Why I’m concerned about climate change, from a materials scientist

Around 50% of the world’s population is voting in 2024. But despite 2023 being the hottest year on record, with 2024 on track to be even hotter–despite historic droughts in Brazil, lethal heat waves in India, and the hottest day ever recorded on July 22–can anyone point to a country where climate change is at the top of the campaign issues?

I say this not to point fingers, but rather to be realistic. Societies face many challenges: the economy, war, polarization. Why is climate change important?

I asked myself that question in 2020. I had been working on low-carbon energy since 2006 when I read Elizabeth Kolbert’s Field Notes from a Catastrophe (I recommend Kolbert’s many books). But covid upended my world, like everyone’s, and made me ask again: there are so many challenges, is climate change really the one worth my time? I decided to dig even more into the physics of climate change.?

What I learned is why I decided to double down on working to prevent climate change for my children’s generation.?

Disclaimer: although I have broad training in physics, chemistry, and materials science, I am not a climate scientist. But perhaps explaining the science from my perspective may help others connect the dots. I have also had a climate scientist friend graciously review this writing.

Let’s start with the greenhouse effect (a misleading name for it, but the name stuck). Outer space is very cold, at a temperature of -270 C. Although the interior of the Earth is hot, huge layers of rocks insulate that heat, and that heat has little effect on surface temperatures. The reason that Earth’s surface is warm enough for humans is the incoming energy from the sunlight that Earth absorbs; and on the flip side, the reason that Earth’s surface is not way too hot for humans is the outgoing energy that Earth radiates back to space (as infrared radiation). The balance between incoming sunlight and outgoing radiation is very important in order to keep Earth in the healthy temperature range for planetary life. Now we get to the greenhouse effect. If Earth did not have an atmosphere, physicists can easily calculate its equilibrium temperature based on these two effects: -18 C, a world of frozen oceans. The atmosphere, through greenhouse gases such as CO2 and water vapor, traps some of that outgoing infrared radiation, thereby shifting the balance and allowing Earth’s temperature to be livable for us.

The problem, which you, reader, might have heard before, is that by burning fossil fuels (and also doing things like cutting down forests and releasing methane), humans have added more and more greenhouse gases to the atmosphere, thereby throwing off the balance of energy and causing the Earth to warm up.

A change in the surface temperature of the Earth is not new, in fact fluctuation is the norm. There is plenty of evidence for the Earth going in and out of ice ages in the last million years. These ice ages are understood to have been initiated by orbital changes (so-called Milankovitch cycles), which affect the exact amount of sunlight that hits the Earth. (These are variations in: eccentricity (degree of ellipticity) on a 100k yr cycle; obliquity (tilt of the axis) on a 41k yr cycle; and precession, on 23k and 19k yr cycles.) We have good measurements of CO2 from ice cores going back 800k years. In the past 800k years, the CO2 concentration in the atmosphere has varied between ~180-300 ppm. Interestingly, the variation of CO2 correlates with the variation in temperatures, however, temperature starts to rise before CO2 does. It is understood that the orbital variations initiate the changes, and atmospheric CO2 is a feedback that causes increased warming during the cycles.

Human emissions of CO2 have flipped this, in a radical way. Since David Keeling began measuring atmospheric CO2 concentrations at Mauna Loa in 1960, we have directly measured CO2 concentrations in the atmosphere rise from ~315 ppm to ~425 ppm. This change is consistent with the CO2 we released from the burning of fossil fuels and land-use/land-cover changes (with some of it being captured by the ocean and plants). The rate of this increase, now >200 ppm/century, dwarfs any rate of change in at least the last million years, where the typical magnitude of change was ~10 ppm/century. This means that we are causing change in the climate at a >20x faster rate than any biology is used to, one reason we are creating an era of mass extinctions, what Tom Chi describes as the era of “big loss.”?

But what if one doesn’t care about plants or animals. Why should humans care? There are many reasons that a rapidly warming world is bad for human societies. I’ll name two. First, the heat itself. As the temperature rises, the risk of deadly heat waves increases, which we have already seen this summer in India. If the heat index (a combination of temperature and humidity) rises above 72 C, healthy individuals will experience hyperthermia, heat illness. A recent analysis showed this would affect 60% of the global population if global warming rises 10 C above preindustrial levels (Lu, Romps, 2023). Second, sea level rise. The average sea level has already risen 0.2 m since 1880. Because of the CO2 emissions to date and the time it takes for oceans to absorb heat, another ~0.5 m rise by the end of the century is pretty much baked in, even if drastic emissions reductions happen tomorrow. And if progress is slow to stop global warming, the sea level could rise 1 or 2 m by 2100. For comparison, Miami sits about 2 m above the current sea level, and the Mekong River Delta, home to 17 M people and the richest rice-producing region of the world, is mostly 1 m or less above the current sea level.

These concern me–a lot. Our activities today are destabilizing the world for our future generations, in ways we can already see.

But on a deeper level, my greatest concern arises not from what we know, but from what we don’t know. We have climate models that predict the range of global temperatures under different scenarios. According to these models, the likely estimate of global warming (“equilibrium climate sensitivity”) if CO2 concentrations reach ~560 ppm (the target that climate goals are aiming to get below) is 2.5-4.0 C. This will already cause dramatic changes everywhere. But these general circulation models cannot capture everything, because the Earth is so spatially vast with so many different processes, and important effects like cloud formation or permafrost melting are not well incorporated. If we go back in geologic history, the accuracy of our data gets lower as we go further and further back, but we can see times with dramatically different global temperatures and sea levels different by 100 m. The Paleocene-Eocene Thermal Maximum, around 55 M years ago, is a period that scares me as a comparison. It’s the high temperature point in the graph above and it is a geologic example of CO2 emissions leading to warming. In that period thousands of gigatonnes of carbon were released into the atmosphere (from Antarctic permafrost/peat, thermogenic methane, and/or something else? we're not sure), and temperatures rose 5-8 C (McInerney, Wing, 2011). Even so, the timespan of the CO2 release was ~20,000 years, meaning the rate of change was much slower than the rates we are experiencing now.

We are running a huge experiment on our planet, our only home.

This year the average temperature will be close to 1.5 C higher than pre-industrial levels. And humans will continue to burn fossil fuels to provide ~80% of their energy demand. But it need not be so. The sunlight hitting the Earth provides more energy in an hour than humans use in a year. We have come a long way in harnessing this energy. We can stop climate change. In future articles I will dive into the details of the efforts and challenges to fully switching our global energy sources to renewable energy.