Sea Level Rise Projections and Policy

One blog I follow is TrustYetVerify. The latest post – Projecting sea level 300, nah, 1000 years in the future – is straightforward and highlights some significant issues for climate policy.

He compares claims of an activist in a Belgium newspaper that unmitigated climate change will result in sea level rise of 5 metres in 300 years, with a graphic from UNIPCC AR5 WG1 Chapter 13 on sea level rise that showed a at most around a 3 metre rise.

There was a good spot by Michel in relation to a graphic from a December 2017 presentation on the impacts of an 8 metre rise in sea levels by the year 3000. In was originally from a 2004 Greenpeace document. Only the earlier document also had the impacts of current sea inundation and a 1 metre sea level rise.

There are some lessons that can be learnt.

Marginal Difference of policy

The current sea coverage is of large areas of the Netherlands that are not currently covered by sea water. To create the graphic, they have removed the dykes that have enabled the Netherlands to vastly increase its land area. This not only vastly exaggerates the impact of sea level rise, but contains the assumption that people are too dumb to counter the impact of sea level rise by building dykes higher. Given that even the exaggerated claims are 5 metres in 300 years, that means an average rate of rising of 17mm per annum and a maximum rate of maybe 30mm. What is more, any rise is predictable over maybe decades. Decisions can be made over 20-50 year timescales, which are far less onerous than taking the long-term perspective. Even if a 5 metre rise over 300 years was accurate, either building dykes now assuming sea levels are 5 metres higher, or abandoning areas that will be inundated will cause needless costs for this generation and the next few generations.
The is an even greater policy assumption, that I repeatedly point out. Climate mitigation through reducing greenhouse gas emissions requires that global emissions are reduced.  It does not matter whether Belgium, and the Netherlands make massive cuts their emissions, if most other countries do not follow similar policies. As a graphic 3.1 from the UNEP Emissions Gap Report 2017 clearly demonstrates, the net impact of all proposed policies is very little compared to doing nothing, and a long way from the 1.5°C or the 2°C targets. This is after over 20 years of annual COP meetings to obtain much bigger reductions.

The marginal impact of sea-level rise is therefore exaggerated by

  • Assuming that the existing flood defences vanish.
  • Assuming people do not build any more defences.
  • Exaggerating the projected rise.
  • Looking at a far greater timescale than rational planning ought to take place.
  • Falsely promoting emissions reductions to combat sea level rise impacts, knowing that whatever a few countries do will not make a difference to overall emissions. If significant warming is caused by human GHG emissions, and this leads to significant sea level rise, then current emissions policies are largely a waste of time.


Checking and Interpreting Forecasts / Projections

Consider the sea level rise graphic from UNIPCC AR5 WG1 Chapter 13 .

Consider the projections for the year 2500.

The High Scenarios shows sea level rise of 1.5 to 6.5m in 2500 for >700ppm CO2.
Medium scenarios show sea level rise of 0.2 to 2.3m in 2500 for 500-700ppm CO2.
Low scenarios show sea level rise of 0.5 to 1.0m in 2500 for <500ppm CO2.

How can the medium scenarios project a lower bottom end than the low scenarios?

The explanation probably lies in different modelling assumptions. After all the greater the scenario from the current state of affairs, the greater the uncertainty range, unless you assume that the structure of the model contains truths not revealing from any observations.

Further note the High scenarios lower limit is only 30cm a century, and the top end is 1.3m a century, whilst the medium scenarios bottom end over five centuries is roughly the rate of sea level rise per century for the last few centuries. That is, well within the medium scenario uncertainty range is the possibility that some global warming will make no difference to the rate of sea level rise.

What I also find interesting is that under the medium scenarios, Antarctica is gaining ice, hence reducing sea levels, but under the low scenarios has no impact whatsoever. Again, this shows the different modelling assumptions used.

Concluding note

Suppose a pharmaceutical company promoted a product with clearly exaggerated claims of its effectiveness, false alarm for the need for the product, and deliberately played down the harms that the product could cause to the patient? There would be an outcry, and the company being sued in a world without regulations. In most countries, strict regulations mean that to market a new product, the onus is on that company to demonstrate the product works, and that side effects are known. But it is alright to promote such falsehoods to “save the plant for future generations“. Indeed, to shout down critics as deniers of climate change. 

Kevin Marshall

“Were going to miss the 2°C Warming target” study and IPCC AR5 WG3 Chapter 6

WUWT had a post on 22nd January

Study: we’re going to miss (and overshoot) the 2°C warming target

This comment (from a University of Southhampton pre-publication news release) needs some explanation to relate it to IPCC AR5.

Through their projections, Dr Goodwin and Professor Williams advise that cumulative carbon emissions needed to remain below 195-205 PgC (from the start of 2017) to deliver a likely chance of meeting the 1.5°C warming target while a 2°C warming target requires emissions to remain below 395-455 PgC.

The PgC is peta-grams of Carbon. For small weights, one normally uses grams. For larger weights one uses kilograms. For still larger weights one uses tonnes. Under the Imperial measurement system, one uses ounces, pounds and tons. So one peta-gram is a billion (or giga) tonne.
Following the IPCC’s convention, GHG emissions are expressed in units of CO2, not carbon. Other GHGs are expressed in CO2e. So 1 PgC = 3.664 GtCO2e.

So the emissions from the start of 2017 are 715-750 GtCO2e for 1.5°C of warming and 1447-1667 GtCO2e for 2°C of warming. To make comparable to IPCC AR5, (specifically to table 6.3 from IPCC AR5 WG3 Chapter 6 p431), one needs to adjust for two things – the IPCC’s projections are from 5 years earlier, and for CO2 emissions only, about 75% of GHG emissions.

The IPCC’s projections of CO2 emissions are 630-1180 GtCO2 for 1.5-1.7°C of warming and 960-1550 GtCO2e for 1.7-2.1°C of warming.

With GHG emissions roughly 50 GtCO2e a year and CO2 emissions 40 GtCO2 a year, from the IPCC’s figures updated from the start of 2017 and expressed in GtCO2e are 570-1300 GtCO2e for 1.5-1.7°C of warming and 1010-1800 GtCO2e for 1.7-2.1°C of warming.

Taking the mid-points of the IPCC’s and the Goodwin-Williams figures, the new projections are saying that at current emissions levels, 1.5°C will be breached four years earlier, and 2°C will be breached one year later. Only the mid-points are 1.6°C and 1.9°C, so it makes no real difference whatsoever. The Goodwin-Williams figures just narrow the ranges and use different units of measure.

But there is still a major problem. Consider this mega table 6.3 reproduced, at lower quality, below.

Notice Column A is for CO2 equivalent concentration in 2100 (ppm CO2eq). Current CO2 levels are around 405 ppm, but GHG gas levels are around 450 ppm CO2eq. Then notice columns G and H, with a joint heading of Concentration (ppm). Column G is for CO2 levels in 2100 and Column H is for CO2 equivalent levels. Note also that for the first few rows of data, Column H is greater than Column A, implying that sometime this century peak CO2 levels will be higher than at the end of the century, and (subject to the response period of the climate system to changes in greenhouse gas levels)  average global temperatures could (subject to the models being correct) exceed the projected 2100 levels. How much though?

I will a magic equation at the skeptical science blog, and (after correcting to make a doubling of CO2 convert to exactly 3°C of warming) assume that all changes in CO2 levels instantly translate into average temperature changes. Further, I assume that other greenhouse gases are irrelevant to the warming calculation, and peak CO2 concentrations are calculated from peak GHG, 2100 GHG, and 2100 CO2 concentrations. I derived the following table.

The 1.5°C warming scenario is actually 1.5-1.7°C warming in 2100, with a mid-point of 1.6°C. The peak implied temperatures are about 2°C.

The 2°C warming scenario is actually 1.7-2.1°C warming in 2100, with a mid-point of 1.9°C. The peak implied temperatures are about 2.3°C, with 2.0°C of warming in 2100 implying about 2.4°C peak temperature rise.

So when the IPCC talk about constraining temperature rise, it is about projected temperature rise in 2100, not about stopping global average temperature rise breaching 1.5°C or 2°C barriers.

Now consider the following statement from the University of Southhampton pre-publication news release, emphasis mine.

“Immediate action is required to develop a carbon-neutral or carbon-negative future or, alternatively, prepare adaptation strategies for the effects of a warmer climate,” said Dr Goodwin, Lecturer in Oceanography and Climate at Southampton. “Our latest research uses a combination of a model and historical data to constrain estimates of how long we have until 1.5°C or 2°C warming occurs. We’ve narrowed the uncertainty in surface warming projections by generating thousands of climate simulations that each closely match observational records for nine key climate metrics, including warming and ocean heat content.”

Professor Williams, Chair in Ocean Sciences at Liverpool, added: “This study is important by providing a narrower window of how much carbon we may emit before reaching 1.5°C or 2°C warming. There is a real need to take action now in developing and adopting the new technologies to move to a more carbon-efficient or carbon-neutral future as we only have a limited window before reaching these warming targets.” This work is particularly timely given the work this year of the Intergovernmental Panel on Climate Change (IPCC) to develop a Special Report on the Impacts of global warming of 1.5°C above pre-industrial levels.


The basic difference between IPCC AR5 Chapter 6 Table 6.3 and the new paper is the misleading message that various emissions policy scenarios will prevent warming breaching either 1.5°C or 2°C of warming when the IPCC scenarios are clear that this is the 2100 warming level. The IPCC scenarios imply that before 2100 warming could peak at respectively around 1.75°C or 2.4°C.  My calculations can be validated through assuming (a) a doubling of CO2 gives 3°C of warming, (b) other GHGs are irrelevant, (c) there no significant lag between the rise in CO2 level and rise in global average temperature.

Kevin Marshall