy.  However, in our current situation, outright elimination of carbon emissions is required to avoid some level of catastrophic consequences. 

Globally, we need to reduce emissions to 50% by 2035, and to zero some time between 2050 and 2070.  I am very pessimistic that we have the public understanding or political will to reach these goals.  As Bill Gates wrote in 2021, "To avoid a climate disaster, we have to get to zero greenhouse gas emissions….The case for zero was, and is, rock solid.  Setting a goal to only reduce our emissions—but not eliminate them—won’t do it.  The only sensible goal is zero.”


CO2, CO2 carbon isotopes, oxygen and methane data, including obsolete CO2 stations

Isaac Asimov, "No More Ice Ages?" prediction and commentary on global warming,
in Fantasy and Science Fiction, Jan. 1959, republished in Fact & Fancy, 1962 and Asimov on Chemistry, 1974. 

Global Warming Potential,uses%20a%20different%20value.).

GWP-20 for methane = 84 to 87; GWP-100 for methane = 28 to 36 (also reported as 25)

Radiative Forcing for various greenhouse gases

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