In part 16 of our temperature trend series we take a better look at one of the main reasons almost everyone still underestimates climate urgency: ‘Thermal inertia’ of the climate system – a delay between the moment of emissions of CO2, and the moment the (majority of) inevitably resulting atmospheric warming manifests itself – a time lag of decades, with very large implications.
Before the onset of new subsequent atmospheric warming records (2014, 2015, 2016) – atmospheric warming showed what some referred to as a plateau. This graph shows the opposite of that cycle: it shows the warming trend not of the atmosphere, but of the world’s oceans, one that has been very steep over the last two decades. It means the oceans have stored a relatively large amount of heat that was generated in the atmosphere. Because the oceans weigh 1.3-1.4 million teratonnes (that should impress you) and the atmosphere only ~5.5 thousand teratonnes (0,004 times as much) it takes a very long time before a higher atmospheric temperature is in balance with a higher temperature of the oceans, and since the initial driver of change is still active (the atmospheric CO2 concentration is still rising) there is a constant time lag between atmospheric and oceanic warming – which in turn delays atmospheric warming.
The currently observed warming is (mostly*) the result of cumulative CO2 emissions up to the late seventies. The implications of this understanding are large: even if we stopped further rise of the atmospheric CO2 concentration completely as per tomorrow, warming would still continue for several decades.
In other words: the actual global temperature trend is significantly higher than the observed temperature trend – possibly 0.6 degrees higher, judging by the one leading study in this area, performed by James Hansen and no less than 14 other climate scientists and published in Science in 2005.
*) How to understand Earth’s climate inertia – and how not to misunderstand:
Sadly it’s a little-known fact, but one that is absolutely crucial to properly understanding the urgency of the 21st century climate crisis: “the climate” does not respond directly to warming CO2 emissions – net warming follows emissions with a significant delay, called ‘thermal inertia’ of the climate system.
But how? Well – you could read the subtext under the above ocean warming graph – or try this additional way of explaining thermal climate inertia:
Of course once an extra molecule of CO2 enters the atmosphere from the chimney of a coal plant or the exhaust pipe of a fossil fuel-driven car, it immediately starts to absorb Earth’s outgoing infrared radiation and therefore immediately starts to add tiny amounts of extra heat to the ongoing build-up in the atmosphere. But the atmosphere is very thin and it connects not only to the cosmos, but also to much cooler surfaces, like ice sheets and oceans – that have a much larger mass, and therefore absorb large quantities of that heat. Meanwhile the oceans also warm up, just like the atmosphere – but they take a far longer time to reach equilibrium. And all that time (especially during La Niña-dominant phases, with a lot of deep-water upwelling) the oceans in turn buffer atmospheric warming – creating a substantial time lag between emissions and net atmospheric warming.
Simply put, this climate inertia is caused by the large ‘thermal mass’ of the oceans. They take a long time to warm – and thereby also delay the warming of the atmosphere. As this means we don’t witness the effects of what we’re doing directly, but the subsequent warming is an inevitability, climate inertia is a very, very bad thing…
Mostly citing the above-linked Science study by Hansen et al Earth’s thermal climate inertia is often quoted as being ’40 years’. The study says something quite different though. It offers a confidence range between 25 and 50 years – with 37.5 years as most likely value.
But for what exactly? “For 60 percent of warming caused by emissions to take place.”
So officially it’s not really correct to say current warming is the result of emissions before the late 1970s – although that is perhaps still the easiest way to explain the implications.
In reality some of the observed warming (<60%) is caused by emissions between 1979 and present (2016) – whereas also some of the inevitable warming (<40%) of emissions before 1979 are still not visible in atmospheric measurements.
All in all we can be sure about one thing: the thermometer does not show the full extent of what we’re doing – not yet, not for decades to come. If we would stop emitting completely as per tomorrow net atmospheric warming would still continue for some 37.5 years, according to the 2005 study – that is, if CO2 concentrations stayed constant afterwards (additional CO2 is largely cumulative).
An additional problem is we have not managed to lower CO2 emissions over the last 37.5 years. To the contrary – we’ve almost doubled them. Hansen’ research group calculated (in 2005) this means we would still have 0.6 degrees Celsius of inevitable warming, once anthropogenic (additional) CO2 emissions stopped.
Understanding thermal climate inertia is understanding mix of amplifying, dimming and delaying climate forcers – and feedback factors. Complicated indeed.
The Earth’s Energy Imbalance is at the very core of physical climate science – and subject of ongoing research, both of the atmosphere and the oceans. A more recent paper, published earlier this year in Nature Climate Change calls investigation of EEI an ‘imperative‘. We especially liked the above graph it contained – reminding us once more of the complexity of amplifying, masking and delaying climate forcers.
It’s not only the medium-term climate inertia – but also the long-term inert warming amplification that should worry us. Like carbon feedbacks
Hats off to the actual scientists who are able to make sense of it all. Let’s try and listen more carefully to what they themselves try to say to us. Like James Hansen, who also published a follow-up study in PLOS ONE in 2013 – called ‘Assessing “Dangerous Climate Change”: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature.’
In that study Hansen and 17(!!) colleague climate scientists warn of long-term climate effects – and additional warming once you add longer-running feedbacks to ‘official’ atmospheric climate sensitivity. These feedbacks might add 1-2 degrees to an equilibrium warming of 2 degrees – implying, if we aim for 2 degrees, we’ll inevitably get 3 or 4 degrees, thanks to carbon feedbacks mostly.
The researchers note the morality of the subject, and write the following:
“Rapid emissions reduction is required to restore Earth’s energy balance and avoid ocean heat uptake that would practically guarantee irreversible effects. Continuation of high fossil fuel emissions, given current knowledge of the consequences, would be an act of extraordinary witting intergenerational injustice.”
Let’s conclude the science may be very complicated – but the choice for human society, the necessity to break with business as usual, is so clear that it almost becomes simple. We have a very large responsibility now for the future, not just because of what we create in terms of direct warming up to the year 2055 (by emitting CO2 today, and the inevitable inert warming that causes) – but also by what we create in the even longer run, if we simply do not stay below the feedback thresholds that we in Paris promised we would respect – 1.5 degrees.
Adding the 0.6 degrees to currently observed global temperatures we can also -very sadly- conclude we are already too late for that, too late to stay below these thresholds – and would need, apart from an immediate end to the age of fossil fuels, start devising effective means for large-scale CO2 sequestration, so that we can get to net ‘negative emissions’.
It’s the biggest challenge mankind has faced thus far.
Now before you go, please also read our next article piece on climate intertia, part 17 of the series. It’s the good news you needed. When discussing climate inertia, we should not be talking about emissions so much, but about CO2 concentrations. Theoretically, there’s no such thing as ‘inevitable warming’. In practice though there is. To prevent the above-discussed additional temperature rise of the current CO2 concentration, we need to actually lower the atmospheric CO2 concentration by about 20 ppm over the next couple of decades. That still requires close to impossible emissions reductions.
© Rolf Schuttenhelm | www.bitsofscience.org