Last week I presented a simple analysis of the temperature data over the last 50+ years which showed that there was good reason to think that the global surface temperature was rising by about 0.18°C per decade. But a further question to ask is when the current upward trend really started in earnest and therefore where are we today against a baseline of the pre-industrial temperature (i.e. 1850s or thereabouts). This is an important question as we have collectively established a desire to keep warming well below 2°C, but with the real prize being to limit this even further and ideally to 1.5°C.
If the current strong warming trend started in the middle of the last century, say in the post-war boom, then at 0.18°C per decade that results in warming over that period of nearly 1.2°C. The 1950s were also presumably warmer than the 1850s as CO2 levels had risen by some 30 ppm (parts per million) over that period, which argues for the current level of warming to be something more than 1.2°C.
Another way of looking at this is to use the climate sensitivity relationship between cumulative carbon and peak warming, which was estimated at 2°C per trillion tonnes of carbon by Myles Allen and his team in their formative paper published in Nature in 2009. A look at the associated Oxford University website will show cumulative carbon now stands at over 600 billion tonnes, which implies associated warming of about 1.2°C.
Yet another way is to seek an answer from a group of climate scientists and I had the opportunity to do just that earlier this week. I am in Boston for the 39th Forum of the MIT Joint Program on the Science and Policy of Global Change. I posed the question and one respondent (Chatham House Rule applies) argued that current warming is around 1.1°C since pre-industrial times, but that there is more to the story than this. The climate system is not at equilibrium, with the oceans still lagging in terms of heat uptake. Therefore, if the current level of carbon dioxide in the atmosphere was maintained at some 400 ppm, the surface temperature would rise by another few tenths of a degree before the system reached an equilibrium plateau. That would take us perilously close, if not over, the 1.5°C goal of the Paris Agreement. This implies that 1.5°C is only possible if we see a fall in atmospheric carbon dioxide, back below 400 ppm; but noting that it is currently rising at 2-3 ppm per annum.
This isn’t to say there are no routes forward to a 1.5°C outcome, with the Joint Program itself publishing one such pathway back in 2012.
MIT analysed four pathways that result in different temperature outcomes, including 1.5°C. These are shown in the chart above against a business as usual trajectory based on the 2010 post-Copenhagen national pledges.
- An immediate drop to net zero by 2015, starting in 2010 (Natural only after 2015).
- A very rapid drop to net zero by 2035, but with growth from 2010 to 2030 (Natural only after 2035).
- A more extended drop to net zero by 2060, with the decline commencing in 2010 (Alternative).
- The IEA 450 scenario, with emissions peaking around 2020 and reaching net zero by 2070 (IEA 450).
Pathway 3 (Alternative) results in peak warming of just over 2°C, but with a return to 1.5°C by the end of the century. Of the three MIT extreme mitigation scenarios, it also represents an outcome that could at least be envisaged, albeit still very challenging to implement.
The ocean also plays an important role here, but in a different way to that described above. Atmospheric CO2 begins to decline once net zero anthropogenic emissions is reached as the ocean continues to take up significant quantities of CO2 from the atmosphere, but with nothing additional being added from human activities. This is because the ocean is also lagging in terms of its ability to dissolve CO2. After some 20-30 years, as the ocean’s upper layer comes into balance with the atmosphere, uptake of CO2 slows. The fall in atmospheric CO2 that results also brings down the global surface temperature by about 0.5°C.
However, this scenario required a very sharp decline in emissions from 2010. Current Paris NDC plans show emissions continuing to rise through to 2030 at which point there are good signs of a plateau but by which time atmospheric CO2 may be at 430-440 ppm. The conclusion from all the above; any pathway that eventually delivers 1.5°C is likely to require a fall in atmospheric carbon dioxide back to 400 ppm or even below.