We would keep the story simpler, helpful real-world paleoclimate experts advise us:

‘Say the Pliocene was 2 to 3 degrees warmer than pre-industrial Holocene – at a CO2 concentration that is about as high as the one that’s currently measured, a CO2 rise that has already led to a global average warming of about 1 degree Celsius. That means, judging from Pliocene paleoclimate, we could still have 2 to 3 minus 1 (the observed warming) = 1 to 2 degrees Celsius additional temperature rise.’

And before we forget it: please do include a large band of uncertainty too – as with ‘close to 400 ppm’ we mean somewhere ‘between 350 and 450 ppm’ – Pliocene CO2 concentrations that is.

Just to have this straight again – what mankind is doing is literally disturbing Geology. Shown here is the Pleistocene-Holocene boundary and the Holocene-Anthropocene boundary, both expressed as a jump in CO2 (top) – and a jump in temperatures (bottom). The point of this article: if we create a miracle on Earth and somehow we manage to keep the CO2 concentration at that black dot (where it says ‘today’) – then (in the long run) we’ll still have another 1 to 2 degrees Celsius temperature rise. As the authors of this Nature publication point out, reality can only be worse – judging by the set of emissions/concentration scenarios – which are all higher than today’s levels.

Different pipelines carry different amounts of additional warming

In our last post we saw in the mid-Pliocene (a little over 3 million years ago – so just before the onset of the Pleistocene, the epoch of the ice ages) summer temperatures in the Arctic were about 8 degrees higher than they are now. This led us to deduce – considering (as current-day climate models suggest) an Arctic amplification of 2 to 2.5 times the global (continental) land temperature rise – we may still have as much as 3 degrees warming in the pipeline, in the very long run, if we stabilise around the current atmospheric CO2 concentration (so excluding carbon feedbacks for that additional warming).

Such additional warming would be way more than the (quicker-acting) ocean thermal inertia that people like James Hansen estimate at about 0.6 degrees (in 2005, so at about 378 ppm CO2 – it may be higher at today’s 404 ppm, closer to +0.75 degrees – assuming (incorrect of course) a linear increase with CO2).

[Beware – this thermal inertia is not the same as ‘inevitable warming’ – it is additional warming the world would get if we stabilise CO2 concentrations where they are now. If we would stop emitting fully per tomorrow – and even though CO2 emissions work (mostly) cumulative – we would (at first) have a slightly decreasing atmospheric concentration. That’s called ocean CO2 climate inertia – a force for good, and something that more or less precisely compensates for ocean thermal inertia – just by coincidence.]

Anyway – once you conclude there could be 3 degrees in the pipeline – you either want to move to one of Jupiter’s moons, or you want to be wrong.

‘Tell us 3C in pipeline @ 400ppm is not possible’

Good point to call in some of the experts. We asked some that are active on Twitter to please say 3 degrees additional warming is wrong:

Please (paleo)climate experts, tell us this not poss: +3C in pipeline at 400ppm -> https://t.co/RJwLI5SfGa #Pliocene pic.twitter.com/0mENkedA0g

— Bits of Science (@BitsOfScience) July 2, 2016

And of course we sent an email to lead author Julie Brigham-Grette, Professor of Quaternary and Glacial Geology at the Department of Geosciences at the University of Massachusetts Amherst. Brigham-Grette is specialised in ‘Arctic paleoenvironments’ – and specifically researches climate evolution since the mid-Pliocene.

Here’s what she has to say about the implications of those elevated Arctic temperatures in the Pliocene:

“One point you might want to make is to clarify climate sensitivity versus polar amplification. […] Arctic amplification per degree of global warming is also very hard to quantify. Many of the Proxies we use for estimating temperatures in the past come from pollen produced by plants that only tell us about summer. So for the Lake El’gygytgyn papers, we are estimating summer temperature increases. Estimating winter in the past is very difficult and the plus/minus value around the mean for winter is much bigger. Therefore, estimating mean annual temperatures in the Arctic are hard.

I published a paper with Gifford Miller in Quaternary Science Reviews that tried to look at Arctic amplification versus global mean temperature. Here we suggest that using paleoclimate data for different times in the past, it seems that Arctic summer temperature tends to be 3-4 times as large as the global change (see figure 4).”

Arctic amplification at 3 to 4 times global average temperature rise! That’s significantly more than the 2 to 2.5 current climate models suggest.

It’s also good news – in an odd way. It means on the one hand that the Arctic is more sensitive to future warming – with possible dire consequences for tundra and offshore methane and the Greenland icesheet – but it also offers an explanation for a part of that 3 degrees pipeline temperature rise we feared in our previous article.

Julie Brigham-Grette continues, telling there is likely additional warming if we stayed at the current CO2 concentration. But delayed temperature rise is just one long-term climate concern, she reminds us:

“I agree with the statement of Hansen that even if we stop increasing CO2 at 400 ppm (dream on!), we are committed to some more increase in temperatures. More importantly, even if we stop CO2 at 400 ppm now, we are committed to much more sea level rise. Ice sheets have a long response time and what we see going on today in the ice sheets is a response to warming that started decades ago. The glaciers are not in equilibrium with today at all and will catch up to today long into the future. That is, the loss of ice mass will definitely continue into the future — so we are committed to a lot of sea level rise.”

Yes. Bad. Models seem to agree – even at a warming of ‘just’ 1.6 degrees the threshold for complete melting of the Greenland ice sheet could be passed. And from the Pliocene we learn 2-3 degrees warming can translate to as much as 32 meters sea level rise.

The recent papers by Clark et al, 2016 and DeConto and Pollard, 2016 are really good at documenting this future commitment. Both are very sobering papers.

I would not likely hazard a guess about rates without talking to a climate modeler. The Pliocene was 2 to 3 degrees Celsius warmer when CO2 was in the range of 350 to 450 ppm (hence people say “like today at 400 ppm”).”

Julie adds that although the current warming is accelerating (at 0.2 degrees per decade) she is an optimist by nature. ‘We can get off fossil fuels and move to clean energy’ – clearly the big challenge for mankind.

Two paleoclimate experts agree: Pliocene might imply there’s 1 to 2 degrees in pipeline

We were also helped to better understand Pliocene lessons by Gavin Foster, Professor of Isotope Geochemistry within Ocean and Earth Science, National Oceanography Centre Southampton at the University of Southampton – someone who is specialised in Palaeoceanography.

About possible ‘pipeline warming’ Gavin Foster answers the following:

Yes, there is the thermal inertia, for example we only see ~60% of equilibrium warming after 100 years in response to an instantaneous CO2 doubling (fig S7 of Hansen et al., 2008). In addition, the ice sheets themselves are what is known as a “slow feedback” and, along with vegetation change, may be responsible for causing the rest of the warmth of the Pliocene. Ice albedo is likely the most important of those. Also note that Pliocene was 2 to 3 degrees Celsius warmer than pre-industrial. Now we are 1 degree warmer, so the long term global climate will likely be 1 to 2 degrees warmer as the system fully adjusts to the anthropogenic forcing.”

Scientists can be wonderfully clear. Even more so when they seem to perfectly agree, as Gavin Foster and Julie Brigham-Grette do on the possible long-term warming we could expect, following our understanding of the global climate of 3 million years ago.

What is confusing to us attention-deficit mortals is that our CO2 emissions do not only cause a lot of climate damage in our own lives – but also, in sadly increasing amounts, over the lifespans of future generations. Foster concludes:

“One thing people seem to be forgetting is the world will continue past 2100 – and so will the warming!”

Explaing the ‘paleo pipeline’: The difference between climate sensitivity and ‘Earth system sensitivity’

Remember when we asked 15 leading climate experts about their best-‘guess’ climate sensitivity value? We learned that almost all name a number close to 3 degrees Celsius for a doubling of CO2 – 3 degrees, or just a bit more, a majority added.

We also learned there is such a thing as ‘warming beyond climate sensitivity’. It depends on how you define climate sensitivity – the warming you get for a doubling of atmospheric CO2. Although ‘equilibrium climate sensitivity’ (ECS, the value we asked for) does take into account some important fast-acting climate feedbacks, like water vapour coupling, it still excludes many (possible, often quite uncertain) slower-acting climate feedbacks*.

[Bare in mind feedbacks* that act on the carbon cycle, which are also many, can increase warming projections for our energy emission scenarios, but cannot elevate (or lower) climate sensitivity, as by definition climate sensitivity is constrained to fixed rises in CO2. Feedbacks that influences albedo (like sea-ice decreases, vegetation changes) or other aspects of Earth’s energy balance – like the thermal mass of melting (high) versus a fully melted (low) icesheet, and possible changes in ocean currents (decreasing currents lead to increased atmosphere heat build up) can work ‘beyond’ climate sensitivity.]

These slower-running climate feedbacks, many of which do not occur directly in the atmosphere, but elsewhere in system Earth, are the reason why in our same survey several climate experts named different, additional metrics. Like Michael Mann, head of Earth System Science Center at Pennsylvania State University.

Mann came up with the following distinction:

“While the canonical (“most likely”) estimate of the “fast feedback” ECS is around 3C, I feel that more recent evidence suggests it might very well be higher than that, between 3C and 4C (say, 3.5C).

The long feedback “Earth System Sensitivity” (ESS) is almost certainly higher, closer to 5C.”

Now if we do a quick and dirty calculation (for the moment ignoring the fact that climate sensitivity acts logarithmically) we see that this Michael Mann’s value for Earth System Sensitivity (say at a CO2 concentration around 560 ppm – in the very long run, and of course highly uncertain) shows clear comparison to numbers that his paleoclimate colleagues Julie Brigham-Grette and Gavin Foster mention (2 to 3 degrees around 400 ppm – again, with a large uncertainty margin, also in that CO2 concentration).

Also Ken Caldeira, a climate scientist at Carnegie Institution for Science, mentioned Earth system sensitivity in our expert survey, in addition to climate sensitivity. Caldeira said the following:

“Earth System sensitivity is a bit more complicated as it might make more sense to think about sensitivity to an emissions rather than a concentration. The idea of doubling or quadrupling CO2 and then holding it constant for many thousands of years is a bit artificial, especially if you are considering biogeochemical feedbacks to be included in your definition of climate sensitivity.”

Earth System Sensitivity is a different question. If you are including carbon cycle feedbacks, does it make sense to ask what is the sensitivity to a CO2 concentration with carbon cycle feedbacks? Isn’t the point of carbon cycle feedbacks that it can change CO2 concentrations?

That is why I suggest that it may be better to think about Earth System Sensitivity in terms of a response to an emission rather than a response to a concentration change.”

Hans Joachim Schellnhuber, Director of the Potsdam Institute for Climate Impact Research, agrees the uncertainties are high – calling it a highest research prioritity:

“As for the full Earth-system response including all feedbacks, we do not know enough to provide any reasonable number. This is highest-priority research terrain.”

A climate model comparing the mid-Pliocene to the preindustrial climate, using PRISM2 palaeoclimate reconstructions and the Hadley Centre atmospheric GCM. Paleoclimatologists say the Pliocene was about 2 to 3 degrees warmer at CO2 levels that were close to the current (raised) atmospheric concentation. Also sea levels were about 25 meters higher than they are today. If the Pliocene analogue holds out, we may have a lot of warming ‘in the pipeline’ – warming that lies ‘beyond regular climate sensitivity’.

This is the 25^th and final part of our ‘Real’ Global Temperature Trend series here at Bits of Science. Stay tuned – because our next piece will (finally!) be the Grand Conclusion, with a hopefully very illustrative & shareable graph!

[And after that – if you keep pressing F5 – you can be the first to know about our NEW climate series! – one that will not focus on the temperature graph as much, is all we can give away now…]

© Rolf Schuttenhelm | www.bitsofscience.org