1 Everything Off

One day in the near future, everything on Earth just turns off. All the lights, all the engines, all the factories power down. Every coal power plant and every gas heating system stops, and vehicles roll to a halt. All the agriculture, livestock and crops alike, also disappear. Anything on the planet that creates pollution of any type just ceases to exist.

Not really, of course. This is just a thought experiment, an imagined scenario that lets us isolate critical aspects of a system and consider how they might change under extreme conditions. An empty planet is a great setting for science fiction, and for scientific analysis too, provided it’s not taken too seriously. There’s a lot of prior environmental thinking that expresses a desire to eliminate certain kinds of humans from their pristine visions of nature. These attitudes are still around today, and need to be called out. Don’t be an eco-fascist.

Whether or not the near future holds anything close to this complete of a cessation of fossil fuel usage (as we’ll see is necessary to stop global temperature rise), or as rapid of a change (which would certainly be catastrophic for much of humanity), imagining Earth without industrial activity is a useful extreme case for determining how climate will evolve in the future. It’s also a good introduction to what we’ll learn in the next few chapters, and why it’s necessary to know.

Days to Years

Within a few days, air pollution all across the world falls from the sky. The most polluted industrial centers shed their gray haze and skies revert back to deep blue. When combustion of fossil fuels and biomass stops, the air quality benefits are immediate. Health outcomes improve right away too. We’re breathing much more easily in this post-industrial world. In the public health context, this hazy pollution is called particulate matter, or PM, while from a climate or atmospheric chemistry perspective, they’re called aerosols.

The darker side of aerosol cleanup is that more sunlight now reaches the ground. Because of all the additional absorbed sunshine, over the following months and years, temperatures rise even more. It might be surprising that the immediate effect of industrial activity on global climate is more warming. But the aerosol haze was reflecting a lot of sunlight back to space. We’ll later discuss the concept of radiative forcing, which quantifies the effect of different factors on the Earth’s energy budget.

It’s important to note that aerosol effects on climate have significant scientific uncertainty. It’s difficult to know how much sunlight is being reflected compared with the 1700s. But the warming that’s resulted has now pushed up global temperatures by about 0.3^o C, hitting the critical threshold of 1.5^o C with 33% probability in this ensemble. The extra warming is associated with some very familiar changes in global climate.

Historical global averaged temperature change (from NASA GISS) and in the future assuming zero emissions (using FaIR). 

With the rising global temperatures, heat waves are more frequent, with record highs being set across the globe. Extreme downpours increase, as does persistent drought: the two opposite side effects of an amped-up hydrologic cycle. Tropical cyclones are intensifying ever more rapidly. Sea ice and snow cover area decrease, with each of these melts causing more sunlight to be absorbed in high latitudes.

Decades

But then global temperatures start to slowly trend back downward. The culprit here is methane, a powerful greenhouse gas that is also chemically reactive. Methane has a timescale of only around a decade in the atmosphere. So by 2030, concentrations are lower than they were in 1960. Its heat-trapping effect, measured by radiative forcing, is reduced too.

Concentrations and radiative forcing of carbon dioxide and methane in the historical period (data from NOAA and radiative forcing from Etminan et al 2016 expression), and in the future assuming zero emissions using a model similar to FaIR. 

By 2050, methane concentrations are close to what they were before the Industrial Era began. And temperatures are finally drop back down to what they were when everything powered down. Since the methane-induced warming in 2021 was roughly similar to the aerosol-induced cooling, the temperatures drop back down to a similar level as today.

2070 and Onward

Carbon dioxide has always been the biggest beast in the climate crisis. Fossil fuels and deforestation caused the largest emissions of any pollutant by far, and it has always had the biggest heat-trapping effect. Now in the 2070s, another aspect of carbon dioxide is felt: its extreme persistence.

We’ll show later that CO₂ has a number of timescales, each associated with different reservoirs of carbon. A decent fraction of the emitted carbon dioxide stays in the air for centuries to millennia. Thus in our thought experiment, CO₂ never decreases back down below its 2002 concentration. Its heat trapping effect remains strong as well.

Global temperatures end up stabilizing at just below 2020 levels, around 1.1^o C over pre-Industrial. It wouldn’t have been a surprise if this eventual temperature was a bit colder or a bit warmer. A lot has to do with how clouds respond to persistent warmth. Some sea ice has grown back since its minimum in the 2030s; it responds quickly and reversibly. Hydrologic cycle extremes are also less severe than in the 2030s.

Some climate variables, though, are changing just as rapidly as ever. Sea levels are continuing to rise. They’re about 60 cm above 1900 levels in 2100, and a full 1.5 meters above 1900 levels in 2300.

The “everything off” thought experiment has hopefully shown us that we have a lot to study in terms of future climate, even under rapid emissions reductions. We’ll begin by getting into detail about “radiative forcing” — how pollutants and natural factors can be put on the same scale in terms of how much heating they cause. We’ll then cover topics that are critical for both global temperature and regional impacts, like water vapor. We’ll explain both the fundamental physics of climate and the expected changes with warming.

Climate science is a scary topic to study because it’s not just a thought experiment. This is really happening. It’s hard to reckon with all the changes that are coming in future decades. But we’re here at an incredibly unique time in history where actions matter more than ever, both in terms of stopping fossil fuel burning and preparing for impacts. I hope that along the way you’ll be able to find joy in studying the beauty and complexity of Earth’s climate. May Fundamentals of Climate Change be “Fun” and be a good “Mental” workout.