Can Humans (and CO2) Really Cause Global Warming?

There’s a lot of fuss about climate change these days, especially human-caused climate change. And sometimes I hear that scientists are “in agreement” that humans are causing climate change or global warming, particularly through carbon emissions. But for a long time no one ever bothered to explain to me what scientists knew that I didn’t, or how they could be so sure. This article is the result of asking what they knew until I found a satisfactory answer. If you’ve ever asked the same question, you may be in the right place.

Is Global Warming Even a Thing?

First of all, it’s important to establish that the world has been getting warmer. I found that was fairly undeniable across the board. Ice is melting at unprecedented rates in the Arctic and Antarctic zones; oceans are rising as a result. And newspapers report that each successive year is “the warmest year on record.” Essentially, the average global temperature has been increasing at a relatively high rate since at least 1900.

It’s not that the earth has never been warmer than it is now — average global temperatures were almost certainly higher during the time of the dinosaurs, when tropical forests covered much of the world. But many scientists are worried because this current warming is happening faster — much faster — than any previous warming or cooling period in the history of the world. The following graph shows good evidence of this fact, using data from ice cores, lake sediment, and thermometers:

See that red/blue spike at the end? That’s how average global temperatures have changed since 1870 or so. The graph is an estimate, like most historical data, but it corresponds well with other estimates of earth’s temperature in the past. Basically, the rate of temperature change in the past 150 years appears to exceed anything seen in at least 20,000 years of life on earth. But how can scientists explain this?

The Greenhouse Effect

We’ll get to the answer. I promise. But too often, the answer is simply “carbon emissions” or “the greenhouse effect.” And that doesn’t help anyone understand why global warming is occurring. So let’s start with the greenhouse effect: what is it?

You may have known this already, but the sun does not emit enough heat to keep water in liquid form on earth. Since liquid water is necessary for life as we know it, the earth compensates by a mechanism which “traps” heat from the sun (“solar radiation”). The following illustration introduces this process:

There are a couple of key things to understand here:

  • At the risk of oversimplifying, we can say there are two different types of heat: visible and invisible. That’s why infrared goggles can be useful at night — they can “see” the heat being radiated by humans or other things which our natural eyes can’t observe. The earth gives off small amounts of this “invisible heat,” much like a human being, only less; the sun gives off “visible heat,” like a bar of metal when you pull it out of a fire.
  • “Visible heat” (solar radiation) comes in short wavelengths which pass right through all molecules. “Invisible heat” (infrared radiation) comes in longer wavelengths which are absorbed by larger, more complex molecules in the atmosphere. These molecules are the “greenhouse gases.”
  • Greenhouse gases only hold infrared radiation for so long, as if they are playing “hot potato.” When they release that heat, some of it comes back down to earth and warms our planet.
  • Thus, the more greenhouse gas molecules there are in the atmosphere, the more heat is absorbed by the world’s air, water, and soil. (See for more details.)

For the most part, this is a beautiful process. Without it, the earth would be a spinning ball of ice and rock. But right now, we have too much of a good thing.

“It’s Getting Hot in Here…”

There are several types of greenhouse gases, but the major ones are carbon dioxide (CO2) and methane (CH4). In our atmosphere, these gases are far outnumbered by oxygen and nitrogen, so they are typically measured in parts per million (ppm) or parts per billion (ppb)— carbon dioxide, for example, is around 400 ppm currently.

How does this relate to earth’s rapidly increasing temperature? Well, for thousands of years — before the year 1800 C.E. — carbon dioxide in the air stayed below 300 ppm (see Then, during the 1800s, human beings began producing increasing amounts of carbon dioxide by burning fossil fuels such as coal and oil. The amount of carbon dioxide increased slowly at first, but as fossil fuels spread across the globe this greenhouse gas began to accumulate in the atmosphere in unprecedented quantities.

In addition, levels of methane in the air are increasing as well, from about 722 ppb in the 1800s to 1,800 ppb today (see IPCC report). Our best evidence says that humans are contributing to this trend as well. Primary human-based sources of methane include livestock farming, rice cultivation, and energy production.

What Are the Skeptics Saying?

There are sincere people who don’t believe humans are causing climate change. I can’t possibly address all their concerns here, and I imagine there are some which haven’t yet been addressed. For any who are interested in this back-and-forth discussion, you may be interested in the Skeptical Science website. But a few arguments stood out to me as deserving particular consideration: the recent slowdown in temperature change, solar variance, and historical rates of carbon dioxide.

1) The effects of carbon dioxide have become increasingly scrutinized. Climate change skeptics vigorously point out that, for the past 1–2 decades, the rise in average atmospheric temperatures has almost leveled out even while human beings pumped out increasing amounts of carbon dioxide each year.

However, this slowdown in air temperature change has been offset by a drastic increase in ocean temperatures (see The Guardian). Thus an unusual quantity of heat is still being trapped by greenhouse gases — it is just going into the oceans rather than the air, disrupting ecosystems there and changing weather patterns like El Niño. Additionally, scientists are concerned that if oceans cycle out of this absorption mode, the earth’s surface temperature will begin rising much more quickly.

2) Some scientists have suggested that “solar variance” is causing most global warming — the sun could just be going through a natural ebb and flow, giving off more energy in recent years and thereby heating the earth.

However, recent studies of the sun indicate that this is unlikely. In the last fifty-five years — while the earth’s temperature has gone up 1°F — average yearly solar radiation has actually decreased slightly (see The Guardian). Thus, solar radiance is unlikely the primary driver of global warming in the last century.

3) Our best models indicate that carbon dioxide levels, while high in comparison to the last few millenia, are actually relatively low compared to many periods in the earth’s billions-of-years history. Therefore, skeptics say, carbon dioxide can’t be the real cause of our current warming trend.

However, the earth was most likely hotter during those times, perhaps even 25°F hotter when dinosaurs roamed. Unsurprisingly, that meant that the oceans were more than a hundred feet higher due to melting ice caps and climate patterns were drastically different (read: tropics in Antarctica). In addition, there is increasing evidence that the sun was up to 30% cooler in the early years of the planet, based on the observed life cycles of stars like our sun (see Science). Since the sun now gives off more radiation, greenhouse gas levels must be lower to allow more of that heat to escape.

The Upshot

What does all this mean for us? The global climate is changing much faster than at any time in earth’s history, primarily due to global warming, and humans appear to be the main drivers of that process. How? The greenhouse effect. But now that phrase packs a punch — infrared radiation and carbon absorption explain how a few puny primates are altering such an enormous system.

Wherever you stand on the political spectrum, it’s time we dig for a deeper understanding of the science of climate change. Always giving proper attention to developing nations, to free markets, and to plant and animal life, we nevertheless cannot “go gentle into that good night.” If we fight to put reasonable limits on greenhouse gas emissions, thereby slowing down the rate of climate change, we may give this resilient earth and our resilient species time to adapt to the warming we are causing.

“I do not understand one thing in this world. Not one.” — Marilynne Robinson, ‘Gilead’

“I do not understand one thing in this world. Not one.” — Marilynne Robinson, ‘Gilead’