Human-Induced Warming (HIW)

Fig. 1. The relationship between the concentration of CO₂ in the atmosphere and increases in global mean temperature. Data sources and analysis can be found here.

Fig.1 summarises a simple analysis we have just published here which applies linear regression to global CO₂ and temperature data to provide a new and perhaps improved definition of the pre-industrial baseline we measure the global warming against. We think it is an improvement on the 1850-1900 baseline currently being used because that baseline derives from the most uncertain temperature data and likely has warming baked into it given the Industrial Revolution was well under way then. Our pre-industrial baseline is for pre-1700 and derives from the ice core CO₂ data which stretch back well before the Industrial Revolution. We use this same relationship to estimate the size of the warming humans are responsible for, or the 'Human-Induced Warming'.

The data we use for the regression shown in Fig. 1 are annual averages of both the global temperature anomaly and atmospheric CO₂ concentration. However, these same data are also issued monthly and so we can plug these monthly updates into the regression to estimate the HIW right up to the lastest month we have the data for. I've chosen to keep the analysis operating on the annual averages, so I use the monthly temperature anomaly data for 2024 from here to produce an updated annual temperature change estimate for 2024. The more months I have the better that annual estimate. For CO₂, I simply use the monthly trend estimate issued here. My code is here, and apologies for not having tidied it yet.

Fig. 2. Estimated 2024 HIW as of October 2024

In Fig. 2 I am comparing the estimate for the 2024 HIW made using the pre-1700 baseline with that made using the more traditional 1850-1900 temperature anomaly baseline. Here I am being a little kind to the 1850-1900 baselined estimate because I am assuming its baseline uncertainty is the same as it is for the pre-1700 estimate when in reality it is somewhat bigger because the early temperature anomaly data are pretty uncertain (see Fig. 1). I make it that using the 1850-1900 baselined HIW there is a ~10 % chance the 1.5 °C guardrail has been crossed. Using the pre-1700 baseline this rises to 74 %.

Fig. 3. How the 'observed' CO₂-temperature sensitivity has changed in the last 20 years

Finally, Fig. 3 shows a simple test to see if the slope of the line in Fig. 1 has changed in the last 20 years. Here I estimate this slope using all the CO₂ and temperature data up to 2005, and then estimate this slope using the observed temperature anomaly and CO₂ data from 2006 - 2024. Details of this can again be found here. It appears we are still in a linear regime - for now.

Andrew Jarvis, November 2024