WHO: Michiel Schaeffer (Environmental Systems Analysis Group, Wageningen      University, The Netherlands),
William Hare (Potsdam Institute for Climate Impact Research, Potsdam, Germany),
Stefan Rahmstorf (Potsdam Institute for Climate Impact Research, Potsdam, Germany),
Martin Vermeer (Geoinformatics, Aalto University School of Engineering, Finland)

WHAT: Estimates of sea level rise with different levels of CO₂ emission reduction

WHEN: 24 June, 2012

WHERE: Nature Climate Change, Vol 2 Issue 7 2012

TITLE: Long-term sea-level rise implied by 1.5 °C and 2 °C warming levels

One of the consequences of climate change is going to be sea level rise as the globe slowly warms and ice sheets and glaciers continue to melt.

The issue is trying to predict exactly how much sea level is going to rise by and how much we can do about it. This paper was published in June in the journal Nature Climate Change, which is an offshoot of the journal Nature, one of the most highly respected peer reviewed scientific journals in the world.

The researchers used several different scenarios for looking at how sea levels rise with rising temperatures and used what’s called a semi-empirical model to predict the numbers out to 2100 and 2300.

A semi-empirical model is one that combines observed and recorded data (thermometers, tide records) with paleo data (ice cores, fossil records). Since there is only recorded tide gauge data from the previous 130 years, this is not a large enough sample to use for long range predictions. It would be like using one week of training to determine whether I could run a marathon. So the researchers combined the historical data with the recorded data and calibrated to test how realistic the combination was.

Calibrating is where you test something for a single result several times and work out what your error margin is. For example, you could measure three different bottles of Coca Cola you bought at the store, and one might have 600mL, one might have 603mL and the third might have 598mL. They’re all supposed to be 600mL, but since most things have a margin of error, not all are exact.

Same thing for science experiments. The researchers calibrated their data for the years 1000 through to 2006 and found that their results were similar to other scientists who had done similar experiments.

Several scenarios were used; two with no reductions in carbon emissions (from the Copenhagen Accord agreement), two where the earth stays within 2C of warming (one that looks at all polluting gasses – carbon, ozone and sulphur based and one that looks at the delay of emission reductions until 2100), and one where the earth stays within 1.5C of warming (with early carbon emission reductions keeping pollution under 400 parts per million (ppm) in the atmosphere).

Three of the scenarios (delaying until 2100 and two in between) used data that will be included in the IPCC 5^th Assessment Report which is the newest data on climate change. The finalised 5^th report won’t be published by the UN until the end of 2014.

One of the scenarios for 2C (the one with the different polluting gasses) was developed by an international scientific group and published in Climactic Change. The scenario for 1.5C was developed by a group working for the International Energy Agency and published in the journal Energy. Another hypothetical scenario where global carbon emissions are zero by 2016 was also used to see what effect historical emissions might have on sea levels.

So what did they find? Already, the data for the 20^th Century (1900-1999) has higher sea levels than any time in the past 1,000 years.

Using the scenario from the Copenhagen Accord (the equivalent of doing nothing) a sea level rise of 72 – 139cm by 2100 was predicted.

Even with zero emissions by 2016 there’s a predicted sea level rise of 40 – 80cm by 2100. The reason for this is because climate systems are inert. Which means they can absorb a lot of change before you can see it and that there’s a time lag between putting the pollution into the atmosphere and seeing the consequences.

Sea level rise precitions (MERGE400 = 1.5C, Stab2 and RCP3-PD = 2C, CPH = no action, others are variations in between). Dark lines are the median (middle) predictions, shaded areas show the uncertainty range.

So what does this mean in reality for climate change and people? It means that even if we limit carbon pollution in the atmosphere to 400ppm the conservative estimate is for a 54cm rise in sea level, which is knee-deep on me. And if 400ppm doesn’t sound like a very big number, just remember the US EPA recommends that more than .002ppm of mercury in your drinking water is dangerous. You don’t need large amounts of compounds for them to be dangerous to your (or the planet’s) health.

The good news is the research shows that even though there’s uncertainty regarding non-linear (unpredicted) changes in ice sheet melt, and we’re going to experience sea level rise continuing over the next 50 years from pollution already in the atmosphere, emissions reductions do show less sea level rise than the ‘do nothing’ or ‘delay’ scenarios.

So while we are going to have around half a metre of sea level rise in the next 50-100 years (I’d hold off on purchasing that beachfront property), with strong emission reductions, the sea level might peak sometime before 2100 and hopefully head back towards normal.

Why the huge range of numbers and years? Firstly because the researchers can’t say what and how much the world is going to do about climate change in the next few decades, and secondly because you can’t be absolutely (as in 100%) certain that anything is going to happen until it actually does. But the data gives us a pretty clear indication that the year 2100 will see somewhere between .5 – 1m of sea level rise.