“The good thing about science is that it’s true whether or not you believe in it.”
—Neil DeGrasse Tyson, physicist

The Greenhouse Effect and Sources of GHG Emissions

What is the ‘greenhouse effect’? Just as a greenhouse allows sunlight to enter but prevents heat from escaping, certain gases (greenhouse gases or GHG) in the Earth’s atmosphere work in the same way, a phenomenon called the ‘greenhouse effect’. These gases come from both natural cycles and human activity. In addition to the carbon dioxide (CO2) produced by burning fuel, humans are also responsible for some other emissions, including methane (CH4) from natural gas extraction, nitrous oxides (NOx) released by synthetic fertilizers, and refrigerants used in appliances and air conditioning systems.

Many of these GHG have a global warming potential much greater than carbon dioxide. According to the EPA, methane has over 25 times more global warming potential (GWP), nitrous oxide has a GWP of 300, and some refrigerants have GWP measured in the thousands.

If it were not for greenhouse gases trapping heat in the atmosphere, though, the average Earth temperature would be about -18°C (0°F). The greenhouse effect has kept Earth in a range that supports life—but due to human activity, more greenhouse gases are being produced, resulting in additional warming that’s changing the climate. (When an activity pushes warming beyond the natural cycles, it is called ‘forcing’.) The greenhouse effect is a case where more of a good thing is not desirable—increasing GHG causes a stronger greenhouse effect and produces global warming.

Where do Greenhouse Gas Emissions (GHG) come from? Carbon dioxide (CO2) moves through the environment in natural cycles of photosynthesis, respiration, and decomposition. Much of the carbon has been safely held by plants, oceans, and soil that serve as carbon sinks. Human activity is now generating more greenhouse gas emissions than the system can handle.

Big Ideas

  • Labeling something as ‘good’ or ‘bad’ can obscure the truth about complex systems
  • Natural systems can be thrown out of balance by human activity

Sources and links to additional resources.

Earth’s Carbon Cycle

Carbon is one of the most abundant elements on Earth, found in almost every living thing on this planet. Green plants take up carbon dioxide in photosynthesis, plants and animals release it through respiration as well as when they die and decompose, and it is also exchanged directly between oceans and the atmosphere.

In natural ecosystems, matter and energy flow throughout the system, and the outputs from one organism or process are inputs to another; there is no waste. The ‘waste’ CO2 from respiration and decomposition is the raw material for the next cycle of growth by green plants. These cycles were in equilibrium for centuries, with atmospheric levels of carbon dioxide ranging from 250 to 350 parts per million (ppm). A December 2013 report by James Hansen and 15 other scientists concluded that 350 ppm is a ‘safe level’ to avoid a climate crisis.

Human activity is having profound effects on the carbon cycle. Current practices are producing more CO2 and other GHG than the system can handle; our activity has thrown the system out of equilibrium. And global warming itself also affects the carbon cycle through feedback loops, in which warming releases additional carbon dioxide and methane into the atmosphere.

Three major sectors lead the way, despite the availability of viable alternatives:

Agriculture and the industrialized food system use far more energy and generate far more CO2 than the alternative, organic growing methods. Organic growing actually sequesters carbon into the soil, creating rich topsoil and reducing atmospheric CO2.

Deforestation is the result of the demand for wood and land for development, resource extraction (mining, drilling, etc.), or agriculture. In turn, this reduces the amount of carbon that forests can absorb and also releases carbon as the cut trees decompose. Instead, we need to practice sustainable forestry and eliminate clear-cutting and destruction by extractive industries such as mining.

Fossil Fuels produce CO2 + water vapor as the natural result of burning any carbon-based fuel. (Typically, plants also incorporate minerals from the ground, which is why burning wood also yields ash.) The alternative, obviously, is renewable energy such as solar, wind, micro-hydro, geothermal, and tidal.

Big Ideas

  • Life is a complex pattern of interdependent biological and chemical processes that must remain in balance to maintain appropriate conditions for life.
  • Human activity seems very small in relation to the whole planet, but our actions have pushed the level of greenhouse gases in the atmosphere past the safe level of 350 ppm.

Sources and links to additional resources.

“The effects of green house gases could have disastrous results for the Earth and the people who live on it, including me.”
—Student ‘Kiana’ (Comment to article in the New York Times, March 2014)

Carbon – Climate Connections

Early in the 20th century, scientists began to suspect that atmospheric CO2 concentrations were increasing due to fossil-fuel combustion, but the lack of data made research difficult. Charles David Keeling started recording daily CO2 levels in Pasadena, California, and found consistent levels of about 310 ppm, with daytime levels lower than at night, due to photosynthesis. Keeling repeated these measurements in many locations with almost identical results. In 1957, he proposed a system of infrared gas analyzers in Antarctica and at Mauna Loa, Hawaii, where they are run by the Scripps Institute of Oceanography. Only after these data became available did Keeling realize that CO2 levels had a seasonal cycle in addition to the daily cycle.


Using ice core samples from Antarctica, scientists have now been able to trace atmospheric CO2 concentrations over 800,000 years. The National Oceanographic and Atmospheric Administration (NOAA), tracks levels of major greenhouse gases and calculates effects on global warming.

According to the ‘Fifth Synthesis Report’ from the Intergovernmental Panel on Climate Change (IPCC), “Anthropogenic greenhouse gas emissions…, together with those of other anthropogenic drivers, have been detected throughout the climate system and are extremely likely to have been the dominant cause of the observed warming since the mid-20th century.” As noted in the preceding section on the carbon cycle, the principal anthropogenic factors causing global warming are agriculture, deforestation, and burning fossil fuels. Some of the known and already visible effects are melting of land and sea ice packs, rising sea levels, more evaporation from the ocean, changes in air flows, and more intense storm and weather patterns.

In the next section, we will look at fossil fuels, the most well-known source of GHG emissions.

Big Ideas

  • Human activity may seem small compared to all of Earth’s natural processes, but our actions have pushed GHG in the atmosphere far beyond levels scientists consider safe.

Sources and links to additional resources.

The Energy – Climate Connection

Our demand for energy is one of the most significant factors in anthropogenic GHG emissions because it releases carbon that’s been sequestered beneath the Earth’s surface for hundreds of millions of years. Taken together, these fossil fuels—coal, oil, and natural gas—contain gigatons of carbon, and burning these fuels for heating, electricity, and transportation puts all that carbon back into the carbon cycle.

In addition to the carbon contained in the fossil fuels, the processes of extraction, transmission, and distribution add to their GHG emissions. Even natural gas, which has less carbon than other fossil fuels, releases methane (CH4) into the atmosphere at every step of the production and distribution process: the drilling site, pipelines, and compressor stations. Methane is a more powerful GHG than carbon dioxide, so these fugitive emissions, added to the CO2 from combustion, make the total carbon footprint for natural gas comparable to burning coal.

Some say nuclear power is the answer. While it’s true that existing nuclear power plants do not generate significant greenhouse gas emissions (GHG) in operation, the construction of a new plant has an enormous climate impact that takes decades to offset. In addition, the mining, refining, and transport of nuclear fuel, including ‘spent’ fuel, do create GHG emissions. Comprehensive Life Cycle Analysis processes can help measure multiple impacts throughout the process. Alternative energy sources also have life-cycle impacts, of course, but preliminary analyses suggest their impacts are far less than those from nuclear power.

But how can we meet our energy needs without fossil fuels?

Our current energy system wastes about half the energy contained in the fuel, and much of our usage is also wasteful. Conservation and improvements in energy efficiency can greatly reduce total power demand (Rocky Mountain Institute refers to this as ‘negawatts’), but there are other energy sources that do not cause global warming.

Here are some proven and effective alternative sources of energy:

Solar energy bombards the Earth every day—many times more than the current energy demand for the entire planet! If we could harness more of this free energy, there would be no need for any other fuel. Solar energy is currently harvested by photovoltaic [PV] panels (which convert solar radiation into electrical current); estimates place the practical recovery potential at over 5,000 TW. Solar energy is also captured by solar thermal systems (where solar energy heats water or some other fluid so it can be used for heating); estimates suggest 2,500 TW recoverable. According to the World Economic Forum, solar is now the least expensive way to generate electricity—especially remarkable when you consider that fossil fuels are subsidized, but solar is not.

Wind is actually produced by solar energy that causes air movement. Estimates suggest that the effective global potential from wind is about 15 terawatts (TW), roughly equal to the current total energy demand throughout the world. (This assumes wind turbines produce, on average, only about 20% of their theoretical capacity.)

Tidal wave power have the potential to generate an estimated at 1.2 terawatts (TW).

While these sources of renewable energy do not all produce at the same level at all times, combinations of solar, wind, and tide generation offset some of these problems. We need to proceed with caution, however; if we are too eager to harvest all this renewable energy, we risk further disruption of natural patterns. (Our past experience should be a clear warning of how much harm can result from unintended consequences.)

Perhaps we asked the wrong question at the beginning of this section—instead of asking how we can meet our energy demand, maybe we need to ask how we can adjust our demand to align with available resources.

Big Ideas

  • Our ever-increasing demands for energy throw natural cycles out of equilibrium.
  • We can meet our energy needs without fossil fuels, using a combination of conservation, improve—ments in efficiency, and a palette of renewable sources.

Sources and links to additional resources.

“We should be using nature’s inexhaustible sources of energy — sun, wind, and tide…”—Thomas Edison, 1931

Impacts Are No Longer Theoretical—They Are Happening Now

Climate scientists developed computer models to estimate future impacts, but some of the major impacts are already happening. It’s true that some of the models have been wrong—but in almost every case, the actual change has been greater than predicted. Even with a global temperature increase (so far) of only about 1°C, we are seeing many changes! And the oceans serve as a huge heat sink that has camouflaged the extent to which Earth is heating.

The pace of change has reached the point where many people now refer to it as anthropogenic climate disruption (ACD). Some of the measured impacts are changing too quickly for a document like this to keep up. What we can do is recap the major impacts, and include links to sites that track these phenomena and make current information available.

Extreme Weather events are a direct result of warming. It accelerates evaporation and increases the air’s capacity to hold water vapor, and warmer temperatures at the poles change the jet stream and other major airflow patterns. We’re already seeing more severe heat waves, intense storms, flooding, prolonged droughts, and desertification.

Deforestation is not only a contributor to global warming, it also reflects the impact of drought, heatwaves, insect infestation, and wildfires. Recent peer-reviewed studies put over 60% of U.S. forests at risk.

Desertification occurs when fertile areas experience warmer temperatures and less precipitation; the combination can result in a loss of vegetation and that further reduces available moisture.

Loss of Biodiversity & Mass Extinction of Species are two levels of the same problem. Changes in temperature, vegetation, and water availability can force species to move or bring new predators into their habitat; many do not survive. The current extinction rate is estimated to be over 1,000 times higher than normal, meaning up to 30% of all species could be extinct by mid-century.

Ocean Acidification is caused by higher levels of CO2 in the atmosphere; when CO2 dissolves in the ocean, it reacts with water to produce carbonic acid. The lower pH prevents coral from forming the reefs they need to survive; it also prevents oysters and other sensitive marine organisms from forming shells or skeletons. (As these organisms fail, the entire marine food chain may be affected.)

Polar Ice Loss is becoming dramatic. At the beginning of January 2017, Arctic ice was at a record low, and major rifts had appeared in Antarctic ice sheets. The Antarctic situation is especially troubling because the sea ice holds back the land ice, and if that slides into the ocean, it will cause a major increase in sea level.

Sea-Level Rise results from two main factors: simple thermal expansion and melting of land-based ice sheets and glaciers. When water from glaciers and from the polar ice caps finds its way to the ocean, the sea level rises. If the Greenland and Antarctic ice sheets both melt, it could raise sea level by over 60 meters (close to 200′).

Spread of Disease can increase under conditions where some species have to expand or move their range northward, because this includes insects that may be vectors for serious and deadly diseases, and animals and people in the new area probably do not have any resistance.

global temp change [[NOAA].jpg

Big Ideas

  • Even small changes, such as a 1°C change in temperature can have profound impacts on living things.

Sources and links to additional resources.

International Efforts on Climate – IPCC, UNFCCC, & COP

Sources and links to additional resources.

Climate Justice — and Injustice

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