Climate Change
Our Diminishing Carbon Budget

Our Diminishing Carbon Budget

By: Chandu Visweswariah


With each passing year of under-achievement in reducing greenhouse gas (GHG) emissions, our planet’s carbon budget is dwindling. To avoid truly catastrophic and irreversible damage from climate change, this blog argues that the world now has to achieve net zero by 2034, if not sooner.

What is a Carbon Budget?

A carbon budget is an amount of GHGs expressed in gigatons (or billions of tons) of CO2e (carbon dioxide equivalent). It represents the GHG budget we have to stay under (from the start date of the budget until eternity) in order to stave off the worst impacts of climate change. Global emissions are accumulated year-after-year until “net zero” is achieved – this total has to be within the budget.

In this blog, the budgets we discuss will be those that are required to keep global warming under 1.5oC with 50% probability, a standard way in which these carbon budgets are calculated.

The Old Budget and the Old Plan

In 2017, the Intergovernmental Panel on Climate Change (IPCC) issued a special report on global warming of 1.5oC. They spent a great deal of effort calculating our carbon budget and the consensus budget was 580 gigatons of CO2e starting January 1, 2018. Referring to the lead graphic, the first orange bar tells us that the actual global emissions in 2017 totaled up to 52.2 gigatons. The blue line represents the “Old Plan” whereby there would be steady GHG reductions year over year until emissions were cut in half by 2030 and net zero was achieved in 2040. This plan became part of CURE100’s mission statement.

The blue line represents the Old Plan, and if we add up the emissions in each year starting in 2018 until net zero is achieved as per this plan by 2040, that total would be exactly 580 gigatons.

The New Budget and the New Plan

Unfortunately, we have not lived up to the Old Plan. In the years 2018, 2019, 2020, 2021 and 2022, our actual global emissions (orange bars) were higher than the Old Plan (blue line). Except for a COVID-caused reduction in 2020, emissions have continued to rise every single year. In other words, we have been eating up our carbon budget faster than desired.

The IPCC recently came out with a new report (see page 114), publishing a new budget for the same goal of restricting global warming to 1.5oC with 50% probability. This new budget gave us leeway to produce 500 gigatons of emissions starting January 1, 2020, until we achieve net zero.

The two plans are entirely self-consistent. From the old budget of 580 gigatons, if we subtract actual 2018 and 2019 emissions, we get a new budget of 473.4 gigatons, and if anything, the new budget of 500 gigatons is a tad more generous.

So Where Do We Go from Here?

Our global carbon budget is diminishing rapidly. From the new budget of 500 gigatons, if we subtract actual emissions (orange bars) for 2020, 2021 and 2022, we see that we have only 333.8 gigatons left starting January 1, 2023, which amounts to less than 6 years at the emissions rate of 2022. To live within this diminished budget, we have to reduce emissions at a faster rate than the previous plan, as shown in the red line. This is like going on a shopping spree in the first week of the month, thus requiring extraordinary belt-tightening in the remaining three weeks to stay within a monthly budget. In other words, to avoid the worst impacts of climate change, we need to achieve net zero not by 2040 (blue line), but by 2034 (red line).

In fact, the longer we wait, the more rapid (and therefore impossible) will be the required pace of decarbonization, as shown in the dotted grey lines in which we stay at 59 gigatons for 1, 2, 3, 4 and 5 years before starting the reduction. Each year that we delay the start of rapid decarbonization (grey lines), the time to achieve net zero shortens by two years.

While the numerical analysis yields 2034, the news is a bit worse than that for a few reasons:

  • Scientists have been appalled at the recent rate of warming, particularly in the oceans, and they are in the process of updating their mathematical models.
  • The models do not take into account certain environmental tipping points, so it would be much safer for us to over-achieve on the carbon budget than the other way.
  • The methane component of greenhouse gases has been underestimated thus far, both in quantity and harm. Due to satellite technology, we will soon have more accurate accounting, which is expected to reduce our budget further.
  • Not every part of the world will achieve net zero at the same time, so it is incumbent on wealthier countries (who are more responsible for historic emissions) to achieve net zero earlier than the global deadline.

What is a Good Net Zero Target? Is it Doable?

The simple answer is 2034, if not sooner. Not 2050 (the U.S. goal), not 2060 (China’s) and certainly not 2070 (India’s). The good news is that we have the necessary technologies like solar panels, wind turbines, batteries, electric vehicles, heat pumps and industrial electric furnaces to achieve this goal. The bad news is that we simply have not had the will to do so. The lack of urgent action by ourselves as individuals and by our politicians, courts, religious organizations, educational institutions, scientific communities and companies (particularly those in the fossil fuel industry) constitute a colossal moral failure. More on that in my next blog, in the form of an open letter to the human race.

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