In my last post on “coming to terms” with climate commitments, I focused on the terms and the meaning of various climate commitments. It is important to understand what they mean but it is equally important to understand the magnitude of the commitments. To make it easy, let’s look at California’s target to achieve carbon neutrality as soon as possible but no later than 2045, and then to achieve net negative emissions thereafter. I will focus only on the carbon neutrality part here. California’s Air Resources Board (CARB) defines carbon neutral as sources equaling sinks. And, even though the target specifies carbon, it covers all greenhouse gas (GHG) emissions. So, once we have done everything we can to reduce or avoid GHGs, which is no small feat, we need to remove a quantity of emissions from the atmosphere equal to the GHGs that still remain. I will not be discussing the actual process of or options for removing emissions — that is a topic for the future — but rather the amount of the remaining emissions in California that will have to be canceled out by removals. This post provides estimates for remaining emissions at the state level and for the San Diego region.

The Scale of Remaining Emissions in California

Remaining GHG emissions — also known as residual emissions — are those that are hard to reduce or avoid and would need to be canceled out through carbon dioxide removals to achieve carbon neutrality. So, what level of emissions will remain in California as we approach our target years? Well, the short answer is that it depends on many factors, including how successful we are at reducing and avoiding emissions and what year we hope to achieve carbon neutrality. Nonetheless, here are some estimates for consideration. If we interpolate between California’s statewide statutory emissions targets for 2030 in SB 32 (40% below 1990 levels) and those for 2050 in Executive Order S-3-05 (80% below 1990 levels), we would need to cancel out about 130 million metric tons of carbon dioxide equivalent (MMT CO₂e) in 2045. If we want to reach carbon neutrality earlier, as Governor Newsom has indicated, that number would be about 170 MMT CO₂e in 2040 or about 215 MMT CO₂e in 2035 (Figure 1). And, we would have to continue to remove from the atmosphere that (or some) amount of emissions annually into the future until we can eliminate all GHG emissions or atmospheric concentrations reach acceptable levels. If we do not meet statewide emissions targets, we would have to remove more. Also, to meet the second part of California’s target, net negative emissions, we would have to remove even more.

Figure 1 Estimated Remaining GHG Emissions in California

There are other estimates, similarly based on achieving some specified level of emissions in future years. The Achieving Carbon Neutrality in California report by E3  presents a range of estimates for remaining emissions based on three scenarios used in its PATHWAYS model to demonstrate how California could achieve at least an 80% reduction in greenhouse gases from 1990 levels by 2045. In the scenario with zero carbon energy, about 33 MMT CO₂e would remain, while the scenario with high carbon dioxide removal, 80 MMT CO₂e would remain. And somewhere in the middle, the balanced scenario would leave 56 MMT CO₂e to be removed by 2045.  

Lawrence Livermore National Laboratory’s Getting to Neutral report also used statewide targets to estimate remaining emissions at 125-150 MMTCO₂e. The report states that “[t]he system total quantity of CO₂ removal, 125 million tons per year, has been chosen to roughly match the expected residual emissions in California in 2045…” Figure 2 compares these two estimates.

Figure 2 Estimate of Remaining Emissions in 2045

Using the high end of these ranges to be conservative, California would have to remove between 80 – 150 MMT CO₂e in 2045 to reach its target. Based on the California Air Resources Board’s 2019 statewide GHG inventory, this is like removing an amount of carbon dioxide roughly equal to the entire industrial category in on the low end to just less than the entire transportation category on the high end (Figure 3).

Figure 3 California Air Resources Board Statewide GHG Inventory (2019)

The high end of the range for remaining emissions in the E3 Achieving Carbon Neutrality in CA report is based numerous assumptions, including the following:

• 100% sales of electric appliances by 2040
• 100% battery electric vehicle sales for light-duty in 2035
• 100% battery electric vehicle sales for medium-duty vehicles by 2040
• 45% battery electric vehicle sales and 48% of compressed natural gas sales for heavy-duty vehicles by 2035
• 95% zero carbon electricity generation

So, if we achieve these levels of activity, among others, we would have to remove about 80 MMT CO2e in 2045. If we can do more to reduce emissions, the amount to be removed would be lower.

Focusing on the San Diego Region

A similar approach can be applied to estimate remaining emissions in the San Diego region. To clarify there are no specific targets for regions, but as an illustrative example, I simply applied the statewide targets to regional emissions to estimate the amount of emissions that would have to be canceled out to achieve carbon neutrality at the regional level. Estimated target emissions levels are based on the most recent regional GHG inventory for data year 2016. Conveniently, at the state level, the 2016 inventory was approximately equal to the emissions target for 2020 (1990 levels). So, using the 2016 value to estimate target emission levels is reasonable and consistent with methods summarized in the Regional Climate Action Planning Framework (Technical Appendix I).

Based on a linear interpolation between 2030 and 2050 targets, remaining emissions would be about 7 MMT CO₂e in 2045. Following the results of the statewide estimates, it is reasonable to assume a range of 5-10 MMT CO₂e for the San Diego region. To put this into perspective, 2016 regional emissions from electricity were about 5 MMT CO₂e, while on-road transportation (i.e., cars and trucks) represented about 10 MMT CO₂e. This means that if we can reduce emissions enough to be on track to meet the 2050 targets, the San Diego region would have to remove from the atmosphere an amount of CO₂ equivalent to the emissions from electricity or on-road transportation in 2016. And, as noted above, if we don’t reduce emissions enough or want to achieve net negative emissions, we would have to remove even more.

Figure 4 Estimated GHG Emissions for the San Diego Region

Removing Emissions

While the amount of emissions we will need to remove to reach carbon neutrality may be daunting, the next step is to explore how and whether we could actually do it. This will be the topic of a future post.