The climate is sometimes compared to a huge ship, in that it takes some time to turn it in a new direction, meaning that actions to limit global warming produce very gradual results. While the lack of instant gratification is certainly frustrating, having some indications of progress could at least sustain patience with the energy transformation needed. The problem is that Earth’s climate system differs from that metaphorical huge ship in a key way—there is a significant amount of natural variability that can also mask a change in trend.

So before we see any change in climate trends from our present actions, we have to both wait for them to start and wait for them to become large enough to be detectable against a background of natural variability.

A new study led by Bjørn Hallvard Samset takes on the question of how long it will take to clearly see the effects of reducing emissions. “This paper is about managing our expectations,” the authors say in their new work. Failure of that management could mean that undertaking the work of climate mitigation would lose support if people are expecting instantaneous progress that doesn’t materialize.

This important reality check has been studied in climate model simulations before. The new study checks the usual scenarios with a slightly different methodology but also zeroes in on what it would look like to reduce individual gases (like methane) against a backdrop where other emissions aren’t reined in as quickly.

Variability

The variability of global temperature largely results from oscillations like back-and-forth El Niño and La Niña patterns in the Pacific Ocean. These patterns can even extend to decades with trends above or below the long-term behavior, as we saw in the 1990s and 2000s. There are also less predictable sources, like the occasional major volcanic eruption that cools the planet for several years. All this means that it’s difficult to evaluate a short-term temperature trend record and confidently state what it would have looked like with slightly higher greenhouse gas emissions.

To handle this, the researchers started with simulations from a major global climate model to get an estimate of the range of natural variability. They then used a much simpler model that could quickly simulate the temperature trends in many different scenarios of emissions of greenhouse gases and aerosols between 2020 and 2100.

The goal in each scenario was to calculate the year when temperatures will become statistically distinguishable from a different emissions scenario used as a baseline for comparison. There are multiple ways one could do that, but in this case, the researchers used the simple t-test to indicate when the two groups of data points were clearly distinct.

First, the researchers compared broad scenarios of emissions—low, medium, and high scenarios that represent warming ranging from less than 2°C by 2100 to over 4°C at that time. They calculated that the temperature difference between even the low and high scenarios isn’t clear until the mid-2030s. But given that the world is currently on track to stay south of that high emissions scenario, the medium vs. low comparison is perhaps most relevant to our future. In that matchup, temperatures don’t clearly separate until around 2046. That’s a long time to wait before seeing that our actions are yielding benefits.

Name that gas

The researchers also ran scenarios where a single type of pollution was reduced while other emissions followed the medium scenario. These generally showed smaller impacts and therefore longer timelines for differences to appear. For CO₂, methane, nitrous oxide, sunlight-reflecting sulfur aerosols, and sunlight-absorbing soot, they simulate the effects of dropping emissions to zero, dropping them five percent each year, and dropping them to their pathway from the overall low emissions scenario.

Dropping CO₂ to zero would obviously be huge, and the researchers’ calculation shows that global temperature would clearly reflect that cut by 2033. The less extreme reductions still are measurable by the mid-2040s.

Things like methane and soot are a little more interesting. Targeted reductions have been suggested as ways to significantly limit near-term warming due to their potency. Immediately eliminating soot emissions (unlikely though that may be) would be measurable by 2028, while five-percent annual reductions would have a clear impact by 2048. Eliminating methane alone would have a measurable impact on temperatures by 2039, and five-percent annual reductions would show up by 2055.

But if you compare the amount of avoided warming in 2100, the long-term effect is small. Zeroing out methane reduces 2100 temperatures by a little less than 0.2°C, and a similar treatment of soot only lowers temperatures by a little less than 0.1°C. That’s because both had a limited long-term effect to start with—a sharp contrast with CO₂. Methane does produce more warming, pound for pound, but it breaks down in the atmosphere, turning into CO₂ and water vapor. And soot only lasts a few days in the atmosphere before washing out, limiting its influence.

So while targeting less abundant but potent sources of warming might yield quicker results with less effort, it obviously takes an “all of the above” strategy to produce large and lasting benefits.

Just as climate scientists undertake studies to see if warming trends or extreme weather events can be attributed to human-caused greenhouse gas emissions, there will (hopefully) soon be a need to analyze the effects of emissions cuts. And that will be necessary to show that the effort is paying off.

“Concurrent, multicomponent mitigation,” the researchers write, “[…] has the potential to be detectable around 2040. These are expectations that need to be clearly explained and communicated to policy makers, and to the public, if we wish to avoid a backlash against perceived ineffective mitigation policies.” In the meantime, they point out, we’ll have to keep our eye on other metrics, like emissions trends and concentrations of the greenhouse gases that continue to accumulate in the atmosphere.

Nature Communications, 2020. DOI: 10.1038/s41467-020-17001-1 (About DOIs).

https://arstechnica.com/?p=1690270