Who touched the thermostat?!

(5 min) How humans fiddled with Earth’s temperature

How do we know greenhouse gases work? Previously, we covered the basics of Earth’s climate & crossed out other sources (1 & 2) of warming.

Earth’s climate has always changed over a long history of billions of years from boiling cauldron to frigid icebox.

But, the climate of 400 millions years ago doesn’t matter for us young squishy mammals. Modern humans burst on the scene in just the last ~0.3 million years, so any rapid deviations from this environment spells trouble for us and our biosphere.

Suspiciously, in the last 0.00017 million years, atmospheric CO2 concentrations took a sharp turn riiight around when humans started setting dinosaur juice on fire. That matters, because CO2 is a greenhouse gas that regulates Earth’s temperature.

In lockstep, global average surface temperatures began to climb. While over a 100,000x longer time horizon, something like the sun could be responsible for warming. On a shorter horizon, global catastrophes like volcanic eruptions could cause a short disruption.

But those natural forces (and others) are not to blame. So how are we so sure that CO2 is the root cause of the warming?

How CO2 acts as Earth’s thermostat

We previously learned there are many greenhouse gases, with CO2 being the largest and most worrisome. CO2 is what we’ll focus on here, but the same mostly holds true for other GHGs as well.

Greenhouse gas theory says that Earth warms due to trapping heat (aka infrared aka IR) trying to escape to space leading to higher radiative forcing.

More specifically, they work by absorbing a portion of the IR radiating off the Earth, then returning some1 of the IR back towards Earth. The net effect is more heat energy being trapped near our surface.

If CO2 was reflecting the absorbed IR back to Earth, we should be able to detect that same wavelength coming down from the atmosphere. Sure enough, that’s exactly what we find. ‘The sky’ is emitting IR radiation at the same wavelengths where CO2 emits.

Similarly, as CO2 intercepts IR as it heads towards space, we observe gaps in the emission spectrum with satellites precisely where CO2 absorbs infrared! (1, 2) Suggesting the heat was blocked on the way out to space.

However, the amount of heat trapped by CO2 and greenhouse gases through direct radiative forcing only accounts for less than half of the warming we’ve observed. So where does the rest come from?

The Feedback Furnace

Turning up the thermostat doesn’t heat your house on its own; it’s the natural gas on fire in your furnace that does the trick. Similarly, CO2 triggers feedback loops that do most of the warming.

As CO2 warms the Earth, other things heat up too2. Higher surface temperatures evaporate water vapor, an even more potent greenhouse gas, into the atmosphere, 2-3xing the warming effect of CO2. Water vapor concentrations are usually kept in check by condensation (rain), but higher temperatures enable more water vapor to stay in the atmosphere before raining. 

More heat also triggers other feedbacks like melting sea ice and glaciers. As it melts, the ice reflects less heat away leading to even more warming. There is evidence (more) to show that CO2 is the main driver of these feedback mechanisms. In other words, CO2 is the spark and the feedback loops are the fire.

However, this isn’t the whole story. Warmer surface and ocean temperatures trigger third order effects, clouds and currents, that are notoriously hard to simulate and also affect climate. Meaning we don’t have a clean “CO2 + CO2 feedbacks = observed warming” formula. Scientists have been trying to pin this down into a simple metric called climate sensitivity, but there’s still a broad range.

So, we know that CO2 warming triggers even more warming (like water vapor and ice melting). However we don’t know if those feedbacks are 100% caused by CO2 or can 100% explain the rest of the warming. Scientists are highly confident that this is directionally correct, in absence of other evidence, but it’s currently not a complete science.

Why humans are guilty for CO2 levels

We can directly measure the increase in CO2, but how can we prove that it is coming from humans? Like any good CSI episode, we must look for the fingerprints...

Carbon molecules have different isotopes which is a fancy way of saying different weights. Plants, for instance, favor lighter carbon (carbon 12) for photosynthesis over the heavier variants (carbon 13, 14). Fossil fuels are made from compressed dead plants so they contain almost entirely carbon 12.

If CO2 was rising because of natural changes in the carbon cycle, we’d expect the ratio of C12 to C13 to stay roughly the same. But if it was from burning fossil fuels, more light carbon (from plants) would be in the atmosphere. We can measure this change in contribution as a negative trendline with lower and lower proportions of heavy carbon in the atmosphere which is exactly what scientists detect3. Therefore, we can conclude that the burning of fossil fuels is responsible for rising CO2 levels.

Congrats! Humans did it.. So what?

Before we get to that… tell me what you want to learn! I’ll select the most popular topics for future editions >>

Research Survey

We now know:

  1. Why the Earth is nice for humans, but getting worse

  2. Why outside forcing or natural causes aren’t to blame

  3. Why GHGs bear most of the burden & humans are responsible

So, the climate is clearly going awry and the human fingerprint is unmistakable. Why should we care? What does a changing climate mean for us? 

Next time, we’ll dig into the impact of the Climate Change ‘threat multiplier’.

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Until next time,


Want more?

The base material abstracted from this NASA Study and the latest IPCC Report. I recommend the exec summary if you are into long science papers like me. 

Chomping at the bit for more climate change content? Here are some of my favorites:

  • The Carbon Curve: Learning about the frontiers of carbon dioxide removal and carbon tech - and how to scale it up

  • Going Green: What problems need to be solved on the path to a carbon-free economy (digital and physical)?

  • 🤷🏽‍♂️ Not My Problem: Startups across the world that are working towards improving the health of the planet

  • Evergreen: The main climate challenges and opportunities in front of us. Breaking down the particular causes of emissions within our economy and what solutions will get us to net zero.

  • Climate Money: How climate and money (finance, currencies, market opportunities) intersect

  • Climatic Thoughts: Solutions towards keeping the world under 2 degrees of warming

  • Climate Pioneers: Exploring low- and high-tech climate solutions.

Extra disclaimer: Science is never done and we’re always learning more. What’s presented here is the current global understanding from an ensemble of climate scientists across leading government organizations pulling from a range of peer-reviewed research (i.e. NASA, IPCC). 

Usual disclaimers: I’m not an expert and will never claim to be. I’ll probably be lacking context, too vague, or flat-out wrong frequently & I hope folks will hold me accountable. After all, the fastest way to find the right answer is to post the wrong answer visibly on the Internet. Lastly, there is a ton of great information online already (e.g. Drawdown, Breakthrough Energy playbooks). I will synthesize and cite as I go.


Also bump into other molecules in the air, heating them up


Any source of radiative forcing (warming) will do this too


Anywhere from 6-17% of this is due to deforestation