Is global warming accelerating? Not relative to greenhouse gas emissions.
One of the hottest debates in climate science right now is whether global warming is speeding up. Some scientists say yes, others say no—or more precisely that we don’t have enough information yet. In my view this is somewhat of a moot point: The basic facts of human influence on the climate system are incontrovertible at this point, and the rapid reduction of greenhouse gas emissions is the only antidote to warming regardless of its precise speed. But discussions over the rate of recent warming have captured significant media attention, and if we are truly missing something in our understanding of recent climate change, the future might be even more dangerous than we currently understand.
As defined in most recent discussions, the acceleration of global warming refers to an increase in the rate of change of global temperature over time—that is, that global mean temperature (GMT) is rising more quickly year-over-year than it did before. This is a bit of a separate debate than the discussions over what caused the off-the-charts global temperature anomaly in 2023, another phenomenon that we haven’t fully resolved. Here we’re talking about the long-term rate of warming, not individual hot years. One of the obvious candidates for the causes of this acceleration is the “unmasking” of warming due to reductions in aerosol pollution, an explanation favored by the legendary Jim Hansen. But I would argue that it’s still not clear whether we’re witnessing a fundamental shift in climate behavior, or simply random variations around a consistent trend.
In this post I want to raise a point that I haven’t seen foregrounded in some of the debates over accelerating warming, which is simply the role of accelerating greenhouse gas emissions. We know that the level of global warming is directly proportional to cumulative carbon dioxide emissions, an incredibly powerful relationship that allows us to do things like set remaining carbon budgets for global temperature targets (see, for example, Figure 5.31 of the most recent IPCC report). Therefore, we’d expect the rate of change of warming to accelerate if the rate of change of cumulative CO2 emissions changed. The rate of change of cumulative emissions is simply the annual amount of emissions. Have those increased over time? Well…yes.
The plot at the left shows the annual rate of CO2 emissions from fossil fuels and land use change from the Global Carbon Budget project. Clearly, the rate of emissions has gone up—we’re emitting more now than we used to, more than 11 GtC in 2023, despite the small blip during the COVID-19 pandemic.
I should note that I’m not analyzing non-CO2 emissions here, either other greenhouse gas emissions such as methane or other substances such as aerosol precursors. But CO2 has the biggest impact on the climate of all of these gases, and warming is proportional to CO2 specifically.
So, the rate of CO2 emissions has steadily climbed over the last 50 years. What does this mean for the debate over the acceleration of global warming? Personally, I wondered whether the observed acceleration in GMT might simply reflect the response to increasing greenhouse gas emissions, not anything fundamentally different in the climate response to those emissions.
The plot above shows my attempt to understand this issue. On the left, I’m showing adjusted GMT anomalies over 1974-2023, using the GISTEMP v4 dataset. (When I say “adjusted,” I mean that I’ve used a statistical model to remove the influences of El Niño, volcanic eruptions, and solar variations. I used Tamino’s regression-based approach to do this.) We see that global temperature anomalies have steadily increased over the last 50 years—but critically, the rate of change appears to be faster in the more recent period. If we fit a linear trend to the first 25 years of the data (1974-1998) and extrapolate it forward, we’d significantly underestimate the rate of warming in the more recent 25 years (1999-2023). The red dashed line shows the actual trend line over this more recent period, and it’s clearly above the extrapolated trend.
The right panel above, by contrast, shows the same GMT time series as a function of cumulative CO2 emissions, not time. Here we see a different story: The last 25 years have seen a trend that is nearly identical to the 25 years before that. That is, the relationship between CO2 emissions and global temperature is not accelerating. It has maintained its linearity and proportionality over time.
We can be more rigorous than this and run a formal statistical test of the change in trends over time. On the right, I’m showing the results of regression models that calculate the relationship between GMT and time (left panel) and GMT and emissions (right panel). The black dots show the relationships over 1974-1998 and the red dots show the relationships over 1999-2023. For GMT as a function of time, we see a clear jump up in the 1999-2023 period, with a trend that is statistically distinguishable (p = 0.003) from the trend over the previous 25 years. (For the technically minded audience, I should note that adjusting for autocorrelation does not meaningfully change the standard errors or the statistical significance of this difference.) For the emissions-GMT slope, there is no distinction between the 1974-1998 and 1999-2023 values (p = 0.699).
This is an extremely simplified analysis, and I’ve neglected many other important factors, such as aerosol unmasking, as I mentioned previously. But I think there is a rather profound point to understand here: The implication of global warming’s “acceleration” depends critically on whether you account for accelerating greenhouse gas emissions. If warming was increasing faster than the rate of emissions would lead us to expect, it would imply that something has fundamentally changed in the relationship between human activities and our climate. By contrast, this admittedly back-of-the-envelope analysis seems to indicate that recent increases in GMT change may simply reflect the steady march of human influence on global temperature. In some ways, this is good news! We know what we need to do—reduce emissions of greenhouse gas as fast as possible—and we know how to do it—roll out solar, wind, and other renewable energy sources across our economy and assist other countries with doing the same. It’s unfortunate that the second Trump regime has decided to light the climate on fire in the interest of cronyism and corporate profit, but our basic task hasn’t changed.
You can find all the code and data used for this analysis here: https://github.com/ccallahan45/Climate_Acceleration
