Focusing only on carbon dioxide (CO2) emissions underestimates how much faster the planet is warming. And focusing only on reducing CO2 emissions misses how much warming already is baked into the system, as well as how much extra cooling we’d enjoy if all greenhouse gas emissions stop.
While CO2 gets a lot of attention as a greenhouse gas (GHG) contributing to rising global temperatures, it’s among one of several other key GHGs that collectively trap more and more heat in Earth’s atmosphere. Methane, nitrogen oxide, and aerosols like sulfur and soot represent about one-third of all emissions. This new research is the first to calculate how global temperatures, and the remaining headroom for avoiding 1.5 and 2.0°C temperature thresholds, would be affected by stopping all GHG emissions, as opposed to stopping only C02 emissions.
I discussed these findings with my esteemed colleague and lead author of the study, University of Washington School of Oceanography doctoral student Michelle Dvorak, and its implications for climate policy.
This conversation has been edited for clarity and length.
George: For years and years, the focus has been on CO2 — that it’s a giant blanket making the planet hotter and we all need to reduce our carbon footprint, etc. The EPA’s most recent estimate is that CO2 is about 76 percent of the greenhouse gases globally, so why was it necessary for your team to include other greenhouse gases in your calculations?
Michelle: We didn’t only include other greenhouse gases, but what are called aerosol precursors as well, and our motivation for including that in our analysis was because aerosols are known to have a cooling effect on the climate. But there’s quite a lot of uncertainty as to how strong they are and how much global warming they have masked in the industrial period — with the potential that they’ve actually masked a significant amount of warming. And that leads to the question of how might the climate respond if we were to remove all of those aerosols and their cooling effect.
George: And what meets the definition of aerosols for your research?
Michelle: Aerosols are sulfate particulate matter created through the combustion of fossil fuels, primarily burning coal. So we consider interactions that aerosols have in the atmosphere. That is, through their effect on creating clouds and also their radiative effects. Both of them acting to cool the atmosphere.
George: And these particulates would include the shipping industry’s emissions and burning coal and also when we drive around in our gas-powered vehicles — we’re also contributing aerosols.
Michelle: Yeah, probably more so from the diesel.
Understanding ‘radiative forcing’
George: And can you elaborate on that aspect of your research that there’s already some additional warming kind of baked into the system that the aerosols are masking?
Michelle: The IPCC (Intergovernmental Panel on Climate Change) has done a very comprehensive study of the current assessment of climate change: the Sixth Assessment Report. And in that they endeavored to estimate radiative forcing from all these different anthropogenic sources, among them being aerosols. And their best estimate through satellite imagery and through modeling is that aerosols may have cooled the climate by up to 0.8°C.
George: Can you clarify what radiative forcing is?
Michelle: Forcing would be how much heat per square meter is either coming or going. So it could be “cooling forcing” or it could be “warming forcing.”
George: So for clouds, their radiative forcing can be ether cooling or warming because some types of clouds trap heat and other types have a cooling effect by reflecting the sun’s energy (heat) back into space.
Michelle: Exactly, and aerosols in particular act to create more reflective clouds, which are in turn a negative forcing on the climate.
George: And so do your findings mean that that most recent IPCC report already is outdated?
Michelle: No, we have used those findings to inform our research because that’s considered the absolute best available science.
Climate models are improving
George: The Washington Post reported last year that countries could be under-reporting their emissions by as much as 16- to 23 percent. They based their estimate using independent sources and found discrepancies of how apparently there’s no standard for reporting greenhouse gas emissions, which led to their findings that emissions could be under reported. And if that’s accurate how does that affect your findings?
Michelle: I don’t think it will affect it because we use modeling outputs to gather our emissions data, rather than actual reported emissions. These are the same economic modeling outputs that the IPCC uses to make their global warming projections.
George: And how can we trust the modeling you use that makes that accurate?
Michelle: A few things. First, our simple model is constrained by the best available science for radiative forcing of these aerosols and greenhouse gases, which in turn has been narrowing over the years due to advancements in satellite instrumentation and modeling.
George: And the modeling is improving with satellite imagery and how else?
Michelle: Complex global climate models now have better representations of clouds, carbon cycling and other climate dynamics, which improve their fidelity. Second, our model is constrained to reproduce observed warming, and that’s a big constraint on past and future temperatures. There is an instrumental record of surface temperatures since about 1850, and we constrain our model to closely match that historical record of warming.
The thick black line reflects how Dvorak et al’s climate model closely matches the historical record of global heating since the industrial revolution. The different colored SSPs represent future heating scenarios for the various Share Socioeconomic Pathways used by the IPCC, which vary based on future GHG emissions. The light blue SSP1-1.9 is a best-case scenario of eliminating all GHG emissions today, and the maroon SSP5-8.5 is a worst-case scenario of “business as usual” GHG emissions. (Source: Dvorak et al, 2022)
Michelle: Lastly, we constrain the model to match observed trends in ocean heat content.
Michelle: There’s a lot of ocean-heat data thanks to the ARGO floats that provide a globally comprehensive estimate of these ocean temperatures, in the surface ocean and the deep ocean. So we are able to use that to estimate how much energy the oceans have taken up over the industrial period, and we constrain our model to produce that same estimate of heat content.
George: Do we know how much more heat the ocean can absorb?
Michelle: Well, the oceans have already absorbed over 90 percent of the excess heat due to global warming, but its the ecological effects that we start to worry about if we continue to warm the climate. I mean the water can warm as much as — it can become a bathtub really if we have a real greenhouse planet. But the ecological effects would be profound long before that ever happened.
George: Like for coral bleaching.
Michelle: Exactly, yeah, we’d have a very acidic ocean that was too warm to sustain any kind of (desirable) life.
George: So methane is getting more attention lately and I’ve seen estimates that, regarding the greenhouse gases, that methane is somewhere between 30-percent and 80-percent stronger than carbon dioxide. Can you help narrow that range?
Michelle: My understanding is that methane is about 30 times as potent a GHG as CO2, but its potency relative to CO2 decreases when we consider timescales longer than approximately 20 years because methane decays much faster than CO2.
Rethinking how we interpret global emissions
George: How would you define the global carbon budget?
Michelle: I would define it as the amount of allowable CO2 emissions or the amount of CO2 that can be emitted while remaining below a certain temperature threshold. And what we really wanted to do with our research was to figure out how that estimate would be revised up or down if you included non-carbon emissions, because we knew that those had a non-negligible effect on climate.
George: And how does your research impact that estimate?
Michelle: The 1.5 degree temperature assessment published by the IPCC in 2018 estimated 500 gigatons of carbon dioxide remaining, and then they had a plus or minus 200 gigatons associated with non-CO2 emissions. We wanted to make a more precise estimate. We knew aerosol forcing to be quite strong, and that would almost certainly be a downward revision of that estimate. So it wasn’t a plus or minus, it was just minus.
George: And since 2018, the carbon budget is down to about 400 gigatons now – that much left to burn before we reach the 1.5 degree C threshold.
Michelle: Yes, but if we account for all of our emissions, assuming you’re combining these aerosols and non-CO2 gases with CO2, then you need to revise that carbon budget downward.
George: So if all greenhouse gas emissions continue, we’ll use up our carbon budget sooner than the IPCC estimates, which were based solely on CO2 emissions. So how much sooner?
Michelle: We have a 66-percent confidence estimate was that we would surpass one-and-a-half degrees Celsius of post-industrial warming by 2029.
George: Why a 66-percent confidence level?
Michelle: The IPCC does use this 66 percent to reflect the likelihood a target is reached, so we wanted to stick as closely to their terminology as possible. Also, with 50-percent confidence something is as likely as not to happen, so we wanted to provide a higher level of confidence for our findings.
George: Okay, so if all emissions stopped today, then we’re due for another 0.2 degrees Celsius increase, is that right? The additional warming already is baked into the system?
Michelle: Yes, and that temperature overshoot would last about two decades.
George: So I guess what would be your message to policymakers on how to prioritize reducing greenhouse gas emissions, where should policy focus in the immediate future?
Michelle: Deep cuts In carbon emissions need to begin immediately.
More warming, and then more cooling
George: I’m looking at the figure where you’ve got the cessation of all emissions, and the warming continues another 0.2 degrees Celsius, and then it goes down and actually falls below the level compared to the cessation of CO2 emissions only.
Michelle: Right.
George: And so that’s a best-case scenario, even given the short term hit of that bump in temperature.
Michelle: Right, if we care more about long-term warming, then, yes, it is better to just remove all the non-CO2 forcing as well, because you are getting rid of the warming associated with shorter-lived GHGs, such as methane and nitrous oxide.
George: Is there anything else about this research that you want to emphasize that we haven’t discussed yet?
Michelle: There is a prevalence of thought outside of the scientific community that it’s already too late for change. And I just can’t really wrap my head around that at all, because now we’re finally seeing the effects of a 1.2 degree warmer world and it’s disturbing people.
George: Even Congress finally took some action, albeit a small step forward.
It’s not too late to reverse global warming
Michelle: Our projections across the board show median warming above two degrees Celsius and some scenarios reaching 4 or 5 degrees of warming by the end of the century. So to me that says we need to change right now. We know how bad it is now at above one degree, then surely, it’ll be much, much worse with more warming. So if we do stop it now, then we could avoid much worse impacts. So I’m not sure I understand that psychology of thinking that it’s already too late, when it’s really like we’re only one-quarter or one-fifth of the way to how bad it can get if emissions continue.
George: That’s a great closing optimistic message – that it’s not too late.
Michelle: Yeah, one optimistic message we had that you pointed out earlier, was that we do cool below present-day temperatures by the end of the century if we remove all of those non-CO2 forcings. So that’s good. I mean it does suggest that there’s some amount of reversibility to the warming.
George: And we all could use good news these days. Thanks again for your time today.
