Reconstructing crime scenes is more or less what most geoscientists do for a living. Sometimes the “whodunnit” revolves around a mass extinction event 66 million years ago, and sometimes it’s about an extreme weather pattern just last week. But as with a homicide investigation, geologists also have to consider natural causes.

A new study led by the University of Oxford’s Karsten Haustein takes a look at the influence of natural causes on the temperatures of the last century. While natural variability inherent to the climate system was thought to play a role in some features of our temperature record, the new results suggest that the record is dominated by external forces—though some of those are natural, too.

Explaining wiggles

It’s well-established that human activities are the dominant cause of recent climate change. But looking at the instrumental temperature record, which goes back to the late 1800s, there are significant wiggles that look curious. Why, for example, did global temperatures drop for a time after World War II before resuming their upward ascent in the late 1970s?

This has generally been understood to be the result of both human emissions of sunlight-reflecting aerosols and natural oscillations of the oceans. By sloshing and shifting back and forth, ocean currents can move regional weather patterns around and influence the global average temperature. (Aerosol emissions have since reduced by pollution controls and aren't as strong an influence as they were.)

Climate scientists refer to factors that can change the total amount of energy in Earth’s climate system (like the Sun or greenhouse gases) as “external forcings,” while these sloshing ocean currents and weather patterns are referred to as “internal variability.” A good analogy is that a dog walker’s path is like an external forcing, while the tail-wagging wandering of the dog on the leash is like internal variability.

The new study looks at how much of the 20th century temperature wiggling was dog walker and how much was dog. Surprisingly, the researchers find that those wiggles can be almost completely explained by the dog walker.

Uneven influences

The study hinges on a couple of things that have been learned in the last few years. The first is that not all external forcings can be represented using a simple global average. Sunlight-reflecting aerosol pollution, for example, doesn’t mix evenly throughout the Earth’s atmosphere. It’s concentrated around the industrial areas emitting it—which means the cooling effect has been concentrated over land areas.

Anyone who has walked out of cool water and onto hot beach sand understands that the continents warm up and cool off more than the slow-changing oceans. As a result, aerosol pollution over land will have a bigger influence on the average global temperature than aerosol pollution over the ocean. Applying this knowledge to the past shows that the aerosol pollution after World War II had a stronger impact than previously recognized—the dog walker took a sharper turn than we thought.

The second key relates to global temperature data during World War II. The sudden deployment of vast naval forces brought with it a change in how ocean temperature data was being collected. Measurements were made at engine water intake pipes rather than in buckets hauled up onto the deck by hand, causing them to be slightly warmer than the old data.

A lot of work has been done to correct for differences like these, but there are signs that the effect hasn’t quite been fully removed from the mid-1940s yet. In this study, the researchers compared coastal and island temperature data to open ocean data, finding that the two wandered slightly apart during this time. Averaged over the globe, it’s a difference of just 0.08°C, but that turns out to matter to the crime scene investigation.

Following forcings

To account for all this, the researchers used a simple mathematical model for global temperatures based on the external forcings of solar activity, volcanic eruptions (which can cool the climate for a couple of years), aerosol pollution, and greenhouse gas emissions. The model includes both the immediate effect of these external forcings, as well as the slower response that comes as the ocean equilibrates with atmospheric temperatures.

Plugging in all the external forcings produces temperature changes that look almost exactly like the real-world data. In fact, it leaves very little room for the internal variability of the oceans to have had a role (other than the small year-to-year variability of El Niño and La Niña conditions). Without that slight bit of extra warmth in the 1940s, the trend in the first half of the century matches the gradually increasing external forcings well. Additional factors are not required. And the stronger effect of aerosol pollution after the war is also sufficient to explain the dip in global temperature.

One interesting implication of their analysis is that it implies we have been overestimating the effect of internal variability in the oceans. In fact, we may have been looking at some changes caused by external forcings and incorrectly chalking them up as internal variability.

A number of studies have tried to assess the influence of oscillations in the Atlantic Ocean on global temperatures, for example. In this study, the researchers found that the Atlantic was largely in sync with external forcings—it's as if the dog walker dragged the dog over to sniff a fire hydrant. That’s useful information if you want to understand where the dog will end up next.

Overall, the study shows that our knowledge of the climate system can explain the major 20th century temperature changes without much contribution from natural ups and downs, contrary to uninformed claims that global warming is “just a cycle.” Doing so also provides yet more evidence that climate scientists have accurately estimated how much warming you get when you emit greenhouse gases—even if that’s a good news/bad news kind of thing.

Journal of Climate, 2019. DOI: 10.1175/JCLI-D-18-0555.1 (About DOIs).