Climate change
- brianaull
- Jan 11
- 4 min read
Updated: Jun 21
Climate change has been a subject of public debate, and the purpose of this blog is science education, not political advocacy. But this is exactly why I am posting on this topic. Whether or not human beings are changing the climate is a scientific question, not a political one. Then, if we are, our response to that situation involves political decision making. The case for human-caused climate change is based on a growing body of scientific research. A recent climate publication study reviewed 88,125 climate-related publications in peer-reviewed scientific journals. Over 99.9% of them agree that climate change is primarily caused by humans. This is up from the 97% reported in 2013. That's an overwhelming consensus. But what are the key pieces of evidence that non-scientists could understand? I'll try to answer that in a nutshell.
Greenhouse gases
My post on greenhouse warming discussed how greenhouse gases trap heat from the sun that the Earth would otherwise radiate back into space. Water vapor (H2O) is the most powerful greenhouse gas, but carbon dioxide (CO2) and methane (CH4) are important as well. Without this heat trapping we'd be living on a much colder planet, around 0 F average temperature rather than the 59 F we have. Some amount of greenhouse warming, then, is a good thing for us. But this leads to another important point: the greenhouse gases don't trap all the heat, just a certain fraction of it. In other words, we are somewhere in between no greenhouse warming and the severe warming that would occur if all the heat were trapped. If we modify the atmosphere, we move away from that happy medium.
Drilling into the past
Scientists have developed techniques to look far back in time and figure out how temperatures and CO2 concentrations have varied. One technique uses ice cores. By drilling down through ice layers in Antarctica and Greenland, they are drilling back in time. This is somewhat like studying the history of a tree by looking at the tree rings. They can analyze the water molecules and the bubbles containing trapped CO2.
The water molecules tell us about temperatures that prevailed when that water fell as snow. Water molecules can have different weights depending on isotopes, that is, how many extra neutrons the oxygen and hydrogen atoms have. Most water molecules are of the lightest kind; we'll call that "light water." Some fraction of the water molecules are heavier; we'll call that "heavier water." Antarctic snow comes from water that evaporated from the ocean and made the journey to the Antarctic without falling as rain. When the climate is cold, evaporation of the heavier water is suppressed, and its condensation into rain is enhanced. Both of these effects make it less likely that heavier water molecules will make it to Antarctica. So, the water that falls as snow in the Antarctic is relatively depleted of the heavier kind. Conversely, a higher concentration of heavier water in an ice layer is the signature of a warmer climate.
There has always been natural warming and cooling
When we drill back 450,000 years, we see natural cycles of warming and cooling. These are caused by well understood variations in the Earth's orbit around the sun, which in turn lead to variations in the amount of sunlight we get. We can see the roughly 100,000-year cycle in the picture below.
We also see that the temperature can change a lot in a geologically short period of time. Notice that the emergence from the last ice age had a warming of about 7 C over 7000 years, or about 1 C per 1000 years. Why so fast? The answer is feedback mechanisms. For example, the initial warming by the sun causes melting and retreat of snow and ice, exposing darker land surfaces that absorb the sunlight rather than reflecting it. This enhanced absorption then accelerates the warming.
Why then should we be worried about a mere 1 C or 2 C temperature rise since the Industrial Revolution? Look at the figure below and notice the pace of warming. Since 1970, the planet has been warming at a rate of roughly 1C per 50 years. This is 20 times faster than the geologically rapid natural warming that ended the last ice age.
Not all carbon dioxide is created equal
We also see natural cycles in CO2 concentration that track the temperature cycles. We come to the Industrial Revolution and the CO2 concentration shoots up in a geological instant to a level not rivaled at any time in the last 800,000 years.
How do we know that this CO2 spike is not from natural sources such as volcanic eruptions? For one thing, we can look at isotopes, just as we can for water. Most CO2 molecules are of the lightest variety, but there is also "heavier" CO2. The carbon in fossil fuels was originally incorporated into plants in photosynthesis reactions. These reactions preferentially use the lightest carbon while releasing the heavier carbon back into the atmosphere. The CO2 that has been released into the atmosphere during the past century has this fossil fuel fingerprint of being enriched in light carbon.
In a nutshell
There are good reasons to believe that human-caused climate change is real. Here are the two we've discussed: • Extraordinarily rapid changes in temperature and carbon dioxide concentration during the post-industrial age • The isotope composition of the recently released CO2 that is characteristic of fossil fuel burning
If you want to learn more, go to MIT's Climate Primer website.
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