The climate has changed in the past, long before humans, so how do scientists know our recent warming is caused by man?
MAINE, USA — The earth is warming. At this point, very few people, even in the depths of the internet, deny this fact. But what I still hear over and over is this: “How do we know it’s us?” “How do we know this is not just a natural cycle?”
I get it. After all, at one point Manhattan was under a mile of ice. Climate change has happened before.
But scientists have been sure since about the 1960s that THIS change in the climate is caused by humans.
How do we know?
Let’s start with the most obvious possible driver for a warming of the earth: the sun. There are a few reasons we know the sun is not responsible for our recent warming. First, the measure of the incoming sun energy to the earth, solar irradiance, has remained flat or actually DECREASED as our temperatures have rapidly increased. This is especially evident since the 1970s.
In addition to the solar irradiance trend, we know the sun is not responsible for our current warming trend because the air high up in the atmosphere, known as the stratosphere, has been COOLING since the late 1970s.
That stratospheric cooling has been taking place while the troposphere, where we live, has been warming rapidly.
Think about it like this. If it was the sun, how could the layer closer to the sun cool while the layer beneath it warmed rapidly? It’s not possible.
That’s how we’ve eliminated not only the sun, but any external influences in space.
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Carbon dioxide is a greenhouse gas. We’ve known that since the 1850s. And since the 1850s, we’ve known that greenhouse gases trap warmth in the lower part of atmosphere. This generally has a net positive effect for life on earth, as it keeps it inhabitable.
But too much of a good thing can have negative impacts. Using ice cores, which contain trapped air bubbles from hundreds of thousands of years ago, scientists have recreated the history of carbon dioxide in our atmosphere, and the graph is striking.
So, while there have certainly been natural variations in carbon dioxide in the past, nothing approaches the scale and speed of our recent run up to over 400 parts per million.
Still not convinced humans are responsible for the recent surge in carbon dioxide? What if I told you we caught fossil fuels red handed, with a fingerprint?
There are three naturally occurring Carbon isotopes on earth, C 12, C 13 and C 14, the last of which decays radioactively to nitrogen with a half life of about 6,000 years.
Well, because of that half life, carbon that is pulled out of fossil fuels that have been in the ground for so long prior to being burned that the C 14 isotopes is almost entirely gone. In addition, plants hold a lower percentage of C 13 compared to the overall atmosphere. So, by measuring the percentage of C 13 and 14 isotopes in the atmosphere’s carbon dioxide over time, and seeing them plummet, scientists can easily conclude that not only is CO2 increasing, but that CO2 is definitively coming from the fossil fuels.
This phenomenon even has a name: The Suess Effect.
CO2 is a greenhouse gas, greenhouse gases warm our atmosphere, and fossil fuels have added CO2 to the atmosphere at an unprecedented rate. It’s pretty much a “1 +1 = 2” equation.
And last but not least, the speed of warming.
Although the climate has changed in the past it has NEVER warmed or cooled at this speed.
Now you might say, how do we know what the temperature was so long ago? Scientists use a complex process of temperature proxies such as something we see a lot here in Maine: snow.
There are measurable chemical differences in snow formed at different temperatures, therefore ice cores can be used to provide a record of temperature going back around 250,000 years.
In addition, yearly banding is found in fossilized corals and lake sediment. Each band has a chemistry that reflects specific temperature ranges. Tree rings are also used, as they get wider or thinner based on temperature.
So scientists can say with a relatively high degree of confidence, the earth has never warmed this quickly before.
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