A Hiding to Nothing

Guest essay by Clive Best

Summary

The currently held belief that we must  decarbonise the world’s economy in order to to stop climate change is a dangerous illusion. It is a ‘hiding to nothing’. I have come to the conclusion that if emissions were held constant for 30 years then the airborne fraction of CO2 emissions would reduce to zero. In other words if the world can hold emissions constant at say 30GT CO2/y then sinks will increase to balance all annual emissions. Thereafter CO2 levels would remain at below 440ppm indefinitely, so long as emissions remain constant.

Introduction

I will argue below that in order to stop global warming all we really have to do is simply stabilise CO2 emissions, not reduce them to zero!  This alone will stabilise CO2 levels within less than 30 years.  The origin of the ‘myth’ as promoted by nearly all IPCC climate scientists is that we have to stop burning all fossil fuels i.e. we must ‘keep it in the ground’. This is a fallacy and I will try to explain why in this post.

The origin of this belief that we must stop burning any fossil fuels by ~2050 can be traced back to Figure 10 which appeared in the AR5 ‘Summary for Policy Makers’. Here it is again.

Figure 10 from SPM AR5

Figure 10 was intended to send a simple message to the world’s political leaders. Namely that there is a finite total amount of fossil fuel that mankind can ever safely burn, and that we have already burned half of it.  Therefore unless all major industrialised countries stop burning fossil fuels altogether by 2050, the world will warm far above 2C (the red curve) causing a global disaster. This message worked, but I find that there is so much wrong with the hidden assumptions and even subterfuge used to produce Figure 10 that I wrote a post about it at the time.

The principal hidden assumptions, as I see it, in Fig 10. are as follows:

1. Carbon sinks are saturating (they are not)

2. ECS (Equilibrium Climate Sensitivity)  is 3.5C (Very uncertain – and could even be as low as 1.5C)

3. The subtle replacement of logarithmic forcing of CO2  with a linear forcing.

4. The assumption that past emissions stay in the atmosphere essentially forever.

As a direct consequence of IPCC successful lobbying based essentially around Figure 10, the Paris treaty now proudly “sets the world on an irreversible trajectory on which all investment, all regulation and all industrial strategy must start to align with a zero carbon global economy“. Does anyone really believe that this is even feasible, let alone realistic? It simply is not going to happen because well before then, their citizens will revolt and kick them out. The best we can hope for in the short term is a stabilisation of global CO2 emissions. The minimum condition needed is that annual growth in emissions needs to be brought to zero.

Carbon Cycle

To understand the carbon cycle means understanding the difference between CO2 decay time and CO2 residence time. The decay time for an individual CO2 molecule emitted by man is only about 5-10 years (based on C14 measurements in both bomb tests and those produced by cosmic rays). Every CO2 molecule in the atmosphere is rather quickly absorbed either by photosynthesis or by the ocean. However on average most of them are simply replaced by another CO2 molecule entering the atmosphere through evaporation from the ocean surface or by biological respiration. The residence time however, is the e-folding time needed for a sudden net increase in CO2 to decay back to normal as the carbon cycle reacts. At equilibrium the total CO2 content of the atmosphere remains constant over centennial time scales. Currently though, as a result of our emissions, slightly more net CO2 molecules are being absorbed than are being returned to the atmosphere each year. As a consequence the atmosphere is not quite in equilibrium with the rest of ‘natural’ life and the oceans. We have given the carbon cycle a kick, and as a result it is reacting to return back into balance. The problem is that we have continued to kick a little harder each year so that balance is never reached.

If you sum up all the sources and sinks since 1960 then you find a remarkable fact, which was unexpected by climate scientists. About half of man-made emissions are being absorbed each year. This means that just one half of the net CO2 emitted by humans remains in the atmosphere each year. This is called the airborne fraction. The strange thing is that this airborne fraction hasn’t changed at all in 60 years, despite exponentially increasing human emissions.

AR4 plot: The fraction of Anthropogenic CO2 retained in the atmosphere (b) is unchanged in  50 years, despite increasing emissions (a). Note how the annual change stalled after the 70s oil crisis

Plot of Carbon content of air versus Cumulative Carbon emissions produced by Nick Stokes.

This means that today we are emitting about twice as much carbon dioxide as we did 30 years ago, yet still only half of it survives a full year. Or putting that another way, the equivalent of 100% of 1990 emissions are now absorbed each year. This ratio of 50% airborne fraction has been true for over 100 years while emissions have been forever increasing.

Comparison of Carbon emissions and that retained in the atmosphere. Increases in Emissions consistently remain about twice the levels of increases in CO2. Flat periods occur when growth stalls.

Natural carbon sinks are increasing dynamically to offset our emissions. The problem is that they don’t have enough time to catch up with the ever increasing rate of emission. The best they can do in a single year is offset half of them. Why is that the case and what does it really mean?

There is a ‘concentration effect’ acting on ocean sinks due to the increasing partial pressure of CO2 in the atmosphere.  Similarly land biota (plants and soil)  react to increased partial pressure by absorbing more CO2. While we are still increasing emissions then CO2 levels in the atmosphere will always continue to rise. If instead we can stabilise emissions at some fixed number of Gtons/year then CO2 levels would also stabilise, albeit at a slightly higher level than now and in the future. This is because the sinks will finally be able to catch up to balance our CO2 source.  The atmospheric faction will decay to zero.

Dissolution/Absorption of CO2 at  Ocean surfaces.

In stability there is a balance of CO2 Partial Pressures between the surface of the ocean and the atmosphere. At any given temperature the exchange of carbon dioxide molecules between the atmosphere and the ocean surface always reaches an equilibrium. This equilibrium is controlled by the partial pressure of CO2 in the atmosphere equalising to the partial pressure of CO2 in the surface of the ocean. Then the number of carbon dioxide molecules that escape from the sea surface is  balanced by the number that enter the sea from the atmosphere.

If the temperature of the ocean rises then the kinetic energy of the carbon dioxide molecules in the seawater increases and more carbon dioxide molecules will leave the ocean than would enter the ocean. This continues until the partial pressure of carbon dioxide in the atmosphere increases to balance the new pressure at the sea surface.

If instead the ocean were to cool then the reverse of the above would happen, and CO2 levels would fall. Consequently carbon dioxide is more soluble in cold water than in warm water. This is Henry’s law. One consequence of this effect is that the oceans “inhale” carbon dioxide from the atmosphere into cold sea surfaces at high latitudes and “exhale” it from warm sea surfaces at low latitudes.

Increasing the carbon dioxide concentration of the atmosphere therefore causes the oceans to take up (inhale) more carbon dioxide. Because the oceans surface layer mixes slowly with the deep ocean (hundreds of years) the increased carbon dioxide content of the surface ocean will be mixed very slowly into much larger carbon reservoir of the deep ocean. The rate of our adding carbon dioxide to the atmosphere has been too fast for the deep ocean yet to be a significant reservoir. So as the carbon dioxide content of the atmosphere rises, so too does the concentration in the ocean surface, causing short term acidification of surface waters. If  atmospheric carbon dioxide remains constant then a ph balance throughout the ocean volume can be reached.

I argue that by simply stabilising emissions, we can halt global warming because CO2 levels will stabilise as the sinks will then be able reach equilibrium with emissions. Clearly the lower total ‘stable’ emissions become then the cooler the planet will be, but even if we only managed to stabilise emissions at current values, then the net warming will still be <2C and CO2 levels will soon stop rising and stabilise at <440 ppm.

Atmospheric CO2 levels must always reach an equilibrium as the natural carbon sinks catch up to balance emissions. For the last 40 years about half of man-made emissions have been absorbed mainly into the oceans, but also into soils and biota. The reason why CO2 levels have been continuously increasing since 1970 is that  we have been increasingemissions each year, so the sinks never get a chance to catch up. Sinks will rather quickly balance emissions and CO2 levels will stop rising once emissions stop increasing. This fact is obvious because run-away CO2 levels have never happened before in the earth’s long history. Such a balancing mechanism has always stabilised atmospheric CO2 over billions of years during intense periods of extreme volcanic activity, ocean spreading and periodic tectonic mountain building. Fossil fuels in this context are an insignificant fraction when compared to  the buried carbon contained in sedimentary rocks.

Simple Model

CO2 levels rise when the rate of change of the sources – S exceeds the rate of change of sinks – K. Without human emissions  then S = K, averaged over one year. However with ever increasing human emissions the situation becomes dynamic

If C is the yearly value of CO2,  S  the net sources of CO2 and K the net sinks, then at time t.

However it has been measured for at least the last 60 years that

If      then  

Now let’s assume that the world manages to stabilise annual emissions at current rates of 34 Gtons CO2/year  indefinitely.  CO2 sinks currently absorb roughly half of that figure – 17 Gtons and have been increasing proportional to the increase in partial pressure of CO2 in the atmosphere – currently that of 400ppm. Stabilising emissions now results in a decreasing fractional uptake by carbon sinks as the partial pressure imbalance between the surface and atmosphere begins to fall. The simplest assumption is that the sink increase depends only on the partial pressure difference for a given year. Therefore  if this pressure difference is reduced by half in one year then the next year it will be reduced by one quarter, then one eighth  and so on. The same argument applies for the case that it takes longer to reduce pressure difference by a half.

Year 1: 50%  Year 2: 25% Year 3: 12.5% Year 4: 6.25% etc. which is simply equal to the infinite sum

So in this simplest of models, CO2 levels in the atmosphere will  taper off after just ~10 years to reach a new long term value equivalent to adding an additional one year of emissions 34 Gtons of CO2 to the atmosphere. The atmosphere currently contains 3.13 x 10^12 tons of CO2 so the net increase at equilibrium would in this simple model be just   1%. Therefore for the years following 2016 the resultant CO2 curve would look like the red curve below. If instead it takes say 4 years for the sinks to increase  by then we get the blue curve. In this case it would take 30 years for CO2 levels to to stabilise and the increase would be 5 times larger.

CO2 stabilisation curves for different time constants. The red curve assumes sinks match half the imbalance in 1 year while the blue matches it in 4 years.

Currently there is also a  good chance that the world will achieve a fixed level of annual emissions, but there is no chance that it will meet an impossible target of zero emissions this century. This  does mean that CO2 levels will remain at  ~410 ppm indefinitely, which is far higher than a planet without human beings, but it buys us time to replace fossil fuels with say new nuclear energy. If I am right then CO2 levels will begin to level off within the next 10-20  years. This would also save trillions of dollars by trying too soon now to replace all fossil fuels, and then probably failing.

Reducing emissions in the future will slowly cause such ‘stable’ CO2 levels to fall. In the long term we will have to develop non-carbon energy sources anyway, most likely nuclear, since fossil fuels must run short. However using remaining fossil fuels to control  CO2 levels may one day have another advantage. It could mean that we can eventually use ‘enhanced global warming’ as a thermostat, thereby avoiding another devastating ice age otherwise due to begin within the next 5000 years.