First we need to determine the Mean Residence Time or Atmospheric Residence Period of the greenhouse gas. This is not a measure of how long a molecule of greenhouse gas resides in the atmosphere but how long the net flow in a given volume remains positive.
Each individual molecule of CO2 doesn’t spend that long in the atmosphere, it will be sequestered by the natural carbon cycle and replaced by another molecule that is released, for as long as this happens there will be a positive influence.
The natural carbon cycle releases about 211.6 gigatonnes of CO2 each year, at the same time it sequesters some 213.8 gigatonnes, there is therefore a net sequestration of approx 2.2Gt per year. If our CO2 emissions were below this figure then natural processes would safely remove them from the atmosphere.
Last year we emitted 34.5Gt of CO2 and a further 14.5Gt as CO2 equivalence in the form of other greenhouse gases – far more than natural processes can handle. This leads to an accumulation in the atmosphere of CO2 and other greenhouse gases.
If we reduced emissions to zero as of today then warming would continue until such time as our historic emissions had been removed from the atmosphere.
In the case of CO2 the atmospheric residence period is approx 115 years, for methane it’s about 12 years, for nitrous oxide it’s 114 years and for dichlorodifluoromethane it’s in excess of 1,000 years. Note: this does not mean that after 115 years (or however long) the gas is removed from the atmosphere, in the case of CO2 there will still be 30% remaining after 115 years; even after 1,000 years some 19% will remain. These periods are when the flow through the unit volume can be considered to be in equilibrium with natural processes and therefore no further anthropogenic warming will continue.
We also have to consider the global warming potentials – a measure of the effectiveness of greenhouse gases at contributing to warming. There is no simple linear relationship with time. Methane for example weakens over time, sulphur hexaflouride gets more effective with time, nitrous oxide is most effective in the medium term (100 years or so).
If equal volumes of all greenhouse gases were released then the warming would get more and more pronounced for about 4,000 years, only then would the effects start to diminish. However, the GWP time horizons of the primary gases (by volume) see the peak GWP occurring on decadal timescales rather than millennial ones.
If, for each of the hundreds of greenhouse gases, we take into account…
? Quantities of the gas released and when
? Sequestration rates
? Amplification and attenuation (chemical influences etc)
? Fluxes in the atmosphere (box flow)
? Atmospheric residence periods
? Global warming potentials
? Radiative forcing capacities
? 28 other variables
Then we crunch all the numbers in a computer, the answer is that after about 84 years our historic emissions will cease to cause any further anthropogenic warming.
If you were to take only very recent our emissions and run the same programme then the point of equilibrium would be further into the future, about 108 years from now.
The figure of 84 years is an approximation. I have a computer programme that specifically calculates this figure and it requires about 6,000 datapoints to be entered (from which the prog creates millions more before running the calculations). Most of the figures we know with some degree of accuracy but the inherent uncertainties and degree of estimation means you should allow a 25% margin of error either way.
Also, this is purely from the anthropogenic perspective. It may be that after 50 years the magnitude of natural cooling is sufficient to cancel out the residual human warming – allow a second 25% margin of error for this.
The earliest we could expect to see our historic emissions no longer adding to global warming is 47 years, the latest would be about 137 years. Most likely it would be between 63 and 105 years from now with 84 years being the mean and median.
EDIT: TO PFLUMPHTELLY (AND OTHERS)
There’s an important distinction to be made between natural and human emissions. Natural emissions are part of the carbon cycle. The amount of CO2 released by natural sources is far greater than human emissions (211.6Gt per annum against 34.5Gt). But… unlike humans, nature also removes CO2 from the atmosphere. Nature not only removes ALL the CO2 it releases but it also removes some of our emissions as well.
Another important distinction – the amount of a greenhouse gas present in the atmosphere is less important than how much it contributes to warming. Water vapour accounts for about 96% of greenhouse gases by volume (globally averaged) but as greenhouse gases go it’s really quite pathetic. To put it into perspective, you need about 12 molecules of water vapour to equal the warming caused by one molecule of carbon dioxide. Although CO2 accounts for about 4% of greenhouse gases by volume, it causes about half of all warming (very approx figure, could be as little as 39% or as much as 72%).
It would take about 40 years to stop warming; that is, we have already caused increased warming that will not be fully felt for 40 years.
To return to natural temperature levels would require many thousands of years if CO2 emissions totally stopped today.
http://nas-sites.org/climate-change/qand...
Pflumphtelly:
You have no idea what the numbers you are playing with are mean. Natural carbon emissions are more than offset by natural carbon sinks. If we look at measurements of increasing atmospheric CO2, which is currently rising at an average rate of 2 parts per million (ppm) or 15.6 billion tonnes annually, and estimates of human emissions, which stands at over 33.5 billion tonnes annually we see this.
http://scrippsco2.ucsd.edu/data/in_situ_...
http://cdiac.ornl.gov/ftp/ndp030/global....
You should also be made aware that molecules of different greenhouse gases in the atmosphere are not equal as various factors are involved with how much each molecule can retain heat.
http://acmg.seas.harvard.edu/people/facu...
http://forecast.uchicago.edu/archer.ch4....
It depends what natural cycles do. Even though I suspect that most of the warming that would occur during a Maunder minimum has already happened, I suppose that there could still be enough cooling to cancel the "warming in the pipeline," But Earth will warm back up when a Maunder Minimum ends. But if solar activity returns to where it was during the mid, or even the late 20th Century, we would get warming on top of the "warming in the pipeline."
We contribute around 0.2-0.7% of co2 emissions, co2 accounts for around 4% of green house gasses in the atmosphere.
In other words if we stopped out co2 output completely there would be no effect.
The oceans and the sun control global warming and good luck controlling those
Unbelievable - one person thinks ocean temperature is independent of atmospheric temperature and the other thinks that the planet is a god-like feminine being that is independent of the physical universe.
And I get criticized for saying that Deniers are stupid. Go figure.
How long would it take? I don't know; at least centuries.
Planet Earth has decided to warm up. When she's warm enough she will stop.
We have no say in the matter.
the truth is this no matter what socalled scientists and liberal agendas attempt to tell everyone. the truth is the earths core warms on a consistent basis and the suns recent activity are the major contributors. this has been going on for tens of thousands of years and is nothing new. the earth will always fix itself. I would be more concerned that the earths core is warming and could cause one or many of the calderas to explode(ie. yosemete, etc.) which would cause worldwide catastrophes.
