A Rhetorical Ambiguity That Propagates Climate Misinformation

Conflating the direction of a trend with the direction of an impact misleads our understanding of the consequences of climate change

By Patrick Brown

If you hear that climate change is causing something to “increase” or “decrease,” what does that mean to you? The words increasing and decreasing are relative terms that have an unspoken baseline, so an important question is increasing or decreasing relative to what? Most people, most of the time, interpret these words to mean increasing or decreasing relative to the past.

This is the straightforward and correct interpretation of the phrase “increasing greenhouse gas concentrations from fossil fuel burning is increasing global temperatures.” We can measure that global temperatures are increasing relative to the past, and physics-based climate models tell us that that increase is being driven entirely (technically, more than entirely because of offsetting cooling by aerosols) by increasing greenhouse gas concentrations.

Source: IPCC

The above represents climate science's traditional “detection and attribution” framework. Here, the word detection refers to the idea that a change relative to the past is detected in the data. That is, some longer-term changes have risen above the noise of variability caused by other factors. After that change has been detected, we can move on to attributing the change to some underlying cause, like increasing greenhouse gas concentrations.

However, for many phenomena that we know are affected by increasing greenhouse gas concentrations (or “affected by climate change” in common parlance), we also know that the effect is small relative to natural variability (i.e., the signal-to-noise ratio is small). In such cases, like for meteorological drought at most particular locations, we don't expect any clear trend to be detectable from the amount of warming that has occurred historically, but it may still emerge in the future.

We can still estimate the influence of increasing greenhouse gas concentrations on some phenomena with a low signal-to-noise ratio using climate models to simulate the counterfactual scenario without increasing greenhouse gas concentrations. But now we are moving from the realm of first detecting and then attributing a trend to the notion that we can skip detection of the trend and go straight to attribution.

Here is an example of this applied to the recent southwestern North American “megadrought,” as quantified by how much moisture is in the soil in the summer:

captionSource: Park Williams from Williams et al., 2022...

As an aside, the LA Times covered the above difference between the black and green lines with this headline:

There is nothing technically wrong with the headline, but I’ll leave it to readers to assess whether the impression left by the headline is consistent with the impression left by the data.

Regardless, using model-generated hypothetical counterfactuals, we can make claims about whether increasing greenhouse concentrations are “increasing” or “decreasing” that thing, but notice how the meanings of the words “increasing” or “decreasing” in this context now apply to a change relative to a hypothetical situation without increasing greenhouse gas concentrations instead of necessarily referring to a change relative to the past. Technically, the words increasing or decreasing can always be said to be relative to a counterfactual without climate change, and if the signal-to-noise ratio is large enough, then those words can also refer to changes relative to the past (as is the case for global temperature in the first figure).

However, things get a little stranger when it comes to discussing climate change's impacts on societal outcomes. In these cases, the outcomes are often trending in the exact opposite direction that climate change is thought to be pushing them. In other words, things are trending in a good direction, and climate change is calculated to be making things worse than they would be otherwise but, importantly, not worse relative to the past. In this case, the notion of ‘detection and attribution’ is turned on its head because any detected trend would be in the opposite direction of the attribution.

In these cases, using the words “increasing” or “decreasing” can be very misleading if the reader doesn't understand exactly what is being claimed. In fact, I would go so far as to call it climate misinformation.

Take, for example, the statement, “Climate change is decreasing crop yields.” This could mean that we are seeing crop yields decrease over time, and this trend is being driven by climate change (left side), or it could mean that crop yields are increasing over time, but they would have been increasing faster if the climate wasn't changing (right side).

So, which is it? For crop yields, the situation is much more like the right side than the left side. However, the IPCC repeatedly states in its reports that climate change is “decreasing” crop yields, misleadingly conveying the message that the situation is more like the left side.

Climate change impacts on agriculture have been calculated to make up the largest negative societal impact of climate change as quantified in the social cost of carbon, while heat impacts on mortality represent the second largest. So, how does the IPCC report on mortality associated with heat? In its technical summary (B.5.3), the IPCC reports that,

“Increasing temperatures and heatwaves have increased mortality and morbidity (very high confidence).”

Most readers will interpret this to mean that heat deaths are increasing relative to the past (they will infer a direction of trend). But only those who read the technical chapters will understand that only a direction of impact is being reported because there you will read that heat deaths are decreasing over time:

“Heat-attributable mortality fractions have declined over time in most countries owing to general improvements in health care systems, increasing prevalence of residential air conditioning, and behavioural changes. These factors, which determine the susceptibility of the population to heat, have predominated over the influence of temperature change.”

This type of conflation is ubiquitous in climate change impact reporting. In addition to crop yields and heat mortality, I have previously drawn attention to this issue in the context of global wildfire, emissions, hunger, and climate-sensitive diseases like malaria.

It is especially pervasive in studies and reports on the economic impact of climate change. In that subfield, it is conventional to neglect to show the background economic growth completely and only show the calculated change relative to the background growth. This takes the misleading language and turns it into a misleading visual, leaving the visual impression that climate change is a large driver of a negative trend when it is actually a relatively small drag on a positive trend.

From Callahan and Mankin (2022).

This all gives a misleading impression of the magnitude of climate change's negative impact, which is a problem if the goal is accurate scientific communication. More importantly, however, it distorts how we assess the desirability of decarbonization because the framing leaves out the important fact that the background progress in said societal outcomes is often driven by fossil-fueled industrialization.

The importance of fossil fuels in driving the background progress in some of these outcomes is illustrated in the paper Anthropogenic climate change has slowed global agricultural productivity growth. To the authors' credit, they do not state that climate change is “decreasing” global agricultural productivity, and they show the calculated negative impact of climate change in the context of background progress:

Source: Ortiz-Bobea et al. (2021)

Also, to the authors’ credit, they attribute the background progress in agricultural productivity to fossil-fueled industrialization:

“Our estimates should not be interpreted as the effect of a world without fossil fuels on global agricultural production. Agriculture has benefited tremendously from agricultural research and carbon-intensive inputs that would not have been as available without fossil fuels. The counterfactual in our study only removes the effect that fossil fuels and other anthropogenic influences have on the climate system.”

But this brings up a more central question at the heart of so much research on the negative impact of climate change: Is imagining a world without climate change all that useful? It almost goes without saying that if we could keep the benefits of fossil-fueled industrialization and jettison the negative side effects of climate change, we would do so. But what makes decision-making so thorny is that for most climate-sensitive societal outcomes (e.g.,  Crop yields, calories available per person, death rates from malnutrition, the share of the population with access to safe drinking water, the rates of climate-influenced diseases like malaria and diarrheal disease, death rates from natural disasters, death rates from non-optimal temperatures (hot and cold), and the fraction of people in extreme poverty) the net effect of fossil-fueled industrialization and technological change has been good.

Thus, “without climate change” is not always the most relevant hypothetical counterfactual, and often “without fossil-fueled industrialization and technological progress” is more relevant. This framing gives a more honest and holistic picture of the state of the climate change problem, and it does not misleadingly paint the current systems as being less attractive than they actually are. When we assess the best course of action going forward, we must compare alternative systems and weigh the benefits of avoided climate change against the costs of transitioning to alternative energy and agricultural systems over time. This is the only way to be accurate and forthright on the tradeoffs we face.

In the meantime, the climate science and climate communication communities should strive to accurately inform the public and decision-makers on the state of climate change impacts. This means ending the use of the words “increasing” or “decreasing” when they do not refer to trends in time, and it means being forthright about the net effect of fossil-fueled industrialization on various societal outcomes.