Sea ice melting at Arctic Sea 2: Future projections

Last time, we looked at the present decline in Arctic sea ice. The decline rate has become faster and faster, and this is a very serious issue in climate change. It is necessary to find out how sea ice would change in the future. This is dominantly relied on future projections, which are based on model simulations under different scenarios. This time, we are going to look at model simulations and future projections about Arctic sea ice. 

At the beginning, let’s clarify some definitions and knowledge that’ll  help you to understand what I am going to write below. If you've already known these, please skip this part. (Since I'm studying modelling now and my undergraduate dissertation was related to modelling and projections, I list some extra academic journal articles here which I think are worth reading if you are interested in.)

• What is a model? And what’s a climate model?

 In one sentence, a model is a representation of the reality, helping to predict or understand something. It is a series of rules and principles to convert inputs (the data we have now) to outputs (what we want). 

A climate model uses quantitative methods to investigate how the climate system responses to variety of forcing, to predict how the future climate would be with different time scales and to make projections of future climate towards 2100 and beyond.

• What are CMIP3 and CMIP5?

CMIP3 and CMIP5 are the two phases of the Coupled Model Intercomparison Project (CMIP) for evaluation in Assessment Report  by Intergovernmental Panel on Climate Change (IPCC). IPCC reports summarise most advanced outstanding studies and researches in the world related to climate change. 

I think you just need to know that CMIP contains a series of climate models that simulate past and future climate, and the future projections are based on multi-model mean and variations among model simulations. That’s enough to understand what I am going to write below. If you want to know more about it, there is an excellent journal article providing the scientific explanation of CMIP5, i.e. Taylor et al. (2012).

•  What are RCPs?

RCP is the Representative Concentration Pathway, and there are four types of it : RCP2.6, RCP4.5, RCP6.0 and RCP8.5. You can simply think that the numbers at the end represent how much the radiative forcing would increase by the end of this century in W/m². e.g. in RCP8.5, radiative forcing reaches 8.5  W/m² by 2100, have highest temperature warming among the four. If you are interested in this, you can look at van Vuuren et al. (2011) for further information.

Okay, now let's go back to today’s topic. CMIP5 models under RCPs provide most comprehensive and advanced projections in the world, and my evidence and examples below are derived from CMIP5 model simulations in IPCC AR5 Chapter 12. The projections can be divided into near-term and long-term time scales. The near-term covers time from now to the mid of this century. IPCC (2013) stated that the near-term projections are not specific and precise enough as the result of changes in external forcing, so I’m not going to look at the near-term projections here.

I’m going to focus on the long-term (towards the end of this century) projections instead. Firstly, we have to determine the basis of model simalations. Sea ice melting in the future is very likely caused dominantly by further rise in surface temperature (IPCC, 2013). Figure 1 shows the relationship between decline in sea ice extent and annual global surface warming in CMIP5. Within the figure, it is obvious that there is a functional relationship (nearly linear) between sea ice extent decline and surface temperature rise before reaching nearly ice-free status (the black horizontal line at 1 × 106 km2  ). Nearly ice-free is that sea ice extent is continuously less than 1 × 106 km2  for no less than 5 years. 

Figure 1. Relationship between annual mean global surface warming and September Arctic sea ice extent relative to the period 1986-2005 as simulated by CMIP5 models. (Source: IPCC, 2013)

Then, let’s look at the future projections. Based on CMIP5, the decline in sea ice shows in all four RCPs but with different rates. According to Figure 2, a general decline in sea ice extent is simulated under each RCP, both for winter (Feb.) and summer (Sep.). I’m more interested in the changes in September and will focus on this in the following sentences, because the condition in September may change to nearly ice-free within this century. I’ve added a green horizontal line in the September one to show the nearly ice-free status. All four RCPs show the possibility of the nearly ice-free before 2100. Under RCP8.5, some model simulations show that the Arctic sea may reach nearly ice-free before 2040. By the end of this century, nearly all models (around 90% according to IPCC) under RCP8.5 reach the nearly ice-free status, while that is about half under RCP2.6 (around 45% according to IPCC). This means that if climate change keeps happening in the future, sea ice in Arctic sea will keep melting and the sea will become nearly ice-free within few decades. 

Figure 2. Changes in ice extent for period 1950-2100 as simulated by CMIP5 models under different scenarios. (Source: IPCC, 2013).