Understanding Sea Level Rise, p2: A short chronology of SLR forecasts for the year 2100 (showing they increase with time)

Sea level rise is a slow process. Other consequences of climate change are generally felt much sooner. But there is something odd about the forecasts. They seem to be catching up with us, bringing a distant future closer to our doorsteps.

In part 2 of our special short series about global sea level rise we thought we’d try to create an overview of leading reports and studies published in the last ten years – all of them offering a different forecast for the extent of sea level rise by 2100:

Overview sea level rise forecasts for 2100 - increasing
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Understanding Sea Level Rise, part 1: Thermal Expansion – a Physical Foundation of several metres, irrespective of ice melting

In between our series about the global temperature trend and our (upcoming) series about climate & biodiversity, let’s do a short series about sea level rise, shall we?

Thermal expansion of water and ice
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Do the math! Because climate sensitivity is logarithmic, 1.5 degrees target was already breached at 400 ppm – if you look at CO2 only (& assume ECS = 3C)

According to ‘conventional climate science’ the currently already emitted amount of CO2 (404 ppm) leads to a committed warming of 1.56 degrees Celsius. To keep ‘the promise of Paris’ – the CO2 concentration must go down, down to below 400 ppm on the decades timescale, and (yes, Hansen was right there too) closer to 350 ppm to also prevent ‘the slow climate catastrophe’.

Climate inertia, the timescale of different climate feedbacks
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Observed vs ‘Real’ Global Temperature. What thermometers do & don’t yet show!

After a perceived ‘temperature plateau‘ of about a decade, global temperatures seem to be rising faster than ever before. First 2014 broke the global record for hottest year (then held by 2010). Then 2015 broke that record. And 2016 in turn is set to completely smash that record. We’ve just had the 15th hottest month in a row globally, and counting.

So, what’s going on? We decided to properly investigate – and wrote a 25-piece series about (what we call) the ‘Real’ Global Temperature Trend. Central question: Are these ‘insane’ peaks on the global temperature graph really peaks? Or is half the story hidden still – hidden in masking factors, hidden in oceans, hidden in climate science…?

Today we finish the series and present our 5 main conclusions here at Bitsofscience.org – introducing a new quantity (RGT), supplemented with a hopefully very illustrative graph, and a summary of the entire series below:

What we conclude about the global temperature trend: 

  1. A large part of atmospheric warming is still masked by (shorter-lived) cooling factors and by climate system inertia – therefore the CO2-coupled ‘Real’ Global Temperature is (much) higher than currently observed temperatures.
  2. Recent global temperature records were no ‘peaks’, but rather corrections to a climatic temperature trend line that is (much) higher than the statistical trend line.
  3. If atmospheric CO2 is stabilised around the current level (404 ppm) there is an uncertain, but possibly large amount of ‘pipeline warming’. This warming in the pipeline may lead to an additional temperature rise of more than 1 degree Celsius – additional warming that will manifest itself after stabilisation of the CO2 concentration. The final temperature rise of the current CO2 concentration could be up to 2 or 3 times as high as the warming that is currently observed(!)
  4. The current atmospheric CO2 level is a dangerous overshoot – to stay below internationally agreed climate targets (both 1.5 & 2 degrees) the CO2 concentration (that is currently still rising year by year) should not be stabilised, but should in fact be lowered.
  5. If we keep measuring climate change by the observed rise in live temperatures and the Earth & climate system responses this temperature rise causes (including extreme weather events) we keep underestimating the real scientific climate urgency.
The below graph shows 4 different temperature trends, against the observed rise of the atmospheric CO2 concentration: 1) observed temperatures (plus annual & 30-year average), an RGT trend based on ‘consensus climate sensitivity’, an RGT trend filtering ocean thermal inertia, and an RGT trend based on long-term ‘Earth System Sensitivity, deduced from Pliocene & Eocene paleoclimate. It shows that at the current CO2 concentration, atmospheric warming could still double:

'Real' Global Temperature - 4 different climate trend lines in one graph, compared to atmospheric CO2 concentration. Please link to Bitsofscience.org
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Real Global Temperature Trend, p25 – Paleoclimate experts to the rescue! Closer to 1-2 degrees Celsius in pipeline from long-term climate inertia

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.

Holocene-Anthropocene: this jump in CO2 puts warming in the pipeline
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Real Global Temperature Trend, p24 – Paleoclimate tells we may have 3 degrees additional warming in pipeline at current CO2 concentration!

Either the entire world is set to experience dramatic additional warming once we stabilise at the current (400+ ppm) CO2 concentration – or we are still dramatically underestimating the local climate sensitivity of the Arctic – a region that might in that case not warm 2 or 3 times as fast as the global average, but rather about 6 to 8 times. (Perhaps most likely it’s something in between.)

In any case, it’s bad news that’s being dredged up from the bottom of Lake El’gygytgyn.

Pliocene Arctic climate - 400ppm CO2 - yet 8 degrees warmer!
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Real Global Temperature Trend, p23 – Climate sensitivity has large geographical spread: 2-3 degrees global average equals 6+ in the Arctic

Why climate change is such a killer? Because disruption is never a linear process. We see that with ecosystem disintegration – where beyond a certain threshold cascading damage sets in – leading to shifts and collapse scenarios that are easy to succumb to, yet almost impossible to predict.

We also see this with weather extremes. A seemingly innocent shift in the climatic average (precipitation, or temperature – likely both) leads to an ‘unexpectedly’ [unless of course your name is Thomas Bayes or Carl Friedrich Gauss – then this would be intuitive we guess] large increase in the incidence of the old category of weather extremes – and a submerging new category of equally rare but now far extremer weather extremes. These extremes of course are responsible for a lot of the (ecological, agricultural, economical) damage of the (average) change.

Average climate change leads to rapid increase of weather extremes
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Real Global Temperature Trend, p22 – World still on course for dramatic 3 degrees warming, need to more than double reduction speed

Let’s keep this one short, because in essence there’s nothing new. Before the start of the Paris climate summit we saw that if you combine all the world’s nations’ 2025 & 2030 emission reduction targets you get to a pathway of around a 860 ppm CO2 equivalent greenhouse gas concentration. By the year 2030 that’s already a 19 gigatonne emissions gap compared to the 2 degrees climate target – and a 25 gigatonne emissions gap compared to the newly adopted 1.5 degrees target.

That is if countries actually deliver on their reduction pledges. During the climate conference there were no increases in national targets – although the world did agree on a higher ambition for the whole: “holding global warming to well below 2 degrees Celsius and to “pursue efforts” to limit it to 1.5 degrees Celsius.”

Well – the below graph sums up where exactly we’re standing:

3 degrees warming
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Real Global Temperature Trend, p21 – Science is ruthless. We have ‘about zero’ years of emissions to stay below 1.5 degrees

That is if you correct for one important piece of scientific criticism to the below graph. But even if you prefer to ignore what may seem the nitty gritty of climatology and accept a maximum global carbon budget of 8.9 years one conclusion should resound across our planet: The time of coal is over.* Yesterday. We cannot build any new coal lock-in infrastructure – and we have to shut down all the existing coal plants, and stop all the existing coal transport – both exports and imports.

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Real Global Temperature Trend, p20 – Combining thermal inertia & carbon cycle inertia: 1 decade extra warming after emissions stop

Apart of course from the amount of greenhouse gases we keep pumping into the atmosphere, there are mainly three factors that determine the amount of warming we will experience in the near future: CO2 climate sensitivity, ocean thermal inertia, and carbon cycle inertia. Here we try to make better sense of their combination.

Climate inertia atmospheric warming
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