We have all heard about greenhouse gases (GHG’s) and how they are the culprit behind a warming Earth. But what exactly are they, how did they get here, and why are they so important to understand? To begin, greenhouse gases are those gases within our atmosphere which trap heat (also known as infrared radiation) within the Earth’s atmosphere and reduce the rate at which that heat is released back into space. In short, GHG’s act like an insulator in the Earth’s atmosphere. There are 5 major GHG’s, listed in decreasing relative abundance, which include: water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases. The effect that each gas has on climate change is dependent on three factors: atmospheric concentration, residence time, and global warming potential. Water vapor, however, is naturally occurring and mostly independent of human activity; we will touch on this concept later in this article.
Most of the greenhouse gases found in our atmosphere today are naturally occurring, with the exception of fluorinated gases. Fluorinated gases are synthetic gases made for industry and they include chlorofluorocarbons (CFC’s), hydrofluorocarbons (HFC’s), perfluorocarbon, sulfur hexafluoride, and nitrogen trifluoride. You may be familiar with chlorofluorocarbons (CFC’s) and their use in refrigeration (also known as freon). CFC’s were phased-out by the international community in 1987 after passage of the Montreal Protocol. Nitrous oxide is both naturally occurring in the atmosphere, and a byproduct of industries like agriculture and transportation.
While carbon dioxide and methane are naturally occurring greenhouse gases, they were typically released at a much slower rate than today. Historically, (i.e. pre-industrial revolution) the emission of CO2 and CH4 would have only been released through natural processes such as forest fires, weathering of rock/sediment, volcanic activity, geologic uplift and exposure, and of course through biologic respiration. At pre-industrialized rates of emission, carbon dioxide and methane were being absorbed back into the carbon cycle at approximately the same rate that they were released. This created an atmospheric steady state, or equilibrium, which allowed climate to be driven by Milankovitch cycles (click to learn more). To summarize, it is not the simple abundance of carbon dioxide within our Earth system that drives climate change today, it’s the fact that the rate of emission is greater than the rate of sequestration. If carbon sequestration could occur at the same rate of today’s global emissions, then excessive carbon emissions would have very little affect on global climate.
“Water vapor does not control the Earth’s temperature, it is itself controlled by temperature.”
Water vapor, by far, makes up the majority of greenhouse gases and has been cited to account for as much as 60% of the warming within the atmosphere. Hold your brakes! Before you take that previous sentence out of context and claim that an unbalanced carbon budget is not responsible for climate change, let me explain why water vapor is not responsible. Water vapor does not control the Earth’s temperature, it is itself controlled by temperature. This brings us back to something called the dew point, or the temperature (and pressure) at which the atmosphere is fully saturated. If you have ever watched a rain storm begin, you have witnessed water vapor condense into liquid water, as the surrounding air temperature dropped to meet the dew point. From this we can conclude that the temperature of a surrounding air mass controls the maximum amount of water vapor that can be held by the atmosphere. Without the addition of the other non-condensable greenhouse gases, the amount of available water vapor would stay relatively constant. Scientists consider water vapor to be part of a positive feedback loop, such that: as global temperatures increase more water is evaporated and this additional water vapor coupled with other GHG’s, increases global temperatures…which leads to the evaporation of even more water, creating a cycle that feeds off of itself.
Atmospheric concentration is pretty simple, it is the abundance or volume of gas which is present within Earth’s atmosphere. The relative abundance of each gas changes in it’s own way. For example, the increase in carbon dioxide comes from sources like burning fossil fuels, cement production, deforestation, and even chemical weathering and other geologic processes. GHG’s are measured in parts per million (ppm), parts per billion (ppb), and sometimes parts per trillion (ppt). This is because the presence of GHG’s in the atmosphere is still relatively small when compared to the entire composition of Earth’s atmosphere. (figure 1.) If the abundance of these GHG’s are so low, then how can they possibly have an effect on climate? To answer this we must consider the following variables: residence time, and global warming potential. Think of it like this: a very small amount of hydrogen cynide can be deadly to most living organisms, however, it is not simply the quantity present, but the physical/chemical properties of the substance that make it dangerous.
Figure 1: The global atmospheric concentration of gases on the left, and the composition of trace gases found in the atmosphere on the right. While trace gases make up less than 1% of our atmosphere’s composition, it is dominated by carbon dioxide, a very powerful greenhouse gas. *Ozone makes up such a small percentage that it is not able to be illustrated on this chart. (adapted from climate.nscu.edu, 2013)
Residence time is the length of time a substance remains in suspension within a system. Some GHG’s only remain in the atmosphere for a few years, while others can stay for thousands of years. However, all GHG’s remain long enough to become well mixed within the atmosphere, meaning measured amounts are roughly equal around the world. To quantify residence time, you can take the total atmospheric sink of a constituent and divide it by the sum of all sources of input or uptake for that constituent. However, calculating the residence time for carbon is much more difficult than dividing ~720 gigatons (sum of all sources) of carbon by ~230 gigatons/year (input/output) . This method would result in a calculated residence time of only about 3 years. So when we refer to residence time, what we really want to understand is how long will the total volume of greenhouse gases remain in the atmosphere in a substantial amount to have an effect on climate? We can set a benchmark of pre-industrialized levels of carbon at ~300 ppm, and then calculate how long it would take to return to pre-industrialized levels of atmospheric carbon, and what we would have to do to meet that criteria. The reason we want to answer this question is because we know that pre-industrialized levels of atmospheric carbon allowed the Earth to maintain a climate system driven predominantly by astronomical forcing (milankovitch cycles), or the natural variability the Earth normally experiences.
Global Warming Potential
Not all GHG’s have the same characteristics, and some are much more effective at absorbing infrared radiation than others. Global warming potential (GWP) is a metric that uses a gases radiative efficiency, and residence time to measure how much energy is absorbed by 1 metric ton of a gas over a given period of time (usually 100 years), relative to 1 ton of CO2. By setting carbon dioxide as the base, we can then compare other greenhouse gases to CO2 to determine which gases have a greater effect on warming. Carbon dioxide has a GWP of 1, no matter what time scale is used, because it is the standard. Methane, however, has a GWP of about 28-36 over 100 years, yet has a much shorter residence time than CO2. Nitrous oxide has a GWP of approximately 265-298, and fluorinated gases are so good at trapping heat that their GWP is usually in the thousands! Ultimately GWP becomes useful in writing policy to help mitigate GHG emissions, and potentially offset the effects of climate change.
Mitigating Greenhouse Gases
Recent international deals, like the Paris Climate Agreement, have been designed with the goal of reducing global greenhouse gas emissions in hopes of curbing the detrimental effects of climate change. While politicians tend to see this as a set back in economic prosperity by limiting the burning of fossil fuels, reducing industrial emissions, and cutting out coal-powered plants, scientists maintain that even such an agreement might not be enough to buffer an already changing climate. With progressive plans to cut emissions by 2050, there still remains a lag time between when emissions are cut, and when the reductions will actually make a difference on the global stage. As climate science continues to pave a path for understanding and reversing the already damaging effects of climate change, it becomes more and more important to be as proactive as possible. Although, you don’t have to wait for policy makers to write into law the changes that we need because we all know how long that can take. Mitigating climate change starts at the very bottom, with you and I making small contributions that come together to make a big difference. Whether you use low energy light bulbs throughout the house, recycle/re-use, or even install solar panels which are becoming more accessible and less expensive every day, you are helping to reduce the global carbon footprint, and create a cleaner more sustainable future for generations to come.
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