Mauna Loa carbon dioxide forecast for 2021

Despite being a La Niña year with a relatively small CO₂ increase compared to recent years, the central estimate of the forecast CO₂ rise of 2.29 ppm for 2021 would be the tenth largest annual increment in the Mauna Loa record of 64 years (Figure 3). Considering the uncertainty range of ± 0.55 ppm, it would be between the 4th and 28th largest annual increments. The relatively high ranking in the series is because annual increments have generally been increasing over time as a result of rising anthropogenic emissions.  

Figure 3. The central estimate of the CO₂ rise forecast for 2020-2021 in the context of the frequency distribution of the observed annual rise for each year in the Mauna Loa record. The horizontal red bar shows the forecast uncertainty range of ± 0.55 ppm.

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 and 2020.  We also issued an updated forecast for 2020 once it became clear that the response to the pandemic would cause global CO₂ emissions to be much smaller than expected. 

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 forecasts included the assumption that the emissions in the coming year would be the same as the previous year, as normally the year-by-year changes in emissions are 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.

Table 2 compares the original and updated 2020 forecasts of annual average CO₂ rise, and annual, May and September concentrations with measurements at Mauna Loa, along with forecasts and observations from previous years. Figure 4 and Table 3 compare the original and updated 2020 forecasts with observations for each month.

Table 2. Summary of forecast and observed CO₂ concentrations and rises for 2016, 2017, 2018, 2019 and 2020, and the forecast for 2021. For 2020, both the original forecast and the revised forecast accounting for the Covid-related emissions reductions are shown.   Where observations were within the error estimates of the forecast, this is underlined. Where observations are outside the error estimates, this is shown in italics.

Both the original and updated 2020 forecasts agree with the observed annual mean within the uncertainty of 0.6 ppm, but the central estimate of the updated forecast was closer to the observed value. The updated forecast was also more accurate for monthly values from March to June and October to December. In August and September, the original forecast was more accurate, with the observed value falling outside the published uncertainty range for the updated forecast but within the range for the original.

Unusually, the observed minimum monthly CO₂ concentration in 2020 occurred in October rather than September.  This may have been a result of the large wildfires in California and shifts in the wind direction. In August and early September, air arriving at Mauna Loa is typically dominated by the easterly trade winds, so the winds would have been carrying air from western North America which could have been anomalously high in CO₂. This would have contributed to higher CO₂ concentrations at Mauna Loa than would normally have been expected in the typical seasonal cycle. Although the California wildfires continued through October and November, Mauna Loa is dominated by westerly winds by then, so would not be immediately affected by wildfire emissions in North America.

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. Quantification of these additional sources of uncertainty at monthly levels are a topic of ongoing research.

Figure 4. Comparison of original (dashed red line) and updated (solid red line) forecasts of monthly CO₂ concentrations at Mauna Loa with measurements from the Scripps Institution for Oceanography (black line).

Table 3. Forecast monthly average CO₂ concentrations at Mauna Loa over 2020. The uncertainty in the forecast values was ±0.6 ppm  

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 about 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 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, 2017, 2018, 2019 and 2020, 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).

Consideration of the large El Niño event that developed in 2015 allowed the record rise in atmospheric CO₂ from 2015 to 2016 to be successfully forecast. El Niño conditions were linked with hotter, drier conditions in many parts of the tropics. These temporarily weakened tropical land carbon sinks, causing less CO₂ to be removed from the atmosphere by natural processes and temporarily accelerating the build-up in the atmosphere. The 2016 rise would have been underestimated without including this.  The annual rise in other years was also more accurately predicted when taking into account the effects of ENSO (Figure 5).

Similarly, the 2020 CO₂ rise would have been underestimated in the updated forecast if the effects of the mild El Niño conditions had been neglected. This appears to have largely counteracted the effect of the emissions reductions that occurred in the first part of 2020 as a result of the pandemic.

We can estimate the potential contribution of La Niña conditions to the forecast CO₂ rise 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 2020 to 2021 would be 2.49 ppm (Table 4). 

We can also estimate what the CO₂ rise would be if affected by emissions at 2020 levels.  Using the 2020 total of 10.6 GtC and including the impact of La Niña, the forecast CO₂ rise for 2019-2020 would be 2.10 ppm, 0.19 ppm smaller than the actual forecast (Table 4). From this it is clear that the year-to-year variability in the rate of rise of CO₂ in the atmosphere is affected as much by the strength of carbon sinks as by the unexpectedly large reduction in anthropogenic emissions that occurred in 2020.

Nevertheless, the anthropogenic emissions are still the overall driver of the long-term rise in concentrations.

Table 4. CO₂ rise (ppm) for 2020-2021 calculated with different combinations of El Niño-like sea surface temperature (SST) and emissions influences

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.  

Note added 4th April 2021: The Scripps Institution measurements quoted here are those reported prior to the move to a revised CO₂ scale on 21st February 2010. As a result, the numbers quoted here differ from those now reported on the Scripps Institution Primary Mauna Loa CO₂ Record by approximately 0.08 ppm.