Saturday, December 11, 2021

SCIENTIFIC STUDY: Global CO2 Emissions Have Been Flat for a Decade Despite Political Rhetoric

Source: Carbon Brief

Published: November 4, 2021

By: Zeke Hausfather
Global carbon dioxide (CO2) emissions from fossil fuels and cement have rebounded by 4.9% this year, new estimates suggest, following a Covid-related dip of 5.4% in 2020.

The Global Carbon Project (GCP) projects that fossil emissions in 2021 will reach 36.4bn tonnes of CO2 (GtCO2), only 0.8% below their pre-pandemic high of 36.7GtCO2 in 2019.

The researchers say they “were expecting some sort of rebound in 2021” as the global economy bounced back from Covid-19, but that it was “bigger than expected”.

While fossil emissions are expected to return to near-record levels, the study also reassesses historical emissions from land-use change, revealing that global CO2 output overall may have been effectively flat over the past decade.

The 2021 GCP almost halves the estimate of net emissions from land-use change over the past two years – and by an average of 25% over the past decade.

These changes come from an update to underlying land-use datasets that lower estimates of cropland expansion, particularly in tropical regions. Emissions from land-use change in the new GCP dataset have been decreasing by around 4% per year over the past decade, compared to an increase of 1.8% per year in the prior version. 

However, the GCP authors caution that uncertainties in land-use change emissions remain large and “this trend remains to be confirmed”.

The GCP study, which is not yet peer-reviewed, is the 16th annual “global carbon budget”. The budget also reveals:

  • China and India both surpassed their 2019 emission peaks in 2021. Chinese emissions grew by 5.5% between 2019 and 2021, while Indian emissions grew by 4.4%. 
  • Chinese coal use was a particularly large driver of the global rebound in emissions, with the power and industry sectors in China the main contributors. 
  • Coal, oil and gas all fell during the pandemic, but both coal and gas emissions have already surpassed their pre-pandemic levels, with a 2% increase in gas emissions and a 1% increase in coal emissions between 2019 and 2021. 
  • Oil emissions remain around 6% below 2019 levels and this persistent reduction is one of the main reasons 2021 emissions did not set a new record.

The new updates to global CO2 emissions in the GCP substantially revise scientists’ understanding of global emissions trajectories over the past decade. The new data shows that global CO2 emissions have been flat – if not slightly declining – over the past 10 years. 

However, falling land-use emissions have counterbalanced rising fossil CO2 emissions, and there is no guarantee these trends will continue in the future.   

Major changes due to revised land-use emissions

The GCP has always reported on emissions from both fossil CO2 and from land-use change (LUC). Fossil CO2 emissions represent upwards of 90% of current global emissions and understandably tend to get most of the attention. However, the GCP researchers have long pointed out that the largest uncertainties in understanding of CO2 emissions comes from LUC, despite its relatively small contribution to the total.

The figure below shows global CO2 emissions from both fossil and LUC. The dashed light blue line shows the prior GCP estimate of global CO2 emissions, while the solid dark blue shows the new estimate. The shaded area represents the combined uncertainty from land use and fossil CO2 emissions in the new GCP estimate.

Annual total global CO2 emissions – from fossil and land-use change – between 2000 and 2021 for both the 2020 and 2021 versions of the Global Carbon Project’s Global Carbon Budget. Shaded area shows the estimated one-sigma uncertainty for the 2021 budget. Data from the Global Carbon Project; chart by Carbon Brief using Highcharts.

Previously, the GCP data showed global CO2 emissions increasing by an average of 1.4 GtCO2 per year between 2011 and 2019 – prior to Covid-related emissions declines. The new revised dataset shows that global CO2 emissions were essentially flat – increasing by only 0.1GtCO2 per year from 2011 and 2019. When 2020 and 2021 are included, the new GCP data actually shows slightly declining global emissions over the past decade, though this should be treated with caution due to the temporary nature of Covid-related declines.

The new GCP dataset also puts historical (1750-2020) cumulative emissions around 19 GtCO2 lower than in the prior 2020 version, roughly equal to half a year of current global emissions. 

This would represent a slight (~4%, or half a year of current emissions) increase in the remaining “carbon budget” of around 460GtCO2 from the start of 2021 (which is 11.5 years of current emissions) to limit warming to 1.5C with a 50% likelihood. 

Historical global fossil emissions are largely unchanged from their prior values in the new GCP data. There are some minor upwards revisions of fossil CO2 emissions of around 0.3 GtCO2 for most years over the past decade – and a somewhat larger upward revision of 0.7 GtCO2 for the year 2020. 

The revision in global CO2 emissions is almost entirely due to revised land-use emissions. The figure below shows land-use emissions in the prior 2020 GCP data (light blue dashed line) and in the new 2021 data (dark blue solid line), as well as reported uncertainties. 

Annual global CO2 emissions from land-use change between 1959 and 2021 for both the 2020 and 2021 versions of the Global Carbon Project’s Global Carbon Budget. Shaded area shows the estimated one-sigma uncertainty for the 2021 budget. Data from the Global Carbon Project; chart by Carbon Brief using Highcharts.

While some modest changes have been made to LUC emissions estimates between 1959 and 2000, much larger changes are evident over the past two decades. In the prior report, LUC emissions were increasing notably between 2000 and 2020, growing by around a third. 

The new dataset almost perfectly reverses this trend, suggesting that LUC emissions have actually declined by around a third since 2000. Over the past decade, LUC emissions went from increasing by 1.8% per year to decreasing by 4% per year in the latest version of the GCP data.

GCP uses the average of three different observational-based land-use change datasets, known as “H&N”, “BLUE” and ”OSCAR”. In the prior year’s GCP report these three approaches showed notable disagreements over the past decade, with H&N showing emissions declines, while BLUE and OSCAR showed emissions increases. However, revisions to both BLUE and OSCAR datasets in the past year have brought them more in-line with recent H&N trends.

All three datasets now show notable declines in emissions over the past decade – though differences remain in the magnitude of estimates between H&N and the other two datasets, as shown in the figure below.

Annual global CO2 emissions from land-use change between 1959 and 2021 for the three bookkeeping methods used by the Global Carbon Project – H&N, BLUE, and OSCAR – for both 2020 and 2021 versions. Data from the Global Carbon Project; chart by Carbon Brief using Highcharts.

The fact that all three different datasets now agree on the declines in emissions is noteworthy. The GCP paper suggests that “there is a decrease in net CO2 emissions from land-use change over the last decade, in contrast to earlier estimates of no clear trend across LUC estimates”.

These revisions are primarily due to changes in underlying land-use data in the History database of the Global Environment (HYDE). HYDE now uses updated estimates of agricultural areas and land cover maps from satellites. This results in lower estimates of cropland expansion, particularly in tropical regions. The updated data also removes spurious interannual variability in forest cover that resulted in increased emissions due to assumed fast decay (e.g. clearing by fire) and slower regrowth.

The authors caution, however, that their new estimates may not fully capture the rise in Brazilian deforestation in the past few years. It also does not include forest degradation – deterioration of forest ecosystems that does not involve a reduction in forested area – that may be contributing to some additional LUC emissions. 

As Prof Julia Pongratz – the director of the Department of Geography at the Ludwig Maximilian University of Munich and a member of the GCP team – explains to Carbon Brief:

“It is too early to infer robust trends. More regional analysis is needed and accurate, high-resolution monitoring of land-use dynamics. Only then can we reduce the uncertainty around land-use emissions and their trends and their contributions to emissions reduction targets.”

China and India lead rapid rise in fossil CO2 emissions

Global fossil CO2 emissions declined rapidly during the height of the Covid-19 pandemic in 2020. While there were hopes that a “green recovery” could help keep emissions down, the world has seen a rapid rebound in fossil CO2 emissions in 2021 as the global economy has recovered. The rebound in global emissions has been led by China and India, who have both already surpassed their previous 2019 record highs.

Speaking at a media press briefing, Dr Glen Peters – research director at the Center for International Climate Research (CICERO) – said the researchers “were expecting some sort of rebound in 2021”, but that it was “bigger than expected”. He added:

“You could say the recovery packages have delivered more in emissions than we were hoping – a little bit too dirty in the recovery packages and not enough low-carbon expenditure.”

Prof Corinne Le Quéré – Royal Society research professor of climate change science at the University of East Anglia – noted that the decrease in emissions in 2020 was not a “structural decrease”. She explained:

“It is the difference between parking your car for a year and changing to an electric car. [The decline in emissions] was not [because of] measures that were put in place to tackle climate change and [so] they were never going to last.”

The figure below shows global CO2 emissions from fossil fuels, divided into emissions from China (red shading), India (yellow), the US (bright blue), EU (dark blue) and the remainder of the world (grey).

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