As we saw in Chapter 6, the amount of carbon dioxide in the air rises and falls on an annual rhythm, highest each year in the month of May, and lowest in September. In May 2023, the monthly mean value was 423.78 ppm. This year it was 426.7 ppm. The rise of 5.9 ppm over those two years was the biggest since the measurement program started in 1958. The annual increase from March 23 to March 24 was the biggest single rise for that metric; and calendar year 2023 at Mauna Loa was a record at 3.37 ppm. For cmparison, the growth rate averaged for the decade 2013-2022 was 2.42 ppm. The diagram showing this below is from a new analysis of the carbon budget, focussed on the reasons for this concerning trend. [Piyu et al, 2024. pre-print copy]

The little squares are the measuremnets at the Scripps station at Mauna Loa; the blue columns are from the combined measurements of stations sampling air from pristine NOAA sites close to sea level. Each quantity is the increase of CO₂ concentration for that year. Horizontal lines show the mean for each decade. 

All this has been happening while the rate of global emissions growth has slowed. You can see this on the chart here, in the top right corner:

These facts raise a question about the capacity of Earth’s carbon ‘sinks’ - the ocean and terrestrial processes that each take up roughly a third of the emitted greenhouse gases from human activity. According to experts at NOAA, the El Nino in 2023 temporarily reduced the land sink - but it was a big effect, and there are reasons for thinking some of it might not be reversible. The new study calculated the total global land sink to be 0.44 billion tonnes of carbon for last year - the lowest since 2003. The year included the huge Canadian fires, and a severe drought in the Amazon. The authors reckoned the Amazon basin switched from net sink to source, contributing 0.31 billion tonnes. Altogether, the contribution from land sources under extreme heat in 2023 was 1.73 billion tonnes of carbon.

So far, Earth’s carbon sinks have soaked up something like two-thirds of the carbon dioxide we’ve released into the air. Without those sinks, the atmospheric concentration would already be over 1,000 ppm. The big reason for concern is this: if a single moderate El Nino can almost abolish the land sink, then how will that sink behave in a world that is never going to be as cool as it used to be? Even now, La Nina years are warmer than El Ninos of 20 years ago. We will be a bit wiser when we see what happens to the graph above in the next few years. If the blue and grey move further apart, then we have yet another urgent reason to stop fossil fuel burning - ALL of it, and fast.

The study can be read [Here]