The Journey not the Destination: A Carbon Budget Simulator

We often hear that it’s the journey and not the destination that really matters. Nowhere is this truer than when thinking about climate change. In this blog, CUSP co-investigator Peter A. Victor is introducing his illustrative carbon budget simulator, that can be used to explore different approaches to achieving an emissions target and remaining within a target budget. (This blog was updated on 30 March 2020 to include an addendum on the Corona crisis.)

We often hear that it’s the journey and not the destination that really matters. Nowhere is this truer than when thinking about climate change. Until recently, policy commitments about anthropogenic greenhouse gas emissions have usually been expressed in terms of a target percentage reduction in emissions from some base level to be achieved by a target year, often decades in the future.

The main problem with this approach is that it says nothing about the cumulative emissions that will occur between the present and the target date. This might be acceptable for an environmental problem caused solely by the flow of emissions and not the stock. But climate change is all about the stock which is commonly expressed as the concentration of CO₂ or several greenhouse gases in the atmosphere. Of course, stocks increase with inflows and decrease with outflows, so flows are not irrelevant. It is by changing the flows that we change the stocks. However, if we focus too much on reaching a target flow in some future year and neglect the emissions path that gets us there, we are likely to discover that the stock of greenhouse gases becomes dangerously high.

This was brought home very clearly in the IPCC Special Report Global warming of 1.5⁰C which ‘… suggests a remaining budget of about 420 GtCO₂ for a two-thirds chance of limiting warming to 1.5°C, and of about 580 GtCO₂ for an even chance (medium confidence).’ (p.96) The report discussed other greenhouse gases but did not include them in the estimates of the remaining carbon budget. In 2018, the year the report was written, global CO₂ emissions were 42 ± 3 GtCO₂ which if maintained would exhaust the remaining carbon budget in 10 to 14 years, and that was two years ago.

The IPCC report showed many emission pathways ‘compatible with 1.5⁰C in the Context of Sustainable Development’. See Figure 1 below. The differences in the pathways relate largely to different socio-economic assumptions on which the pathways are based. However, they all share the common features of a very steep decline in emissions with negative emissions mostly in the second half of the century.

One result of this IPCC report is the adoption of net zero targets by a number of countries such as the UK and Canada. This is an improvement on an 80% or 90% reduction though it still leaves open the possibility that global and national carbon budgets will be exceeded on the way to reaching net zero. The path to net zero really matters. This becomes crystal clear in the simple to use carbon budget simulator that is now online.

Figure 2 shows how you set values for the key variables that determine the emissions and targets. The values shown are for a target reduction of 80% in emissions in 30 years. The carbon budget is set at 10 times current emissions and the assumed annual rate of economic growth is 2%. No delay in reducing emissions is assumed but this can be changed as can negative emissions which are set at zero. Figure 3 shows the implications of these settings.

Figure 2 shows that to reduce emissions in 30 years by 80% requires an annual reduction in the emissions intensity of GDP (emissions/GDP) of 7.1% every year for 30 years. And even then, the carbon budget, set at 10 times the current level of emissions, would be exceeded in only 15 years.

Carbon Budget Simulator

Other emission pathways can be simulated. You can select a linear reduction pathway, or you can draw your own. It really matters which pathway you choose. As the saying goes, it’s the journey not (just) the destination. Do try the Carbon Reduction Simulator. It works for the for the globe or for an individual country or region. The results will surprise you. https://exchange.iseesystems.com/public/petervictor/carbon-budget-simulator/index.html#page1