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# Are We the Reason?

In this chapter, I’ll give you the key numbers—just three!—that make it abundantly clear that the planet is in trouble; and then I hope to show you that the climate crisis has unquestionably been caused by human behavior.

### Give me the info!

It is easy to overload folks with statistics, which is one reason why the horror of climate change is often minimized or ignored. For those interested in more detail and additional data, my “climate facts” blog post may be worthwhile.

#1. Carbon Dioxide

The first number is the one that gets the most press, the concentration of $CO_2$, or carbon dioxide, in Earth's atmosphere. Having more of this greenhouse gas in the atmosphere is a very bad thing: the more of it there is, the hotter our planet gets. $CO_2$ is measured in parts per million, or ppm for short.

In March 2021, the concentration of carbon dioxide in Earth's atmosphere was 417 ppm. That is 48 percent higher than pre-industrial levels—and the highest it has been in at least 800,000 years (and possibly 3 million!).

#2. Global Average Temperature

So how much does this concentration actually matter? This question brings us to our second figure—the global average temperature. It is 1.27 degrees Celsius warmer than the 1850-1900 average. A one-and-a-quarter degree increase may not seem like much, but the impact of that change has been enormous—the 10 warmest years on record have all been from the 21st century. More importantly, that number is a world average. There are places where the rise has been much greater, including on the Mediterranean coasts, and in the central regions of the Americas, Asia, and Africa.

Because global temperature has crept up slowly, we are sort of like the proverbial frog in a boiling pot—by the time we sense anything is amiss, it's way too late.

#3. Sea-Level Rise

The third and last number is the measurement of sea-level rise. Sea levels have risen 24 centimeters since 1850, or almost 10 inches. And they are rising at the fastest rate in 6,000 years. This sea-level rise, as it continues, is going to wipe out coastal cities and island nations from Miami to Mauritius.

Understandably, these numbers can seem like mere words on a page. And a common reaction is…

### BUT is it really our fault?

Yes, it is.

Today, 97 percent of climate scientists agree that climate change is the product of human activity.

The study of the human influence on climate change is called attribution science, and as the field grows, its findings are getting more and more robust. While many have suspected for some time that humans are the cause of climate change, scientists are continuing to find more evidence to support this belief.

Carbon Dioxide Concentrations

The most compelling evidence? Carbon dioxide concentrations only started skyrocketing recently.

Ice core reconstruction indicates that pre-industrial levels of atmospheric carbon dioxide ranged between 275–285 ppm, with the highest historical carbon dioxide level peaking at around 300 ppm 325,000 years ago.

Then, right around the time that humans started burning fossil fuels to power the Industrial Revolution—circa 1850—worldwide concentrations of $CO_2$ started to go up dramatically. That's because the carbon released from burning fossil fuels combines with oxygen and forms carbon dioxide, contributing to the greenhouse effect that we talked about earlier. Since the Industrial Revolution, emissions have grown exponentially— more than half of all $CO_2$ emissions since 1751 were produced in the last 30 years.

Burning fossil fuels is not the only activity that can contribute to the greenhouse effect, though. Humans engage in many of the other activities that do, like deforestation and certain agricultural activities, including the industrial-level farming of livestock.

Carbon Isotopes

If that's not proof enough that humans are responsible for climate change, the specific forms of carbon (known as isotopes) that have been found in the atmosphere since the Industrial Revolution have increasingly been of the carbon-12 variety. What does that mean? Carbon-12 comes from organic matter, the main ingredient of the fossil fuels we are burning at ever-higher rates. In comparison, carbon-13, another carbon isotope produced by volcanic activity, has decreased as a percentage of the total atmospheric carbon in the same time period.

More Evidence

As the years pass and we accumulate data that spans decades, findings increasingly suggest that it is unlikely that the warming we're seeing is anomalous in either duration or degree. Scientists have begun to draw strong links between human actions and specific extreme weather events, like the heat waves that struck New Zealand in 2017 and Japan in 2018.

"It's getting colder where I am!"

Climate is different from weather. "Weather" is short-term while "climate" refers to changes in atmospheric conditions over relatively long periods. Although short-term fluctuations in weather might make it seem like one's region is colder, long-term climate trends clearly show global warming.

Additionally, climate change is responsible for the rising frequency and intensity of extreme weather events—for example, a warmer Arctic means a weaker jet stream, which allows cold air to move south and cause unusual weather events. So that blizzard that made winter seem particularly bad is very likely due to climate change.

Lastly, human activities today will have effects on the climate for decades or even millennia, as greenhouse gases persist in the atmosphere. So the effects we are seeing now are just a fraction of the scope and scale of effects we will see in the coming years, even if we were to stop emitting carbon completely today — which is all the more reason to help reduce the effects on future humans now.

"It's the sun/volcanoes/the Milankovitch cycles!"

It is true that there are natural factors that contribute to climate change, like fluctuations in solar energy, volcanic activity, and ocean circulation—but climate scientists have shown that these factors are not the ones causing the dramatic changes to the climate that we are seeing today.

For example, while the amount of energy Earth receives from the Sun can change the climate, measurements of what is known as solar irradiance have actually gone down in the last few decades. Similarly, volcanic activity of the previous two centuries has actually mitigated warming trends, not produced them.

The Milankovitch cycles—kudos if you already knew what these are; I sure didn't before I began my research—in which the changing shape of Earth's orbit and the tilt and wobble of Earth's axis produce a cyclical climate effect, have also been blamed for global warming. But the cyclical pattern is actually supposed to be steering Earth into an ice age. Unfortunately, as with volcanoes and the sun, the Milankovitch cycle is not a get-out-of-jail-free card for global warming. The greenhouse gas effect is likely to outweigh the cooling effect of all three of these natural climate influences.

"The scientific community is conspiring to scare us!"

In the 2009 Climategate scandal, emails stolen from researchers at the University of East Anglia were purported to show corruption in the peer-review process of several climate research papers, leading skeptics to allege that the human influence on climate change was wholly fabricated. However, reviews of the emails that fueled the scandal ultimately concluded that they did not, in fact, reflect corruption at the level of the university or of the scientific community.

If anything, many climate scientists are conservative in their public communications about the scope and timeline of climate change. That itself is a problem, but not the one alleged by climate change skeptics.

"Okay, maybe climate change is real— but we still have plenty of time to stop it!"'

Unfortunately, this could not be further from the truth. Climate scientists have predicted that going past 1.5 degrees of warming would be a dangerous turning point for the planet's natural systems, which we will talk about more in the next chapter. Again, we're already looking at 1.27 degrees of warming. To make matters worse, global warming is a positive feedback loop—the warmer it gets, the faster Earth will continue to warm. In reality, the delayed effects of human activity on the climate may mean the 1.5-degree goal is already unrealistic—but it's not too late to prevent the even more serious consequences of two-, three-, or even six-degree warming (and remember, US readers, this is in Celsius!).

Hopefully at this point what should be clear is that the climate is changing dramatically for the worse and that it is our doing. But so what?