In 2016 two influential new publications raised the possibility of a rapid acceleration of sea level rise in the 21st century – to ±2 metres (DeConto & Pollard) or more (2-5m, Hansen et al).
In this background article we take a look at both these studies – but also at 30+ other publications that we think are helpful to show the broader context: an increasing pile of evidence, coming from a world of science, indicating ice sheet dynamics are never linear and can be surprisingly rapid.
Looking at the key processes we show that the Hansen and DeConto studies add to a list of no less than 13 different (yet highly entangled, often synergistic) ice sheet melting feedbacks, feedbacks that can promote a rapid acceleration of global sea level rise:
Today we show ‘the full story of anthropogenic sea level rise’ – a story that requires us to zoom out to much larger timescales, to learn to respect the immense thermal inertia of oceans and ice sheets while simultaneously coming to terms with the full magnitude of the sea level rise that our CO2 emissions in the 20th and 21st century are causing: that is 29 to 55 metres in total, depending on the amount of fossil fuels we choose to continue to dig up and burn.
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:
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’.
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:
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.
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.
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(!)
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.
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:
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.