Mauna Loa carbon dioxide forecast for 2023

Table 2. Decadal average CO₂ rises in the Mauna Loa record

Past a symbolic threshold: 50% above pre-industrial

Despite the smaller rise expected from 2022 to 2023, CO₂ concentrations are still increasing and will once again reach the highest values on record. For every month of 2023, atmospheric CO₂ will be at levels more than 50% higher than when humanity began large-scale burning of fossil fuels in the industrial revolution in the late 18th Century. Ice core records show that the average CO₂ concentration for 1750 -1800 was 278 ppm.  2022 was the first year with the annual average CO₂ at more than 50% above this (417 ppm), although in some months the level was below this. 2023 will be the first year with monthly CO₂ consistently above this level.

This 50% increase above pre-industrial is of symbolic importance because it represents half-way towards "doubled CO₂", a standard level of climate forcing used for quantifying the response of global temperatures to human-caused increases in greenhouse gases. Although "halfway to doubled CO₂" is not of any physical significance, it can nevertheless be considered a milestone that highlights how much humans have already altered the composition of the global atmosphere and increased the amount of a gas that warms the global climate. 

Contributions of anthropogenic emissions and varying natural carbon sinks to the CO₂ rise

Long-term increases in observed CO₂ are entirely the result of human-caused emissions of carbon dioxide into the atmosphere – more than enough CO₂ has been emitted by fossil fuel burning, cement production and deforestation to account for the increase measured in the atmosphere. Although CO₂ concentrations have now increased by just over 50% since the industrial revolution, this increase would have been almost twice as large if some CO₂ had not been removed from the atmosphere through being absorbed by plants and the oceans. 

These natural sinks of carbon vary in strength from year to year as a result of short-term fluctuations in climate, principally through El Niño Southern Oscillation (ENSO) cycles in the Tropical Pacific Ocean. The annual CO₂ rise is faster during El Niño conditions and slower during La Niña conditions. The Met Office CO₂ forecast therefore takes account of both anthropogenic emissions and the impacts of climate variability on natural carbon sinks. So far, including the effect of ENSO variability has in all years brought the central value of the CO₂ rise forecast closer to the observations than when considering anthropogenic emissions alone (Figure 5).

Figure 5. Observed (black) and forecast (dark blue) annual average CO₂ concentrations for 2016 to 2022, and re-forecast values based on emissions alone, without the effects of ENSO (light blue). Thin black lines show the forecast uncertainty range (2 standard deviations).

Impact of La Niña on the forecast CO₂ rise in 2023

We can estimate the potential contribution of La Niña conditions to the CO₂ rise forecast for 2023 by repeating our forecast calculation without the sea surface temperature change, ie: with "neutral" conditions. This suggests that without a La Niña response in the atmosphere and tropical land ecosystems, the forecast annual mean CO₂ rise from 2022 to 2023 would be 2.32 ppm, a faster rise than the 1.97 ppm forecast when accounting for La Niña (Table 3). 

Similary, the annual CO₂ rises 2020/21 and 2021/22 would have also been higher if La Niña conditions had not been present (Table 3). Although the observed annual CO₂ rise appears to have declined over the last 3 years, the rise calculated without the effects of La Niña shows an increase over that time, consistent with the emissions. This shows that for short periods, the observed rate of build-up of CO₂ in the atmosphere does not necessarily reflect the changes in emissions – the effects of climate variability on the short-term rise can dominate. However, in the longer term, the ongoing increase in the annual rate of CO₂ rise is quite evident despite large interannual variability (Figure 6). When the effects of ENSO are removed, the calculated CO₂ rise shows an ongoing increase.

Table 3. Annual emissions and the resulting mean CO₂ increment for 2021-2022 calculated with and without La Niña sea surface temperatures (SSTs). The increment is shown both in parts per million (ppm) and gigatonnes of carbon (GtC) to allow comparison with the emissions.

Figure 6: Annual increments (growth rate) in CO₂ concentration at the Mauna Loa observatory from observations (black) and the 2023 forecast (red star), and the estimated increments without the influence of ENSO (green). The forecast uncertainty range (red line) is ± 2 standard deviations.  Observations are from the Scripps Institution of Oceanography, UC San Diego.

Comparison with CO₂ trajectory consistent with limiting global warming to 1.5°C

The Paris Agreement commits nations to pursuing efforts to limit the rise in global temperatures to 1.5°C above pre-industrial levels, and model calculations suggest that achieving this would require the rise in atmospheric CO₂ to slow rapidly within the next few years and cease completely within the next two decades. Figure 7 shows one example of such a scenario, the IPCC's "C1-IMP-LD" scenario which features low demand for energy - other trajectories could also be followed, but all requires the rise in CO₂ to rapidly reach zero. This scenario can provide a benchmark against which the observed atmospheric CO₂ rise can be compared as part of assessing progress towards the Paris Agreement goal. 

Although this year's slower rise (red star) is within the 1.5°C scenario (grey plume), this is only because of the enhanced carbon sinks due to La Niña conditions which will be temporary. When the effects of La Niña are excluded (green star) the anthropogenic emissions would be causing the CO₂ rise in 2023 to be above the rate required to track the 1.5°C scenario. 

If atmospheric CO₂ concentrations are to follow the C1-IMP-LD scenario limiting global warming to 1.5°C, the average CO₂ rise over the 2020s would need to be more than 30% lower than the annual rise forecast here for 2023 (Table 4). This would only be achievable by immediate and substantial cuts in global CO₂ emissions. 

Figure 7. Comparison of recent and forecast annual CO₂ increments with scenario limiting global warming to 1.5°C. Black: Annual increments in CO₂ concentration at the Mauna Loa observatory from observations. Red star: the 2023 forecast increment. Green: Estimated increments without the influence of ENSO. Grey plume: simulated CO₂ concentrations in scenario limiting global warming to 1.5°C.

Table 4. Decadal average CO₂ rises in the C1-IMP-LD scenario for limiting global warming to 1.5° with little or no overshoot (IPCC 2022).

Verification of previous CO₂ forecasts

The technique used to make this forecast was also used to make forecasts ahead of time for 2016, 2017, 2018, 2019, 2020, 2021 and 2022.  In 2020 we also issued an updated forecast once it became clear that the response to the Covid-19 pandemic would cause global CO₂ emissions to be much smaller than expected that year. 

Our usual methodology uses a statistical relationship between the annual CO₂ rise, human-caused emissions and changes in sea surface temperature (SST) in the equatorial Pacific Ocean as a measure of the strength of the dominant pattern of natural climate variability that is known to affect the strength of carbon sinks.  We use an average of SSTs from ocean observations in recent months and seasonal forecasts for the coming months. 

From 2016 to 2019, our forecast calculations used the annual emissions from the previous year as published in the Global Carbon Budget, as normally the ongoing trend in emissions is not large enough to affect the forecast substantially. Our original forecast for 2020 also made this assumption, while our revised 2020 forecast included an adjustment based on projected emissions profiles across the year applied to an atmospheric transport model. Our forecast for 2021 assumed that global emissions had returned to approximately 2019 levels, having already returned to near those levels at the end of 2020. Our forecast for 2022 used the fossil fuel emissions for 2019 and land use emissions from 2020. Our forecast for 2023 has returned to the usual method of using Global Carbon Budget emissions from the previous year, ie: 2022.

Table 5 compares the 2022 forecasts of annual average CO₂ rise and annual CO₂ concentrations with measurements at Mauna Loa, along with forecasts and observations from previous years. As with previous years, the 2021 forecast agreed with observations within the forecast uncertainty (2 standard deviations).

Table 5. Summary of forecast and observed annual CO₂ concentrations and rises for 2016 to 2022. Note that observations are not available from 28^th November 2022 onwards due to the eruption of the Mauna Loa volcano cutting off power supplies to the observatory, so for 2022 the comparison of forecast and observed mean concentration is given for January-November, and the forecast and observed increases are calculated relative to the January-November mean for 2021. For 2020, both the original forecast and the updated forecast accounting for the Covid-related emissions reductions are shown.

Figure 8 and Table 6 compare the 2022 forecast with observations for each month. In our forecast we had assigned the same uncertainty to monthly and annual values, but this ignores the role of within-year impacts such as periods of fire activity in areas not typically affected by ENSO, or anomalous wind directions at the measurement site at Mauna Loa. Quantification of these additional sources of uncertainty at monthly levels are a topic of ongoing research and our stated uncertainty on the monthly averages will therefore be an underestimate.

Figure 8. Comparison of Met Office forecasts of monthly CO₂ concentrations in 2022 at Mauna Loa (dashed blue line) with measurements for 2022 (solid black line) from the Scripps Institution for Oceanography UC San Diego. Measurements are only available until 28 Nov. The vertical blue lines show the forecast uncertainty range (2 standard deviations).

Table 6. Forecast and observed monthly average CO₂ concentrations at Mauna Loa over 2022. The uncertainty in the forecast values was ±0.5 ppm. Observed monthly averages for January to October are those published by the Scripps Institution for Oceanography at UC San Diego. The observed monthly value for November is the mean of daily values up to 27^th November posted on social media by the Scripps Institution for Oceanography. Observations at Mauna Loa are not available from 28^th November onwards.

Note: definitions of annual CO₂ rise, increment and growth rate 

We define the annual CO₂ rise or annual increment for a particular year as the difference in annual average concentration for that calendar year and that of the previous calendar year.  This is different to the definition of annual 'growth rate' as published by NOAA  which is the average change across an individual calendar year.  

Published 26th January 2022