Real Global Temperature Trend, p17 – Climate System Thermal Inertia is lower when you don’t assume CO2 flatline

Oceans, oceans, oceans. You thought the atmosphere was complex? Well, just take a look at the oceans. Oddly shaped features with disturbing cycles and conveyor belt currents. Home of the octopus, the blue whale and a Mariana Trench full of complicated science.

ocean CO2 climate inertia & ocean thermal climate inertia
Combining ocean CO2 climate inertia & ocean thermal climate inertia leads to an optimistic conclusion: climate change may not be reversible – but it is stoppable, if anthropogenic CO2 emissions go to zero. This may still ignore a couple of other (positive) carbon feedbacks though…

What’s worse: they’re connected – the oceans and the atmosphere – as an integral part of Earth’s climate system. It means if you change the one, you also change the other – and as these are delayed processes, it means you start up all sort of feedback processes, both positive and negative.

And sometimes one of such feedbacks might actually even out another, Earth scientists Damon Matthews (Concordia University) and Susan Solomon (MIT) wrote in Science in 2013. Their message is hopeful. Even though the climate change we are currently creating is irreversible – it is stoppable. (‘Just’ stop emitting.)

In our previous article about the thermal inertia of the climate system we looked at another publication in Science, performed by a large research group led by climatologist James Hansen. This group concluded that if we would manage to stabilise the atmospheric CO2 concentration excess heat stored in the oceans would inevitably create a new and higher equilibrium temperature, meaning atmospheric warming would continue for somewhere between 25-50 (~37.5) years.

Damon Matthews responded to our article on Twitter [we're called @Bits of Science if you want to be friends!] stating ‘carbon cycle inertia opposes thermal inertia’, ‘because the long-term ocean uptake of atmospheric CO2 creates a cooling that offsets the delayed additional warming’.

His below graph illustrates it well. We always say that atmospheric CO2 is cumulative – because the closed carbon cycle of the biosphere (that will – to make matters worse – turn into a net emitter of CO2 under continued warming!) cannot compensate for the disturbance of the geological carbon cycle, that we are causing by digging up and burning enormous quantities of fossil fuels.

But although that is very helpful to remind people of the fact that we are indeed creating irreversible changes to our planet – it is not entirely true.

Climate inertia: declining CO2 concentration
Top graph shows the difference between drawing a CO2 flatline and assuming the oceans will continue to compensate for part of our emissions. Important requirement to note: we need a global 100% CO2 emissions reduction first.

The atmospheric CO2 concentration is rising due to our increased – and since two years constant – CO2 emissions – and much of that increase will be with us for millennia. But once we reduce our additional fossil CO2 emissions to zero the atmospheric concentration will not remain constant, but rather decrease slowly.

The reason is those same oceans that will return heat – thereby creating the thermal inertia. These oceans will also continue to absorb a portion of atmospheric CO2.

So if you assume a constant atmospheric CO2 level from the day you decrease emissions to zero – you have some 37.5 years of continued ‘inevitable’ warming still in the pipeline. But if you assume Matthews’ model calculation you also have a cooling effect of slightly decreasing atmospheric CO2 concentrations – and those two almost even out. That’s proper Good News(!)

It is also exactly the scientific nuance that we’ve been looking for the day we embarked on our mission to unmask the “real” global temperature trend. This is the trend line you get when use proper statistics on observed global temperatures – and then try to remove all masking factors.

Yes – there is still a substantial ‘thermal inertia’. There are just other feedbacks too. Some make the equation a lot better – and some again worse…

Despite the substantial good news that Matthews’ research offers, this trend line is still above the observed statistical trend line. Factors like global dimming – and all forms of short-lived aerosol reflection – these create their own thermal inertia.

Matthews also does not disagree with the thermal inertia that Hansen had coined in 2005 – only with a ‘wide (journalistic) misrepresentation’ of that study. Hansen has calculated we have about 37.5 years of additional warming once we manage to stabilise the atmospheric CO2 concentration. Matthews adds that we can do better than stabilising – we can actually see a decrease of that concentration – starting the day we make the world economy entirely fossil free.

But it’s probably also good to note we’re at the cutting edge of climatological research here. Where actual debate happens. Where the one climate model may still have difficulty replicating the other. And where different research groups keep adding different carbon feedbacks to the same equation.

Like Andrew MacDougall, Christopher Avis and Andrew Weaver from Victoria University, who in 2012 published a paper in Nature Geoscience, in which they calculated the Earth’s atmospheric CO2 concentration would actually keep rising once we would have managed to stop emitting.

The reason? Arctic methane (which turns to CO2 in a couple of years) and direct tundra CO2 emissions.

According to these Canadians atmospheric warming is set to release enormous amounts of carbon from (underneath) thawing permafrost – 68-508 gigatonnes of pure carbon – that’s 1864 gigatonnes of extra CO2 to the atmosphere, in the worst case. Possibly more than the oceans could compensate for.

Let’s not think of our oceans as a carbon dump. They too have positive feedbacks lurking:

The oceans in turn – we should not forget – will have diminishing capacity to dissolve CO2 due to ocean warming – and diminishing capacity for structural CO2 sequestration, due to ocean acidification. If ocean acidification would continue beyond biological thresholds, leading to plankton collapse, we are in for another positive carbon feedback. And there’s not just methane in the Arctic – there’s also clathrates in the deep sea, a methane bomb (that caused the end-Triassic Mass Extinction - and may have played a triggering role in the end-Permian Extinction) that could be triggered once ocean warming reaches the very deep waters down below.

Grand Conclusion: on all accounts we should stop adding extra CO2 to the atmosphere. And that means we should stop adding coal plants – that will continue these emissions for decades. Yet another example of ‘thermal inertia’ there – fossil lock-in.

© Rolf Schuttenhelm | www.bitsofscience.org

Comments are closed.