As a term, ‘semi-empirical’ has been around since 1935, according to Merriam Webster’s Dictionary. It’s not a term I have come across during my years involved in the climate conversation. It kind of sounds like using your favored assumptions to help you get the answer you want.

adjective semi·em·pir·i·cal \-im-ˈpir-ə-kəl, -em-\

: partly empirical; especially : involving assumptions, approximations, or generalizations designed to simplify calculation or to yield a result in accord with observation

However, that’s what climate activist site Real Climate is using to publicize their paper on 2,500 years of sea level rise. Astonishingly, their graph of sea level rise over the past 2,500 years has the shape of a Hockey Stick.

The paper is Kopp et al, found here. The Real Climate co-author of the paper is Stefan Rahmstorff (introduced in greater detail below).

Astonishingly, the paper claims to have charted sea level rise for the past 3,000 years, not the 2,500 claimed by Real Climate. I don’t know if that’s modesty or just an example of semi-empiricism.

Anyhow, here’s how they did what they did: “We present the first, to our knowledge, estimate of global sea-level (GSL) change over the last ∼3,000 years that is based upon statistical synthesis of a global database of regional sea-level reconstructions. …”We assess the relationship between temperature and global sea-level (GSL) variability over the Common Era through a statistical metaanalysis of proxy relative sea-level reconstructions and tide-gauge data.”

Okay… proxies again. I start to see where the term ‘semi-empirical’ is relevant here.

Here’s the money graph:

Why, that’s… that’s… more than 5 inches! It’s 5.1 inches, to be more precise.

They continue: “A significant GSL acceleration began in the 19th century and yielded a 20th century rise that is extremely likely (probability P≥0.95P≥0.95) faster than during any of the previous 27 centuries. A semiempirical model calibrated against the GSL reconstruction indicates that, in the absence of anthropogenic climate change, it is extremely likely (P=0.95P=0.95) that 20th century GSL would have risen by less than 51% of the observed 13.8±1.513.8±1.5 cm. The new semiempirical model largely reconciles previous differences between semiempirical 21st century GSL projections and the process model-based projections summarized in the Intergovernmental Panel on Climate Change’s Fifth Assessment Report.”

Well, we’re all in favor of reconciliation, right? And Real Climate wants to share the wealth–they cite other papers as they begin to talk about future sea level rise:

“This brings us to a second study, also published this week in PNAS (Mengel et al. 2016). In this paper a new method of sea-level projections is developed. It is not based on complex models, but also on simple semi-empirical equations. But not for sea level as a whole but for the individual causes of sea-level changes: thermal expansion of sea water, melting of glaciers and mass loss of the large ice sheets. These equations are calibrated based on data from the past and then extrapolated to a warmer future. It is a kind of hybrid between the semi-empirical and the process-based approach (favored by the IPCC) to sea-level projections. The following table compares the projections of the various methods.

Scenario IPCC 2013 Kopp 2016 Mengel 2016 Horton 2014 RCP 2.6 28–60 24–61 28-56 25–70 RCP 4.5 35–70 33–85 37-77 n.a. RCP 8.5 53–97 52–131 57-131 50–150

Table 1 Global sea level rise in centimeters over the 21st century according to various studies for different emission scenarios. The first scenario (RCP 2.6) assumes successful climate policies limiting global warming to about 2°C; the last (RCP 8.5), however, unabated emissions and heating by around 5°C. (The ranges indicate the 90 percent confidence intervals except for the IPCC, which only provided a 66 percent confidence interval.)”

Note again the use of the term ‘semi-empirical.’ Perhaps that’s appropriate because using RCPs for this type of forecast is similar to examining the entrails of chickens, as the Representative Concentration Pathways started off with the conclusion and worked backwards to find a plausible scenario for reaching said conclusion.

Update: (I slipped a decimal point. Corrected figures here.) Using empirical observations without the need for a ‘semi’ gives us this. So far during the 21st Century, sea level rise has totaled 6.2 cm. Should sea level rise continue at this breathtaking rate we will reach 10 centimeters of global sea level rise by 2025, nine years from now. That will give us 75 years to achieve the other 120 centimeters Stefan Rahmstorff thinks may befall us by 2100.

You can sort of drive a semi through some of these projections.

However, I must be careful. Stefan Rahmstorff was convicted of libeling a German journalist a few years ago when she dared question some of his more alarmist scribblings. He actually drove her out of the profession, despite his being found guilty of libel. As the German newspaper Der Spiegel noted, “Time and again he has not only gone after journalists, but also after scientists who have openly expressed views that Rahmstorf didn’t like.” Sure would hate to annoy this master of ‘semi-empiricism.’ He might tell some ‘semi-truths’ about me.